Heartworm is a preventable, but serious and potentially fatal, parasite

Heartworm primarily infects dogs, cats, and ferrets. It can also infect a variety of wild animals. Dogs are extremely
susceptible. Cats appear to show a very high level of resistance, but not 100%.

Mosquitoes Transmit Heartworm Disease

Heartworms can only be transmitted from animal to animal by mosquitoes. When a mosquito bites an infected animal, young heartworms called microfilariae enter into the mosquito’s system and begin their journey toward infecting unprotected pets with heartworm disease. Here is the journey:

Infected animal with microfilariae bitten by mosquito microfilariae develop into infective larvae (2wks) in mosquito uninfected animal bitten by infected mosquito larvae migrate through animal’s body (3 months) larvae settle into the blood vessels of the lungs and the heart and mature (3 more months) within 6 months of the animal being bitten, heartworm disease rears its ugly head if adult worms of both sexes are present they mate and produce new microfilariae and the journey starts all over! Note: infected cats do not often have microfilariae circulating in their blood so are not likely to transfer the infection to another mosquito.

Heartworm Prevention

Heartworm infection is almost 100% preventable in dogs and cats. We prescribe Heartgard Plus, Interceptor® Flavor Tabs, Iverhart Max, or Revolution® Topical, FDA-approved heartworm preventatives. These are once-a-month preventatives and are given by you as directed by one of our veterinarians. We recommend all dogs be given monthly heartworm preventatives year-round.

Heartworm Testing

Testing for existing heartworm infection is by a blood test.

The test we use, which is very accurate in dogs, detects adult heartworms, which take 6 months to mature from larvae. A heartworm test for your dog is recommended before beginning a prevention program to assess your dog’s current heartworm status. The next test should be performed about 6 months after starting the preventative treatment to confirm your dog was not infected just prior to beginning prevention. Heartworm tests should be performed periodically to ensure your dog does not become infected. The recommended frequency of heartworm testing will depend on you giving your pet the preventative. Please let us make the appropriate recommendation.

Testing in cats often requires a series of tests to give the best chance of detecting the presence of heartworms.

If blood test results are positive, further tests such as chest X-rays, a blood panel and an ECG or ultrasound of the heart are recommended to evaluate the severity of the disease.

Treatment for Heartworm Disease

Dogs: It is better to prevent than to treat! However, there is an FDA-approved treatment available for dogs. There is substantial risk involved in treating a dog for heartworms and it requires hospitalization with complete rest during and for some time after the treatment.

Cats: There is currently no effective and safe medical treatment for heartworm infection or heartworm disease in cats.

Surgical removal of heartworms from dogs and cats is a high-risk procedure and typically reserved for severe cases.

Symptoms of Possible Heartworm Disease

Dogs may cough, become lethargic, lose their appetite or have difficulty breathing, or tire rapidly after only moderate exercise.

Cats may cough, show respiratory distress or vomit. Rarely, cats may suddenly die from heartworms.


Some images were taken from Hill’s Atlas of Clinical Anatomy, Published by Veterinary Medicine Publishing Company, Inc.