Canine Heartworm Disease

Heartworms (Dirofilaria immitis) are parasites that are transmitted through the bites of mosquitos. Microscopic heartworm larvae called microfilariae circulate in the bloodstream of infected dogs where they can be picked up by a mosquito and injected into another animal. The larvae move around the body and go through several stages of maturation, eventually becoming adults that live in the heart and breed, producing microfilariae and completing the parasite's life cycle. Adult worms can grow to be up to 14 inches long and severely disrupt the flow of blood through the heart and lungs. Heartworm disease can also go by the name dirofilariasis.




Dogs with heartworm disease frequently cough and may tire easily, lose weight, and have difficulty breathing and a pot-bellied appearance due to fluid accumulation within the abdomen. Laboratory tests for heartworms can quickly diagnose most cases of the disease using only a small sample of blood. These tests cannot diagnose early infections because adult worms must be present in the heart and lungs for a dog to test positive, and it takes approximately six months for larval heartworms to mature into adults. Chest x-rays, blood work and urine testing are necessary to determine the disease's severity and plan appropriate treatment.




Canine heartworm disease can be treated with injections that kill adult worms, medications that kill microfilariae, and rest. Some dogs develop additional problems that need to be addressed as the worms die and are absorbed by the body. Most animals with only mild clinical signs at the time of treatment can be expected to recover, but if a large number of worms are present and/or permanent heart and lung damage has occurred the prognosis is less favorable. Dogs can be easily protected against heartworm disease by regularly giving them one of the preventive medications that are available through veterinarians.


Written by: Jennifer Coates, DVM
Last reviewed: October 2, 2008