Serologic assays, predominantly immunofluorescent antibody (IFA) tests, have been the mainstay of diagnosis for most tick-borne
infections. However, serologic testing has limited ability to identify acute infection or to differentiate infection from
prior exposure or vaccination (e.g. Lyme disease). A high degree of serologic cross-reactivity exists among species within the same genogroup; thus, for example,
seroreactivity to E. canis may in fact represent exposure to Ehrlichia chaffeensis or Ehrlichia ewingii. Serologic cross-reactivity occurs among other pathogens.29 Patterns of cross-reactivity are summarized in Table 3.
Table 3. IFA Cross-reactivity
Further complicating serologic testing is the fact that chronic, but not acute, infection with E. canis causes cross-reactivity with A. phagocytophilum. In one study, six of six dogs experimentally infected with E. canis became seropositive for A. phagocytophilum within 150 days after infection with E. canis.37 Infection with A. phagocytophilum can also cause cross-reactivity with E. canis.21 It has been proposed that the appearance of A. phagocytophilum genogroup antigens in dogs infected with E. canis may indicate persistent E. canis organisms, and, in treated cases, their appearance may be a serologic marker of treatment failure.37
The development of enzyme-linked immunosorbent assays (ELISAs) for identification of ehrlichiosis and borreliosis has allowed
for quicker in-house testing. Comparing a diagnosis of ehrlichiosis based on positive results of the ELISA-based Snap 3Dx
Test (IDEXX Laboratories) with IFA showed an overall agreement of 91% (61/67) in experimentally and naturally infected animals,
with all disagreement occurring in samples found to have IFA titers of 1:320 or less.38 Sensitivity and specificity of the Snap 3Dx Test were 0.71 and 1.00, respectively.38 In a later study comparing the Snap 3Dx Test with a commercial IFA, similar results were found. Stored serum samples known
to be nonreactive, low-reactive (titer 80 to 160), medium-reactive (titer 320 to 2,560), and highly reactive (titer 5,120
to > 20,480) by IFA were consistent with ELISA results in the nonreactive, medium-reactive, and highly reactive groups.39 Discordance was only identified in the low-reactive group, indicating ELISA sensitivity is poor for this group; however,
the authors postulated that poor specificity of the IFA could result in this discordance as well.39 Unfortunately, in each study, naturally infected cases were not evaluated with PCR testing to exclude coinfection or assess
cross-reactivity. Because ELISAs give clinicians no information about titers, the tests have somewhat limited use in assessing
therapy for which decreases in serum titers would be considered a positive response to treatment. Although the IFA is subject
to cross-reactivity, because of better sensitivity as compared with ELISA, it remains the test of choice for diagnosing ehrlichiosis.
The Snap 3Dx Test detection of borreliosis was highly sensitive and specific.40 In addition, the ELISA was able to distinguish vaccinated from unvaccinated individuals.40 Disadvantages of this test relate to the high prevalence of exposure to Borrelia burgdorferi in the population in addition to limitations that parallel those described above for detecting ehrlichiosis. Despite these
disadvantages, the Snap 3Dx Test is a viable option in place of IFA for diagnosing borreliosis when there is an appropriate
history and physical and laboratory abnormalities.