With Heartworm Medicine, Do Not Follow Package Directions

Giving heartworm preventives during warm months only is not going to cut it.


Heartworms are parasites that can infest a dog’s pulmonary artery — the artery that brings blood from the heart to the lungs. That can result in severe illness and even death, which is why our pets need to take a heartworm preventive from the time they are puppies throughout their lives.

Previous guidance suggested heartworm preventives need to be administered only during the warm months of mosquito season, as heartworms are transmitted to dogs through mosquito bites. In fact, some package directions still recommend giving heartworm medicine only at certain times of the year. For instance, the prescriber information from the manufacturer of Heartgard says that “Heartgard Plus should be given at monthly intervals during the period of the year when mosquitoes, potentially carrying infective heartworm larvae, are active. The initial dose must be given within…30 days after the dog’s first exposure to mosquitoes. The final dose must be given within…30 days after the dog’s last exposure to mosquitoes.”

The American Heartworm Society disagrees, making clear in its guidelines that “heartworm transmission does decrease in winter months, but the presence of microenvironments in urban areas suggests that the risk of heartworm transmission never reaches zero.” In other words, even in the dead of winter in northern states, conditions can sometimes be suitable for heartworm transmission.

Tufts weighs in

Veterinary internist Michael Stone, DVM, of Tufts’ Foster Hospital for Small Animals, agrees that heartworm preventives should be given year-round. “Doses must be administered every 30 days to be effective,” he says, not only because a mosquito could end up biting your dog in a normally cold month that has an unusual spell of warm weather but also because when mosquito season begins and ends is not a science — it’s an estimate. If you miss a dose and your dog gets bitten, it takes three to four monthly doses of the medicine for its efficacy to catch up to the problem, when the heartworm inside your pet’s body has already reached a later stage of development and is not as easily destroyed.

Another reason to give heartworm prevention medicine once every 30 days is that your dog could end up vomiting the pill without your being aware, particularly if he does so outside. “Continuous, year-round administration of heartworm preventive is a partial safeguard in” such an event, the American Heartworm Society says.

The bottom line: always have this medicine on hand — it’s available only by prescription from your veterinarian — and keep the 30-day dates on your calendar so you never miss a dose.


  1. My 5 year old Golden Retriever has had sensitivities to Heartworm preventatives as well as Flea and tick collars , pills etc. Recently….. after giving him his Tri heart plus pill he started having issues mild at first and then started vomiting continually throughout the day. My GR rarely has vomited prior to receiving this pill. One has to wonder why the very things we are told to give our pets to help them ends up making them sick.


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