Heartworm Disease in Dogs and Cats

Heartworm disease affects dogs and, less commonly, cats throughout the entire continental United States and many other areas throughout the world. Heartworm disease is mosquito-driven and thrives in warm and temperate climates. While originally existing mostly in southern and south-eastern portions of the country, it is believed that cases now originate in all 49 contiguous states. In the following article, we will discuss heartworm disease in dogs and cats and the testing and treatment options available.

Heartworms are worms that live in your dog’s pulmonary arteries and heart, although migrant heartworms can also be found in other places, including the eyes and under the skin. Transmission occurs through a mosquito that bites an infected dog. The microfilariae (baby heartworms) develop into infectious larvae inside the mosquito.

The next time a mosquito bites a dog, the larvae are passed to the dog and continue growing under the skin. Over the next 2-3 months, the larvae migrate from under the skin and into the circulatory system. By the moment they reach your dog’s pulmonary arteries, they have become L5 Larvae and by 5-7 months mature, adult, heartworms.

Heartworms damage a dog’s pulmonary arteries through direct contact and resulting in inflammation. Over time, this inflammation can lead to enlargement of the heart, congestive heart failure, and pulmonary hypertension (high blood pressure). Additionally, they can damage the valves in the heart and arteries and, in high worm burdens, can cause a condition called caval syndrome, which is rapidly fatal.

Most dogs with heartworms will still have normal physical examinations. In some cases, heart murmurs may be detected but are not always present; this is why annual heartworm testing is important! A rapid antigen test taken from just a few drops of blood is the most common method of diagnosis.

While this test is extremely accurate, it can only identify adult female worms, so infections present for less than 5-7 months may not be identifiable. False-negative tests can occur when there are only a few worms present, all-male worms, or only baby heartworms present.

In positive heartworm cases, additional tests to stage how far the disease has progressed are required before treatment. Routinely, a confirmatory heartworm test and a blood smear or other test to look for microfilariae are ordered. Radiographs, bloodwork, and/or echocardiography can help characterize the stage of the disease.

Treatment for heartworm disease first centers on eliminating microfilariae safely. Allowing time for juvenile stages to mature into adults and then killing the adult worms. To do this, a series of visits to the veterinarian are required. After staging the disease and placing your dog on exercise restriction, your dog should be placed on medication to limit inflammation. And to kill the bacteria inside of the heartworms, called Wolbachia. Next, the baby heartworms are eliminated using routine Preventatives, although a veterinarian should monitor your dog for 8 hours for allergic reactions during administration.

The current heartworm protocol then involves giving a series of injections to kill adult heartworms. First, a single injection, and then 30 days later, the injection is repeated for two consecutive days in a row. Using the American Heartworm Society Treatment guidelines gives the best chance of limiting the disease’s transmission, minimizing complications from treatment, and eliminating all worms inside your pet.

As with most diseases, an ounce of prevention is worth a pound of cure. Fortunately, numerous prescription products have extremely high efficacy at preventing heartworm infection. For this reason, all dogs should be kept on monthly heartworm prevention all year long.

If you think your pet may have heartworm disease or is not currently on heartworm prevention, please schedule an appointment with our Veterinary Hospital so that your dog can be examined, tested, and started on Heartworm Prevention. For further information, please refer to the American Heartworm Society.

Jeffrey Stupine, V.M.D.
Medical Director of World of Animals Veterinary Hospitals