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With over 250,000 known cases across the United States, canine heartworm disease continues to plague our pets, causing emotional distress to the owners and financial worries to their pocketbooks. The saddest part of all: this disease is completely preventable.

We know what causes heartworm disease, we know how to treat it, and we even have safe, effective medications to prevent the disease. So, why are more than a quarter of a million dogs and cats still getting this terrible disease?

According to a survey recently released by the American Heartworm Society over 250,000 dogs and cats tested positive for heartworm infection nationwide in 2004. Since these cases only included dogs that routinely see the veterinarian, some estimates of the true incidence of heartworms in dogs range as high as 11 million canines infected with the parasite. Throw in coyotes and foxes and one can easily see the huge reservoir of potential cases.

Heartworms are a parasite that reside in the vessels leading from the heart to the lungs of many different mammals, but are primarily suited for life in a canine. The immature larva of the adult heartworms are taken in during feeding by mosquitoes and then spread from mosquito back to dogs after a short, 2 week maturation period in the mosquito’s stomach and salivary glands. After returning to their natural host, the heartworm larva migrate through the dog’s body over the next four to six months, growing in length until they reach the heart. Upon reaching the heart, the foot long parasite becomes sexually active, producing large numbers of larva, which, in turn, wait to be picked up by a feeding mosquito, continuing the disease cycle. Infected dogs might have as few as 5 or 6 adult worms or as many as 250!

Adult heartworms absorb nutrients from the blood stream of the dog. In an attempt to rid the body of the parasite, the dog’s immune system fights the invader, often causing collateral damage to the blood vessels and lungs. In severe cases, large numbers of heartworms can block the major vessels entering and leaving the right side of the heart, causing high blood pressure, bleeding into the lungs, kidney and liver problems, and even death. Treatment of the disease itself involves the use of an arsenic compound. Although deadly side effects with the medication have been extremely rare, many dogs succumb to blood clots in the lungs as the adult heartworms die. And the cost of treatment is also a concern. Appropriate diagnostics, medications, and re-testing of the heartworm positive dog might run as high as $500 to $1,000, depending on the size of the pet.

“Many people are just not aware of how deadly heartworms can be, especially to active pets.” says Dr. Tom Nelson, President of the American Heartworm Society. “Heartworms can live 5-7 years and the owner may not see of any of the symptoms. Many of our pets might be considered less active and these pets will not show the signs of heartworm disease until it becomes severe.”

Keeping your pet indoors will not insure that your pet will not get exposed. It only takes one mosquito getting into your house or one potty trip outside to be bitten by an infected mosquito. Even a few worms can cause severe damage to the heart, lungs and kidneys. Now even cats are presenting with heartworms and we are recommending both dogs and cats use a heartworm prevention all year round.

On a more positive note, veterinary medicine has a wide variety of options available to the pet owner for prevention of this disease. Easy to give monthly chewables are the most convenient way to prevent infection. The most commonly prescribed monthly chewable is called Heartgard. Administration of these preventives at the appropriate time intervals can virtually guarantee protection for your pet. In fact, manufacturers of heartworm preventive will stand behind their product and reimburse any medical treatments necessary should a dog develop heartworms while on their product.

Newer products, such as Revolution and Advantage-Multi, are applied on the skin and also help protect against fleas and internal parasites. New Trifexis is a chewable tablet that covers heartworms, fleas and intestinal parasites.

It is vitally important to test your dog prior to starting heartworm preventive or extreme allergic reactions could develop. Your veterinarian will draw a small amount of blood from your pet and, in many instances, you might know the test results prior to leaving the veterinarian’s office. Due to the extreme prevalence of this disease, the American Heartworm Society strongly encourages annual re-testing of all dogs.

According to Nelson, pet owners seem to be likely to switch products, with or without the knowledge of their veterinarian. This product and brand switching has the FDA concerned about a perceived lack of protection, or even potential product failure. “We need to make sure we catch this disease as early as possible, thus the strong recommendation for annual testing.” says Nelson.

Also to be considered is how society has changed in the last 20 years. As people and their pets move from the wetter regions of the Midwest and Southeast to the sunshine of southern California and Arizona, they often bring along these unwelcome parasites. Nelson says “If you have mosquitoes where you live, heartworms, even if they aren’t native to the area, will be there as well.”

Hurricane Katrina caused many heartworm positive dogs to move into all parts of the country thus accelerating the spread.

As spring time approaches, we all welcome the return of the bright sunshine, the longer days, and the blooming of nature. Just remember, the return of warmer days will mean the return of mosquitoes and the potential for heartworm disease spreading. Make sure your best friend protected! Call your veterinarian today and schedule a heartworm test. For more information, visit the American Heartworm Society at www.heartwormsociety.org.

Dr. Debra Garrison is a veterinarian at the Treaschwig Veterinary Clinic