This form of anaplasmosis can be difficult to distinguish from Lyme disease, but new diagnostic capabilities are now available.
1. An intracytoplasmic Anaplasma phagocytophilum morula in a toxic band neutrophil in the peripheral blood of an infected dog (Wright's-Giemsa; 100X).
2. An intracytoplasmic Anaplasma phagocytophilum morula in a neutrophil in the synovial fluid of an infected dog (Wright's-Giemsa; 100X).
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).
Anaplasma phagocytophilum morulae cannot be distinguished from those caused by E. ewingii, the causative agent of canine granulocytic ehrlichiosis.12 Clinical signs of E. ewingii infection are also identical to those seen with canine anaplasmosis, and these two infections cannot be distinguished by
clinical (fever, lethargy, lameness, or reluctance to stand) or routine laboratory (complete blood count or synovial fluid
3. An intracytoplasmic Anaplasma phagocytophilum morula in a neutrophil (above center) in a buffy coat smear prepared from the peripheral blood of a dog. An intracytoplasmic
Döhle body is seen in the neutrophil in the upper left corner (Wright's-Giemsa; 100X).
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.