Heartworm disease, which is transmitted through mosquito bites, is caused by a parasitic roundworm named Dirofilaria immitis. Dogs are the animal most susceptible to heartworm disease, but it also affects cats and ferrets, and certain wild animals. The disease is called "heartworm" because, once the roundworms mature, they live in the heart, although they can also reside in the lungs and associated blood vessels. Once an animal is infected, it takes the parasites 6 months to develop into mature heartworms.

What Harm do Heartworms Cause?

Mature heartworms can reproduce within the animal, and their numbers can cause severe lung disease, heart failure and death. Dogs are much more susceptible to the disease than cats and ferrets. Heartworm is almost 100 percent preventable with medication provided by your vet. Prevention is also much easier, less traumatic and more successful than treatment after infection.

What are the Symptoms of Heartworm?

Heartworm disease may not be apparent in its early stages, developing gradually over a period of months and, sometimes, years. An animal typically shows symptoms when the infection becomes severe.

Symptoms in Dogs

In dogs, symptoms of heartworm disease include:

  • Persistent cough
  • Fatigue
  • Loss of appetite
  • Weight loss
  • Reluctance to move

Symptoms may not be obvious in dogs that have been recently infected, have relatively few worms, or are not very active.

In severe cases, dogs may acquire caval syndrome, in which the heart is physically blocked by a large mass of worms. Caval syndrome is life-threatening, and quick surgical removal of the heartworms is the only treatment option. The surgery is difficult and, even with treatment, most dogs with caval syndrome die.

Symptoms in Cats and Ferrets

In cats, heartworm disease may be difficult to diagnose, because its symptoms are similar to other feline diseases. Ferrets exhibit symptoms similar to those of cats. Symptoms can include:

  • Vomiting
  • Gagging
  • Weight loss
  • Difficulty breathing
  • Reluctance to move

In cats, early symptoms are sometimes mistaken for allergic bronchitis or feline asthma, when they are actually caused by an associated syndrome called Heartworm Associated Respiratory Disease (HARD).

In the United States, heartworm disease is most commonly found along the Atlantic and Gulf coasts, and the Mississippi River, but it has been reported in every state.

Diagnosis of Heartworm Disease

Heartworm disease can be diagnosed with a blood test, which looks for microfilariae (the parasite in its early stages), but it is not considered a reliable indicator until approximately 7 months after the time of infection. Antigen testing is considered more reliable than blood testing, and is now used more often than blood testing to diagnose heartworm disease.

Pets older than 6 to 7 months should be tested for heartworms before starting heartworm-prevention medications, which should then be administered annually. In animals known to be already infected, ultrasound and/or X-ray images of the heart and lungs may be used to locate the infection, and to determine its severity and whether organ damage has occurred.

Treatment of Heartworm Disease

Heartworm disease can be prevented with United States Food and Drug Administration (FDA)-approved medication. Medication, either in the form of a topical liquid applied to the skin, or as an oral tablet, should be administered monthly. Both chewable and non-chewable oral tablets are available, as is an injectable medication that is administered every 6 months by a veterinarian. In ferrets, an approved dog or cat treatment may be prescribed.

Dogs with heartworm disease may receive one of two FDA-approved injectable medications: thiacetarsamide sodium and melarsomine dihydrochloride. Medications are administered in a series of injections, and the dog may be kept in a veterinarian's office overnight. There are currently no FDA-approved drugs to treat microfilariae in the bloodstream; however, monthly preventive medications are sometimes used "off label" in this manner.

Currently, there is no FDA-approved drug for the treatment of heartworm in cats or ferrets. Symptoms, however, can be managed with medication. For a ferret with heartworm disease, drugs that are approved for other animals may be used off-label.


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