Consultant on Call Virology / Infectious Disease Peer Reviewed Canine Distemper Virus Somporn Techangamsuwan, DVM, MSc, PhD Chulalongkorn University Melissa A. Kennedy, DVM, PhD, DACVM University of Tennessee Profile Definition n Canine distemper virus (CDV) is a mor- billivirus in the Paramyxoviridae family. o The causal viruses of rinderpest and measles are also included in this family. n CDV is an enveloped virus. o CDV is relatively easy to inactivate and requires only the removal of the lipid outer membrane. o Any disinfectant with detergent activ- ity effectively inactivates this virus. n CDV, an RNA virus, has a significant mutation rate that is much greater than that of DNA viruses. n *Domestic dogs are considered the reser- voir species. n★ CDV also affects multiple wildlife species (eg, raccoons, skunks, foxes, ferrets) and can infect and cause disease in large felids (eg, lions). n Antigenic drift and strain diversity have A CDV-infected dog with purulent nasal discharge. been increasingly associated with out- breaks in wild species, domestic dogs, and exotic animals in zoos and parks. n Respiratory, GI, and urogenital epithe- Domestic dogs are o New strains can emerge, so monitor- lium; CNS; and optic nerves are also ing is necessary to ensure current vac- infected. considered the cines are fully protective against reservoir species. predominant circulating strains. Incidence & Prevalence n The virus occurs globally, but it is a rare Systems disease in typical pet populations. n The infection is associated with all n Disease can be common in areas of low lymphatic tissue. vaccine coverage. MORE CDV = canine distemper virus November 2014 • Clinician’s Brief 19 Consultant on Call Geographic Distribution Signs disease, conjunctivitis, vomiting, n The H-gene of CDV exhibits the n Signs most often involve the respira- diarrhea) with encephalitis caused by highest variability within the genome. tory tract. direct viral replication. n Based on H-gene alignment, CDV is o GI signs may also occur. o Alternately, neurologic disease classified into 11 lineages: Asia-1, n Affected dogs are listless and have a may occur several weeks after res- Asia-2, Asia-3, Asia-4, Europe, Euro- decreased appetite. olution of epithelial invasion. pean wildlife, vaccine (America-1), o In milder cases, signs may be simi- n A significant amount of pathol- America-2, South America, South lar to other agents of canine infec- ogy results from virus immune Africa, and Arctic-like. tious respiratory disease complex. response, as well as the virus o Subclinical infection with shed- itself. Signalment ding may also occur, depending on o Seizures and myoclonus are two of n The disease has been reported in the level of host immunity. the more common signs. dogs, bears, raccoons, ferrets, civets, n Systemic signs are most common in n The latter may affect limbs or red pandas, elephants, and large or unvaccinated dogs (eg, puppies) as manifest as chewing motion of exotic cats. maternal immunity wanes. the jaw. n There is no breed or sex predilection. n Conjunctivitis, nasal discharge, n Ocular disease may also occur. n Animals <6 months are particularly cough, and fever are classic signs. o Lesions include anterior uveitis, vulnerable. n Respiratory infection may involve the optic neuritis, and retinal lower respiratory tract with possible detachment. Pathogenesis primary viral pneumonia. n Infection during pregnancy may lead n The main route of infection is via n Secondary bacterial infection may to abortion or stillbirth. aerosol droplet secretions from the occur. o Puppies infected before permanent oral or nasal cavities of infected n Vomiting and diarrhea may be dentition may have enamel animals. present. hypoplasia. o Can also spread by direct contact. n Neurologic signs may be concurrent n Digital hyperkeratosis may be noted. n The virus initially replicates in the with epithelial signs (ie, respiratory epithelia and lymphoid tissue of the upper respiratory tract. n Depending on the level of immunity, CDV may spread via the bloodstream Pitfalls in an infected host, replicating in n CDV should not be ruled out simply because there is no history of con- mononuclear WBCs. tact with other dogs. o The virus may then target epithelia of the respiratory system as well as – Wildlife (eg, raccoons, foxes) can be an important source of CDV. the CNS. n Although current vaccines appear to be protective against most circu- n In cases where CNS invasion lating strains, vaccine breaks may occur. occurs, neurons will become infected. n PCR can detect virus from the vaccine for a few weeks postvaccination. n The degree of viremia and the extent – It is important to know if the dog has recently been vaccinated when of viral spread to other tissue are testing samples for CDV. moderated by the level of specific humoral immunity in the host during – The testing laboratory can help discern whether a positive result is the viremic period. from natural infection or vaccine. n Canine distemper virus, including the canine distemper vaccinal virus, kills ferrets. It is critical never to use the canine vaccine in this species. – Use vaccines licensed for use in ferrets only. CDV = canine distemper virus, CSF = cerebrospinal fluid, PCR = polymerase chain reaction 20 cliniciansbrief.com • November 2014 Diagnosis Distemper is sometimes confused with other systemic infections, including leptospirosis, canine infectious hepatitis, rabies, canine infec- Definitive Diagnosis n Diagnosis is established via virus tious respiratory disease complex, or Rocky Mountain spotted fever. identification in a clinical sample through use of reverse-transcriptase PCR of whole blood, a swab of con- junctiva or tissue, CSF, or urine. o Urine is a good choice for PCR testing in patients with CDV encephalitis after resolution of epithelial signs. o CDV may be detected in urine for *a longer period than other sample types.1 ★o CDV detection in urine and CSF was equivalent in one study of neurologic cases.1 n Postmortem evaluation and micro- scopic findings confirm infection. o The specific lesion of CDV is eosinophilic intranuclear/intracy- toplasmic inclusion bodies in glial cells, neurons, epithelial respira- 1 tory cells, and cells of the GI and urogenital tracts (Figure 1). Eosinophilic intranuclear inclusion bodies in glial cells in the cerebrum of a dog infected with n Virus isolation is the gold standard CDV. for diagnosis and is useful in low lev- els of viral infection through observa- tion of typical syncytial cell formation (Figure 2). n Immunocytology can be used to enhance the visibility of inclusion bodies by fluorescein-conjugated CDV antibodies. o Fluorescent color confirms dis- temper infection, but the lack of color does not rule it out. Differential Diagnosis n Distemper is sometimes confused with other systemic infections (eg, leptospirosis, canine infectious hepatitis, rabies, canine infectious respiratory disease complex, Rocky Mountain spotted fever). MORE 2 Typical cytopathic effect of CDV isolation shows numerous small and large syncytium cell formations. November 2014 • Clinician’s Brief 21 Consultant on Call Laboratory Findings be negative unless exposed to bacterial infection. n Clinical findings may include infection with a field strain. n In cases of acute to peracute death, lymphopenia. n Detection of antibodies in CSF is sig- respiratory abnormalities may be o Inclusions in WBCs and RBCs nificant if the blood–brain barrier is found exclusively. may be noted, especially in early intact. n Histologically, CDV causes necrosis infection. o This does not occur unless CNS of lymphatic tissue; interstitial pneu- n Analysis of a CSF specimen in dogs invasion is present. monia with cytoplasmic and intranu- with neurologic disease shows n Immunohistochemistry using specific clear inclusion bodies in respiratory, increased protein and WBCs— antibodies against CDV is helpful urinary, and GI epithelium; and neu- primarily lymphocytes. when typical lesions are not evident rologic complications. n Serologic studies can be helpful in (Figure 3). o Neurologic complications include: unvaccinated dogs, particularly if neuronal degeneration, gliosis, IgM is detected. Postmortem Findings noninflammatory demyelination, o In dogs with unknown history or n Thymic atrophy is a consistent find- perivascular cuffing, nonsuppura- in vaccinated animals, the useful- ing in infected puppies. tive leptomeningitis, intranuclear ness of serology is dubious. n Hyperkeratosis of the nose and foot- inclusion bodies (mainly in glial o Measuring CDV-specific IgM can pads is often found in dogs with neu- cells; Figure 1, previous page). be useful if the vaccination history rologic manifestations. is known. n Bronchopneumonia, enteritis, and o If the dog has not been vaccinated skin pustules may also be present, Treatment in the past 1–2 months, IgM should depending on the degree of secondary Inpatient or Outpatient n Treatment depends on severity of clinical signs. n If mild respiratory distress occurs, animals may be treated as outpatients with supportive care (ie, nebulization, antibiotics for secondary infections, mucolytics, cough suppressants). n *If signs are more severe, dogs may require hospitalization and should be ★housed in isolation. n If neurologic disturbances are present (eg, tetraplegia, semicoma, seizure), euthanasia should be considered. Medications Drugs & Fluids n Treatment is supportive only. 3 n Antiviral medications are not rou- tinely used. Diagnosis by immunohistochemistry using