What are Heartworms


Heartworm infection is a serious, progressive, and fatal disease in pets. It affects dogs, cats and ferrets, but heartworms also live in other animals, including wolves, coyotes, foxes. These wild animals often live close to urban populations and thus are considered important carriers of heartworm disease.

Heartworms live in the heart, lungs and associated blood vessels of affected pets. It causes severe lung congestion and damages the heart as well as other bodily organs. 

The heartworm lifecycle involves a mosquito bite. The blood taken from an infected animal contains baby worms, or microfilaria, which develop into infective larvae in up to 2 weeks. 

The infected mosquito then bites another dog, cat, or wild animal, the infective larvae are deposited onto the surface of the animal’s skin and enter the new host through the mosquito’s bite wound. Once inside a new host, it takes approximately 6 months for the larvae to develop into sexually mature adult heartworms. Once mature, heartworms can live for 5 to 7 years in dogs and up to 2 or 3 years in cats. 

In the beginning, dogs may show few symptoms or no symptoms of heartworm infection. Symptoms develop the longer the infection stays and the bigger the worms grow.

Signs of heartworm disease in dogs:

  • a mild persistent cough
  • reluctance to exercise
  • fatigue after moderate activity
  • decreased appetite
  • and weight loss

As heartworm disease progresses, pets may develop heart failure and the appearance of a swollen belly due to excess fluid in the abdomen. Dogs with large numbers of heartworms can develop a sudden blockages of blood flow within the heart leading to a life-threatening form of cardiovascular collapse. This is called caval syndrome, and is marked by a sudden onset of labored breathing, pale gums, and dark bloody or coffee-colored urine. Without prompt surgical removal of the heartworm blockage, few dogs survive.

Heartworm disease in cats is very different from heartworm disease in dogs. Cats are not the “normal” host for heartworms, therefore most worms in cats do not survive to the adult stage. Heartworm infected cats typically have just one to three worms, and many have no adult worms. This means heartworm disease goes undiagnosed in cats many times, and it’s important to know that even immature worms cause real damage in the form of a condition known as heartworm associated respiratory disease (HARD). Moreover, the medication used to treat heartworm infections in dogs cannot be used in cats, so prevention is the only means of protecting cats from the effects of heartworm disease.

Signs of heartworm disease in cats:

  • coughing
  • asthma-like attacks
  • periodic vomiting
  • lack of appetite
  • weight loss
  • sudden collapse
  • sudden death

Occasionally an affected cat may have difficulty walking, experience fainting or seizures, or suffer from fluid accumulation in the abdomen. Unfortunately, the first sign in some cases is sudden collapse of the cat, or sudden death.

The earlier it is detected, the better the chances the pet will recover. There are few, if any, early signs of disease when a dog, cat or ferret is infected with heartworms, so detecting their presence with a heartworm test administered by a veterinarian is important. The test requires just a small blood sample from your pet, and it works by detecting the presence of heartworm proteins. Some veterinarians process heartworm tests right in their hospitals while others send the samples to a diagnostic laboratory. In either case, results are obtained quickly. If your pet tests positive, further tests may be ordered.