Breaking down how parasite preventives work

Parasiticides need to be administered properly and consistently to prevent heartworm disease. Michael W. Dryden DVM, MS, PhD, DACVM (Parasitology), drove the point home in a keynote lecture that launched Day 2 of the 2025 Fetch dvm360 Conference in Kansas City, Missouri.

Dryden is a professor (emeritus) at Kansas State University in Manhattan, Kansas; author of numerous articles, book chapters and books; and frequent lecturer on parasitology at industry events around the world. He has also won multiple awards including the distinctions of 2015 American Association of Veterinary Parasitologists Distinguished Parasitologist of the Year, and the 2017 American Association of Veterinary microbiologists Distinguished Microbiologist of the Year.

Photo: Kristen Coppock Crossley, MA/dvm360

Michael W. Dryden DVM, MS, PhD, DACVM (Parasitology), discusses heartworm prevention and research on resistance during his Day 2 keynote address at the Fetch dvm360 Conference in Kansas City, Missouri.

Dryden’s lecture, sponsored by Boehringer Ingelheim, first emphasized the importance of understanding the lifecycle of heartworm (Dirofilaria immitis). This lifecycle is key because products on the market that target this parasite are effective at killing heartworm in a certain range of time before they reach adulthood.

Produced by female D immitis, larva in the first stage (L1) need to be ingested by a mosquito feeding on an infected dog. The larva then molts to L2 and L3, while hosted by a needed vector mosquito. Dogs become infected with L3 through a mosquito bite and the larva then molts to L4 in subcutaneous canine tissues. Finally, L4 migrates to a thorax muscle and molts into immature adulthood before migration to pulmonary arteries occurs.

“All currently available heartworm preventives administered to dogs are macrocyclic lactones (ivermectin, milbemycin oxime, moxidectin and selamectin). These drugs have exceptional anthelmintic activity and were dose titrated during development against L3 and L4,” Dryden said.

Dryden noted that adult D immitis rarely inhabit the heart. “This is really the pulmonary arterial disease worm. That's where it grows and develops,” he said.

Monthly parasiticide products are essentially “deworming” tools used to kill heartworm once in the subcutaneous stage (L3 and L4), according to Dryden. “They must be administered before L4molt to the immature adult to be consistently effective.”

Oral products administered monthly can kill L3 and L4 from up to 30 days prior with a short term “pulse” of macrocyclic lactones anthelmintic that lasts a few days. Other products with different formulation of pharmacokinetics can also “reach back” 30 days. Injectable moxidectin is a slow release formulation that provides an additional 6 to 12 months of protection, and transdermal moxidectin/imidacloprid features a slow dermal absorption and slow elimination from the blood, which allows it to achieve protection 30 days back and 30 days forward, according to Dryden.

Dryden also addressed the possibility of drug resistance with heartworm preventives. He noted that interest in this notion has grown, possibly because of ‘lack of efficacy’ reports issued by the FDA’s Center for Veterinary Medicine. However, although true resistance may occur, he said, there may be other factors causing canines to be left unprotected by parasiticide products that have been proven to work. Possible causes may be improper administration of these drugs, such as timing, dose or duration; or failure of a dog to retain a dose or to absorb an active ingredient as expected.

As an example, he cited a study conducted in the Lower Mississippi Delta region that found heartworm in 301 dogs that had documented treatment with a heartworm prevention product. However, data showed many of these dogs had breaks in their parasiticide coverage when the clinics did not have the preventive product available to them.

“In 80.7% of the [301] cases there wasn’t even have enough product on hand [to give these dogs] to prevent heartworm…In the other 19%, did they even give it? So when I say we have no idea how much resistance is out there, we have no freaking idea because we don't do a good job. We've got to do a better job,” he said.

References

Dryden MW. Heartworm preventive resistance, where do we go from here? Presented at: Fetch dvm360 Conference; August 22-24, 2025. Kansas City, MO.