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1082 Main St. / Kingfield, ME 04947 / (207) 265-2214


June 8, 2023

Heartworm disease is a serious and potentially fatal disease in companion animals. Dogs, cats, and ferrets can get heartworms, but the disease is different in each of these animals. Many wild animals also get heartworm. Wild animals are important carriers of the worm. Occasionally humans can get heartworm disease.

Mosquitos play an essential role in the heartworm life cycle. Adult female heartworms living in dogs and other canines, such as wolves, coyotes, and foxes, produce microscopic young that circulate in the canine bloodstream. When a mosquito bites an infected animal, it picks up these baby worms. The worms mature into larvae in 10-14 days. When that mosquito bites another animal the heartworm larvae are deposited onto the animal's skin and enter the new host through the mosquito bite wound.

Canines are the natural host of the heartworm. Once the heartworm larvae enter the canine's bloodstream the heartworm matures. In about 6 months it will reach one foot in length and may live for up to 7 years in the heart, blood stream, and lungs. With more and more mosquito bites experienced over a lifetime, a dog may accumulate 100 or more worms. Heartworms kill by causing heart failure and severe lung disease.

Cats are not natural hosts of heartworm, so the disease is much less common and appears different, mostly because the worm cannot mature to its adult stage in cats. Cats generally are infested with just a few immature worms. But since cats are smaller animals it takes fewer worms to cause severe disease; heartworms damage by causing respiratory problems.

Ferrets experience a mixture of the problems in dogs and cats. The ferret may have a larger number of worms than cats, but due to the small size of the ferret's heart, even just one worm can cause severe disease.

Dogs can be treated for heartworm disease, but the treatment is difficult for the animals to undergo.

Neither cats nor ferrets have a treatment available. Thus, prevention is the most important way to protect our companion animals. It is important to test for the presence of heartworm in the dog's bloodstream prior to starting heartworm prevention medication as it is dangerous to use the preventative in infected animals. In New England, veterinarians recommend using the preventative from late spring to December.

However, as our climate is warming, mosquitos are becoming a problem year-round and many pet owners are keeping their animals on preventative medication continuously. In that case the veterinarian will advise when and how often testing should occur. Also, for our pets who winter in warm climates, heartworm preventative needs to be used year-round.

One other advantage of heartworm prevention medications is that many of them also prevent intestinal worms. This is important anywhere, but especially so for pets who travel to places where the ground isn’t covered by snow all winter.

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