An update on anaplasmosis in dogs
This form of anaplasmosis can be difficult to distinguish from Lyme disease, but new diagnostic capabilities are now available.
Light microscopy. Canine anaplasmosis is often diagnosed by microscopic identification of morulae in circulating neutrophils in the peripheral blood (Figure 1) and sometimes the synovial fluid (Figure 2). These findings are most often identified during the acute phase of the disease. Experimentally, organisms appear in the peripheral blood between four and 14 days after inoculation and usually persist for up to eight days.5 Anaplasma phagocytophilum morulae may be seen in 1% to 27% of circulating neutrophils. The ability to identify morulae in the circulating neutrophils of infected dogs is enhanced by preparing and microscopically evaluating a buffy coat smear (Figure 3). In animals presenting with polyarthritis, synovial fluid analysis would reveal decreased viscosity and an increased leukocyte count (> 3,000 cells/µl) with a predominant neutrophil population. Organisms may also be found in a small number (≤ 1%) of neutrophils in synovial fluid (Figure 2).
Serologic testing. Anaplasma phagocytophilum infection can be diagnosed serologically at most commercial laboratories with indirect fluorescent antibody (IFA) testing as well as with the in-house ELISA, SNAP 4Dx (IDEXX Laboratories).
The IFA test uses whole organisms grown in cell culture as the source of antigens. With this test, experimental studies have shown that seroconversion in dogs may occur as soon as two to five days after morulae appear in the peripheral blood.8 In another study using sera collected from confirmed cases of A. phagocytophilum infection in horses, dogs, people, and cattle, all serum samples were positive at titers of 1:80 or greater, and most had titers of ≥ 1:320.13 For this reason, and because nonspecific binding of antibodies can occur using this assay when serum samples are at dilutions of 1:40 or less, a titer of ≥ 1:80 is considered positive for antibodies to A. phagocytophilum. In cross-reactivity studies, serum samples from animals or people infected with Neorickettsia risticii, Ehrlichia canis, E. ewingii, Ehrlichia chaffeensis, Ehrlichia sennetsu, Rickettsia rickettsii, Ehrlichia typhi, Bartonella henselae, or Bartonella quintana did not contain antibodies that cross-reacted with A. phagocytophilum.13 Serum samples from animals infected with A. platys were not evaluated in this study.