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Heartworm: Frequently Asked Questions

Environmental changes are paving the way for the spread of heartworm, but veterinarians and scientists know more about the disease than ever before

The rescued and fostered pets of Katrina and Rita already had plenty to overcome: fear, confusion, injuries, separation from their families. But an estimated 60 percent of those pets were also contending with something else: heartworm. The prevalence of the disease was a surprise to many northern animal welfare agencies and veterinary clinics that spent weeks or even months nursing the animals back to health. But while heartworm is endemic to warm, wet regions of the country, it is on the move—and has been for decades.

Humans have known about heartworm for 400 years, so it’s easy to get complacent. “Everybody thinks they know all they need to know about heartworm disease,” says veterinarian Tom Nelson, president of the American Heartworm Society. “And we’re learning more about the effects of heartworm every year. What we know now from what we knew five years ago is just astronomically different.”

Nelson recently answered a few common questions about the disease.

Are pets from Katrina and Rita spreading heartworm to areas where it wasn’t a problem before?

Scientists won’t know the real effects of the transport of these heartworm-positive animals for at least another year or so, says Nelson. But even without monster hurricanes in the picture, heartworm is spreading. By transporting heartworm-positive dogs into areas where the parasites haven’t been an issue, we are probably just accelerating a process that’s already in progress. Despite the preventatives available, heartworm cases have increased over the past 30 years.

After all, Katrina and Rita pets aren’t the only infected animals on the move. “Because we’re such a mobile society and only 59 percent of the households give preventative anyway, there are heartworm-positive dogs being transported across the country … every day,” says Nelson. Groups that transport animals in need of fostering or new homes may also shift positive dogs from one area to another.

Humans are doing their part to extend the parasite’s reach, too. Our society’s manipulation of the natural environment doesn’t just create holes in the ozone layer or raise global temperatures. We’re producing more subtle changes by taking areas that have naturally been inhospitable to mosquitoes and therefore heartworm (microfilariae, produced by adult worms, cannot reach maturity without first passing through mosquitoes) and changing them so much that we may as well put up signs that proclaim, “Heartworms Welcome.”

Rural regions with temperatures that have consistently dipped below the microfilariae-maturation threshold of 57 degrees are becoming hotter as people build tall buildings, large developments, concrete structures, and parking lots, says Nelson. Areas that were once stark desert are changing as residents build golf courses, plant non-native plants and trees, and install sprinkler systems to maintain the new greenery.

“Where historically we may not have had an issue before, we will have an issue later,” says Nelson. “We continue to see the numbers in California, in Oregon, in Washington, go up. Yes, they’re small numbers … but they are going up.”

Can a heartworm-positive dog be treated with a preventative? After all, isn’t traditional heartworm treatment painful and harmful for dogs?

Giving preventative to a heartworm-positive dog is not necessarily safer than typical heartworm treatment, although some people think so, says Nelson. “I hear so frequently, somebody adopted a dog that was heartworm-positive and they were told that all they had to do is put the dog on preventative and everything would be fine, that the worms would die out within a couple years,” he says. “There are cases where that would be highly acceptable: the 12- year-old cocker spaniel that’s a couch potato. But [for] somebody [who] adopts that three-year-old Labrador that’s going to run all the time, this is not the way to go.”

Nelson says the culprit of misinformation in this case is research that’s been taken out of context. The preventative might kill all of the worms if they are seven months old, but the effectiveness comes into question with older worms; treating some heartworm-positive dogs with preventative may simply shorten the lives of some worms without killing them all—“accelerating the natural death process” of the worms, says Nelson.

Treating heartworm with preventative can be presented as an option if necessary, says Nelson. “If somebody’s adopting [a heartworm-positive dog], you can tell them, ‘Okay, here’s an alternative if you can’t afford the other treatment.’ If nobody can afford the other treatment, then yes, that’s a much better way to go than not doing anything, but it needs to be informed consent.”

To determine whether a dog needs treatment, the heartworm antigen test is the “gold standard” and the most accurate, says Nelson. Testing for microfilariae is less expensive, but microfilariae aren’t present in 20 percent of positive dogs. Nelson says shelters can screen upon intake with the microfilaria test; a positive result indicates heartworm infection, while a negative result warrants antigen testing. Cats are a different story; very few infected cats will have microfilariae present, and heartworm tests often produce false negatives in cats. Nelson recommends shelters omit testing for felines and get them on preventative.

So if a shelter or rescue adopts out a heartworm-positive dog, what treatment is ideal?

First, the dog should be put on a preventative, says Nelson. This will prevent the dog from picking up additional worms. The adopter should be encouraged to meet with their veterinarian to decide on heartworm treatment that is tailored to the animal’s individual risk factors.

Factors to consider are the dog’s breed, size, age, health, and activity level, says Nelson. “If you look at the Heartworm Society’s guidelines, they say guidelines for the diagnosis, prevention, and management of heartworm disease,” says Nelson. “It doesn’t say treatment of heartworm disease; it says management of heartworm disease, because you have to manage that case based on the parameters of that particular case.”

A dog who lounges around on his dog bed all day is at lower risk: as the worms inside his bloodstream gradually die, they probably won’t create problems for him as his body clears them out naturally. For more active dogs, risks can come into play as the worms die. “We find that the majority of the clinical signs and pathology we see with heartworms comes from when a worm dies,” says Nelson. In a dog under treatment, the increased blood flow pumping through his body as he runs or plays can rupture capillaries that are blocked by remains of worms killed by the treatment.

“Horror stories” about heartworm treatments are frequently passed around, says Nelson. But 95 percent of heartworm-infected dogs can be treated successfully, he says, and there are ways to minimize the risk of embolisms and decrease injection pain. “This is like chemotherapy,” he says. “Everyone’s so afraid about chemotherapy and cancer because they hear about somebody’s [experience] like 10 or 15 years ago. Well, the agents have changed and how they use them has changed. … But we have long memories, and we don’t realize that three to five years from today in medicine, half of what we know is obsolete.”

Can cats get heartworm?

Yes. Even indoor cats can get heartworm disease—anyone who’s been bitten by an uninvited blood-sucker inside their home can understand how it happens.

Cats suffer from heartworm disease differently than dogs do. Mention heartworm disease in dogs, and the first thing many people think of is adult worms in the dog’s heart and the resulting cardiovascular effects, says Nelson. That’s not the case for heartworm’s feline hosts, whose main risk is getting pulmonary disease from the parasites. “In the cat, you can have an area where 30 to 40 percent of the cats can be infected, but of those, only three or four percent may have an adult worm in the heart,” says Nelson. “But you’re still having disease that the immature worms are causing in the lungs.”

Although cats may not always show symptoms, Nelson compares a subclinical heartworm infection in a cat to the effects that smoking cigarettes produces in people’s bodies. “They may smoke, and they may look perfectly normal, but we know things are happening in those lungs,” he says.

What should shelters and rescues tell adopters about preventative?

Heartworms occur naturally in every state but Alaska, says Nelson, and shelters should advise year-round preventative to adopters who take dogs and cats home. “If you’re in an area where you’re seeing heartworms in dogs, those cats need to be on preventative,” says Nelson.

Heartworm and the intestinal parasites that preventatives help fight are most prevalent where population density is highest—the East Coast, Gulf Coast, and Mississippi River Valley.

Although dogs in other areas of the U.S. may have a lower risk for heartworm infection, they can enjoy the other benefits of the preventatives, like prevention of dog roundworm (Toxocara canis). (When it’s transmitted to humans, this roundworm can lead to blindness.)

The Heartworm Society used to recommend that pet owners in the northern part of the U.S. use preventative for just six months of the year, but it now urges year-round prevention. Studies show that pet owners give only 60 to 70 percent of prescribed doses, Nelson says.

Whether a pet owner forgets to give a dose or a dog vomits after he’s given one, missed doses can leave the door open for parasites. “By giving these products on a year-round basis, you increase what we call the reach-back effect, or safety net effect,” he says. “If you have an immature worm develop, if you have 12 months [of preventatives] consistently after that, it will eliminate it so it never makes it to an adult worm.”

Nelson discounts claims that heartworm preventatives are too costly for pet owners. “You can take a 100-pound dog, and you can put him on a heartworm preventative for half the cost of a can of soda a day,” he says. “I hear all the time, ‘Well, some people can’t afford it, can’t do that.’ Now, it’s not having to give up a whole lot. I give up a couple Cokes a week and my dog’s protected from heartworms.”


American Heartworm Society

UC Davis School of Veterinary Medicine Shelter Medicine Program

Shelter Medicine for Veterinarians and Staff,
by Lila Miller and Stephen Zawistowski, Blackwell Publishing Professional, 2004, $79.99



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