Anaplasma phagocytophilum is an obligate intracellular, gram-negative, aerobic bacterium that infects neutrophils and, more rarely, eosinophils. It was first documented in humans in 1990, and has received considerable attention in the past several years due to its ability to cause disease in humans, dogs, and horses. It is transmitted by the Ixodes scapularis tick in the midwest and the northeastern U.S., and by the Ixodes pacificus tick in the northwest. Reservoir hosts include the white-tailed deer and small rodents, such as the white-footed mouse. Transmission seasonality may exist, and transmission may be associated with the feeding behaviors of the Ixodes vector (early summer and early fall). Dogs seropositive for A phagocytophilum from August 2004 to March 2008 were included in this study (n = 26). Positive titers for both A phagocytophilum and Borrelia burgdorferi were identified in 45% of dogs, and 3 of 10 dogs with positive B burgdorferi titers were vaccinated prior to documentation of positive titers (7 were not vaccinated). The most common clinical signs associated with presentation included lethargy, inappetence, lameness, and fever. Other clinical signs and physical examination findings included cough, epistaxis, lymphadenopathy, joint effusion, vomiting, splenomegaly, and hepatomegaly. Interestingly, neither of the 2 dogs with epistaxis had platelet counts low enough to expect spontaneous bleeding, suggesting an alternate mechanism, such as vasculitis, for the bleeding. Mild to severe thrombocytopenia was the most common laboratory abnormality and was more severe in dogs coinfected with B burgdorferi. Immune-mediated hemolytic anemia (IMHA) was diagnosed in 3 dogs, and 2 of 3 were seropositive for other agents, including Ehrlichia risticii, Rickettsia rickettsii, and B burgdorferi. All 3 IMHA patients died, and follow-up and response to treatment were impossible to assess. Overall, some A phagocytophilum seropositive dogs had relapse of clinical signs after a therapeutic course of doxycycline, which averaged 10 mg/kg Q 24 H for 21 to 28 days.

Commentary: Anaplasmosis continues to elude us as we try to determine the clinical significance of the disease. The most difficult aspect of elucidating this disease (as with other tick-borne diseases) is the lack of repeatable data concerning the fulfillment of Koch’s Postulates (where the injection of a pathogen produces treatable or reproducible disease in a consistent fashion). Further data evaluating disease prevalence and treatment are required; however, this study does highlight the importance of A phagocytophilum testing, particularly if a patient presents with vague and nonspecific clinical signs of illness.—Heather Troyer, DVM, Diplomate ABVP (Canine & Feline Practice)

Clinical presentation of 26 Anaplasma phagocytophilum-seropositive dogs residing in an endemic area. Mazepa AW, Kidd LB, Young KM, Trepanier LA. JAVMA 46:405-412, 2010.