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Heartworm, Mosquitoes and Pets

- A serious parasitic infection in dogs -

Summary: Heartworm is a parasitic worm that occurs mainly in dogs and can be fatal. The worms are transmitted by the bite of mosquitoes. In parts of the country where heartworm is common dogs should be routinely treated with medications that prevent the disease.

Jack DeAngelis, PhD
OSU Ext. Entomologist (ret.)

What is heartworm disease?

Heartworm is a very serious often fatal disease caused by a nematode (roundworm) that infects the heart, lungs and blood vessels of certain mammals. Unlike most nematodes which are tiny, heartworm nematodes grow to be large (up to 14") and can physically clog the heart and arteries of infected animals (right). Infection causes severe heart, lung and liver injury and often results in death.

How are animals infected with heartworm?

Immature heartworms are transmitted by the bite of mosquitoes. Once introduced into the blood the tiny worms grow rapidly and produce new offspring. A mosquito bite is the only way an animal can become infected with adult heartworm. Dogs (all breeds) are highly susceptible to heartworm infection while cats are less affected. See Related Articles below for ways to reduce mosquitoes in your yard.

heartworm-infested heart

Heart infested with heartworm. Image from the American Heartworm Society.


Where does heartworm occur?

In the US heartworm is endemic in all states except Alaska, and including Hawaii. The highest incidence of infection occurs within about 150 miles of the Atlantic coast, especially south of New York to Florida, the Gulf Coast and along the Mississippi and Missouri River systems. You should ask your veterinarian if heartworm has been found in your immediate area. Heartworm occurs worldwide in all temperate regions as well.

How do I prevent heartworm?

Where heartworm is common, dogs should be regularly treated to prevent the disease. Fortunately disease prevention is relatively easy and inexpensive. There are a number of medications that can be given once a month that will completely prevent heartworm disease. Some of these medications control fleas and ticks as well, however I prefer to separate the two because monthly flea and tick treatments may not be needed (see Flea Control for Pets).


Heartgard or Interceptor medications

Both Heartgard (tm) and Interceptor (tm) are effective for monthly heartworm treatments. The active ingredient in Heartgard is ivermectin while the active ingredient in Interceptor is milbemycin oxime. All heartworm medications require a prescription. An initial vet exam is required because giving these medications to an animal that is infected with heartworm already may result in a fatal reaction.

Some heartworm medications include ingredients to treat other parasitic worms such as hookworms and tapeworms, this is the "Plus" part of some products. Normally these intestinal worms do not need monthly treatment so save your money and get the single ingredient versions, then use a "de-wormer" when needed.

Related Articles

Mosquito Bites Transmit Heartworm


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