Heartworm Disease Continues to Cause Heartbreak for Pet Owners

by Steve Dale

Squashing heartworm disease has been quite the challenge. For starters, a surprising number of dog owners (over half, according to one survey) and far more than half of cat owners fail to purchase a heartworm preventative in the first place. Of those who do, a surprising number forget to use it. That's all bad enough. Now, the news gets worse.

In the Mississippi Delta area, there are places where it's possible that the heartworms themselves are building resistance to products used to destroy them.

What's more, the mosquitoes responsible for delivering heartworm disease seem to be thriving, and are even on the rise in many parts of the country. Heartworm has now been identified in all 50 states.

Mosquitoes deliver heartworm by sharing a microscopic organism called a microfilaria when they bite dogs, cats or ferrets. This organism eventually grows into a 6- to 12-inch adult spaghetti-shaped worm, which ultimately settles in the lungs and chambers of the heart of affected pets. Heartworm may be deadly. In cats, sometimes the only symptom is sudden death.

"Of course, we're very concerned that so many dogs are unprotected, either because their owners don't purchase a preventative or because their people forget to use the product," says Dr. Wallace Graham, of Corpus Christi, TX, president of the American Heartworm Society. "That's not to mention all the dogs in some places in the country who aren't exactly strays and aren't exactly owned, but people feed them. Still, there's no one who gives them heartworm preventatives."

Graham concedes that the economy plays a role, since heartworm preventatives do cost money, and people may not understand the investment they're making.

"Not treating (for heartworm) may potentially create a financial burden," says Dr. Sheldon Rubin, immediate past president of the American Heartworm Society. He explains that treatment of the disease, at least $1,000 per dog, is far more expensive than many years of using a preventative. In cats, there currently is no treatment for heartworm disease.

Graham concedes that some cats probably don't require a preventative, such as indoor-only cats living in high-rises where the windows don't even open. "However, I've had an eye-opening experience in my own practice," he adds. "I used to not worry about heartworm for indoor cats, but I've recently seen six cases of heartworm in cats, and half the cats were indoors-only. Of course, mosquitoes can get indoors."

Heartworm disease in cats may cause a recently-discovered syndrome known has heartworm-associated respiratory disease (HARD), which remains commonly misdiagnosed as asthma. "This can be very serious, with cats being very uncomfortable, even dying," Graham says.

New research about HARD was one of the many topics discussed at the State of the Heartworm, the 13th Triennial Symposium, April 16-18 in Memphis, TN. Over 300 veterinarians and scientists from around the world attended. The hot topic of conversation was potential resistance to some or all heartworm preventative products. "No one really knows what's happening," says Graham. "We didn't walk away with a silver bullet answer; it's a complex issue."

Why is it that in some communities in the Mississippi Delta -- along the Gulf Coast and up through to Tennessee -- veterinarians at one clinic are confident clients are using appropriate products as directed, yet animals come in and are diagnosed with heartworm, while just a mile away at another clinic there's no such problem? How can resistance develop in one place, close to another where resistance doesn't seem to occur? This may link with another hot topic: microclimates. It seems that miniscule variations in temperature and humidity within tiny geographic sectors create different responses in mosquitoes, and maybe in the microfilaria, which develop into adult heartworms. No one knows for sure, but researchers are working to unravel the mystery.

Rubin, based in Chicago, said the symposium also delivered news about a mosquito-delivered disease called subcutaneous dirofilaria repens, which is related to heartworm. After the mosquito bites a pet or a person, a nodule resembling a boil eventually appears on the skin, which a worm grows inside. This problem exists in many countries throughout Africa and Asia, seems to be on the rise in Europe, and may eventually make its way here.

There was good news at the symposium. Rubin says scientists are optimistic that early diagnosis of heartworm disease in dogs or cats may soon be possible with a blood test to measure a biomarker. Experts may be close to developing a treatment for cats with heartworm disease. In addition, for dogs with heartworm disease, a common antibiotic, combined with the standard treatment, seems to decrease the likelihood of adverse reactions.

"Heartworm is an exciting and complex issue, but for most pet owners, prevention is the key word to remember," Rubin adds.

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