Prevalence of canine dirofilariasis in Taiwan

Between 1993 and 1997, 837 stray dogs from North Taiwan were necropsied and examined for dog heartworm infections. A thick smear from 20 ml of peripheral blood from each dog was also prepared and examined for microfilariae (mf). The overall prevalence of adult worms in the dog population was 57%. The prevalence of mf in 1228 house dogs from different parts of Taiwan was also determined from 20 ml of peripheral blood in the same way. The overall prevalence of mf was 25%, with a value of 30% in the main island of Taiwan, this being 15 times higher than that in the offshore islands (2%). In Taiwan, the prevalence ranged from 4% in Hualien County, East Taiwan, to 41% in Nantou County, Central Taiwan. The mf prevalences on offshore islands were 1% on Liuchiu and 2% on Lanyu. The mf density per 20 ml blood in 82 house dogs was found to be 23 mf per dog, with a range of 3-97 mf per dog. A total of 477 stray dogs were found to be infected with adult worms of Dirofilaria immitis. The mean number of 7 worms per dog was obtained, with a range of 1-55 worms per dog. These results indicate that the prevalence of canine dirofilariasis has increased in Taiwan over the past 10 years. Moreover, the prevalence may be related to the wind speed, temperature, relative humidity, and altitude in the different areas surveyed.