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

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Don't Get Stung by Heartworm Disease


Pet Column for the week of April 23, 2007


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

Mosquito season is buzzing. Mosquitoes' itchy bites are not only annoying for humans and animals alike, but they also have the potential to transmit life-threatening illnesses. In pets, the mosquito-borne heartworm is extremely dangerous. Luckily, this disease can easily be prevented.

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

The heartworm disease cycle begins when a mosquito bites an infected animal. As the mosquito feeds on the blood of the infected animal, immature heartworms, known as microfilaria, enter the mosquito. During several weeks of warm weather the microfilaria begin to mature within the mosquito, then are passed to another animal when the mosquito feeds again.

From the time of infection, it takes approximately six months for the microfilaria in the pet's bloodstream to mature to adulthood. The adult worms like to live near and in the heart, as the name heartworm implies. The majority of the adult worms are located in the pulmonary artery, the blood vessel that carries blood from the heart to the lungs.

The heartworms can grow up to 12 to 14 inches in length, causing irreparable damage to the heart. The extreme stress that the adult heartworms place on the animal's heart and other internal organs can cause coughing, difficulty breathing, fainting, fatigue, and weight loss. If left untreated, heartworm disease eventually causes death from heart failure and other complications.

Though heartworm disease can be deadly, pet owners can easily and safely prevent heartworms in their animals. Among the most common methods of prevention are daily or monthly tablets and chewables that are prescribed by a veterinarian based on the animal's weight. Topical monthly heartworm preventives can also be used. An advantage of many of the oral medications is that they also prevent intestinal parasites, such as hookworms, roundworms, and whipworms.

The heartworm preventives prescribed by a veterinarian are effective against the heartworms during the larval life stages. The danger comes when your pet is not on a preventive medication and an infection is allowed to progress to the adult heartworm stage.

Whenever possible, heartworm prevention should begin at a young age. Puppies and kittens should start receiving heartworm medication at between 4 and 8 weeks of age. In order to ensure the lowest chance of infection, it is also recommended that the pet then stays on preventive medication year-round for life.

Dr. Paul explains that although the preventive medications on the market have proven to be extremely effective in the prevention of heartworm disease, they can not be 100 percent effective. As a result, yearly heartworm tests are recommended for dogs older than 7 months; this simple blood test detects only the adult worms.

If your dog does test positive for adult heartworms, your veterinarian will likely recommend staging the disease in order to classify the severity of the infection and then prescribe the treatment to kill the adult heartworms based on this classification.

While the incidence of heartworm disease in cats is lower than in dogs, cats are indeed susceptible to the disease. Unfortunately for cat owners, accurate testing and treatment for heartworm disease in cats is not available at this time, which makes year-round prevention key for the health and well-being of both indoor and outdoor cats.

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