- 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.
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:-
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.
- Duration of clinical signs from initial acute illness to presentation – usually > 2 months.
- 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.
- Spontaneous bleeding, anemia, and generalized lymphadenopathy.
- Splenomegaly, hepatomegaly.
- Uveitis, hyphema, retinal hemorrhages and detachment with blindness, corneal edema.
- 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.
- 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.
- 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.
- Elevated BUN and creatinine – glomerulonephritis and renal interstitial plasmacytosis.
- 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.
- 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.
- 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 with Borrelia or Rickettesia. A positive Snap® 4Dx™ Diagnostic Test in southeast US is most likely due to A. platys; in the northeast US, it is most likely A. phagocytophilum.
- Also, by using the Snap® 4Dx™ Diagnostic Test, if there is a positive E. canis and A. phagocytophilum reaction, the dog could have both infections, or Anaplasma with a cross reaction with E. canis (and likely an acute active infection and not convalescent), but unlikely to have E. canis alone because there is little cross-reactivity between E. canis antigen and A. phagocytophilum-positive serum.
- PCR – more sensitive than finding morulae and provides a very species-specific test in the right hands – not well reported in diagnosing naturally-infected cases.
- Only E. canis cases might need hospitalization; stabilize medical for anemia and hemorrhagic tendency (due to thrombocytopenia) – fluids, blood transfusion, platelet-rich plasma (in emergency), or treat for the results of pancytopenia (sepsis) with broad spectrum antibiotics if necessary.
- Short-term therapy of glucocorticoids (2 to 5 days of 2mg/kg/day of prednisone) may be beneficial early in acute disease when life-threatening thrombocytopenia is present, and may help other immune mediated associations of the disease (polyarthritis, vasculitis, meningitis).
- Androgenic steroids (oxymetholone or nandrolone decanoate) or granulocyte colony-stimulating factor (Neupogen®, Amgen) – to stimulate bone marrow production in chronically E. canis-affected dogs with hypoplastic marrows may be indicated.
- Doxycycline is the drug of choice for both E. canis and A. phagocytophilum (5 mg/kg, PO, q12h for 3 to 4 weeks) – see rapid (within 72 hrs of onset of treatment) improvement of platelet count in acute cases; recovered dogs still able to become re-infected.
- Imidocarb dipropionate (Imazol®, Schering-Plough Animal Health – 5mg/kg, IM, once then repeat in 2 to 3 weeks); reasonable alternative to doxycycline; pretreatment with atropine may lessen anticholinergic adverse effects (salivation, serous nasal discharge, diarrhea).
- Both doxycycline and imidocarb give ~ 95% cure rate – not improved if used together.
- Other effective antibiotics include: minocycline, tetracycline, oxytetracycline, and chloramphenicol; enrofloxacin is not effective.
- A seropositive (by the 4Dx test) asymptomatic dog for A. phagocytophilum - could treat with doxycycline with hope (unknown if it does) that it removes carrier state, but as carriers seem to remain subclinical, this practice is controversial.
- Prognosis in acute E. canis and A. phagocytophilum infections is excellent; response may take months in chronic ehrlichiosis.
- Control tick infestations with dips or sprays containing dichlorvos, chlorfenvinphos, dioxathion, propoxur, or carbaryl; flea and tick collars may reduce reinfestation but reliability unproven; selamectin topically (Revolution®, Pfizer) monthly; a combination of imidacloprid and permethrin is effective; avoid tick-infested areas.
- Examine dogs for concurrent infections that exist in the region frequented by the pet.
- Zoonotic potential – serologic evidence indicates that E. canis (or possibly a related species) occurs in people; probably not directly infected from dogs; tick exposure thought to be necessary; R. sanguineus probably not the vector in humans.
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