Tick-borne diseases: Lyme borreliosis and anaplasmosis (Proceedings)


Tick-borne diseases: Lyme borreliosis and anaplasmosis (Proceedings)

Apr 01, 2010

Interpreting test results for agents of Lyme borreliosis and anaplasmosis

Testing for Lyme disease and anaplasmosis often involves detection of antibodies. Antibodies may be detected on a patient-side assay such as the 3Dx/4Dx SNAP tests or using IFA at a reference lab. The SNAP test uses C6 as the target antigen and thus the B. burgdorferi result is very specific. Only antibodies to wild-type B. burgdorferi will be detected and there is no reaction (no false positives) in vaccinated dogs in the absence of exposure. Because vaccines are not 100%, vaccinated dogs that are exposed to wild type organisms in nature may still seroconvert, and a low level of antibodies indicating exposure but not necessarily disease may result in a positive test. Positive SNAP tests for B. burgdorferi should be followed up with a quantitative C6 assay to determine the level of antibody present. If significant and if clinical disease is present, then treatment is indicated. In a seropositive dog with no evidence of clinical disease, a urinalysis can be performed to determine if proteinuria is present, which could suggest that Lyme nephritis may be developing. However, approximately half of all positive dogs in endemic areas will have significant levels of antibody. In non-endemic or very low-endemic areas like the southeastern US, the majority of B. burgdorferi SNAP-positive dogs have levels of antibody below that considered clinically relevant.

A positive result on the A. phagocytophilum component of the 3Dx/4Dx assay also indicates that antibodies are present, and thus infection, but not necessarily disease, has occurred. There is not a direct follow-up quantitative assay for SNAP-positive A. phagocytophilum samples as there is for B. burgdorferi positives. 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 should also be pursued in any dog with clinical signs of anaplasmosis. The A. phagocytophilum analyte is less specific than that targeting B. burgdorferi; for example, a positive may be generated on the A. phagocytophilum spot when exposure to A. platys has occurred. 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.

Other tests for tick-borne diseases include immunofluorescent antibody assays (IFA), polymerase chain reaction, examination of stained blood smears, and culture. The IFA tests are less specific than the patient side 3Dx/4Dx tests. However, IFAs can be performed quantitatively to determine a titer and sequentially to detect a change in titer, thus suggesting a recent, active infection that requires treatment. Positives for B. burgdorferi on IFA must be followed by a Western blot to both confirm the antibodies detected are indeed to B. burgdorferi and to distinguish from vaccine antibodies; for this reason, the more specific C6 testing is largely preferred to IFA for diagnosis of Lyme disease. Testing by PCR, which detects DNA of causative agents, can be quite useful to diagnose acute ehrlichiosis or anaplasmosis, so long as circulating organisms are present. Because B. burgdorferi is not present in circulation, PCR testing of whole blood is not used to diagnose Lyme disease. Anaplasmosis can also be diagnosed by finding morulae within neutrophils and occasionally eosinophils on stained blood smears. However, a negative result on PCR or blood smear examination does not rule-out the possibility of infection. Culture can be performed on skin biopsy or joint synovium (B. burgdorferi) or buffy coat (A. phagocytophilum) but is challenging and largely reserved for specialized research laboratories.

Geographic distribution of Lyme borreliosis and anaplasmosis

Lyme disease

Lyme disease is commonly recognized in both people and dogs in hyperendemic areas of the United States, which includes northeastern, upper midwestern, and West Coast states. Borrelia burgdorferi is the only etiologic agent of Lyme borreliosis identified in people or dogs in North America to date. The organism is maintained in nature through a cycle involving rodents, primarily white-footed mice, as reservoir host and Ixodes spp. ticks as vector. Infection is transmitted to people or dogs via the bite of nymphal or adult Ixodes spp. ticks that acquired the spirochetes when feeding on rodents as larvae or nymphs. Although Ixodes scapularis, the primary vector of Lyme disease in the eastern half of the US, is common in southern states, confirmed cases of Lyme disease in people or dogs are considered rare or non-existent in this region.