Transmitted by mosquito bites, heartworms, Dirofilaria immitis, are common parasites of domestic dogs and other canine species. Less commonly, they infect cats, ferrets, muskrats, otters, and sea lions. Mosquitoes sometimes transmit the parasite to humans, but immature larvae are not able to develop to maturity in a human host—they never reach the heart.
Heartworm life cycle
Heartworms require two hosts to complete their life cycle: a mosquito and, usually, a dog or other canine species:
- A mosquito becomes infected when it draws a blood meal from an infected host. The blood contains microfilariae, long thin embryos of D. immitis that look like tiny worms in the blood.
- The microfilariae develop to the infective larval stage inside the mosquito and migrate within the insect’s body to its mouth parts.
- The next time the mosquito takes a blood meal, heartworm larvae drop onto the host’s skin and migrate in through the bite lesion.
- Under the surface of the host’s skin, and in muscle tissue, the larvae continue to mature for eight to ten weeks, attaining a length of about 25mm, after which time they move to the right side of the lungs and the pulmonary artery, where they become adult in about two months. Adult females are about 25 – 30cm long; males are about two thirds that size. They live two to five years.
- Dirofiliaria immitis adult worms mate and females produce eggs, which develop into the long thin embryos, or microfilariae, that are released into the bloodstream. Microfilariae are more common in the circulating blood at night than they are during the day.
- Mosquitoes take in the circulating microfilariae when they draw blood from the infected host, starting the cycle all over again.
Heartworm symptoms
Less than 25 worms in a dog's heart may cause no symptoms at all, so a dog can be infected and passing the parasite on to mosquitoes without anyone knowing there’s anything wrong. If there are 60 or more worms in the heart and pulmonary artery, there are likely to be circulation problems with damage to the heart, liver, and kidneys, and more than a 100 heartworms is associated with blood circulation blockage. Typical heartworm symptoms in animals include:
- Respiratory difficulties
- Cough
- Difficulty with exercise
- Vomiting
- Fatigue
- Heart failure
- Death
Where is heartworm found?
Dirofilaria immitis is a parasite of canines all over the world. Where there are large numbers of mosquitoes, cases of heartworm disease tend to be more common—the parasite larvae in mosquito hosts can develop to the infective stage anywhere where the daily temperature rises above about 14° C (57° F). In North America, heartworm infection has historically been highest along the Atlantic and Gulf coasts, with up to 45% of dogs infected in some areas. Heartworm appears to be spreading and becoming more common in the rest of the United States and in Canada. Because heartworm disease is very difficult to treat, many people choose to give their dogs antifilarial drugs to prevent larvae from maturing.
Related content:
Heartworm Can be Deadly for Dogs
Read about other parasitic worms:
Read about another warm weather health issue for dogs:
Sources:
College of Agricultural and Environmental Sciences (UC Davis). “Dirofilaria immitis.”
Garcia, Lynn S. and David A. Bruckner. Diagnostic Medical Parasitology 3rd ed. Washington: ASM Press, 1997.
Roberts, Larry S. and John Janovy Jr. Foundations of Parasitology 6th Ed. Boston: McGraw Hill, 2000.
