Heartworm disease is a potentially fatal condition transmitted by the bite of a mosquito that is carrying infected larvae of the heartworm parasite. Dogs, cats, and ferrets can get heartworm. Heartworm preventives, given daily, monthly, or semiannually, depending on the product, kill the larvae before they become adult worms.

The American Heartworm Society recommends

◦using heartworm medication for dogs year-round, no matter where you live in the United States

◦getting dogs tested yearly to make sure they're not infected with heartworm

"Testing is important even in dogs regularly treated with heartworm preventive products due to the occasional reports of product ineffectiveness," says Hartogensis. An Internet pharmacy veterinarian cannot draw blood from the animal to perform the test. If the test isn't done, a pet owner could be giving heartworm preventives to a dog that has heartworms, potentially leading to severe reactions.