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

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Heartworms in Cats and Dogs

Pet Column for the week of May 4, 1998

Office of Public Engagement
2001 S. Lincoln Ave.
Urbana, Illinois 61802
Phone: 217/333-2907
By Theresa A. Fuess, Ph. D.
Information Specialist

"Heartworms (Dirofilaria immitis) are long, slender parasites than can reach 12 inches in
length. The worms live in the right side of the heart and adjoining vessels of infected cats
and dogs and are capable of causing substantial damage to the heart and lungs before the
animal shows any signs of disease," says Dr. Allan Paul, small animal Extension veterinarian
at the University of Illinois College of Veterinary Medicine at Urbana. "Unfortunately, cats
and dogs of all ages and all breeds are susceptible to heartworms. Fortunately, it is a
disease that is entirely preventable."

Heartworms are transmitted only by mosquito bites. Immature forms of the worm circulate
in the blood of infected animals and are picked up by mosquitoes that bite them. The
mosquito then passes the infection to other animals they bite. Outdoor dogs are more likely
to be infected with heartworms than those that stay inside. However, mosquitoes can slip
through cracks around doors and in screens and every dog and cat may be at risk, whether
indoors or out.

There are differences in the heartworm disease in cats and dogs. Dogs may have as many
as 100 worms that can live in the heart and vessels for up to seven years or as long as the
dog can live with the worm burden. Cats usually have only two or three worms and the
worms live only up to two years.

The general signs of heartwormscoughing, difficulty breathing, lethargy, and weight lossare
common to cats and dogs, but the course of the disease is very different. In untreated dogs,
heartworms will increasingly compromise the circulatory system over time and will
eventually lead to death. In cats the signs of heartworms can be vague and can mimic other
feline diseases, making diagnosis difficult. Many cats are able to clear the infection before
the worms reach the heart. Other cats can appear healthy and then suddenly die from

Treatment of heartworms can be risky in both cats and dogs because of the potential for
severe complications and even death when the worms die.

"The good news is that prevention of heartworms eliminates the need for a cure," says Dr.
Paul. Before dispensing preventative medication, your veterinarian will perform a blood test
to determine whether your pet is already infected.

For more information on animal health, contact your local veterinarian.