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

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No Excuse for Heartworms


Pet Column for the week of April 18, 2005


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

Heartworm season is fast approaching, and pet owners should start preparing for the inevitable onslaught of the dreaded mosquito. Heartworm--a parasite transmitted by these little blood-sucking fiends--should be taken seriously by all dog and cat owners. It can be deadly, yet it is very easy to prevent. With all of the resources available to prevent this disease, no pet should ever contract it.

This parasite is an actual worm that lives in the pulmonary vessels of the heart (the vessels that carry blood to the lungs to pick up oxygen). "They can grow up to 12 inches long," says Dr. Allan Paul, a veterinarian parasitologist at the University of Illinois College of Veterinary Medicine in Urbana, "and if the infestation is severe enough can even live in the heart itself."

As they increase in number the worms begin to take up space in the heart and pulmonary vessels causing high blood pressure, difficulty in breathing, and eventually death due to heart failure.

Heartworm is spread from animal to animal by mosquitoes. When the mosquito feeds on the blood of an infected animal, immature worms (called microfilaria) enter the mosquito. The microfilaria must then undergo an incubation period inside the mosquito during which the temperature outside must be constantly warm for several weeks. Because of this developmental period, danger of infection for pets begins in the spring after a few weeks of warm weather.

After the heartworm has reached the infective stage inside the mosquito, it is passed to a new host animal when the mosquito bites again. After entering the new host, microfilaria must spend up to five months maturing before they migrate to the pulmonary arteries. During this developmental period the worms cannot be detected, so veterinarians recommend testing for heartworm every spring in case infection occurred the previous summer.

Once the worms reach the pulmonary arteries, they grow and reproduce, releasing more microfilaria into the bloodstream. The next mosquito that bites this host animal then carries the heartworm microfilaria to its next victim, starting the cycle of infection anew.

Signs of heartworm include coughing, fainting, fatigue, and difficulty breathing. Animals may have heartworm for several years before showing any sign of the disease, and when signs finally occur, the heart and pulmonary arteries are often so full of worms that treatment becomes very risky.

Treatment for this disease can be just as taxing to the animal as the infection. The primary treatment available for heartworms is a form of arsenic administered at doses designed to kill the worms but not the dog. Although this treatment is safer today than in the past, there is still a risk that the animal may suffer complications, especially in those dogs that have large numbers of worms.

Cats are not as susceptible to heartworm infection as dogs are; however, in areas where incidence of heartworm is high in dogs, some cats will inevitably contract heartworm disease as well. According to Dr. Paul, it is theorized that the lower incidence in cats is due to the cat's ability to fight off the infection. Signs of heartworm in cats include coughing, difficult breathing, and intermittent vomiting. Sudden collapse and death can also occur as a result of even a single worm infection.

Without a doubt, the easiest way to keep your pet from getting this disease is to prevent infection. There are a variety of options for preventing heartworm infection, including daily and monthly tablets and chewables and monthly topicals.

Some preventatives, if given to an animal that already has heartworm, may cause a life-threatening reaction. In addition, if the heartworms are already present in the animal, the preventative will not kill them, leaving them free to cause damage and eventual heart failure. The preventative acts by killing the infective forms of the worm before they have a chance to develop into adults.

With all of the products available to prevent this disease, there is no reason for any pet to contract heartworm. If you have any questions about heartworm, please contact your local veterinarian.