The A. phagocytophilum test spot on the in-house SNAP 4Dx assay uses a synthetic peptide based on the immunodominant p44 protein of A. phagocytophilum as the antigen. Experimentally inoculated dogs had positive test results on the SNAP 4Dx assay as early as eight days after
inoculation.7 These animals remained persistently infected for nearly one year and remained seropositive during that entire time even
though the animals were clinically healthy and light microscopy did not reveal organisms in circulating neutrophils. In performance
trials, the sensitivity and specificity of the SNAP 4Dx test for antibody to A. phagocytophilum was 99.4% and 100%, respectively.14 Dogs infected with Ehrlichia species, including E. ewingii, will not likely cross-react with the A. phagocytophilum test on the SNAP 4Dx. In addition to testing for infection with A. phagocytophilum, the SNAP 4Dx also detects infection with Dirofilaria immitis, Borrelia burgdorferi, or E. canis.
PCR testing. Nucleic acid amplification is the most sensitive method for detecting organism DNA in the peripheral blood of an infected
animal. Several commercial laboratories offer polymerase chain reaction (PCR) analysis of peripheral blood for detecting A. phagocytophilum and E. ewingii. These assays are species-specific and can distinguish between these two infectious agents. Submit EDTA-anticoagulated peripheral
blood to laboratories for PCR analysis. Collect these samples before initiating antimicrobial therapy because recent antibiotic
administration, particularly tetracycline or its derivatives, increases the likelihood of a false negative result.
Although this technique is more sensitive in detecting circulating organisms than is light microscopy, it has been shown to
be only intermittently positive in subclinically infected, persistent carriers of A. phagocytophilum since organisms may circulate intermittently in the peripheral blood of subclinically infected animals.7,15 Therefore, PCR analysis should not be considered a definitive method for conclusively excluding subclinical infection in
clinically normal animals that are seropositive on routine screening.
Treatment and prognosis
The treatment for canine anaplasmosis is the same as that for infection with the closely related Ehrlichia species—doxycycline. The optimal dose and length of therapy have not been firmly established, but an oral dosage of 5 to
10 mg/kg given twice a day for 30 days has been recommended.12,16 In most cases, clinical signs resolve rapidly. Dogs are often markedly improved 24 to 48 hours after therapy is instituted,
and the prognosis for clinical recovery is excellent.12 However, persistent infection and subclinical carrier states have been demonstrated in experimentally inoculated dogs treated
with doxycycline for 14 days and 28 days at the recommended dose.7,15
Although clinical resolution of disease is usually accomplished, it may be difficult to ascertain the effectiveness of therapy
in completely clearing organisms from naturally infected animals. If an animal no longer produces antibodies to the organism,
this may indicate that the organism has been cleared from the body. Since PCR analysis can give false negative results, this
assay is unreliable in detecting all subclinical, persistent carriers.
In light of these diagnostic challenges, once an animal with clinical anaplasmosis has been treated with the recommended course
of doxycycline therapy and is clinically normal, administering a second course of doxycycline therapy or a different antimicrobial
agent, in an attempt to obtain a seronegative status, is not recommended at this time.
Because tetracyclines may result in tooth discoloration, chloramphenicol (15 to 25 mg/kg every eight hours for 14 to 21 days)
has been suggested as an alternative to tetracycline derivatives in dogs under 1 year of age.12 However, it appears that doxycycline, unlike tetracycline, does not result in enamel discoloration in young growing puppies.
In vitro testing has shown that rifampin and levofloxacin are also effective against A. phagocytophilum, but their effectiveness in vivo has not been evaluated in dogs.17