Vector-borne diseases in general, and tick-borne diseases in particular, are increasingly recognized as important in both
veterinary medicine and public health. In recent years, new disease agents have been identified in both dogs and people,
tick populations have increased in number and extent of geographic distribution, and the potential for transmission of disease
agents to people and dogs appears to be increasing. Throughout much of the United States, as many as six major tick species
may be commonly found on dogs, and at least one stage or species of tick is active in any given month, creating a risk for
year-round transmission of a number of different tick-borne disease agents (Table 1). This presentation will review important
features of two common tick-borne diseases of dogs, ehrlichiosis and Rocky Mountain spotted fever, including clinical features,
diagnostic and treatment strategies, where the infections occur geographically, and frequency and importance of infection
and co-infection with these agents.
Interpreting test results for agents of ehrlichiosis and RMSF
Tick-borne disease testing often involves detection of antibodies to different organisms of interest. Antibodies may be detected
on a patient-side assay such as the 3Dx/4Dx SNAP tests (ehrlichosis) or may require a diagnostic laboratory to perform an
IFA (RMSF). The E. canis component of the 3Dx/4Dx assay indicates that antibodies are present, and thus prior or current infection, but not necessarily
disease, has occurred. Thus, when positives occur, a complete blood count with platelet count should be performed. If a
low platelet count is present then treatment is indicated. Treatment is also indicated in any clinically ill dog. A positive
also may be generated on the E. canis spot when exposure to E. chaffeensis has occurred; indeed, recent research shows that the majority of E. canis SNAP-positive dogs in some areas actually have exposure to E. chaffeensis. Antibodies may also be detected on IFA using E. canis infected cells as antigen. This approach allows determination of a titer. However, cross-reactivity among different Ehrlichia spp., including E. canis, E. chaffeensis, and E. ewingii is common. Regardless of the cause of antibody production, positives do indicate that there has been a breach in the tick
control program and that the dog is being bitten by ticks and at risk of contracting a tick-borne disease. For detecting
antibodies to RMSF, IFA using R. rickettsii infected cells as antigen is performed. Rapid patient-side assays for RMSF are not available. The IFA also detects antibodies
generated against other Rickettsia spp. and so specificity for R. rickettsii is not assured. Dogs with RMSF can develop severe disease and die prior to seroconversion; treatment should not be delayed
pending confirmation with serologic testing.
Other tests for ehrlichiosis and RMSF include polymerase chain reaction, examination of stained blood smears, and culture.
Testing by PCR, which detects DNA of causative agents, can be quite useful to diagnose acute ehrlichiosis or RMSF, so long
as circulating organisms are present. Ehrlichia spp. are found in circulating monocytes (E. canis, E. chaffeensis) or neutrophils (E. ewingii). Rickettsia rickettsii may be present in circulation and thus detected by PCR in sloughed infected endothelial cells. Ehrlichiosis can also be
diagnosed by finding organisms within blood cells on stained blood smears. However, the organisms are often present in low
numbers, particularly with the monocytic forms, and specialized training is required to identify them accurately. A negative
result on PCR or blood smear examination does not rule-out the possibility of infection. Culture for Ehrlichia spp. can be performed but is challenging and largely reserved for specialized research laboratories. Culture of R. rickettsii is only performed in high level biosafety laboratories (BL-3) due to the risk of severe human disease.