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

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Heartworm Prevention Should Be a Yearly Event

Pet Column for the week of May 22, 2006

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

While the spring brings long lazy days and hours of romping with the dog in the back yard, it also brings heartworm season. All dog and cat owners should take heartworm disease seriously because it can be devastating. Fortunately, it is very easy to prevent.

Dr. Allan Paul, a veterinary parasitologist at the University of Illinois College of Veterinary Medicine in Urbana agrees. He says, "Prevention is much easier and cheaper than treatment!"

The heartworm parasite is an actual worm that lives in the pulmonary vessels of the heart (vessels that carry blood to the lungs to pick up oxygen). "They can grow to be up to 12 inches long," says Dr. Paul, "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, an immature form of the worm (called microfilaria) enters 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. Therefore, danger of infection for pets begins in the spring after a few weeks of warm weather.

The heartworm becomes infective inside the mosquito and is passed to a new host animal when the mosquito bites again. After entering the body the microfilaria spend up to five months maturing. 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 migrate to the pulmonary arteries, they grow and reproduce, releasing more microfilaria into the bloodstream. The cycle of infection starts anew when a mosquito bites this host animal and carries microfilaria to its next victim.

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. 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. Although incidence of heartworm in cats is lower than in dogs, dogs tend to develop signs of the disease before heart failure ensues, whereas cats may simply die suddenly.

Signs of heartworm include coughing, fainting, fatigue, and difficulty breathing. Unfortunately, infected 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. The treatment for this disease can be just as taxing to the animal as the infection. The only treatment available for heartworm 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 a risk that the animal may die during the treatment process.

Without a doubt, the easiest way to keep your pet from getting this disease is to prevent infection. There are several options available to treat heartworm disease including once-a-month tablets, daily tablets, and an injection that lasts for six months. The preventatives act by killing the infective forms of the worm before they have a chance to develop into adults. Some form of the preventative should be used throughout the warm months of the year and can be purchased through a veterinarian.

If you have any questions about heartworm, please contact your local veterinarian.