Feline Heartworm: Symptoms and Treatment of Heartworm in Cats

Heartworm, a type of parasitic roundworm, is transmitted by mosquitoes. Heartworm in cats is virtually impossible to treat, so prevention is essential.

Feline heartworm is potentially fatal to cats. Heartworm disease (dirofilariasis) affects cats, dogs and other mammals including wolves, foxes, ferrets, sea lions and horses. In rare cases, heartworm can infect humans. Heartworm is a health risk for pets all over the world.

Life Cycle of the Heartworm (Dirofilaria immitis)

Adult heartworms look like spaghetti noodles. The females grow up to a foot (30cm) long, and the males are generally smaller. In cats, a typical infection is about two to five worms. Cat heartworms can live from two to three years.

Heartworms lodge in the right ventricle of a cat’s heart, or in the adjoining vessels. Female heartworms give birth to live young, or microfilariae. The microfilariae travel through the bloodstream of the infected cat, feeding on nutrients in the blood. When a mosquito bites the cat, it picks up some of the microfilariae.

The microfilariae develop inside the body of the mosquito. Over 10-14 days, they become larvae, and are ready for the next stage. When the mosquito next bites a cat or other animal, it transfers the larvae under the animal’s skin. The larvae grow at the site of the bite for about two weeks. Then, they move into the muscles of the chest and abdomen, where they continue to feed.

After a period of growth, they move into the bloodstream and travel to the heart. Three to four months later, they mature into adult heartworms, and begin releasing microfilariae. The microfilariae travel into the bloodstream, and the heartworm life cycle starts again.

The time between infection and maturity of the heartworm, known as the prepatent period, is about nine months in cats. In dogs, it’s about seven months.

Heartworm disease in cats is not as common as heartworm in dogs. Cats are more resistant to heartworm disease, and have a lower incidence of infection. In cats, there are usually no more than five heartworms, while dogs can develop dozens – but it takes only one worm to kill a cat.

Cat Heartworm Symptoms

Heartworm in cats is often hard to diagnose, because some symptoms are similar to those of other conditions, such as feline asthma. Heartworm symptoms may include:

  • difficulty breathing

  • wheezing

  • vomiting

  • loss of appetite

  • weight loss

  • lethargy

  • sudden death

Heartworms block the arteries and obstruct blood flow to the lungs. A cat that seems healthy can suddenly collapse and die.

Cats are also susceptible to aberrant migration of the heartworm larvae. If one of the larvae strays from its intended course, it could move into other parts of the cat’s body such as the eye, brain or stomach.

Detection of Heartworm in Cats

When an infected mosquito bites a cat, the cat’s immune system produces antibodies. Dogs don’t produce these, which is why dogs have a higher rate of infection. Most of the time, the cat’s antibodies kill off the heartworm larvae.

A veterinarian can administer a blood test to detect the presence of antibodies, but the test can’t determine whether or not the cat has heartworm. There are other blood tests, but they’re even less effective. The best ways to diagnose heartworm in cats are with x-rays or ultrasound.

Treatment of Heartworm in Cats

There is no medical treatment for heartworm disease in cats. Because cat heartworms live for only two to three years, many vets recommend treating the symptoms, and allowing the worms to die off naturally. The cat will need medical supplements, and must be under constant supervision, because the dying worms could clog blood vessels, lungs or arteries as they break up and decompose.

The vet may prescribe thiacetarsamide (Caparsolate), an arsenic-based compound, to kill the worms. This is risky, because the cat could have a bad reaction to the medication, and killing all the worms at once can cause medical complications, distress and possibly death for the cat. In extreme cases of feline heartworm, the vet may recommend surgery.

Never give dog heartworm medication to a cat. Common dog heartworm medications can be toxic to cats.

Preventing Heartworm in Cats

Cat heartworm prevention is essential. Although many cats can fight off the disease, there are serious consequences for those who can’t. Cat heartworm can also spread through mosquito bites to other pets, and can be fatal. Even indoor cats are at risk. In a North Carolina study, 28% of cats diagnosed with heartworm disease were inside-only cats.

Medication for the prevention of heartworm in cats is easily available from the vet, or over the counter. The two FDA-approved medications available in the United States are:

  • Interceptor Flavor Tabs

  • Feline HeartGard

Both are oral medications, to be given once a month. In areas with mosquitoes, or where other animals could have heartworm, a cat is at high risk for heartworm disease. It takes only one mosquito bite to infect a beloved family pet.

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User Comments
  1. CaSundara

    On August 23, 2009 at 3:22 pm

    Wow, I’ve never heard of this before!
    We don’t get to many mosquitoes in the UK but I’ll ask my vet if they sell similar medication here, just to be safe.
    Very informative, thank-you.

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