Updates on the current state of heartworm resistance

When a patient is infected with heartworm, it can cause devastating and irreversible damage in dogs. Because of this, compliance is critical in the battle against heartworm and heartworm resistance in veterinary practice. So, how can we improve compliance?

During his lecture Heartworm Surveillance: Staying Ahead of Resistance with New Research at the 2025 WVC Annual Conference in Las Vegas, Nevada, Chris Adolph, DVM, MS, DACVM (Parasitology), shared the latest research on heartworm resistance surveillance to educate attendees on new data and help improve compliance.

“We're going to talk a little program study in the next few minutes, and I'm going to give you some new updates on what's really going on out there. Heartworm is maybe different than the last time you thought about it, and it makes a difference in which prevention you use, for sure,” Adolph kicked off his lecture with.

According to the American Heartworm Society (AHS), most cases of heartworm medication failure can be attributed to compliance issues. Even missing or delaying just one preventive dose can leave an animal vulnerable to infection.

Once a patient is infected with heartworm, the damage is often severe and irreversible. When diagnosing heartworm, veterinary teams typically determine that if a pet is unprotected, it is susceptible to infection. However, when a patient is taking medication as prescribed and still tests positive for heartworm, it signals an evolution in the parasite’s resistance over the past 40 years.

Adolph presented prevalence maps from the American Heartworm Society comparing data from 2001 to 2022, demonstrating the rise of heartworm resistance in the United States. He addressed skeptics—whom he referred to as “heartworm resistance deniers”—pointing out clear patterns emerging across the country.

Adolph also explained that the transportation of dogs across the country for adoption has contributed to the spread of resistant heartworms. Pet owners may unknowingly introduce heartworm-infected animals to regions where resistance was previously uncommon.

“How many of y'all live somewhere other than here, but you see dogs that their book started their life down there. Maybe you guys see these dogs. We're moving them all over the country. When you adopt a dog, guys adopt its parasites, okay, its package deal, right? And if they are resistant heartworms, they get in your ecosystem. And that's the ball game. Okay? Now they live there, right?” Adolph explained.

Between 2013 and 2016, results from a AHS survey on the incidence of adult heartworm infections in adult canine patients in the US showed a 21.7% increase. A study followed the release of the survey results to determine if heartworm testing results available from the Companion Animal Parasite Council aligned with the findings, and whether change in heartworm preventive dispensing accounts for the increase.2

Between 2013 and 2016, results from an AHS survey on the incidence of adult heartworm infections in US canine patients showed a 21.7% increase. A follow-up study examined whether heartworm testing results from the Companion Animal Parasite Council (CAPC) aligned with these findings and whether changes in preventive medication dispensing contributed to the rise.

Researchers analyzed 7-9 million heartworm antigen tests reported annually to CAPC and compared them with the AHS 2016 survey. A state-by-state analysis focused on the southeastern US, where heartworm prevalence is particularly high. National heartworm preventive dispensing data were obtained from Vetstreet LLC and analyzed. The study included oral, topical, and injectable heartworm preventives, with injectable moxidectin counted as six doses.

The results showed a 15.28% increase in positive antigen tests from 2013 to 2016, aligning with the AHS survey. In the southeastern US, the increase was 17.9%, compared to 11.4% in the rest of the country. Nine of the 12 states studied had an increased positive test frequency greater than 10%. Additionally, the findings revealed that two-thirds of canine patients in the US received no heartworm prevention each year.

“I see a lot of low hanging fruit here. I see a tremendous opportunity to have that conversation, and when they say, No, I don't really want to do this and [can I explain for a little bit] why this is super important, and just don't take that person. No, I don't really want any harm or okay, I'm on my 25th patient today. I don't have any gas in the tank for this, but they deserve the best, even if you're on the 25th appointment of the day,” said Adolph.

Taking the time for education and trying to get that third of a third giving their pets preventatives, it can help continue the battle against heartworm resistance and improve the lives of pets across the US.

Reference

1. Adolph C. Heartworm Surveillance: Staying Ahead of Resistance with New Research. Presented at: WVC; Las Vegas, NV. March 2-5, 2025.
2. Drake J, Wiseman S. Increasing incidence of Dirofilaria immitis in dogs in USA with focus on the southeast region 2013-2016. Parasit Vectors. 2018;11(1):39. Published 2018 Jan 17. doi:10.1186/s13071-018-2631-0