General Information for Ontario
The findings for Ontario are presented in one figure and 3 tables and a few comments about these should be made.
1. In 1998, 935 questionnaires were sent out and 593 were returned, but 3 were rejected for reasons as indicated above. The response rate was 63.3%. For those responding, 96% indicated a willing-ness to complete a questionnaire if it was offered again.
2. There were 549 dogs reported with HW in 1998 (676 in 1997). Twelve of these dogs had been diagnosed and reported on in these surveys previously and since then were untreated. More dogs (289,289) were tested in 1998 than in 1997 (273,025) and the prevalence of HW in 1998 was 0.19% (0.25% in 1997).
3. Southwestern Ontario continues to be the major focus of HW infection. There were 2 dogs with HW in Northern Ontario; 1 (Dryden) which had been outside Canada and 1 (Thunder Bay) which had visited an endemic area in Ontario.
4. I sought from practitioners information on whether dogs tested for HW in 1998 had been on preventive medication in 1997. That data is useful to provide an estimate of the prevalence of HW in dogs that are unprotected and the level of risk of infection to dogs in the area. But many practitioners did not have that detailed information and the results reported here should be considered with caution. The indications were similar to that seen previously; (a) most of the dogs tested had been given medication, (b) most of the dogs with HW (465) were not on preventive medication in 1997, (c) the prevalence of HW among dogs tested in 1998 and not on preventive medication in 1997 was greater (0.74%) than those on medication (0.04%). In endemic areas, the prevalence in unprotected dogs, as shown below, was considerably higher. However, the sample size in those subsets is considerably smaller. The prevalence of HW in unprotected dogs for any town can be determined using the data in both Tables 2 and 3. The sample size for a town or city is usually small and again caution should be used in interpreting the results.
5. In 1998, 84 dogs diagnosed with HW had been on preventive medication in 1997; 44 were on ivermectin, 32 on milbemycin and 5 on DEC. In 1997, about 150,000 dogs were given ivermectin as preventive medication and 172,000 milbemycin (annual report on "Heartworm in Dogs in Ontario in 1997"). The owners of 39 of the 84 dogs with HW had advised that they had missed treatments and 4 were given inadequate dosage. The remainder, 41 dogs, were classified in the category "Other". Eleven of these dogs were diagnosed with HW in 1997, were not treated with an adulticide and were placed on preventive medication in 1997; 1 on DEC, 3 on ivermectin and 7 on milbemycin.
6. The percentage of dogs with HW and a history of not having left the province was about 73%. About 4% of the dogs with HW had been outside Canada previously and for about 21% the movements were unknown.
7. HW was most frequent in companion dogs, over three years of age and maintained mainly outdoors in rural areas.
8. There were 191 dogs with HW which were not treated and 42 were euthanized. Many of the dogs that were not euthanized were microfilaremic.
9. In 1998, there were 188 clinics (160 in 1997) and 118 towns (111 in 1997) reporting a diagnosis of HW.
10. I solicited data on the number of dogs using the various preventive medications in the marketplace in 1997, but practitioners did not have that detailed information. About 42% of the dogs were reported to have been given Sentinel, about 39% Heartgard 30 or Heartgard30 Plus, about 19% Interceptor and less than 1% other medications.
11. In 1998, there were 20 dogs with Dipetalonema reconditum in Canada and this is about 3% of all dogs found with microfilariae. In Ontario, there were 2 dogs with D. reconditum.
12. There were 6 cats reported with HW; all in Ontario; one each in Concord, Kingston, Brantford, King City, Hagersville and Simcoe. I had indicated previously that in highly endemic HW areas in several countries cats are being reported with HW.
13. HW was reported for two other animals; a coyote and a fox from Brantford.
14. In these surveys, I had requested respondents to indicate (Question 25) whether the data provided for the several questions in the questionnaire were taken from medical records. The responses ranged from 30-59% for the several parts of that question. You should note that the answers reported especially for Questions 19, 20 and 21 are approximations since only 35% of the respondents provided data from medical records.