VIRUSES & ANTIVIRAL THERAPY PART 2

Eric C. Ledbetter, DVM, DACVO Associate Professor, Comparative Ophthalmology Cornell University Hospital for Animals College of Veterinary Medicine VMC Box 34 Ithaca, NY 14850 [email protected]

Acknowledgements: The following notes are a modified and updated version of an original set graciously provided to me by David Maggs, BVSc (hons), DACVO.

INTRODUCTION Although there is no denying the clinical importance of feline herpesvirus-1 in veterinary medicine, numerous additional viral infections are associated with ocular disease. This discussion will focus on these “other” viral infections of animals and the ocular lesions they may induce.

FELINE CALICIVIRUS

Virus Features FCV is an unenveloped, single-stranded RNA virus in the family Caliciviridae. The family’s name originates from the unique morphologic appearance of the virions which have cup-shaped surface depressions on the capsid (“calix” = cup). FCV possesses moderate environmental resistance and persists for several days to weeks on dried surfaces. Numerous strains of FCV circulate that vary in both tissue tropism and virulence. In general, the virus is wide- spread in domestic cat populations (including both household cats and within shelters) and 10% of pet cats are carriers. Carriers have persistent, active infection in the tonsils and other oropharyngeal tissues with evasion of host immunity (unlike FHV-1, this is not a true latent infection).

Pathogenic Mechanisms Natural routes of infection include the conjunctival, nasal, and oral routes. FCV infection is most commonly a mild, self-limiting clinical disease associated with pyrexia, respiratory tract disease, stomatitis, oral ulceration, conjunctivitis, and +/- skin ulceration or lameness from acute synovitis.1 A variant of this mild, self-limiting infection is FCV-associated virulent systemic disease (FCV-VSD). FCV-VSD is typically observed with confined infection outbreaks and high mortality rates. During FCV-VSD, the virus infects atypical tissues and results in pneumonia; facial and paw edema; and necrosis of liver, spleen, and pancreas.

The classic ocular lesion associated with FCV is ulcerative conjunctivitis. In a study of 99 cats with upper respiratory tract disease and ocular involvement, FCV was detected in conjunctival samples of 30 cats and was the most common ocular pathogen detected.2 Monoinfection with FCV was detected in 11 cats and co-infection (with Chlamydia, Mycoplasma, or FHV-1) in 19 cats. All cats with FCV infection had oral ulceration and conjunctivitis. No monoinfected FCV cat had keratitis. Five of the 11 FCV monoinfected cats had conjunctival ulceration.2

Diagnosis of FCV FCV is readily detected by PCR or virus isolation assay in conjunctival samples of infected cats.

Antiviral Treatment of FCV Specific antiviral therapy is not typically administered to cats for FCV infection. Ribaviran is effective against FCV in vitro, but no effect on clinical disease or viral shedding was demonstrated in experimentally-infected cats.3,4 Ribaviran is also highly toxic to cats, inducing both thrombocytopenia and hepatotoxicity. Phosphorodiamidate morpholino oligomer (PMO) therapy was also studied in natural outbreaks of FCV-VSD.5 These synthetic antisense oligonucleotide analogs were designed to target regions of the FCV genome and block viral RNA translation. In this study,5 PMO therapy reduced viral shedding and clinical disease severity, but it is not currently commercially

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available for routine clinical use. Numerous studies have evaluated interferon therapy for FCV infection, including subcutaneous human interferon alpha-2b, subcutaneous feline interferon omega, topical human interferon alpha-2b, and topical feline interferon omega.6-9 These clinical trials reported inconsistent effects (or a lack thereof) on clinical disease, viral loads, and viral shedding. Many of these clinical trials also lacked appropriate control groups which limited the investigators’ ability to draw definitive conclusions from the study data.6-9

FELINE INFECTIOUS PERITONITIS

Virus Features FIP is taxonomically positioned in the family Coronaviridae. This large enveloped RNA virus possesses the largest of all known viral RNA genomes. For recent comprehensive reviews see the paired articles by Pedersen10,11 and the article by Tekes and Thiel.12 Pedersen succinctly summarizes the frustrations regarding this virus: “After a half century, FIP remains one of the last important infections of cats for which we have no single diagnostic test, no vaccine and no definitive explanations for how virus and host interact to cause disease. How can a ubiquitous and largely non-pathogenic enteric coronavirus transform into a highly lethal pathogen? What are the interactions between host and virus that determine both disease form (wet or dry) and outcome (death or resistance)? Why is it so difficult, and perhaps impossible, to develop a vaccine for FIP? What role do genetics play in disease susceptibility?”

The internal mutation theory, whereby a ubiquitous feline enteric coronavirus (FECV) mutates into a FIPV, is strongly supported by the literature, and at least three specific mutations have now been associated with the FECV- to-FIPV biotype conversion. The working hypothesis is that the FECV-to-FIPV transition involves positive selection for mutants that are increasingly fit for replication in macrophages and unfit for replication in enterocytes. The ultimate target cell is a distinct population of precursor monocytes/macrophages that have a specific affinity for the endothelium of venules in the serosa, omentum, pleura, meninges, and uveal tract. Variability in genetic susceptibility has been identified in at least one breed of cat, but the genetics appear to be highly complex and cannot explain the entire disease incidence. Rather, it seems likely that FIP results from a confluence of numerous viral, host, and environmental cofactors. Tests for feline genetic resistance factors against FIP (e.g., polymorphism in the feline interferon-γ gene, tumor necrosis factor- α gene, and fCD209 genes) are now being offered, but the accuracy of these tests and how they will be used in clinical practice remains to be determined.13,14

Pathogenic Mechanisms Infection with the FeCV results in mild, self-limiting, and presumably often unnoticed gastrointestinal signs. In a small percentage of cats mutation to the FIPV occurs. Although FIP can affect cats of any age, it is diagnosed most commonly in cats less than 3 years of age and especially from 4-16 months of age.15 Diseases is seen most commonly in cats living within multicat settings. Mortality is extremely high although some cats can live with the disease for weeks to months. In cats with relatively mild presenting signs of the dry form of FIP, the 1-year survival rate was 5%.10 The basic pathology involves vasculitis and associated pyogranulomatous perivascular inflammation, both of which are thought to be due to an Arthus-type reaction (virus-antibody-complement).16,17 The immunopathogenesis of FIP is extremely complex and still not well understood. In a recent review, Pedersen provides an articulate summary: “It is widely assumed that immunity, when it occurs, is largely cell-mediated and that the production of antibodies is counterproductive. Antibodies enhance the uptake and replication of FIPVs in macrophages and also contribute to a type III hypersensitivity (antibody-mediated or Arthus-type) vasculitis.15 It is also assumed that much of the pathology occurring in FIP is associated with how macrophages respond to viral infection and how the immune system of the host responds to the infected cells. In this scenario, the effusive form of FIP results from a failure to mount T cell immunity in the face of a vigorous B cell response. At the opposite extreme, cats that resist disease presumably mount a vigorous cell-mediated immune response that is able to overcome any negative effects of antibodies. Cats with the dry form of FIP represent an intermediate state involving a cellular response that is partially effective in containing the virus to a relatively small number of macrophages in a few focal sites within specific target organs. The two forms of FIP are somewhat interchangeable; when it has been observed in experimental infection, the dry form always follows a brief bout of effusive disease. In the terminal stages of naturally occurring dry FIP, immunity can completely collapse and the disease reverts to a more effusive form. Although this scenario fits what is known about FIP, it must be emphasized that much of this scheme awaits confirmation and there are large gaps to be filled.” Ocular lesions occur more commonly with the dry form of FIP, result from the same mechanisms operative systemically, and are estimated to occur in >25% of infected cats. The

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principal target tissue is the uveal tract, particularly the choroid; the optic nerve may also be involved. Characteristic clinical signs are aqueous flare, anterior chamber fibirin, keratic precipitates, iris swelling and color changes, hyphema, chorioretinitis, optic neuritis, retinal hemorrhages, retinal perivascular exudates, and retinal detachment.18-20

A recent immunohistochemical study evaluated pathologic features and the distribution of viral antigens within eyes of 15 cats with natural FIP.21 Macrophages were present in FIP-associated ocular lesions, but they were the most numerous inflammatory cells only within granulomas (2/15 cats, 13%). In cases of severe inflammation of the ciliary body with damage to blood vessel walls and ciliary epithelium (3/15, 20%), some macrophages expressed viral antigens, and immunolabeling for calprotectin on consecutive sections suggested that these virus-positive macrophages were likely to be recently derived from the blood. In cases of severe and massive inflammation involving most ocular structures (4/15, 26%), B cells and plasma cells predominated over T cells and macrophages. The authors concluded that these results indicate that gliosis can be present in FIP-affected retinas and suggest that breakdown of the blood–ocular barrier can allow virus-bearing macrophages to access the intraocular compartments.21

Diagnosis of FIP Due to cross reactivity between the enteric coronavirus, serology is not useful as a sole test for diagnosing FIP infection. Rather, histopathological and immunohistochemical examination of affected tissues has long been considered the only reliable form of diagnosis. A large retrospective study appraised the commonly used diagnostic tests for FIP and calculated their positive (PPV) and negative (NPV) predictive values with respect to histopathologically confirmed cases.22 Individual serologic and hematological tests all had low diagnostic value. However, in cats with clinical signs suggestive of FIP, the presence of lymphopenia, hyperglobulinemia, and an antibody titer of 160 or greater had a PPV of 89%, whereas the absence of all 3 of these findings had a NPV of 99%. Nested RT-PCR directed toward a unique gene coding for peplomer protein E2 of the FIP virus produced sensitivity and specificity of 91.6% and 94% respectively.23 However, no diagnostic assay at this stage reliably differentiates FeCV from FIP virus. This, along with knowledge that FeCV (and FIP) can be detected in circulating blood cells, means that results of so-called “FIP” PCR conducted on aqueous humor samples collected from cats with uveitis must be interpreted with caution.

Antiviral Treatment of FIP Antiviral therapy for FIP is not yet clinically available, but recent advances provide some optimism for future treatments. Chloroquine, an antimalarial drug with in vitro activity against FIPV, was evaluated in cats with experimentally-induced FIP.24 Clinic disease was reduced in chloroquine-treated cats, but the drug was found to be hepatotoxic to the cats. More recently, a 3C-like protease inhibitor called GC376 that blocks coronavirus replication in vitro was reported. GC376 was highly effective against FIPV in vitro and it was demonstrated to be safe to administer to cats in vivo (although CNS penetration was found to be poor).25 The drug remarkably reversed experimental FIP infection in cats after 14 – 20 days of treatment (with subcutaneous BID dosing).25 In a clinical trial of 20 cats with FIP and no CNS disease, 19/20 cats improved with treatment, but 13/20 cats had relapses that were refractory to treatment.26 The investigators concluded that GC376 showed promise in treating cats with certain clinical presentations of FIP.

FELINE IMMUNODEFICIENCY VIRUS

Virus Features The taxonomic classification of FIV is the family - Retroviridae, Subfamily - Lentivirinae, Group - Lentiviruses. FIV consists of a 100-nm diameter, enveloped virion. The genome consists of single-stranded RNA. FIV is highly unstable in the environment and is inactivated within hours.

Pathogenic Mechanisms Transmission may be vertical (in utero and via nursing) or horizontal (via saliva/blood transfer, occurs with fight wounds most commonly). The worldwide prevalence of FIV infection is currently approximately 2.5-5%. Basic disease stages are: 1) Acute infection (days to weeks): lethargy, pyrexia, lymphadenopathy; 2) Subclinical infection (years); and 3) Terminal infection (feline AIDS): clinical disease from opportunistic infection, neoplasia, myelosuppression, and neurologic disease.

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The frequency of ocular lesions in cats infected with FIV is unknown. In one study, 13/15 cats with terminal naturally-acquired FIV had histologic findings of anterior uveitis.27 In these cats, diffuse lymphocyte and plasma cell infiltration of the anterior uvea was observed with no retinal lesions or opportunistic ocular infections identified. In another study of cats with experimental FIV infection that were monitored for 1 year, 3/20 cats developed conjunctivitis and anterior uveitis over the study period.28 Lymphocyte aggregates were histologically present in the conjunctiva, sclera, and choroid of the cats. Studies evaluating the frequency of FIV infection in cats with uveitis suggest that the relative importance of FIV as an etiologic agent of uveitis in cats has slowly and progressively declined over the past several decades. Publications from the early 1990’s reported 21-23% of cats with uveitis were serologically positive for FIV while more recent studies from the period of 2010-2016 reported only 2.1-7.3% of cats with uveitis were infected with FIV.29-34

FIV Retinopathy. FIV can cause geographic areas of thinning of the sensory retina, characterized ophthalmoscopically by hyper-reflectivity. Flame hemorrhages may also be present. Histologic findings are degeneration of all retinal layers with minimal inflammation. The lesion has been seen in naturally and experimentally infected cats, suggesting it is due to FIV and not opportunistic infections. Lesions occur during periods of higher viremia. These lesions have developed during the acute phase of infection (first 6 months) in experimentally infected cats, and during the AIDS-like stages in natural infection. The pathogenesis of this lesion is potentially the same as the microvascular retinopathy that occurs in HIV infection, and the neurological degeneration that occurs in FIV infected cats and HIV infected humans. Two hypotheses have been proposed:

1. Neurotoxin; both FIV and HIV are capable of enhancing excitatory amino acid toxicity in cultured fetal neurons. This toxicity can be blocked using N-methyl-D-aspartate (NMDA) receptor antagonists. Glial cells must be present however for the toxicity to occur, suggesting the effect of the virus is indirect and mediated by glial cells. These viruses do not appear to infect neurons, but can infect microglia, and possibly astrocytes.

2. The second hypothesis is that FIV/HIV results in increased vascular permeability and altered hemodynamics. The finding of cotton wool spots and retinal hemorrhages suggests vascular compromise does occur, but the pathogenesis of these lesions and their importance remains unclear.

FIV Anterior Uveitis. FIV is capable of causing anterior uveitis that can be transient or chronic and may persist despite anti-inflammatory therapy and lead to secondary glaucoma. Mononuclear inflammatory cell infiltrate adjacent to the base of the iris and anterior ciliary body are characteristic (but of course not pathognomonic) histologic features. The pathogenesis is unknown. It has been suggested that opportunistic Toxoplasma infection plays a role.30 It is not known if this is a primary FIV-induced lesion or predominantly the result of opportunistic disease. FIV can be isolated from the aqueous humor of infected cats.

FIV-Associated Anterior Hyalitis (vitritis). An accumulation of WBC's in the anterior vitreous, especially around the lens zonules, may be seen in FIV infected cats, often producing a "snow bank" appearance.35 Although the lesion is not pathognomonic, most cats are FIV positive. The ocular lesion is often an incidental finding and not associated with discomfort or other signs of anterior segment disease. Lens luxation and secondary glaucoma can result. Histologically these cells have been shown to be predominantly plasma cells. The pathogenesis of this lesion is not known but presumably arises from a pars planitis or peripheral retinitis.

FIV-Associated Conjunctivitis. Mild, self-limiting conjunctivitis may be seen in acute FIV infection. FIV infection may also worsen the course of FHV-1 infection.36,37 FIV infection is associated with alternations in the conjunctival microflora of cats, but the clinical relevance of these changes (if any) has not been determined.38

FIV-Associated Anisocoria. Mild to moderate mydriasis may be seen in association with FIV infection, and both motor and sensory abnormalities have been described. The changes are usually permanent and slowly progressive. The pathogenesis is not known, but possibly related to CNS disease. Similar problems have been seen in HIV patients.

FIV-Associated Neoplasia. FIV-infected cats have a much higher rate of B-cell lymphoma than uninfected cats. These tumors tend to occur at atypical sites such as the eye or CNS, and can occur without lymph node or bone marrow involvement. Intestinal and renal lymphomas have developed in experimentally-infected SPF cats. The

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lymphomas examined to date using virus culture, PCR, and flow cytometry have been negative for virus, as is typical for HIV. The pathogenesis is unknown, but may be secondary to chronic type 1 immune activation occurring after HIV infection. A study that utilized nested PCR failed to demonstrate FIV DNA in any of 18 feline globes enucleated due to histologically confirmed uveal melanoma.39

Diagnosis of FIV Point-of-care ELISA testing is the most frequent method to diagnose FIV infection in clinical practice. These tests have high sensitivity and specificity, but potential testing pitfalls include cats < 6 months of age (maternal antibody present), after vaccination, acute infection (antibodies may not develop for > 60 days in some cats). In some cases, confirmatory tests, such as PCR or virus isolation, should be considered.

Antiviral Treatment of FIV Antiviral treatment many not be necessary in most infected cats as FIV often does not cause severe clinical disease and may not shorten life-span. Of the available options, zidovudine (AZT) is the most studied/established treatment for cats with FIV infection.40,41 AZT is a nucleoside analog that blocks lentivirus reverse transcriptase activity. It has been demonstrated to reduce viral loads, improve immunologic status, resolve clinical disease (stomatitis, neurologic disease, etc…), increases quality of life, and prolongs life expectancy in cats with FIV.42

Other nucleoside analogues studied in vivo in cats with FIV infection include didanosine and lamivudine.42 Didanosine suppressed FIV replication, but contributed to antiretroviral toxic neuropathy.43 Lamivudine was associated with severe side effects in cats (i.e., fever, anorexia, and hematologic abnormalities).44 Nucleotide analogue RT inhibitors that have been evaluated in FIV infected cats include adefovir. Adefovir had inconsistent clinical efficacy, but produced severe side effects (non-regenerative life-threatening anemia).45 Interferon therapy (human interferon alpha-2b, feline interferon omega) for cats with FIV infection had mixed results and many of the studies were not placebo controlled.46-48 Numerous other drugs display in vitro efficacy against FIV, but controlled in vivo feline studies are currently lacking. For additional information, the comprehensive review on FIV antiviral by Hartmann et al (2015) is recommended.42

FELINE LEUKEMIA VIRUS

Virus Features FeLV is in the family Retroviridae, subfamily Oncovirinae, Group Mammalian C-Type. It has an enveloped RNA genome. FeLV is highly unstable in the environment and is inactivated within hours.

Pathogenic Mechanisms Transmission may be vertical (in utero, nursing, and via saliva) or horizontal (oronasal route most common via saliva transferred during close contact, grooming, and sharing food/water bowls). The worldwide prevalence of FeLV infection is currently 2-3%. The basic pathogenesis of infection begins with viral infection of local lymphoid tissue. Viremia then develops via infected monocytes and lymphocytes. At this point, interactions between the host and virus may result in either: 1) Regressive/Abortive infection where the cat’s immunity controls the infection and clinical disease is unlikely, or 2) Progressive infection where persistent viremia develops and clinical disease is expected to occur.

Systemic. Viremia seeds virus to lymphoid and epithelial tissues, and the bone marrow. Clinical signs are related to ability of the virus to induce hematological changes, immunosuppression, and tumor formation.

Ocular. The only ocular changes definitively linked to FeLV are retinal dysplasia, tumor formation, retinal hemorrhages, and bizarre pupillary changes.49-55 Uveitis is also frequency reported to result from FeLV infection, but the exact mechanism is unclear. In one study, 7.5% of FeLV positive cats had ocular disease.56 Studies evaluating the frequency of FeLV infection in cats with uveitis suggest that the relative importance of FeLV as an etiologic agent of uveitis in cats has slowly and progressively declined over the past several decades. Publications from the early 1990’s reported 5.9-12.1% of cats with uveitis were serologically positive for FeLV while more recent studies from the 2010-2016 period report 0-2.7% of cats have FeLV.29-34

Retinal dysplasia: Exposure of fetal and neonatal kittens to FeLV can result in retinal dysplasia, which occurs

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secondary to diffuse retinal inflammation. Infection causes progressive disorganization and necrosis of retinal tissues, with subsequent reorganization into cell clumps and dysplastic rosettes. RPE proliferation and migration into the sensory retina were also described. The mature retina demonstrated full thickness folds and tubes associated with folding of the RPE. After experimental inoculation, retinal tumor formation is also possible.

Anemic retinopathy: Retinal hemorrhages have been seen in cats with FeLV and were originally attributed to anemia. However a prospective study of the prevalence of retinal hemorrhage in dogs with anemia, thrombocytopenia, neither, or both reveals that anemia alone is not associated with retinal hemorrhage.57 Thus, further assessment of FeLV-infected cats with this finding is warranted.

Lymphosarcoma: Lymphosarcoma, especially of the anterior uvea, but also involving the conjunctival and retrobulbar tissues reflects the oncogenic effect of this virus. This is sometimes seen in the absence of identifiable tumor elsewhere in the body.

“Spastic Pupil” Syndrome: Although the data supporting this are limited, in all probability this represents FeLV infection of the autonomic nerves (parasympathetic portion of CN III). A wide variety of spontaneously changing pupillary abnormalities is possible.

Corneal Latency: In a study of normal corneas, FeLV DNA was demonstrated (using PCR) in 65% (11/17) FeLV- positive cats (as determined by p27 ELISA) and none of 17 FeLV-negative cats.58 Virus was localized to corneal epithelium using immunohistochemical staining techniques. This information must be considered when selecting suitable donor cats for corneal grafting procedures.

Diagnosis of FeLV Point-of-care ELISA testing for the FeLV p27 antigen is the most frequent screening test for FeLV infection in clinical practice. These assays have a negative predictive value of approximately 100% and a positive predictive value of approximately 80%. Since false-positive test results occur at a relatively high rate (from technical and user errors) and because there is a relatively low prevalence of FeLV infection in the general cat population, confirmatory tests should be pursued for all cats with positive ELISA test results. Confirmatory FeLV tests include IFA, PCR, and virus isolation.

Antiviral Treatment of FeLV In general, there are limited antiviral options with questionable efficacy for FeLV infected cats.59 Nucleoside analogue RT inhibitors evaluated in cats with FeLV infection include zidovudine (AZT) and zalcitabine. An in vivo feline study of AZT in cats infected with FeLV found there were was no improvement in the clinical, virological, and laboratory parameters evaluated.60,61 Zalcitabine was demonstrated to delay/control viremia, but its short half- life makes clinical administration impractical in most scenarios (IV boluses or delayed release SC implants are required) and high doses are toxic to cats (killed 8 of 10 cats in one study).62 Adefovir, a nucleotide analogue RT inhibitor, was evaluated in cats with FeLV infection and prevented development of persistent viremia in an experimental study, but was found to be ineffective vs a placebo in a clinical trial.63,64

More recently, the HIV drug raltegravir (an integrase inhibitor) was show to be effective against FeLV in vitro.65 When administered to cats with FeLV infection, raltegravir was well-tolerated and effectively reduced viremia, but viremia rebounded once administration was stopped.66 Studies of interferon therapy (human interferon alpha-2b, feline interferon omega) for cats with FeLV infection had mixed results and many of the studies were not placebo controlled.46-48 Numerous other drugs display in vitro efficacy against FeLV, but controlled in vivo feline studies are currently lacking. For additional information, the comprehensive review on FeLV antiviral therapy by Hartmann et al (2015) is recommended.59

FELINE PANLEUKOPENIA

Virus Features This virus is in the family Parvoviridae, Genus - Parvovirus. Characteristics: Non-enveloped virions, icosohedral symmetry, 18-26 nm in diameter, virions have 32 capsomeres. Replication and assembly take place in the nucleus. Virions are very stable and may persist in the environment for > 1 year at room temperature.

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Pathogenic Mechanisms Dysplastic ocular development has been attributed to intrauterine infection of kittens with the panleukopenia virus.67 In one case report of a 6-week-old kitten, lesions were characterized by retinal thinning, loss of the normal architecture, and rosette formation.68 The virus was recovered from the cerebrum. Cerebellar hypoplasia was also present. Hydrancephaly has also been reported in association with perinatal panleukopenia infection.69 The optic nerves were small, with increased perineural connective tissue.

FELINE SARCOMA VIRUS (FeSV)

Virus Features Taxonomic classification: Family - Retroviridae, Subfamily - Oncovirinae, Group - Mammalian C-type. Horizontally transmitted oncogenic RNA virus. Replication-defective variant of FeLV.

Pathogenic Mechanisms Experimental Sarcoma Virus Uveitis. In experimental studies, FeSV induced anterior uveitis 37 days after subcutaneous injection.70 Iritis, posterior synechiae, and cataracts were seen. The cellular infiltrate was lymphocytic and plasmacytic. Detection of large quantities of virus in the aqueous humor suggests that intraocular viral replication occurs, but inflammatory changes have been attributed to virus-antibody interactions, or a cell- mediated immune response.

Experimental Sarcoma Virus-Induced Uveal Melanoma. Intracameral injection of FeSV induces formation of uveal melanoma in cats.71,72 Tumors develop between 40 and 60 days after injection, and are composed of spindle and epithelioid cells. The tumors eventually enlarge to fill the anterior chamber. The posterior segment is involved only late in the course of the disease. More recently, nested PCR was used to examine globes enucleated due to feline uveal melanoma. DNA common to FeLV and FeSV was detected in only 3/36 globes.39

FeSV-Associated Fibrosarcomas. Fibrosarcomas of the eyelids in young cats have been associated with the oncogenic FeSV virus.51 However, in a study using PCR and immunohistochemistry, no association between FeLV or FeSV and ocular sarcoma could be demonstrated in biopsies from 6 cats.73

CANINE HERPESVIRUS

Virus Features Taxonomic classification: Family - Herpesviridae (DNA), subfamily Alphaherpesvirinae. Host Range: Canine only. Viral replication is similar to FHV-1.

Pathogenic Mechanisms The virus shows marked host specificity for canidae.74 Traditionally, CHV-1 infection in dogs has been described as causing one of two relatively clearly defined syndromes believed to depend almost exclusively on the age of the affected dog – severe and often fatal systemic disease (sometimes with ocular signs) in young puppies, and mild conjunctivitis, vaginitis, or upper respiratory disease in adults.

Young, immunologically naïve puppies are believed to become infected in utero, or following perinatal contact with contaminated reproductive and respiratory secretions. Experimental infection of newborn puppies is associated with keratitis, uveitis, retinal necrosis and optic neuritis.75,76 The lesions are the result of virally-induced inflammation. Experimentally infected puppies develop panuveitis with the presence of intranuclear inclusion bodies within 4 days of infection. Necrosis, retardation of maturation, and reorganization into folds and tubes occur throughout the retina. Depigmentation and vacuolization of the RPE, with subsequent folding and hypertrophy are also seen. In some animals, ectopic retina was found within cystic spaces in the optic nerve. Death in puppies may occur from disseminated viral replication in parenchymal organs and the CNS. Like FHV-1 infection, lifelong neural latency is believed to occur in many affected animals with immunosuppression causing reactivation. By contrast, clinical signs in adults are typically very mild and include transient vaginitis, mild upper respiratory disease, or conjunctivitis.

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However, reports have suggested that this clear age-related demarcation is not necessarily reliable with older dogs getting the fatal systemic forms of the disease and puppies and older dogs getting mild ocular disease. For example, an adult dog receiving chemotherapy for lymphoma experienced the severe disseminated syndrome usually seen in puppies infected with CHV-1.77 Meanwhile, corneal disease has been described in 3 adult dogs with naturally- acquired CHV-1 infection.78,79 These dogs had classic dendritic ulcers, as well as nonspecific signs of ocular surface inflammation, which responded to cessation of topical immunomodulating drugs and initiation of topically administered antiviral agents. CHV-1 was isolated from ocular swabs. All dogs had altered local and/or systemic immunity (diabetes mellitus, KCS, topical administration of corticosteroids or cyclosporine, etc.). These cases suggest that these unusual presentations may, at least in part, be due to more regular local (ocular) and systemic immunosuppression of pet dogs as a result of or during treatment of other chronic diseases. However it is unlikely to be that simple. For example, the typical fatal form of hepatic necrosis described originally in only young puppies has been described in an apparently immunocompetent adult (9yo) dog.80 Severe, fatal respiratory CHV-1 infections are also described in mature dogs.81 Likewise, a natural outbreak of CHV-1-induced ocular disease was described in a colony of 27 juvenile dogs but in the absence of the usual fatal systemic clinical syndromes and in which no source of immunosuppression was noted.82 Although, in affected dogs within the colony, marked bilateral conjunctivitis was a consistent sign (sometimes with petechiae), many dogs also developed ulcerative (26%) - often dendritic (19%) - or perilimbal stromal nonulcerative keratitis (19%). Antiviral treatment was not initiated. Another adult dog was presented with partial limbal stem cell deficiency and neurotrophic keratitis subsequent to a protracted episode of CHV-1 dendritic ulcerative keratitis (metaherpetic disease) raising the question of the role of previous CHV-1 infection in the pathogenesis of other chronic ocular surface diseases of dogs.83

Despite these striking clinical reports and the susceptibility of the canine cornea to CHV-1 infection in vitro,84 canine herpetic disease and shedding at the ocular surface still appear to occur infrequently when compared with the prevalence of FHV-1 on the feline cornea or conjunctiva.85 For example, samples collected from 50 dogs without surface ocular disease and 50 dogs with non-dendritic corneal ulcers all were negative when tested for the presence of CHV-1.78 In a separate virologic survey assessing the prevalence of CHV-1 (and 6 other viruses) in conjunctival samples from dogs with or without conjunctivitis, no virus (of any type) was ever detected in dogs without conjunctivitis.86 In dogs with conjunctivitis, CHV-1 (5 dogs) or CAV-2 (2 dogs) was detected by VI and/or PCR occasionally but significantly more commonly than in normal dogs. Further evidence of the degree of natural resistance to corneal, systemic, and severe recurrent ocular disease in most dogs is provided by a study in which adult SPF beagles were infected with CHV-1 with or without prior disturbance of the corneal epithelial surface and/or subconjunctival corticosteroid administration.87 Dogs developed mild to moderate bilateral conjunctivitis, shed virus, and seroconverted but corneal disease was not noted, epithelial disturbance or corticosteroid administration did not worsen signs, and all dogs spontaneously recovered without evidence of spontaneous reactivation or recrudescence for 224 days post inoculation.87 Likewise, a single fraction 50 Gy radiation dose administered to the ocular surface by strontium-90 beta radiotherapy did not result in detectable recurrent ocular CHV-1 infection in mature dogs with experimentally induced latent infection.88

By contrast, pharmacologically induced reactivation appears to be important in the pathogenesis of this virus, but highly dependent upon drug chosen and route administered. Dogs latently infected with CHV-1 via experimental inoculation 8 months previously and subsequently administered prednisolone systemically (3 mg prednisolone/kg/day x 7 days)89 experienced bilateral ocular disease (83%), viral shedding (50%), and 4-fold elevations in serum CHV-1 SN titers (100%). The same dogs where stimulated 12 months later (20 months after inoculation) by using topically applied 1% prednisolone acetate (1 drop OU QID x 28 days) and did not experience viral reactivation (detectable by quantitative PCR or SN) or recrudescent disease (detectable by slit lamp examination or ocular confocal microscopy).90 When the potent immunosuppressive agent cyclophosphamide was administered at 200 mg/m2 to mature dogs latently infected with CHV-1, evidence of myelosuppression was noted on Day 7, but no ocular signs suggestive of recurrent CHV-1 were noted clinically or with confocal microscopy, and ocular CHV-1 shedding was not detected by PCR.91 Finally, 8 mature Beagles with experimentally-induced latent CHV-1 infection proven to be reactivatable when prednisolone was systemically administered received 0.2% cyclosporine ointment in both eyes twice daily for 16 weeks. This failed to produce detectable virus at the ocular surface or signs of herpetic disease.85 (For a recent review of the ocular effects of this virus see the article by Ledbetter EC, 2013).92

Diagnosis of CHV-1 Similar to FHV-1 (e.g., PCR, virus isolation, or immunofluorescent detection methods).

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Antiviral Treatment of CHV-1 Clinical descriptions of antiviral therapies thought to be successful in dogs with ocular CHV-1 infections include idoxuridine, trifluridine, cidofovir, and ganciclovir.78,79 Three placebo or vehicle controlled studies of dogs with experimentally-induced CHV-1 ocular disease have been published investigating topical antiviral therapy. In these studies, cidofovir 0.5% solution (q12h) was found to reduce viral shedding, but significantly worsened clinical disease and corneoconjuntival leukocyte infiltration scores as measured by in vivo confocal microscopy.93 Ocular toxicity associated with cidofovir administration also included development of marked conjunctival pigmentation and blepharitis. Trifluridine 1.0% ophthalmic solution (q4h for 2 days, then q6h) was well tolerated by dogs, reduced the duration of ocular viral shedding, and significantly reduced clinical disease scores.94 Ganciclovir 0.15% ophthalmic gel (5 times/day 7 days, then q8h) was well tolerated and significantly reduced the duration of ocular viral shedding, severity of clinical ocular disease, and corneoconjunctival leukocyte infiltration scores as measured by in vivo confocal microscopy.95

In addition to topical antiviral therapy, therapeutic vaccination using a CHV-1 subunit vaccine from Europe (Eurican® Herpes 205, Merial) was evaluated in dogs with latent CHV-1 infections.96 After vaccination, the dogs were subsequently administered corticosteroids to experimentally induce viral reactivation. In this study, a two dose vaccine series induced long-term specific CHV-1 immunity, but was not able to not prevent corticosteroid-induced viral reactivation, recurrent ocular disease, or ocular viral shedding in the dogs.

CANINE DISTEMPER VIRUS (Carre's Disease, 1905)

Virus Features Taxonomic classification: Family - Paramyxoviridae (RNA), Genus - Morbillivirus. Structure: Negative strand RNA virus. CDV consists of 2 structural modules; an internal ribonucleoprotein core (nucleocapsid) containing single stranded viral RNA genome, and an outer spherical lipoprotein envelope. CDV is highly unstable in the environment and is inactivated within hours.

Pathogenic Mechanisms General. Direct cytolysis. Target tissues are epithelial and neural.

Systemic. Route of infection is via aerosolized respiratory secretions. Virus then spreads from oropharynx to regional lymph nodes and tonsil. Viremia disseminates virus to lymph tissues throughout the body and the virus has epithelial and neural tropism. Virus persists in epithelial tissues of dogs unable to mount a protective immune response.

Ocular. Tissues affected are eyelids, conjunctival epithelium, lacrimal gland, retina, optic nerve, and optic tracts. In the earliest report, retinitis, optic neuritis, and destruction of the optic tracts were described.97 Acute retinitis is characterized by congestion, edema of the nerve fiber layer and inner plexiform layer, and perivascular cuffing.98-100 Intranuclear inclusions may be seen in retinal glia. Sequelae are optic nerve degeneration, and focal to diffuse areas of retinal degeneration characterized ophthalmoscopically in the tapetal fundus by hyperreflectivity, and by RPE pigment loss in the non-tapetal fundus. Distemper inclusions have also been found in the ciliary body of a dog.101 In a subsequent retrospective study, 41% (9/22) of dogs with distemper were noted to have retinochoroiditis, and 54% were positive by IFA testing of conjunctival smears.102

Eyelid lesions may include vesicular or pustular blepharitis and eyelid hyperkeratosis. Conjunctivitis is often prominent in infected dogs. Lymphoplasmacytic infiltration, acantholysis, acidophilic inclusion bodies, and keratinization of the ocular surface is seen histologically.103 Dacryoadenitis results in keratoconjunctivitis sicca that is typically transient in dogs that recover from the infection.

Diagnosis of CDV Intracytoplasmic CDV inclusion bodies may be observed histologically or cytologically in conjunctival scrapings from infected dogs (25% of dogs with CDV keratoconjunctivitis had visible inclusions in the conjunctival epithelium in one study103) or more rarely in blood samples. Immunofluorescent assays for detection of CDV antigens in conjunctival scrapings offer improved sensitivity and specificity vs. cytology alone.104 PCR assays and

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antibody assays are also available for CSF samples.

Antiviral treatment of CDV Specific antiviral therapy has not been reported/evaluated for CDV infection.

CANINE ADENOVIRUS 1 (Infectious Hepatitis Virus, Rubarth's Disease, 1947)

Virus Features Taxonomic classification: Family - Adenoviridae (DNA). Adenoviridae are divided into two genera, Mastadenovirus and Aviadenovirus. The former includes canine adenovirus. Structure: Non-enveloped, regular icosahedrons, 65-80 nm in diameter. Capsid is composed of 250 subunits (capsomeres). Replicative Cycle: Attachment, penetration, uncoating, transcription, gene expression, protein synthesis, and assembly of virions. Virions are environmentally stable and persist several days at room temperature.

Pathogenic Mechanisms Systemic. Natural infection occurs via the oropharynx, with subsequent spread to regional lymph nodes. Principal tissues involved are the liver, kidney, lymph nodes, and vascular endothelium.

Ocular. Viral invasion of the eye occurs in acute infection. Mild anterior uveitis results from virus replication in vascular endothelium. Corneal lesions developed between 7 and 21 days after experimental subcutaneous inoculation.105 Moderate to severe iridocyclitis was the consistent finding in all eyes that developed edema. The presence of a loose fibrin clot in the anterior chamber was a consistent finding in opaque eyes. Using IFA, viral antigen was identified in the vascular endothelium of the iris, trabecular endothelial cells, and macrophages. Virus was also found in corneal endothelium. Uveal infiltrate includes lymphocytes and plasma cells. Relatively low levels of virus were recovered from the eyes compared to other organs.

Ocular damage has been suggested to be predominantly mediated by a Type III immune response (Arthus reaction).106,107 Twenty-four dogs were inoculated with ICH; 9 dogs received virulent virus subcutaneously, and 15 dogs received attenuated virus intravenously. After 4-8 days, aqueous humor was aspirated and the anterior chambers injected with 0.2 ml of anti-ICH serum. Fourteen of 24 dogs developed ocular disease, and in 2, corneal edema was severe. Purified viral antigen and anti-ICH serum was then injected into the anterior chamber of 8 dogs; intense iritis with severe edema occurred. The reaction was found to be more severe if complement was added to the immune complexes. In a subsequent study, viral replication was found to occur consistently in the corneal endothelium.108 Virus infection was found to occur not by contagious spread, but as the result of endothelial adsorption of virus that was released into the aqueous humor. It was concluded that parenteral inoculation with ICH does not in itself cause severe or permanent corneal endothelial cell dysfunction.

Prevalence. Ocular disease is estimated to occur in 20% of naturally occurring cases, and in 0.4% of dogs given modified live vaccine. Afghan hounds have been suggested to be predisposed to more severe ocular lesions after inoculation with attenuated CAV-1 vaccine than are beagles.109

Diagnosis of CAV-1 Presumptive in most cases, but serology, virus isolation, and IFA assays are available.

Antiviral treatment of CAV-1 Specific antiviral therapy has not been reported/evaluated for CAV-1 infections. Modified live CAV-1 vaccines have been discontinued in most countries (replaced by MLV CAV-2).

EQUINE HERPESVIRUS

Virus Features Family: Herpesviridae.

To date, five genetically distinct equine herpesviruses (EHV) have been definitively identified: EHV-1 through

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EHV-5. As more accurate molecular techniques are used, it becomes obvious that previous reports often failed to differentiate between some of these viruses. Until relatively recently, EHV-1 and 4 were considered subtypes 1 and 2, respectively of EHV-1, while EHV-2 and 5 were not treated separately in the literature until around 1988, and some papers since then have continued to inadequately differentiate the two viruses.110,111 Interpretation of older data is therefore often difficult or impossible.

Although equine herpesviruses are frequently incriminated as ocular pathogens, little is known about their true role, and much evidence is based upon response to therapy. In fact, Koch’s postulates have rarely been demonstrated for the ocular signs currently blamed on the equine herpesviruses.112,113 A survey of ocular lesions and herpesvirus serology in a herd of 266 Lipizzaner horses in Austria sheds further light on the prevalence of these viruses but reinforces the difficulty of associating them with ocular syndromes.114 EHV-2 DNA was identified in 167 (62.8%) of the horses and EHV-5 in 136 (51.1%) of the horses; 105 (39.5%) were co-infected with EHV-2 and EHV-5. Horses carrying EHV-2 or EHV-5 DNA were significantly younger than the negative group. No significant association between any ocular disease pattern and presence of EHV-2 or EHV-5 was detected. A closely related study in the same population of horses revealed no association of ocular lesions and herpesviral infection status when horses were examined serially every 6 months for 18 months.115 A smaller study in Northern California assessed 12 horses suspected of having herpetic ocular surface disease and 6 normal horses.116 Prevalence of EHV- 1, -2, -4, and -5 DNA as assessed by qPCR did not differ significantly between control horses and those with idiopathic keratoconjunctivitis. Neither EHV-1 nor EHV-4 DNA was detected in any normal or affected horse; EHV-2 DNA was detected in 2/12 affected horses but in no normal horses. EHV-5 DNA was commonly found in ophthalmically normal horses and horses with idiopathic keratoconjunctivitis. Taken together these data suggest that (just as in the cat) diagnostic testing in individual horses may be of limited value.

Antiviral Efficacy Topically applied antiviral agents have been used to successfully treat presumptive herpetic keratitis in the horse, although no drugs are currently labeled for equine use. These drugs have undergone little testing with respect to their efficacy against the equine herpesviruses particularly EHV-2; however a recent review is comprehensive and very helpful for the alphaherpesviruses (EHV-1, -3, and -4) but has little information regarding EHV-2 and -5.117 When equine-specific data are not available, expected efficacy and application frequency are usually extrapolated from data generated in humans infected by HSV-1, cats with FHV-1, and dogs with CHV-1. Some general comments gained from those sources are likely to apply to horses and equine herpesviruses. All of the currently available antiviral agents are virostatic in action and therefore should be applied frequently. Although the treatment regimen often recommended for humans (two-hourly application throughout the waking hours until the ulcer has healed) is usually not possible in horses, as frequent application as possible is recommended and therapy should be initiated early in the disease course. A minimum of four applications daily is probably indicated. The antiviral agents are variably toxic to corneal and conjunctival epithelium. In some cases this is noted as an apparent stinging reaction when the medications are applied. In more marked situations, delayed ulcer healing may occur.

There have been some studies assessing the in vitro antiviral efficacy against equine herpesviruses of some of the agents commonly used for human and feline herpesviruses. These are reviewed by Vissani.117 For EHV-1, these can be summarized (from most to least effective) as: ganciclovir ≅ cidofovir > acyclovir ≅ penciclovir > adefovir > foscarnet > brivudin.117,118 The majority of clinical reports for ophthalmic disease describe topical application of either idoxuridine or trifluridine for the treatment of herpetic keratitis in horses. Some reports suggest antiviral drugs developed for HSV-1 and used in the treatment of FHV-1 are effective.119-121 Acyclovir given orally to horses is 122,123 118 poorly bioavailable and reaches plasma concentrations that do not exceed the in vitro IC50 for EHV-1. By contrast, oral administration of the acyclovir prodrug – valacyclovir – achieves concentrations within the sensitivity range of EHV- 1.123,124 In spite of these promising results, studies in horses experimentally infected with EHV-1 have not yielded consistently positive data.117,125 Plasma penciclovir concentrations achieved after horses received a single oral dose of 126 20 mg famciclovir/kg were sufficient to exceed the IC50 for EHV-1; however, to my knowledge, none of these drugs have been tested in a prospective placebo-controlled therapeutic efficacy trial and tear pharmacokinetics have not been studied as they have in cats.127,128 The latter parameter seems particularly relevant in horses given that equine herpetic keratitis often lacks a robust vascular response. None of the antiviral agents has known antibacterial properties and therefore horses with ulcerative keratitis should probably be treated with a topical antibiotic agent in addition to any antiviral agent.

For additional information, the comprehensive review on antiviral therapy in horses by Maxwell et al (2017) is

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recommended.129

Clinical Syndromes Ocular signs associated with equine herpesviruses may be divided into three broad categories:  Primary ocular disease in the absence of systemic signs - for example, keratoconjunctivitis due to EHV-2 infection, keratitis and blepharitis associated with EHV-3, or chorioretinitis due to EHV-1.  Ocular signs secondary to neurological disease - for example, keratitis secondary to facial nerve paralysis or blindness secondary to cerebral vasculitis seen with EHV-1.  Ocular signs as a minor and self-limiting component of a more serious upper respiratory syndrome - such as mild and transient conjunctivitis seen in horses with severe rhinopneumonitis due to EHV-1 or -4 infections.

Keratoconjunctivitis Herpetic conjunctivitis is seen in association with respiratory disease syndromes following infection with EHV-1 or -4, and alone or with keratitis with EHV-2. Clinical signs are non-specific and all may occur as outbreaks. In the case of EHV-1 or –4, bilateral conjunctivitis is usually a relatively minor feature of a significant respiratory infection. Younger animals are more severely affected.

In horses with keratitis/conjunctivitis or both (in the absence of respiratory signs), some data seems supportive of a role for EHV-2. For example, the prevalence of EHV-2 in horses with keratoconjunctivitis (12 of 27) was significantly higher than in clinically healthy horses (2 of 12).130 Response to treatment with trifluridine in 9 of 16 cases further supported an etiological role for EHV-2. Similarly, herpetic keratitis due to corneal replication of virus has been suggested to occur with EHV-2 only.120,121,131 This syndrome seems to be diagnosed more commonly in the United Kingdom than in other parts of the world, although the reason for this is unclear. Herpetic keratitis, and often keratoconjunctivitis, may occur in individual horses or as small outbreaks, especially in young horses, and is often unilateral. Concurrent respiratory signs are usually absent or very mild. Recurrences are likely. Superficial ulcerative and non-ulcerative keratitis is described and may represent different stages of the same disease process. Corneal opacities range widely in appearance from ring-shaped lesions, a more generalized stippling or “orange- peel” appearance, or reticulated or lace-like (dendritic) networks. In all cases, their multifocal nature is highly suggestive. Variable corneal stromal inflammatory cell infiltration, edema, and neovascularization have been described. Uveitis, if present, appears to represent “reflex uveitis” mediated by the axonal reflex and is usually manifest as a miotic pupil without flare.

Corticosteroids are contraindicated in the treatment of ulcerative disease in the horse and active herpetic disease in all species studied so far. Some reports exist of topical or systemic corticosteroids causing exacerbation or recurrence of herpetic keratitis in horses. For these reasons, it seems wise to avoid the use of topical or systemic corticosteroids in horses with active herpetic keratitis. The use of topical non-steroidal anti-inflammatory agents is controversial but should also probably be best avoided in herpetic keratitis.

Anterior uveitis frequently occurs with herpetic keratitis and is a significant component of the ocular pain seen in this disease. This usually responds very well to one or two applications of topical applications of atropine ointment. Atropine should be given to effect (i.e., until wide mydriasis is achieved) and then given as often as necessary to maintain pupil dilation. Frequently, only once or twice weekly application is required.

Recently, EHV-3 (the viral agent of equine coital exanthema) was reported to cause ocular surface disease.132 In this case report, a 3 month old Quarter Horse filly was presented with bilateral eyelid papules, vesicles, and dermal ulceration. Similar lesions were present on the gingival mucosa as well as the cutaneous and mucosal surfaces of the lips. Severe conjunctivitis, superficial corneal ulcers (multifocal punctate and geographic in appearance), and keratitis were present in both eyes. Intranuclear inclusions, typical of herpesviruses, were present in the in eyelid skin and EHV-3 was detected by PCR and virus isolation in corneal samples. No viral inclusions were found in corneal tissues and it was not clear if the corneal disease was a result of primary viral infection of the ocular surface epithelium or secondary to the eyelid disease and associated eyelid dysfunction.132

Ocular Signs Secondary to Neurological Disease Infection is generally sporadic, but dramatic when it occurs. Ocular signs secondary to EHV-1 myeloencephalitis include nystagmus, blindness, strabismus, ptosis, exposure keratitis, optic neuritis, and retinal hemorrhage. These

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ocular signs represent manifestations of altered neurological function rather than virally induced ocular pathology. The ocular signs may result from CNS or peripheral cranial nerve injury, often secondary to vasculitis. The etiology of the exposure keratitis is presumably related to injury to the trigeminal (CN V) and/or facial (CN VII) nerve with resultant inability to sense corneal dryness, stimulate tear production by the lacrimal gland, and/or blink. Additionally, CN V is probably responsible for the supply and release of various trophic factors essential for normal corneal health. This means that the perceived need for, production of, and dispersion of tears and other corneal neurotrophic factors can all be affected leading to profound exposure keratitis.

Uveitis Anterior uveitis or diffuse chorioretinitis has been described in horses in which EHV-1 infection has been diagnosed or experimentally induced. The exact causal association however remains undefined. In one case,133 a foal experimentally infected with EHV-1 developed severe visual impairment and bilateral, chronic diffuse chorioretinitis approximately one month after experimental inoculation with EHV-1. Anterior uveitis was not seen and a causal association with EHV-1 was not demonstrated. A separate report134 describes an otherwise typical outbreak of neurological disease and neonatal mortality attributed to EHV-1 in which a number of affected foals had evidence of anterior uveitis in addition to more classic pneumonic and/or gastrointestinal signs. Clinical signs sometimes included blindness and hypopyon, both of which resolved. In these animals, uveitis may be a manifestation of a more generalized vasculitis or may relate to super-infection with another pathogen secondary to EHV-1-mediated immunosuppression. An experimental study135 provides more definitive evidence that EHV-1 is a cause of multifocal chorioretinal lesions (typically classic “bullet hole” lesions in the peripapillary fundus) in 50- 90% of experimentally infected horses. The lesions were evident between 3 weeks and 3 months after infection. The authors proposed that this was a vascular (ischemic) event.

Chorioretinitis, optic neuritis, vitritis, retinal vasculitis, and encephalitis have also been described in camelids (i.e., alpacas and llamas) infected with EHV-1.136,137

OTHER VIRUSES CAPABLE OF OCULAR INVOLVEMENT IN THE HORSE

Equine viral arteritis - conjunctivitis, hypopyon, iridocyclitis, and keratitis. African horse sickness - conjunctival and eyelid edema. Equine adenovirus infection - conjunctivitis. Borna Disease - retinal infection, optic neuritis. Rabies - viral replication in retina and optic nerves. Equine viral encephalitis - blindness due to encephalitis. Equine infectious anemia - conjunctival hemorrhages.

BOVINE HERPESVIRUS I (Infectious Bovine Rhinotracheitis, IBR)

Virus Features Taxonomic classification: Family - Herpesviridae, Subfamily - Alphaherpesvirinae. Structure, replication, and tissue damage is typical of that of other alphaherpesviruses.

Pathogenic Mechanisms Systemic. Upper respiratory disease, tissue damage by direct cytolysis.

Ocular. Ocular disease may occur with or without respiratory signs. Conjunctivitis is the predominant feature, with formation of white conjunctival plaques composed of mononuclear cells (plasma cells and lymphocytes).138-140 Keratitis is also possible and is frequently peripheral and nonulcerative

BOVINE VIRUS DIARRHEA VIRUS (BVD; Mucosal Disease)

Virus Features Taxonomic classification: Family - Togaviridae, Genus - Pestivirus (mucosal disease viruses). Virus structure:

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Lipid containing envelope with surface projections surrounding a spherical nucleocapsid. Virions are 40-70 nm in diameter. Replication takes place in the cytoplasm, and assembly is assumed to occur during budding through the plasma membrane.

Pathogenic Mechanism Ocular lesions occur as a result of exposure of the fetus to the virus between days 76 and 150 of gestation. Characteristic lesions are retinal dysplasia, cataracts, and microphthalmia.141,142 Reported histologic findings include swollen and multi-layered lens epithelium, microphthalmia, diffuse cataracts, retrolental fibrovascular membranes containing cartilage, and retinal dysplasia. Retinal dysplasia is described as severe, with absence of normal retinal architecture, diffusely fibrosed nerve fiber layer, and gliotic optic nerves. Mononuclear cell infiltration may also be seen.

MALIGNANT CATARRHAL FEVER (MCF)

Virus Features Taxonomic classification: Family - Herpesvirinae, Subfamily - Alcelaphinae.

Pathogenic Mechanisms General. Pansystemic vasculopathic disease of cloven-hoofed animals worldwide.143,144 Peracute form: fever, hemorrhagic gastroenteritis, and death in 1-3 days of onset of clinical signs. Alimentary form: fever, diarrhea, diffuse exanthema, lacrimation, lymphadenopathy. Head and eye form: typical form seen in Africa, mucopurulent nasal discharge, corneal opacity, edema of head and neck, and neurological manifestations. Mild form: nasal and lacrimal discharge, dermatitis, and lymphadenopathy (usually results in recovery).

Ocular. Lymphocytic vasculitis of retinal, scleral, and uveal vessels. Anterior uveitis is associated with corneal edema and keratitis. Predominantly a lymphocytic infiltrate, contrasting the disease from BVD, TEME, IBR, and IBK.144 Immunological mechanism suspected; Type IV (DTH) mechanism proposed.

MISCELLANEOUS CAUSES OF VIRAL INDUCED OCULAR DISEASE IN CATTLE:

Rabies - non-suppurative retinitis.

BLUETONGUE

Virus Features Taxonomic classification: Family - Reoviridae, Genus - Orbivirus. Virus structure: Non-enveloped spherical virion, 70 nm in diameter, with 2 concentric capsids, the inner being icosohedral. RNA genome.

Pathogenic Mechanism Systemic. Virus replicates in hematopoietic cells and vascular endothelium.

Ocular. Similar to BVD (both calves and lambs).145-147 Vaccination of ewes during the first trimester of gestation (attenuated live vaccine) produces necrotizing retinitis and CNS damage. Target tissues are brain, spinal cord, and retina. The severity of the disease depends on when in gestation the ewes are vaccinated. In lambs infected at 50-58 days, early lesions affect predominantly the retinal vasculature (endothelial nuclei rounded and vesicular).

MAREK'S DISEASE

Virus Features Gamma herpesvirus of chickens.

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Viral Pathogenesis Systemic. Virus is slowly cytopathic but has ability to transform T lymphocytes to cause tumor formation. Systemic disease includes neurolymphomatosis (range paralysis), acute Marek's disease, and ocular lymphomatosis.

Ocular. Severe panuveitis to ocular tumor formation.148

FERRET CORONAVIRUS-ASSOCIATED DISEASE

Virus Features A relatively recently described and emerging viral infection of ferrets. Ferret systemic coronavirus (FRSCV)- associated disease produces a clinical condition that closely resembles the dry from of FIP. It is often referred to as Ferret-FIP. Similar to FIP, it was initially hypothesized that this condition could be associated with the ferret enteric coronavirus (epizootic catarrhal enteritis virus) that mutates within ferrets to produce Ferret-FIP. An alternative hypothesis was that there were separate and distinct strains of coronavirus affecting ferrets and causing the disease. Recent publications have reported the detection of a novel group 1 coronavirus named ferret systemic coronavirus that was believed to be responsible for the condition.149

Viral Pathogenesis Systemic. Similar to FIP in cats. The condition is characterized by pyogranulomatous perivasculitis and peritonitis.

Ocular. Unilateral pyogranulomatous panophthalmitis is described in an infected ferret with systemic coronavirus disease.150

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