Canine heartworm testing in Canada: Are we being effective?

Mass testing of dogs in Canada for the presence of Dirofilaria immitis has been ongoing since 1977. Since that time, there have also been changes in t he diagnostic tests available to detect the presence of heartworm and chang es in the therapy for heartworm, which necessitate a reevaluation of heartw orm screening as currently practised in Canada. The principles of evidence- based medicine were used to determine the prevalence of heartworm infection in various dog populations, and the effectiveness of screening these popul ations. The annual surveys of heartworm testing have shown that Canada is a low prevalence area (0.16%), with most of the test-positive dogs located i n southern Ontario (0.19%), southern Manitoba (0.18%), southern Quebec (0.0 9%), and the southern Okanagan Valley (0.04%). Foci of higher prevalence ar e found within these 4 main geographic areas. Furthermore, the prevalence o f heartworm infection is higher in the population of dogs not on preventati ve medication (0.62%), when compared to the population of dogs on preventat ive medication (0.04%). The evidence indicates that a heartworm diagnostic test applied to an asymptomatic dog on preventative medication contributes little information regarding the heartworm infection status of that dog. Ho wever, testing of a dog characterized as being high risk will provide clini cally useful information. Recommendations regarding the testing of dogs for heartworm in Canada are derived on the basis of available evidence.