Winning the Battle Against Heartworms

Rates of heartworm infection in pets are increasing. Here’s a call to step up prevention and get that number down

Veterinarians and pet owners alike have more heartworm information at their fingertips than ever before. We now know, for instance, that some of the most aggressive mosquitoes that transmit heartworm disease are not just early-morning and late-evening feeders. We know that even in the northern parts of the country, mosquitoes that can transmit heartworms are living and feeding in small microclimates that are much warmer than the cooler surroundings. We know that most of our heartworm preventives also help to control intestinal parasites that could also infect people. And we know that natural disasters and their resulting relief efforts can actually serve to spread heartworms, as was the case after Hurricane Katrina.

Even with all this knowledge, we seem to be losing ground to these parasites that infect dogs, cats, and many wild mammals. Data from the American Heartworm Society show that the number of heartworm disease cases is going up. And it seems sales of heartworm preventives are going down. With all this information at our fingertips, shouldn’t this trend be just the opposite? Yes.

THE CIRCLE OF LIFE
So what’s holding us back? First, understanding the life cycle of the heartworm is critical to winning the fight. Adult female heartworms produce large numbers of first-stage larvae, called microfilaria, that circulate in the bloodstream of the infected host. When a mosquito feeds on the infected host, it takes in a blood meal containing the microfilaria, which soon develops into an infective larva. When that mosquito later feeds on a susceptible host, the infective larva is deposited on the skin next to the feeding site. The larva enters the body through the bite wound and eventually migrates to the arteries that serve the lungs, where the larvae develop into adult heartworms and the cycle begins again.

Why is it important to understand this? Heartworm larvae are susceptible to heartworm preventives only for the short period of time during the migration phase of the life cycle just after they enter the host. Once-monthly dosing of preventive products ensures that the deposited larvae are exposed to lethal doses of preventive medication before they progress to the less-susceptible stages of the life cycle. Missed doses cause a window of vulnerability to heartworm infection.

EVERY MONTH, ALL YEAR
To keep pets safe, the American Heartworm Society recommends year-round heartworm prevention for all cats and dogs—as do most veterinarians. The aforementioned microclimates are one of the most compelling reasons, but they’re not the only reason. Most heartworm preventives also control other dog and cat parasites—parasites that have the potential to infect us and our children.

The cost of heartworm prevention is low compared to the cost—and difficulty—of treatment. Year-round prevention should be viewed as inexpensive insurance. Along with preventive medication, regular heartworm testing is crucial in the battle against heartworms. As with most diseases, early detection and treatment are crucial to successful outcomes.

Some think the poor economy is responsible for the current reduction in heartworm prevention. This may be part of the equation; however, the trends were apparent before the current economic downturn. More than 50 percent of dogs that made a visit to a veterinarian last year had not one single dose of heartworm prevention during the previous calendar year. Your veterinarian may step up efforts to keep you informed about the need for consistent year-round prevention, but prevention doesn’t start and stop at the veterinary practice. Let’s face it: People are busier than ever, and giving your pet a monthly dose of heartworm prevention can fall off the radar. But such a simple mistake can be costly.

The cost of heartworm prevention is low compared to the cost—and difficulty—of treatment. Year-round prevention should be viewed as inexpensive insurance.

Occasionally, despite our best efforts, pets on heartworm prevention become infected. The vast majority of infections are due to human error in administering the preventive medication. There are rare cases where a lack of efficacy—when the medication appears to fail—is the most logical explanation. Most of these are limited to dogs living in the Mississippi delta region of the southeastern United States. Researchers are studying the exact cause of this phenomenon. However, these rare instances are no reason to abandon heartworm preventives.

Cats and dogs depend on the teamwork of you, your family, your veterinarian, and the veterinary staff to safeguard their health. To win the battle against heartworms, turn your knowledge into action—and act now to protect your pet.

Real-Life Lessons
Some of my patients have suffered from heartworm disease. I’ve learned from their experiences, and I hope you do, too.

CASE STUDY 1: SASSY
Knowledge about what products do (and don’t do) to prevent heartworm is key
When Sassy, a Labrador Retriever, was diagnosed with heartworms, her owner was shocked. After all, her owner believed that the monthly flea product she was using also protected the pooch from heartworms. It was a mistake on the owner’s part, but my veterinary team and I were at fault, too. Our practice didn’t pay enough attention to the owner’s buying habits to notice that her purchases didn’t include heartworm prevention. I’m happy to report that Sassy was successfully treated and is doing well, but this was a hard lesson learned all around.

CASE STUDY 2: LUKE
Trying to save money on heartworm prevention may backfire
In order to cut costs, Luke’s owner purchased heartworm preventives in Mexico. She’s certain Luke received his medication every month, and yet he tested positive for heartworms. What happened? Was the medication a knockoff? Was it a repackaged, outdated product? Was the owner mistaken about the schedule to administer the preventive? I can’t be sure. What I do know is that Luke unfortunately didn’t benefit from the money his owner saved—and, in fact, neither did his owner. Fortunately, because we were testing Luke regularly, we caught his heartworm infection early and expect him to do well with treatment. His owner will spend her minor cost savings (plus much more) on his treatment.

CASE STUDY 3: LOU
Heartworm protection is not just for dogs—indoor cats need it, too
My friend Lou has long since passed away, dying at a ripe old age. He was one of my most memorable patients. Lou was a 20-pound Domestic Shorthair cat. Yes, he was heavy, but he refused to go on a diet. If his owner limited his food intake, he would retaliate by misbehaving. Lou also loved to have his teeth brushed with an electric toothbrush. As you can see, Lou was a real character. So you can imagine the desperation when his owner called one night and reported that Lou was suddenly having extreme difficulty breathing. Indeed, Lou almost died that night from feline heartworm disease, which we now refer to as heartworm-associated respiratory disease.

Heartworm infection in cats is easily overlooked and harder to detect than in dogs. What’s more, there is no approved treatment for heartworm infection in cats, so prevention is critical. Signs of heartworm disease in cats can be subtle and misleading. They can include coughing, difficulty breathing, lethargy, rapid heart rate, decreased appetite, weight loss, vomiting, diarrhea, blindness, collapse, and convulsions. Unfortunately, many cats don’t survive the crisis of heartworm infection. Luckily, Lou did. Back then, most indoor cats like Lou were not placed on heartworm preventives, but we now know better.