Heavy infestations of dog heartworm migrate up the pulmonary artery and clog blood vessels of the lungs. Although isolated human infections have been reported, dog heartworm is not presently recognized as a human health problem. The parasite, however, is by no means limited to the dog. The fox, wolf, and raccoon have been shown to be reservoirs, and evidence implies that the skunk, muskrat, and opossum may be infected as well. Although the household cat is capable of harboring Dirofilaria immitis , infected cats are rarely found.
Evidence suggests that mature heartworm adults maybe rendered sterile in animals other than canines.
If this proves to be true, blood tests on household cats would show no microfilarial activity even though the adult worms are present. Infected canines apparently jeopardize only the health of other dogs. One infected dog serves as a focus from which other dogs are inoculated. An infected dog sleeping in an open kennel would be frequently fed upon by mosquitoes. These same mosquitoes might later return to feed upon the same dog, thus building up his own infection, or they might choose to feed upon a neighboring dog, thereby producing a new infection.