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Understanding Heartworm in Dogs and Cats

Heartworm is a potentially devastating disease that can affect both dogs and cats. Because clinical signs often do not appear until the disease is in its advanced stages, screening and prevention are crucial to protect your pets.

How Do Pets Get Heartworm?

Heartworm is caused by the parasite Dirofilaria immitus, which is transmitted by infected mosquitoes. The disease is not spread through direct contact between animals. When a mosquito carrying heartworm larvae bites your pet, the larvae enter the bloodstream and travel to the heart and blood vessels. As the larvae mature into adult worms, they cause inflammation and block blood flow, leading to severe damage to the heart and other organs. Without treatment, this can be fatal.

Symptoms of Heartworm in Dogs and Cats

Despite the significant size of adult heartworms (six to 14 inches long), symptoms often go unnoticed until months or years after infection. In dogs, symptoms include a soft cough, difficulty breathing, weakness, fainting, and weight loss. In cats, symptoms can also include vomiting or gagging. If left untreated, heartworm can lead to heart, liver, or kidney failure.

Treatment for Heartworm

Treating heartworm involves eliminating all stages of the parasite—adult worms, larvae, and microfilaria (the prelarval stage). Treatment usually includes multiple injections to kill the adult heartworms, antibiotics to make the parasites more vulnerable, and steroids to reduce inflammation. A heartworm preventive is often used to kill the microfilaria before starting treatment for the adult worms. After treatment, animals must be closely monitored and have restricted activity for six weeks to prevent complications from dying worms blocking the arteries in the lungs.

Currently, there is no approved treatment for heartworm in cats, so treatment focuses on managing the symptoms.

Preventing Heartworm in Pets

Because treatment is expensive and complicated, prevention is essential. There are several safe and effective monthly preventatives available with a veterinarian’s prescription, such as Heartgard® and Iverhart®. These preventatives work by killing the immature larvae, preventing them from maturing into adult worms. Before starting a preventive regimen, your pet should be blood tested to ensure they are free of existing heartworm. Annual re-testing is also recommended.

Special Considerations for Certain Dog Breeds

Some dog breeds, particularly those with the MDR1 gene (such as collies and Shelties), have a higher sensitivity to ivermectin, which is found in many heartworm preventatives. While the ivermectin dose in commercially marketed preventatives is generally safe for MDR1 positive dogs, some owners may prefer to use alternatives such as Sentinel, selamectin, or ProHeart 12.

Heartworm Prevention During Pregnancy

Most oral heartworm medications are labeled safe for use during pregnancy and in breeding dogs, with the exception of Trifexis, which has not been tested in breeding animals. Injectable ProHeart 12 is also labeled as safe. However, it is advisable to avoid any drug during the first trimester (the first three weeks) of pregnancy, as this is the critical period for fetal structural development. Timing the dose of heartworm medication to a few days before breeding, then repeating it on a four-week schedule, can help bypass this critical period while still providing necessary protection.

In many regions, heartworm disease is a significant threat, so skipping heartworm prevention entirely during pregnancy can put the breeding female at risk. With careful timing, you can protect your pregnant dog and her upcoming litter from heartworm and other parasites.

For more information on heartworm prevention and treatment, visit

Heartworm in Dogs
Heartworm in Dogs


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