The host-parasite relationship in canine heartworm infection in a hyperendemic area of Italy.

From November 1990 to June 1991, 2273 Dirofilaria immitis were collected from 175 stray dogs euthanized in the province of Padua. The breed, sex, age and weight of each animal were noted. The worms were counted, sexed, differentiated as adults or fifth-stage larvae and weighed, and their length was measured. The host-parasite relationship in this epidemiological situation was characterized by a high prevalence (67%) together with a low intensity (19) and abundance (13). The frequency distribution of the parasites in the population of dogs followed a negative binomial pattern (k = 0.3177), showing a good, stable host-parasite relationship. The major risk of infection was in late July and August, some new infections being established in October and, maybe, in November. The majority of dogs that acquired new infections were young animals probably because of an immune response in older dogs. However, the prevalences increased (p < 0.01) in dogs from 1 to 5 years old, in shepherd and hunting dogs compared to the other breeds, and in large as opposed to small dogs. Microfilaraemia (range 1-1,883,000/ml) correlated, as a trend, with the number of adult filariae, which also correlated with the age and the weight of the dogs (p < 0.01). No significant correlations were found between the length or weight of the adult filariae and the data from dogs.