A hookworm that shrugs off three drug classes. Tapeworms that no longer care about praziquantel. A flesh-eating fly that turned up in the US again this year. Those are three of the twelve parasites the American Animal Hospital Association put on its new watch list, released October 29.
The 2026 AAHA One Health Guidelines on emerging parasites in dogs and cats profile twelve species of greatest concern in the US and Canada, plus nine more worth watching. They name four reasons parasites keep finding new ground: heavy antiparasitic drug use and the resistance that follows, changes in environment and host animals, more animals moving between regions, and better lab tests that catch what used to go unnoticed.
The practical half is about stewardship. Use parasite drugs when there is a reason, at the dose your vet set, for the full course. Half doses and leftover pills are how resistance gets a foothold. AAHA also asks clinics to talk about the human side of parasites, because roundworms and hookworms can infect people too, and kids in sandboxes and gardens are the usual route.
What this means for your dog: if your vet suggests a fecal test, say yes. Resistance is exactly the situation where a quick look under the microscope is not enough, and a dog can carry a resistant hookworm while looking perfectly healthy. Bring a fresh sample, keep preventives on schedule, and mention travel, dog parks, and foster dogs at the appointment. Those details change which parasite your vet tests for.