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Ehrlichia Canis/Anaplasma Phagocytophilum: What Do We Do with a Positive? Stephen C

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Ehrlichia Canis/Anaplasma Phagocytophilum: What Do We Do with a Positive? Stephen C Ehrlichia canis/Anaplasma phagocytophilum: What do we do with a positive? Stephen C. Barr BVSc MVS PhD DACVIM College of Veterinary Medicine Cornell University Ithaca, NY. INTRODUCTION • Tick-borne diseases of dogs and cats causing vasculitis leading to multisystemic dysfunction often characterized by thrombocytopenia and hyperglobulinemia. • Recently reclassified – moved from the family Rickettsiaceae to the Anaplasmataceae. OVERVIEW OF ETIOLOGIC AGENTS • 3 pathogenic genera: Ehrlichia, Anaplasma, and Neorickettsia; we are concerned here with the first 2. • Ehrlichia spp. – divided into 3 groups:- o E. canis – intracyctoplasmic morulae found in circulating leukocytes. o E. chaffeensis – like E. canis, found in mononuclear cells; mainly a human pathogen but can causes disease in dog. o E. ewingii – canine granulocytic ehrlichiosis; like A. phagocytophila, infects granulocytic cells in dogs, but differs in geographic distribution (mainly found in southeastern and south-central USA). • Anaplasma spp. – 2 organisms of importance:- o A. phagocytophila – previously E. equi, E. phagocytophila, and HGE; infects mainly horses; infects granulocytic cells of dogs; serologic cross reactivity occurs with other Anaplasma spp. but uncommon with Ehrlichia spp.; same distribution as Lyme dz (same tick vector) - northeastern and upper Midwestern states, and California. No serologic cross reactivity with Borrelia, Bartonella or Rickettsia rickettsii. o A. platys – previously E. platys; tropism for platelets; does not share serologic cross reactivity with E. canis. In this presentation, we will focus on E. canis and A. phagocytophilum. PATHOGENESIS Ehrlichia canis – Canine monocytotrophic ehrlichiosis. • Rhipicephalus sanguineus (brown dog tick) and occasionally Dermacentor variabilis transmits disease to dogs in saliva. • Incubation period – 1 to 3 weeks. • Severity and outcome of disease depends on infecting strain and dose of inoculum; German shepherd dogs seem more clinically affected • After infection – organisms multiply in macrophages then spread throughout hose:- • 3 stages of disease:- o Acute – spreads from bite site to the spleen, liver, and lymph nodes (causes organomegaly); then subclinical with mild thrombocytopenia; mainly endothelial cells affected; vasculitis; anti-platelet antibodies exacerbates thrombocytopenia; variable leukopenia; mild anemia; severity depends on organism. o Subclinical – organism persists; antibody response increases (hyperglobulinemia); thrombocytopenia persists. o Chronic – impaired bone marrow production (platelets, erythroid suppression); marrow hypercellular with plasma cells. • Multisystemic disease – including bleeding tendencies (thrombocytopenia and vasculitis), lymphadenopathy, splenomegaly, CNS, ocular (anterior uveitis), and lung. Anaplasma phagocytophilum – Canine granulocytic anaplasmosis. • Ixodes spp (I. scapularis in northeast, I. pacificus in California). transmit Dz in saliva • Incubation period – 1to 2 weeks (much shorter than for Lyme disease). • Reservoir hosts - white-footed mouse, chipmunks, wood rats, voles, and white-tailed deer; various bird species are also implicated in spread of disease. • After infection, organism binds and enters mainly neutrophils – multiply (morula) – rupture cell to release more organisms which infect more cells. • How organisms cause disease is not known; after ~ 10 days of infection, cellular and humoral immune mechanisms control infection. • Clinical findings are virtually always associated with acute disease period during the bacteremic phase – chronic disease has not been reported. • Mild illness typified by fever, lethargy and thrombocytopenia; polyarthritis occurs but rare in comparison to infection with E. ewingii; most dogs present in the spring, early summer and again in the fall. CLINICAL SIGNS Ehrlichia canis • Duration of clinical signs from initial acute illness to presentation – usually > 2 months. Acute • Bleeding diatheses – resulting in the development of petechia on mucous membranes (conjunctiva, skin), epistaxis, retinal hemorrhage due to thrombocytopenia and vasculitis • Fever – with depression, anorexia, weight loss. • Generalized lymphadenopathy/splenomegaly (in about 25% of cases). • Ticks – found in 40% of cases. • Respiratory – dyspnea (even cyanosis) and increased bronchovesicular sounds. • Diffuse CNS disease – meningitis (seizures, stupor, ataxia, vestibular dysfunction, cerebellar dysfunction, anisocoria). • Generalized or local hyperesthesia. • Polyarthritis can occur but is rare in E. canis infections – much more common in E. ewingii and A. phagocytophilum. • Most dogs recover without treatment – then enter a subclinical state. Chronic • Spontaneous bleeding, anemia, and generalized lymphadenopathy. • Scrotal and limb edema. • Splenomegaly, hepatomegaly. • Uveitis, hyphema, retinal hemorrhages and detachment with blindness, corneal edema. • Seizures – rare. Anaplasma phagocytophilum • Middle aged to older dogs are more likely diagnosed with the disease. • Most dogs present in the spring, early summer and again in the fall. • Mild illness typified by fever, lethargy, depression, anorexia. • Generalized musculoskeletal pain (reluctance to move, stiff, lameness) – 50% of dogs. • Actual joint pain is found in fewer than 10% of cases. • Very rarely – lymphadenopathy, splenomegaly, hepatomegaly, or CNS signs. • Although thrombocytopenia occurs, bleeding diatheses do not (clinically inapparent). • Dogs are susceptible to reinfection (with 5 mths or more between infections). • Serum antibody levels revert to negative levels by 7 to 8 mths post-treatment. • Chronic carrier state may exist although asymptomatic – none to date show chronic Dz. • Unknown if treatment removes carrier status. • Dogs co-infected with A. phagocytophilum and Borrelia (Lyme disease) will have more severe disease. DIAGNOSIS Ehrlichia canis Acute • CBC – thrombocytopenia (before onset of clinical signs), anemia, leukopenia (due to lymphopenia and eosinopenia), leukocytosis and monocytosis (as disease becomes more chronic); morulae (intracytoplasmic inclusions in leukocytes) are rare. • Mild increases in ALT, ALP, BUN, creatinine, and total bilirubin (rare). • Hyperglobulinemia – progressively increases 1–3 weeks post-infection. • Hypoalbuminemia – usually from renal loss. • Proteinuria – with or without azotemia. Chronic • CBC – Pancytopenia typical; monocytosis and lymphocytosis may be present. • Hyperglobulinemia – size of globulin increase correlates with duration of infection; usually polyclonal gammopathy, but monoclonal (IgG) gammopathies occur. • Hypoalbuminemia. • Elevated BUN and creatinine – glomerulonephritis and renal interstitial plasmacytosis. Serology • Indirect FA – positive by 7 days post-infection (some not until 4 wks PI); most clinically useful and reliable method; highly sensitive – poor specificity with cross-reactivity with E. chaffeensis and E. ewingii (not problems in the northeast) but not A. phagocytophilum (in A. phagocytophilum IFA, slight cross-reactivity with E. canis serum). • Titers usually 1:80 are diagnostic; paired titers recommended as single titer can reflect past infection; titer progressively declines over 6 to 9 months after treatment. high seroprevalence compared to disease prevalence. • Canine Snap® 3 and 4Dx Diagnostic Test (IDEXX Laboratories, Westbrook, ME) – an in-house snap test which tests for antibodies against Lyme disease and E. canis, and heartworm antigen (4D included A. phagocytophilum); detects positive titers to E. canis at an equivalent IFA titer of > 1:500; sensitivity of 95%; specificity of 100% (according to manufacturer); also detects antibodies against E. chaffeensis but not E. ewingii; as with any serologic test, predictive value also depends not only on sensitivity and specificity but also prevalence; in low prevalence areas, likely to get a high false positive rate; could get false positives by reactions to non-pathogenics; confirm +ve test with IFA. • Western Blotting – may be used to differentiate mixed infection. • PCR – proving to be very sensitive in early detection of experimental infections (positive as early as 4 to 10 days PI) but sensitivity (using whole blood samples) in naturally infected cases not as clear cut; poor correlation with IFA in some studies; should be used in conjunction with serology to detect early cases; analysis of samples other than blood (joint fluid, CSF, aqueous humor, splenic aspirates) may be more clinically applicable. Anaplasma phagocytophilum • Most reported cases have been found by finding morulae in neutrophils on the CBC; however, more and more cases are being diagnosed in asymptomatic dogs using the Snap® 4Dx Diagnostic Test (IDEXX Laboratories, Westbrook, ME). • Up to ~40% of neutrophils can contain morulae (are present for 5 to 9 days experimentally); about 80% have mild thrombocytopenia and lymphopenia, and most have eosinopenia (but not neutrophilia); some have mild anemia and hypoalbuminemia. Serology • Sero-conversion (using indirect IFA) occurs about 2 to 5 days after appearance of morulae in blood, and titers remain detectable for about 8 months after acute infection. • Need paired samples 2 to 3 weeks apart to show 4 fold increase in titer because 40% of acutely ill morulae-positive dogs can be sero-negative at presentation; low titers may be because of a previous infection. • Serologic cross-reaction does occur with other Anaplasma spp. and E. canis (although mild) but becomes less cross-reactive during convalescences; no cross- reactions
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