Lethargic Cat
Veterinarians and pet owners alike have more heartworm information at their fingertips than ever before. We now know, for instance, 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, though, 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? Absolutely.

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 microfilariae 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 microfilariae, which soon develop into infective larvae inside the mosquito. When that mosquito later feeds on a susceptible host, the infective larvae are deposited on the skin next to the feeding site. The larvae enter the host’s body through the bite wound and eventually migrate 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 progress through several stages, and not all of them are equally susceptible to heartworm preventives. Consistent dosing of preventive products helps ensure 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 can cause a window of vulnerability to heartworm infection.

Every Month, All Year

To keep pets safer, the American Heartworm Society recommends year-round heartworm prevention for 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 many diseases, early detection can be critical to successful outcomes.

Why aren’t more pet owners opting for year-round heartworm prevention? Some experts have blamed the struggling economy for the reduction in preventive care. This may have been part of the equation, but it’s not the only explanation. People are busier than ever, and giving a pet a monthly dose of heartworm prevention can fall off the radar. But such a simple mistake can be costly — both in terms of your wallet and your pet’s health.

Occasionally, despite our best efforts, pets on heartworm prevention become infected. The vast majority of these 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.

Your cats and dogs depend on you and your veterinarian, working as a team, 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

Most veterinarians have seen patients who have suffered from heartworm disease. We can all learn from their experiences. Here are three stories of pets who contracted heartworm disease and the lessons you can learn from each.

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 didn’t. This was a preventable mistake. Fortunately, Sassy was successfully treated and is doing well, but this was a hard lesson learned all around.


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? It is impossible to know for sure. What we do know is that Luke didn’t benefit from the money his owner saved — and, in fact, neither did his owner. Fortunately, because Luke’s vet was testing him regularly, his heartworm infection was caught early, and he is expected to do well with treatment. But his owner will spend the small amount she saved on his heartworm preventives (plus much, much more) on his treatment.

Heartworm protection is not just for dogs — indoor cats need it, too. Lou was a Domestic Shorthair who lived indoors and never ventured outside. One night, Lou’s owner called the vet in desperation, because Lou was suddenly having extreme difficulty breathing. Indeed, Lou almost died that night from a complication of feline heartworm disease that we now refer to as heartworm-associated respiratory disease, or HARD. Owners of indoor cats like Lou may assume that their pets don’t need heartworm preventives, but we know better. 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 — or completely absent. 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.

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