<< Return to Index     < Previous: Where's Fido    Next: Holiday Health Hazards >

Heartworm Disease in Dogs and Cats
By Kathy Schnizler

One annoying little mosquito is all it takes to infect your dog or cat with the deadly heartworm parasite. A mosquito carrying the heartworm larvae bites your pet and the larvae is passed through the bite wound and into the body of your dog or cat. The larvae then travel through your pet's body to the heart, lungs, and associated blood vessels where they mature into adult heartworms and begin to produce offspring called microfilariae. The microfilariae then circulate through the bloodstream of your pet. When a mosquito bites an infected dog or cat, the microfilariae pass into the mosquito where they develop into infective larvae and the cycle starts all over again. Microfilariae cannot develop into adult worms without first passing through a mosquito.

The worms living in a dog can get up to twelve inches long and live mainly in the right side of the heart. They cause inflammation of the heart and the major arteries to and from this side of the heart restricting blood flow. This can cause clots in the dog's lungs and heart failure. It can also lead to kidney or liver failure. An infected dog will die from one or a combination of these problems.

When a cat is infected with heartworms, the worms are generally smaller in size and fewer in number. Cats seem to be more resistant to infection and some cats appear to be able to rid themselves of infection spontaneously. It is assumed that these cats have strong immune systems which are able to fight off the infection. Infected cats can die suddenly without time for diagnosis or proper treatment.

Preventing heartworm disease is safer and more economical than treating the infection. Dogs are the definitive host for the disease and so there are several types of preventatives available. You should discuss with your veterinarian which preventative would work best for your dog and for your situation. You can only purchase heartworm preventative from a veterinarian and it is recommended that your dog be tested for the infection before starting him or her on any preventative program.

While cats can be infected with this parasite it is less common than in dogs. You should discuss with your veterinarian whether heartworm prevention would be a good idea for your cat and what type would be appropriate.

Determining if a dog has heartworms is done with a simple blood test. The most common test looks for female heartworm antigens in the dog's blood. It takes seven months from the time of infection for the antigens to be found in the dog's blood so an early infection will not be detected. The results are very accurate and it is recommended the test be performed prior to starting the preventative for dogs older than six months and once a year after that.

There is not a test which is as accurate for determining heartworm infection in cats. A positive antigen test can confirm the presence of heartworms in a cat but a negative result does not mean the cat is not infected. There is another blood test which looks for antibodies in the blood instead of antigens. A positive result from this test means the cat at some point was exposed to heartworms but may not currently be infected. A negative result confirms the absence of heartworms with 50-90% accuracy according to the American Heartworm Association.

There is only one drug approved for treating heartworm infection in dogs and treatment can be expensive. It usually involves hospitalization and tests to determine if a dog is healthy enough to withstand the treatment and how severe the infection is. If your dog tests positive for heartworms your veterinarian will discuss the tests required, the treatment, and any possible side effects with you.

There is no approved drug for treating a cat that is infected. If your veterinarian determines your cat has heartworm disease he or she will discuss your options with you.

Preventing this disease can be as easy as giving your pet a treat. See your veterinarian to get your pet on heartworm preventative and your pet will love you for it.