What Is Heartworm and How It Affects Your Pets

What Is Heartworm and How It Affects Your Pets

Sep 21, 2021
9 min

Heartworm in Dogs


When a dog is bitten by an infected mosquito, the microfilariae are deposited in a tiny drop of mosquito saliva, adjacent to the mosquito bite. Once in the bloodstream of the new host, the microfilariae will spend the next week or two developing into the next stage of development within the host’s skin. It will live in the skin at this stage for three months or so until it develops to the final stage of development. It will then be ready to enter the host’s circulatory system. In this final stage, the microfilaria migrates to the heart and out into the pulmonary arteries (if there is room) where it will mate, approximately 5-7 months after first entering the new host.

Heartworm in Cats


perrobook heartworm


The cat is not a natural host for the heartworm, which means the migrating larval heartworm is not likely to find its way to the heart when passed from a mosquito. Mosquitoes that carry heartworm definitely prefer to feed on dogs, but cat infections will happen from time to time. While a moderate heartworm infection in a dog would involve 25-50 adult heartworms, infected cats typically have less than six adult worms. Because the feline heart and blood vessels are so small, these few worms can wreak havoc.



Treating Heartworms


perrobook heartworm


The treatment varies from dog to dog. Each animal’s personal condition is evaluated, and the treatment protocol tailored individually. In the end, you and your veterinarian will choose the best treatment protocol for your pet. Treatment involves two basic areas:


1. Patient evaluation and stabilizing for treatment procedure

Patient evaluation and stabilization The veterinarian evaluates the overall health of the animal by conducting X-rays and blood and heart tests, then determines how to best proceed with treatment. Part of this evaluation is staging the severity of the heartworm disease in the dog.


2. Elimination

Elimination of all forms (adult, larvae, and microfilaria) of the heartworm parasite is one way to treat affected pets. 

Some animals need to have certain conditions stabilized before heartworm treatment can proceed. Those in third stage heartworm disease, for example, may require deliberation to decide if it is best to try surgical removal of some worms through the jugular vein before any other steps of parasite elimination are considered.


Elimination of the Heartworm Parasite


perrobook heartworm


This is a two-step process. The adult worms and the microfilaria are eliminated separately. The adults are treated first, then a different treatment is used to kill the microfilaria and migrating larvae. The most serious side effects usually occur with the treatment of the adult heartworms. As the worms die, they lodge in the lung arteries and block even more blood vessels than before treatment. Besides the usual inflammation caused by the presence of the worms, the inflammation is amplified due to the decomposing worms within the blood vessels.

The worm destruction also releases foreign substances into the dog’s circulation as the worms break down, and are eliminated by the immune systems. A large amount of inflammation and swelling generally occurs during this period. The prescription medications used to treat the adult heartworms are called adulticides. The two adulticides used most commonly are derivatives of arsenic.



There is no need for your pet to have to endure heartworm disease or its treatment, because it is so easy to prevent. A heartworm prevention program is effective and simple, and consists of three parts:


1. Regular blood testing for Dogs

This ensures your dog is free from heartworms before he begins or continues on his preventive medication. Your veterinarian will advise you as to the recommended frequency of regular blood tests. Interpreting test results in cats is more problematic and you should discuss testing with your veterinarian.


2. Preventive medication

This means administering a heartworm preventive to your pet year round, regardless of the mosquito season. You need to consult your vet before providing any kind of medicines.

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3. Reducing your pet’s exposure to mosquitoes

This means making your pet’s environment less welcoming to mosquitoes. This decreases the risk of your pet being exposed to mosquitoes which may be carrying heartworms.

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