My Pet World: Preventing heartworm is the best 'remedy'

Only about a third of all dogs, and likely less than 10 percent of cats, leave a veterinary practice with a heartworm preventive. No one knows how many ferrets are protected, but it's likely that number is very small. Dr. Stephen Jones, president of the non-profit American Heartworm Society, calls the situation "frightening."

Heartworm disease, transmitted to pets from mosquitoes, is potentially deadly. Recent research shows even dogs treated successfully for heartworm likely suffer permanent damage as a result.

"Every dog I've necropsied (similar to doing a human autopsy), most after living out a normal life after suffering the disease many years ago, showed evidence of permanent lung damage, sometimes significant," Jones notes.

Treatment for heartworm in dogs is expensive, and the medication, which includes arsenic, is taxing on the animal. For cats, symptoms can be treated, but there's no treatment for the disease itself.

"It seems obvious that prevention is the way to go," says Jones, of Moncks Corner, S.C. "Preventives work extremely well, and they're very, very safe," he adds.

Heartworm exists in all 50 states. The heartworm cycle begins when mosquitoes bite a dog and ingest blood containing a microscopic larvae or "baby heartworm" called a microfilariae.

Later, that same mosquito will infect another dog. About two months after initial infection, the larvae undergo a final molt and become juvenile (sexually immature) worms. These juveniles range from one to three centimeters in length. They enter the dog's vascular system and move to the heart and lungs, where they mature into adulthood, reaching the size of strand of spaghetti.

Adult heartworms can then produce live microfilaria, which are released into the bloodstream and the cycle repeats when another mosquito comes along. Without preventives, nature is on the side of the mosquitoes perpetuating infection.

Dogs can have dozens, even hundreds of these worms in their bodies. No wonder infected dogs get so sick, even die.

In cats, heartworm is a very different disease.

"Today, we know cats are about as susceptible to heartworm as dogs are," Jones says. Even indoor cats can be bitten by mosquitos, and the American Heartworm Society even recommends protection for them.

It's not a good day for the heartworm that's transmitted into a cat. Heartworms in cats may or may not mature into adults. Still, even immature worms can cause damage. The good news is, infected cats typically harbor only one to a handful of worms. The bad news is, even a single worm can be fatal, and there's no treatment.

Some cats develop heartworm-associated respiratory disease (HARD), which has symptoms similar to feline asthma. While HARD can be treated, the cat remains in periodic and often chronic discomfort, and the treatment isn't cheap.

Feline heartworm is the second most common cause of sudden death in cats (next to feline hypertrophic cardiomyopathy, a common heart disease in cats). Obviously, sudden death can't be treated. And, to date, veterinarians have no technology to predict if this sad outcome will occur.

Some cats with feline heartworm show a variety of severe symptoms, such as blood clots in the lungs and lung inflammation, which can occur when the adult worms die in the cat's body. Some cats have more generalized symptoms, such as vomiting. Some lucky cats with feline heartworm have no symptoms, and live out what seems to be a normal lifespan, finally succumbing to cancer or kidney disease at a ripe old age.

"We now understand that just like dogs, all cats with heartworm suffer some damage, and just live with it," says Jones. "Even when cats' immune systems kill off worms before they develop into adults, there may be permanent damage."

There are tests to detect heartworm in cats but they aren't perfect. For dogs, there's an accurate screening test for heartworm, which dogs should get annually, according to Jones.

"There are dogs on preventives who get heartworm, and we to know that," Jones says. While no drug is perfect, heartworm drugs are incredibly effective; the failure rate is exceedingly low. More commonly, a dog may spit out or throw up oral doses in succession, or owner compliance (forgetting to give the heartworm preventative) is poor. Veterinarians report that occasionally dog owners find a secret stash hidden behind a sofa.

Heartworm preventives should not be confused with the medication required to treat heartworm disease. Using a preventative in a dog with active heartworm disease can cause a severe reaction.

There are a variety of heartworm preventives available for dogs, cats, even ferrets. Many also kill intestinal parasites. One product (called ProHeart 6) offers six months of protection; other products are monthly. See your veterinarian for the right recommendation for your pets.

Learn more from the American Heartworm Society: www.heartwormsociety.org.

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