Heartworms are deadly parasites that can infect any mammal, although they much prefer hanging out in canine hosts. Dogs can be infected with more than 100 worms that can live for five to seven years, while in cats, only a couple of worms that live for two to three years, if they reach adulthood at all, can be harmful.

By learning more about this serious threat to your pet’s health, you can do your best to protect them from disease. Read on to discover how heartworms infect pets, and what you can do to prevent infection.

Heartworm disease transmission in pets

Unlike many other worms, heartworms are not transmitted via the fecal-oral route. Instead, this blood-borne parasite is transmitted when an infected mosquito bites. Microfilariae (i.e., heartworm larvae) swim through an infected mammal’s bloodstream, and are ingested by a mosquito that takes a meal. The microfilariae then develop into their infective stage after their trip through the mosquito’s gastric juice. Then, when the mosquito bites another mammal, like your pet, microfilariae are transmitted via the bite to continue their life cycle, growing to adulthood and reproducing inside the animal.

Heartworm disease signs in dogs

Heartworms can thrive in dogs for many months before they cause clinical signs. In most cases, the first, obvious heartworm disease sign is a mild, persistent cough that worsens with exercise, and progresses over time. Other signs include:

  • Exercise intolerance
  • Fatigue after physical activity
  • A decreased appetite
  • Weight loss

Without treatment, heartworm disease will progress and your dog can develop heart failure and a swollen abdomen from fluid accumulation. In some dogs, the overwhelming heartworm population causes a blood-flow blockage inside the heart. This situation, called caval syndrome, is life-threatening and requires prompt surgical removal of the worm blockage.

Heartworm disease signs in cats

Heartworms do not prefer cats as their hosts, and do not thrive as well as in dogs. However, they can still cause massive, irreversible damage to the heart, lungs, and surrounding blood vessels. Immature and dying adult worms can trigger a severe inflammatory response, especially in the lungs, leading to a condition called heartworm associated respiratory disease (HARD). 

Heartworm disease signs in cats can vary greatly in severity and may include:

  • Intermittent vomiting
  • Diarrhea
  • Rapid and difficult breathing
  • Coughing
  • Wheezing
  • Gagging
  • Appetite loss
  • Lethargy
  • Weight loss

Cats with heartworm disease can also go blind, seizure, or experience difficulty walking. In some cases, the first and only sign of heartworm disease in a cat is sudden death.

Heartworm diagnosis in pets

A heartworm disease diagnosis in pets requires a small blood sample to test for adult female heartworm proteins, which must be present for an in-clinic test to achieve a positive result. Confirmation or additional testing is therefore typically necessary, including blood work, such as a complete blood count (CBC) and chemistry panel, to assess your pet’s general health, and chest X-rays to determine infection severity. In some cases, ultrasound imaging may be performed to reveal heartworms in the heart  or pulmonary vessels. Testing through an outside laboratory to confirm heartworm presence may also be necessary before treatment can begin.

Heartworm disease treatment for dogs

If heartworm disease has been confirmed in your dog, treatment should begin as soon as possible to minimize scarring from the worms. Depending on your dog, treatment may include antibiotics or steroids before administering the worm-killing injections. Typically, your dog will receive three arsenic-based injections, with two back-to-back injections administered a month after the first injection. These injections can cause pain and nausea and, as the worms die and begin to degenerate, your dog can suffer from adverse side effects. A dog who is too active, or has a high worm burden, can develop anaphylactic shock or a vascular clot from the dead and dying worms, so their activity must be severely restricted throughout the entire treatment course, plus an additional six to eight weeks following the final injection, to minimize the risks.

Heartworm disease treatment for cats

The drug used in the injections for canine heartworm disease treatment is not approved for use in cats and, unfortunately, no other medication that can safely eradicate adult heartworms in cats is available. Supportive nursing care is the only option for cats.

Heartworm disease prevention for pets

Although heartworm disease can be fatal for your pet,  various simple-to-administer prevention options are available. Oral tablets, chewable “treats,” topical liquids, and long-lasting injections can keep your pet safe from heartworm disease, as well as other parasitic infections.

Administering year-round heartworm prevention to your furry pal is critical for preventing this deadly disease. For help choosing the best heartworm prevention product for your pet, ask our Southwest Vet team.