Ask the Vet

Heartworms a Year-Round Danger

Mark the calendar and give heartworm preventative year round, the same time each month. (DTN/Progressive Farmer photo by Virginia Harris)


I give my dog heartworm preventive every month, but my vet told me he's seen cases where consistently-treated dogs still develop heartworms. How is this possible? I've been told we are to use this preventive every month of the year, but I wonder if that can actually cause resistance in these worms? Using antibiotics too often can create resistant bacteria, so why doesn't heartworm preventive do the same thing? Please explain the difference.


First, let me say there are strains of heartworms resistant to our preventives. Fortunately, these seem to be rare and primarily in certain geographic areas.

As to your question about the 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. The potential for infection with heartworms, however, is continuous.

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

Some have advocated stopping preventive in the winter, but this is not recommended by the American Heartworm Society or me. It was 78 degrees at my house Christmas day and there were mosquitos. There are also micro-environments around heating units, vents and equipment where the higher temperature can allow mosquitos to live even in the dead of winter.

Stopping preventive use also brings human nature into play. We often forget to start back. It's important to remember, too, that many heartworm preventives help control other parasites including hookworms and roundworms (which can infect people). So in this way heartworm preventive offers protection to our pets and those around them.

My recommendation is to use heartworm preventive year-round and to check for heartworms at least once a year. This is the only way we can know how much of an issue resistance is. Most of the companies that manufacture these products will 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.