In the Rostov Region, there have been stable foci of dirofilariasis for a long time (since the 1990s). Two helminth species (Dirofilaria repens, D. immitis) parasitize in definitive hosts. There has been recently a species-specific alteration in the foci, by increasing the proportion of canine D. immitis invasion and mixed invasions associated with the change in weather and climatic conditions and in the species-specific composition of carriers. The smaller number of patients with dirofilariasis is a consequence of the reduced affection of dogs and mosquitos.