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20701 Kingsland Blvd. Suite 105,
Katy, Texas 77450

Heartworm Disease in Dogs and Cats

Susan Randlett, DVM

“Ms. Smith, the results of Fluffy’s heartworm test have come back, and unfortunately, she is positive for heartworm infection. Let’s talk about what that means.”

Most people in our area are at least sufficiently familiar with heartworm disease to know this is not a phrase they want to hear from their veterinarian. But what does it mean for your pet to have heartworm disease?

Heartworms are an endemic parasite in the gulf coast area that are transmitted by mosquitoes. The bite of an infected mosquito can inject the parasite in both dogs and cats. The parasites’ normal host is the canine, and in a dog, the worm causes cardiovascular disease. Occasionally, however, the parasite finds its way into a feline. Since the cat is not a normal host, once inside the cat’s body, the worm cannot recognize the normal migratory markerers that are found in a dog, and the worm wanders around the internal organs of the cat stimulating a tremendous inflammatory reaction. In cats, heartworm disease typically presents itself as a respiratory disease due to this inflammation.

There are many allergy “treatments” available, but initially, we must address secondary infections with various oral and topical antibiotics and/or antifungals and discuss appropriate “adulticidal” flea prevention. Also, as part of our initial goal, we want to decrease the discomfort and itch as much as possible; the medications we use for this vary greatly depending on the severity of the itch and individual pet.

In the dog, the heartworm has adapted to live a long life (5-6 years) in the dog’s heart without eliciting a tremendous immune response, and so many dogs remain asymptomatic for many months. In many cases, the first sign of heartworm disease is when the worm burden reaches such a level as to present an obstruction to blood flow. If untreated, dogs with heartworm disease can begin to exhibit the following symptoms as the disease progresses:

In most cats with adult heartworms, only a few worms (1-3 on average) are found in the heart and these are usually of a single sex, rendering reproduction impossible. Therefore, cats rarely have baby heartworms (microfilaria) circulating in the blood. When an adult heartworm dies in a cat, it causes a tremendous inflammatory response. Unfortunately, many cats do not survive this stage. Heartworms in cats can produce an assortment of clinical pictures:

  • Coughing, wheezing, difficulty breathing
  • Vomiting
  • Disease related to embolism or abnormal clots
  • Extreme nosebleed
  • Neurologic signs (probably associated with larvae accidentally migrating to the brain)
  • 10-20% of cats experience sudden death (probably associated with the death of the adult worms)
  • Many cats never show noticeable symptoms and most of these cats (approximately 80%) clear the infection on their own. How many cats are infected without symptoms is not clear because, unlike dogs, regular heartworm testing in cats is not a typical part of wellness care.

The recommended treatment for heartworm disease in dogs is a months-long process, involving testing to assess the dog’s health for the treatment, multiple drugs and injections, and confinement for the duration of treatment. It represents a substantial investment of time and money for the family.

At this time, there is no treatment for heartworms in cats.

Fortunately, we can prevent this disease from ever compromising the health of our four-legged friends. All dogs and cats living in the gulf coast, whether residing indoors or outdoors, should be put on a preventive product. With so many different types of products that are effective and affordable, a perfect solution is available to all.