What is the significance of a positive result?
PCR. Assuming appropriate laboratory controls are in place, a positive PCR result indicates active infection or the presence of
circulating dead organisms. PCR assay results can be positive at the level of genus or species, depending on the specific
DNA sequence that the primers target. Determining species may have implications with regard to treatment and prognosis, among
other factors, so speciation of a positive result for a genus is important.
Serology. A positive serologic test result may indicate exposure to or active infection with a given organism or an antigenically related
organism. Considering the epidemiology and pathophysiology of the organism can help determine the clinical significance of
a positive result.
For example, a positive titer to B. burgdorferi in a Lyme disease-endemic area may or may not be evidence that infection is causing clinical signs. Anaplasma phagocytophilum and R. rickettsii typically cause acute disease in dogs. Persistent infection with A. phagocytophilum has been documented in experimentally infected dogs.27 Chronic infection has not been documented in naturally infected dogs, but it may occur.8,22 Chronic infection with R. rickettsii has not been documented in experimentally or naturally infected dogs, but chronic disease has been associated with exposure
to another spotted fever group (SGF) Rickettsia, R. massiliae.28 Previous infection with either A. phagocytophilum or R. rickettsii may result in long-lived antibody titers.22,29
The clinical relevance of these observations is that if serologic test results are positive for one of these agents and clinical
signs have been present for many months, coinfection with other vector-borne agents, the presence of cross-reacting antibodies,
or the presence of other underlying disease should be considered as additional explanations for the clinical signs. This is
particularly true if infection cannot be verified by PCR or there is inadequate response to treatment. In contrast, a positive
titer to Babesia or Bartonella species or E. canis is more likely to be clinically significant in a patient with or without clinical signs because longstanding, subclinical
infection is common in dogs naturally infected with these agents.7,10,12
Serologic cross-reactivity among species within a genus and among genera also affects the interpretation of a positive serologic
test. For example, there is extensive serologic cross-reactivity among SFG Rickettsia. Rickettsia rickettsii is the organism used in most serologic testing for SFG Rickettsia in the United States. However, there are other species of SFG Rickettsia that infect people in the United States. It is likely that these organisms also infect dogs and induce disease.28,30 Furthermore, exposure to nonpathogenic SFG Rickettsia, some of which are common endosymbionts in ticks, may be a common cause of positive titers, particularly low and persistent
titers, to R. rickettsii in dogs.31 Serologic cross-reactivity with SFG Rickettsia also appears to occur in dogs infected with Bartonella henselae.32
Cross-reactivity also occurs among other genera and species and must be considered when interpreting any positive serologic