The canine hookworm Ancylostoma caninum is the most common species of hookworm affecting dogs in the US. And they’re only likely to become more common, thanks to the spread of multi-anthelmintic drug resistance (MADR) in the species.

MADR canine hookworms are hookworms resistant to two or more drug classes of FDA-approved dewormers: benzimidazoles (BZ), macrocyclic lactones, and tetrahydropyrimidines.

“Multiple anthelmintic resistance in Ancylostoma caninum is much worse than we feared,” said Ray Kaplan, DVM, PhD, DACVM, DEVPC, a professor of parasitology at the School of Veterinary Medicine at St. George’s University in Grenada, West Indies, and corresponding author of a new study from the University of Georgia that found MADR in hookworms infecting greyhound dogs.

Kaplan and his colleagues analyzed fecal samples from greyhound adoption kennels, three veterinary practices that work with greyhound adoption groups, and an active racing kennel. The researchers found that MADR A. caninum were highly prevalent in the breed: Four out of every five greyhounds they tested came up positive for hookworms.

But how common are MADR hookworms in other breeds? Kaplan told NEWStat that’s hard to say for sure because the research is ongoing: “[B]ut our preliminary data based on a genetic test for BZ-resistance suggests that resistance to benzimidazoles in hookworms-infected pet dogs is quite common. Our genetic test only tests for BZ resistance, though, so the worms could be resistant to the other drugs as well; we just have no direct evidence.” But, he adds: “[M]ost likely, a fair percentage are indeed MADR.”

Pablo Jimenez Castro, DVM, PhD, one of Kaplan’s co-authors on the UG study who currently works as a parasitologist at Zoetis, told NEWStat that benzimidazoles are currently the only class of drug that have known molecular markers we can use to screen for MADR mutations in hookworms: “To date, we don’t have these tools for the other drug classes. Hopefully in the coming years we will.”

Castro also mentioned the risk of MADR A. caninum hookworms to humans: “This parasite is zoonotic and causes cutaneous larva migrans [in humans], most commonly known as ‘Plumber’s itch’ and because they’re MADR, then physicians won’t be able to treat these infections.”

It’s a serious problem that’s only going to get worse. That’s why the American Association of Veterinary Parasitologists (AAVP) recently formed a national task force to address the issue: AAVP K9 Hookworm Task Force.

Antoinette Marsh, JD, MS, PhD, head of diagnostic parasitology at the Ohio State University College of Veterinary Medicine and chair of the task force, told Newstat that the “most important thing” that DVMs can do is to be aware of this potential issue and take steps to anticipate it: “Do centrifugation fecal examinations and do follow-up fecal examinations 12 to 14 days after deworming a dog identified as having a patent hookworm infection,” she said. Suspected cases can also be evaluated using Fecal Egg Count Reduction Tests (FECRT).

Marsh said veterinarians should also make sure their clients are aware of the problem, as MADR hookworms pose a threat not only to their pets, but to their families: “Client education regarding fecal hygiene is critical to avoid spread of this parasite and mitigate the zoonotic risk.”