Sam, a young Australian shepherd, was healthy hours earlier when his owner gave him his first heartworm preventative. But now he’s in an emergency clinic barely clinging to life — and all because of a drug that was meant to protect him.

Sam is one of many herding dogs with a mutation in the MDR1 gene, making them sensitive to ivermectin. This parasite-killing drug is used in products from heartworm preventatives to large animal dewormers. Ivermectin-containing pooch products are safe for most dogs but cause serious neurologic damage in those with the mutation.

After six weeks of intensive care, Sam’s back to normal. But not all MDR1–positive dogs are so lucky.

A simple test identified Sam’s mutation, and now he’s on a different heartworm preventative. Ask your veterinarian if the MDR1 test is right for your dog. It could be a life-saver.