Clinical and laboratory findings

Clinical disease in dogs is most often associated with the acute, bacteremic phase of infection. The duration and severity of the clinical findings during the acute phase of the disease can vary greatly and can last from one to several days.⁵

Chronic, subclinical, persistent infections have been documented for more than five-and-a-half months in dogs experimentally inoculated with a Swedish isolate of A. phagocytophilum and for almost one year in dogs inoculated with a human isolate from New York (NY18).^6,7 However, no confirmed reports of clinical disease occurring in animals known to be chronic carriers exist. It is unknown whether a chronic carrier state can later result in chronic disease.

Clinical signs. A high seroprevalence in people and dogs living in endemic areas¹⁰ suggests that many infections result in a mild, flu-like disease that is self-limiting or in a subclinical infection. Animals with clinical disease associated with acute infection often have vague signs of illness including fever, lethargy, malaise, anorexia, and general muscle pain resulting in reluctance to move.

The most frequently observed clinical findings in dogs that would alert you to A. phagocytophilum infection are joint pain and lameness resulting from polyarthritis. Other less commonly observed clinical signs include gastrointestinal problems such as vomiting, diarrhea, or both or respiratory signs such as coughing and labored breathing. Central nervous system disease (meningitis) can also occur, resulting in seizure activity, ataxia, or neurologic manifestations such as dullness or stupor, but these findings are infrequently observed.

Because many animals present with clinical signs of polyarthritis and possibly a history of tick exposure, clinical signs of canine anaplasmosis may be indistinguishable from those seen with Lyme disease. In addition, both diseases are transmitted by the same tick vectors and have similar geographic distributions.

Laboratory test results. Abnormalities in laboratory test results may vary during the acute phase of the disease. Test results will likely be normal in persistently infected, subclinical carriers.

The most common hematologic abnormality noted in clinically affected dogs is a mild to severe thrombocytopenia, seen in more than 80% of acutely infected dogs.^2,8,11 Additionally, many animals will initially have lymphopenia but later may develop a reactive lymphocytosis. Rarely, neutropenia is seen. Membrane-bound vacuoles of intracytoplasmic organisms (morulae) can sometimes be observed in circulating neutrophils and, rarely, in eosinophils in acutely infected dogs. A mild to moderate nonregenerative anemia is occasionally seen.

Common serum biochemical abnormalities include elevated serum alkaline phosphatase activity and mild to moderate hypoalbuminemia and hyperfibrinogenemia.² The hypoalbuminemia usually resolves quickly once animals are afebrile.