Dear Sir,
Recently, IDEXX Laboratories and Wyeth Animal Health held workshops across Ontario entitled “Lyme disease for the canine practitioner.” At the Kitchener meeting, data were presented from 53 541 dogs screened with the SNAP® 3Dx® Test (IDEXX Laboratories) in 2006. Seventy-five per cent (40 015) were from Ontario, 50 of which were positive for exposure to the Lyme Disease agent; the authors concluded that “This study demonstrates that a significant percentage of dogs in Canada have been exposed to Borrelia burgdorferi” (1). The take-home message was that Ontario dogs should routinely be screened for B. burgdorferi and Ehrlichia canis, and that dogs exposed to B. burgdorferi should be treated with antibiotics.
In our opinion, the risk of exposure to B. burgdorferi in dogs that have not travelled outside Ontario is very low. Adventitious Ixodes scapularis (vector ticks dropping off migrating birds) may be encountered uncommonly anywhere in Ontario, but only 10% to 12% are infected with B. burgdorferi. Exposure to B. burgdorferi in Ontario is most likely to be associated with travel to locations with established I. scapularis populations: Point Pelee, Rondeau, Long Point, and Turkey Point (all Lake Erie), Long Point — Prince Edward County, and the Thousand Islands (2). Ehrlichia canis infection is an extremely uncommon diagnosis in Ontario dogs that have not travelled, since its vector, Rhipicephalus sanguineus, prefers a warmer climate.
The very low risk of infection with B. burgdorferi and E. canis was reflected in a recent study of sera from dogs with suspicion of tick-borne illness submitted by Ontario and Quebec veterinarians; prevalence of antibody to B. burgdorferi and E. canis was 2/108 (1.9%) and 1/271 (0.4%), respectively (3). Unfortunately, the travel history was not available for most of the dogs. Over the last 5 y, there has been no diagnosis of Lyme disease in dogs at the Ontario Veterinary College, and just 1 case of canine ehrlichiosis (travel-associated). This correlates well with the low prevalence of exposure to B. burgdorferi (0.12%) and E. canis (0.02%) in the Ontario dogs screened with the SNAP® 3Dx® Test in 2006 (1).
The reported sensitivity and specificity of the SNAP® 3Dx® Test for detection of exposure to B. burgdorferi are 94.4% and 99.6%, respectively (4,5), indicating that the test is extremely good. However, if the prevalence of such exposure in Ontario dogs is truly 0.12% (1), the predictive value of a positive test is 22% (6); that is, more than 75% of positive test results will be false positives. Thus, unless confirmatory testing is carried out with a different test, unexposed dogs are at risk of being treated unnecessarily with antibiotics. The same concern applies to treatment of dogs with genuine subclinical B. burgdorferi infections, because the benefit of treating such dogs is uncertain (7,8).
In summary, the rationale for routinely screening healthy Ontario dogs for exposure to B. burgdorferi and E. canis is highly questionable. Any testing should be based on an evaluation of the clinical syndrome, risk of infection, and benefit of treatment, on a case-by-case basis.
References
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