Pet Columns, Office of Public Engagement, College of Veterinary Medicine at the University of Illinois at Urbana-Champaign

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Heartworm: Serious but Preventable

Pet Column for the week of May 7, 2001

Office of Public Engagement
2001 S. Lincoln Ave.
Urbana, Illinois 61802
Phone: 217/333-2907
Jennifer Stone
Information Specialist

Every dog or cat owner should be concerned about heartworm. "Ideally, all dogs should be treated with heartworm preventative because infestation by this parasite can cause serious and life-threatening complications," says Dr. Allan Paul, a veterinary parasitologist at the University of Illinois College of Veterinary Medicine in Urbana. "But prevention is as easy as giving a once-a-month pill."

The heartworm is an actual worm that can grow to be twelve inches or longer. It lives primarily in the pulmonary blood vessels (vessels that carry blood to the lungs to pick up oxygen), but in bad cases, the worms can even infest the heart itself. These worms cause high blood pressure, difficulty in breathing, and eventually death due to heart failure.

Heartworms are transmitted between animals by mosquitoes. When the mosquito feeds on the blood of an animal that is infected with heartworm, an immature form of the worm, called microfilaria, enters the mosquito. This immature form is not capable of infecting other animals immediately; it requires an incubation period at a constant warm temperature over several weeks inside the body of the mosquito in order to reach the infective stage. Because of this, the danger of infection for pets begins in the spring after a few weeks of warm weather.

After the microfilariae have reached this infective stage, they enter the new host animal when the mosquito bites again. They spend up to five months maturing in the dog's body before migrating to the pulmonary arteries. During this time, heartworm tests cannot detect the worm. For this reason, veterinarians recommend testing for heartworm every spring in case infection occurred the previous year. Once the worms reach the pulmonary arteries, they grow and reproduce, releasing more microfilariae into the bloodstream. The cycle starts again when a mosquito spreads the infection to yet another animal.

Cats are not as susceptible to heartworm as dogs are; however, in areas where incidence of heartworm is high in dogs, some cats will be infected with heartworm as well. According to Dr. Paul, it is theorized that lower incidence in cats is due to the cat's ability to fight off the infection. Heartworm prevention medication is now available for cats as well as dogs.

Signs of heartworm can include coughing, fainting, and difficulty breathing. The animal may also be easily fatigued. Unfortunately, animals may have heartworm for several years before signs of the disease appear. When signs finally do occur, the heart and pulmonary arteries are often so full of worms that treatment is very risky. The treatment for heartworm is a form of arsenic administered at doses intended to kill the worms but not the dog. Although the treatment is safer today than in the past, there is a risk that the animal may die during the process.

Without a doubt, the easiest way to ensure the health of your pet is to prevent heartworm infection. The preventative should be given to your pet only after a negative heartworm test has been performed. Some preventatives, if given to an animal that already has heartworm, may cause a potentially life-threatening shock reaction. In addition, the preventative will not kill the adult worms already present in the body, and therefore the worms will continue to reproduce and will eventually cause heart failure.

The preventative is available in convenient monthly doses and acts by killing the infective forms of the worm before the have a chance to develop into adult worms. Some form of heartworm preventative should be used throughout the warm months of the year. All of these products are available through your veterinarian.