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Tips from the Vet for Your Pet: Heartworm

Community Contributed

By Stewart Morgan, Ph.D., D.V.M.

Heartworms are parasites that live in the arteries (blood vessels) going from the heart to the lungs. These worms can be between six inches to one foot long and can cause weakness, difficulty breathing, cough, heart disease, liver disease, kidney disease, and even death. Heartworm is spread by mosquitos, including the Asian tiger mosquito, and dogs with heartworm have been found in all 50 states. In short, where there are mosquitoes, there is heartworm!

When a mosquito carrying heartworm larvae bites a dog or cat, the heartworm larvae enters the animal’s bloodstream and matures. It takes about five months for the heartworm larvae to become adults. The adult female heartworm produces more heartworm larvae that can be taken up by mosquitos and spread to another animal. The adult heartworm can live in the bloodstream for up to five years and heartworm larvae can live in the bloodstream for over two years before being taken up by a mosquito and spread to another animal.

Heartworm prevention requires testing an animal to make sure it does not have heartworm as well as a medication given monthly. The heartworm test is a blood test that detects adult female worms. Because the heartworm test detects adult worms, it is not possible to detect heartworm in a dog that might have contracted the disease two, three, four or possibly even five months before. This is because those worms have not become adults yet. That is the reason we do not test young puppies for heartworm. The test the Molokai Humane Society has to detect heartworm also tests for tick diseases that are common on Molokai. This is another good reason for getting dogs tested every year.

The medications used to prevent heartworm are given monthly. These medications work by killing heartworm larvae before they can become adults. Another good thing about these medications is that they also are good at preventing intestinal worms including roundworms, hookworms, whipworms, and, in some cases, tapeworms. Thus, there is no good reason for any dog or cat to not be on a heartworm preventative.

In dogs, heartworm treatment is expensive and involves giving dogs three shots of a chemical containing arsenic to kill adult worms. Other medications are also given to kill heartworm larvae and bacteria that live inside heartworms. When the worms die off in the animal’s bloodstream, there is a possibility of complications, including death.

In cats with heartworm, options are limited to monitoring the cat for signs of illness and supportive care, or surgery to remove worms from the vessels near the heart. The good news is that cats are naturally more resistant to heartworm than dogs, normally have fewer worms infecting them than dogs, and adult heartworms in the cat only live about two to three years.

If your pet is not currently on heartworm preventative, we highly encourage you to get them tested and on a preventative!

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