Updates on heartworm in the United States

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Heartworm disease is not a new challenge for veterinary professionals, but according to Susan E. Little, DVM, PhD, DACVM (Parasitology), now is a great time for heartworms—and a bad time for dogs. The prevalence of heartworm is rising, making it more critical than ever for veterinary teams to take action.

“The challenges we’re facing with heartworm infection and the risk of heartworm infection in dogs in North America have never been higher. This is the worst it’s ever been for dogs because we’ve got the greatest risk of infection that we’ve ever faced. Thankfully, we also have some of the most powerful [preventives] we’ve ever had at our disposal,” Little said during her lecture.

At the 2025 WVC Annual Conference in Las Vegas, Nevada, Little presented "Heartworm Disease: Timely Updates on an Age-Old Problem," where she discussed the impact of rising mosquito populations on heartworm, plus the importance of additional testing.

Why is the risk increasing?

Just as it’s a great time for heartworm, it’s also an optimal time for mosquitoes. In recent years, mosquito populations have increased dramatically. Because heartworm transmission depends entirely on mosquitoes, this change has serious implications for veterinary medicine.

“There’s been really good data about the mosquito populations in North America over the last century, and from the 50-year period, the second 5 decades that they included [1996-2016]. It’s documented that there’s a 10-fold increase in the mosquito populations in North America. That is phenomenal, right?” Little said.

“There are a lot of theories about why that exists, and it’s not just [that] the...sheer number of mosquitoes [has] increased so much. There’s also a 2- to 4-fold increase in diversity of species. So, twice as many to 4 times as many species out there,” she continued.

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One factor believed to contribute to this rise is wetland restoration efforts in the United States. Because mosquitoes thrive in wet environments, these ecological restoration projects have inadvertently fueled an explosion in mosquito populations.

Because of this, Little emphasized the importance of year-round heartworm prevention. She urged veterinarians to discuss these changes with pet owners, ensuring dogs are protected every month, all year long. With mosquitoes actively transmitting heartworm and driving its geographic spread, veterinary teams must be diligent in their recommendations.

Additional diagnostics

Little acknowledged that many veterinarians are frustrated by the challenges of heartworm testing, but she emphasized that early detection and treatment lead to better outcomes.

“We know that early detection and early treatment lead to better outcomes. That's true in most diseases, and it's certainly true in heartworm, but we also know that the antigen tests aren't perfect. False negatives occur,” she said.

“When I graduated vet school in [1993], I knew that early infections [and] all male infections—we were going to have a harder time picking those up, like only an antigen test. Antigen tests are sensitive, they're specific, they're phenomenal. But the sensitivity is usually 90% to 95%, which means you are missing 5% to 10% of the infections,” Little continued.

Because of this, it’s possible for a dog with heartworm to test negative on an antigen test. To improve detection rates, the Companion Animal Parasite Council and American Heartworm Society recommended 10 years ago that dogs undergo annual testing with both antigen and microfilaria tests.

Little explained that this 2-step testing approach increases diagnostic accuracy. For example, if a dog tests antigen negative but microfilaria positive, then heartworm infection is confirmed, and treatment can proceed. To simplify microfilaria testing in practice, she provided a step-by-step method:

“You collect that blood sample in practice, which you already have to do to run the antigen test. Many folks use a whole blood sample, which is what's necessary for a microfilaria test. And [it's] just a drop of blood on the slide, a drop of saline, cover slip, and examine. That's a microfilaria test,” she said.

Conclusion

Preventive care is the key to controlling heartworm disease, and client adherence is essential. Little suggested offering heartworm testing packages to ensure all necessary diagnostics are completed in 1 visit, minimizing the need for additional appointments and improving owner adherence.

By educating clients on rising mosquito populations, year-round prevention, and improved testing methods, veterinary teams can shift the balance—making it a bad time to be a heartworm and a great time to be a dog.

Reference

Little S. Integrating heartworm disease: timely updates on an age-old problem. Presented at: WVC Annual Conference; March 2-5, 2025; Las Vegas, NV.