WHICH ORGANISMS SHOULD BE INCLUDED IN TESTING?
A patient's clinical findings, breed, geographic location, and travel history can help clinicians decide which organisms to
include in testing.
The nonspecific signs of malaise and gastrointestinal abnormalities are often reported in patients with vector-borne disease.
Physical examination findings that more specifically suggest a vector-borne disease include but are not limited to
- Evidence of disordered primary hemostasis (petechiation or mucosal bleeding and epistaxis due to vasculitis, thrombocytopenia,
- Ocular abnormalities (e.g. retinal hemorrhages, uveitis, chorioretinitis, perivascular infiltrates)
- Arthralgia and joint effusion
- Respiratory signs
- Neurologic signs.
Clinicopathologic findings commonly associated with vector-borne disease include but are not limited to
- Synovial fluid analysis results consistent with neutrophilic polyarthritis.
Although immune-mediated disease could theoretically be triggered by any infectious agent, it can be diagnostically useful
to consider that some findings have been specifically associated with particular organisms in dogs. For example, Babesia species, Anaplasma phagocytophilum, Bartonella species, Ehrlichia canis, and hemotropic Mycoplasma species have been associated with immune-mediated hemolytic anemia (IMHA) in dogs.5-9 Therefore, it would be prudent to rule out infection with these agents in a dog with IMHA.
Further examples of signs associated with certain disease agents include vasculitis with Rickettsia, Ehrlichia, and Bartonella species; granulomatous or pyogranulomatous inflammation with Bartonella species; and hyperglobulinemia with Ehrlichia species and other organisms that cause chronic disease.7,10 Comprehensive summaries of clinical signs that have been associated with specific infections are available.11
Breed association can also help clinicians decide which organisms to include in diagnostic testing. For example, infection
with Babesia gibsoni should be ruled out in American pit bull terriers with hemolytic anemia or thrombocytopenia.12,13 Commonly used serologic and PCR tests that target Babesia canis may have negative results in a dog infected with B. gibsoni. Knowing that B. gibsoni is prevalent in American pit bull terriers can help you direct appropriate testing.
Another example is that greyhounds are overrepresented among dogs infected with B. canis,13 presumably because of their exposure to Rhipicephalus sanguineus (the brown dog tick) in kennel environments. In addition to B. canis, E. canis would also be important to rule out in this or any dog breed exposed to kennels because both of these organisms are transmitted
by R. sanguineus.
Additional examples include the fact that vector-borne disease is common in hunting breeds, and leishmaniasis is common in
foxhounds in the United States. Thus, consideration of breed can help guide testing.
Geographic location and travel history
Geographic location and travel history are also important to consider when deciding which organisms to include in testing.
For some organisms, the distribution of exposure is fairly geographically restricted. For example, A. phagocytophilum infection should be suspected in an acutely ill thrombocytopenic dog with arthralgia if that dog lives in or has traveled
to the upper Midwest, Northeast, or Pacific Coast; whereas, Ehrlichia ewingii infection would be considered more likely if a dog lives in or has traveled to the south central or southeastern United States.14-16
Babesia conradae is an important differential diagnosis in dogs with IMHA that have lived in or traveled to southern California, but it has
not been documented in other areas of the United States.17 In contrast, infection with E. canis or B. canis should be considered in a dog with compatible clinical signs in most parts of the United States, as these organisms are transmitted
by R. sanguineus, a ubiquitous tick with an expanding geographic distribution.18-20 Summaries of common clinical signs and geographic distributions of infectious agents are available.11
Although geographic locale helps determine which agents should be included in testing, it is important to be aware the distribution
of vectors and their associated infectious agents is expanding. For example, the incidence of Rocky Mountain spotted fever
(RMSF) in people in the United States has recently expanded beyond the distribution of its historic tick vectors, Dermacentor variabilis and Dermacentor andersoni. Rhipicephalus sanguineus recently caused an outbreak of RMSF in people in a nonendemic area of Arizona.20 Retrospectively, it was shown that infection existed in the dog population before the fatal outbreak occurred in people.21 Thus, although it can help clinicians to decide what to include, geographic locale should not necessarily be used to restrict