Ask the Vet

Heartworms Are A Year-Round Danger

It takes less than two months after being bitten by an infected mosquito for heartworms to develop to a point where preventives can no longer control them.(DTN/Progressive Farmer image by Virginia H. Harris)

Question: I faithfully give my dog heartworm preventive every month, but my vet told me he's seen cases where even consistently treated dogs are developing heartworms. Is this really possible? Are these preventives like antibiotics in that resistance can develop from constant use?

Answer: First, let me say there are strains of heartworms resistant to our preventives, but fortunately, these seem to be rare. As to differences between antibiotics and heartworm preventives, it's important to note that antibiotics should only be used when an animal has a confirmed bacterial infection. With heartworms, however, the potential for infection is continuous.

After a dog is bitten by a mosquito and infected, "baby heartworms" begin their development into adults. By Day 52 after the bite, the developing heartworms are not controlled by our preventives. Even missing a month of preventive can lead to infection. Continuous use of heartworm preventives has not led to resistance. Rather, their improper use has selected for resistance.

Some have advocated stopping preventive in the winter. This is not recommended by the American Heartworm Society or me. It was 78ºF at my house Christmas, and there were mosquitos. Stopping preventive use also brings human nature into play. We often forget to start back. Many heartworm preventives help control other parasites, including hookworms and roundworms (which can infect people). Heartworm preventive doesn't just offer protection to our pets but to us, as well.

I recommend year-round use of heartworm preventive and annual checks. This is the only way we can know how much of an issue resistance is. Most of the companies that manufacture these products will even help pay for heartworm treatment if the owner has a record of consistently purchasing their preventive from a veterinarian, and the dog has had a "negative" annual heartworm test.