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Latest health and behavior news and advice from the veterinarians at Tufts University.

Features April 2019 Issue

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 veterinarian Michael Stone, DVM, and his canine pal, Dandy, who gets heartworm medicine every single month of the year.

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.

Comments (4)

In a furtive article, can you please address the issue of how harmful Heartworm preventative is to dogs of all ages? A lot of people, including myself, are very concerned about the toxicity of all of these insecticides and and other potentially harmful chemicals we are told to give our pets. I'd like to really be able to see the eveidence baseded risk versus reward.

Posted by: MilliesMom | March 26, 2019 9:13 PM    Report this comment

I have a beautiful GSP, Piper Eve. Before her was my beloved "first dog" Abigail, a chocolate lab.
Abigail was happy, and I thought healthy. I abided by my state law to vaccinate for rabies every 3 years. Her last vaccine was her death sentence. Within 2 weeks of vaccination, she stopped eating. I took her to the vet and blood work was done. 3 days later, the vet said she was in kidney failure. After 1 month of trying everything, 20k+ ,to keep her alive, she was diagnosed with "auto immune disorder) ,aka, we dont know, and passed away in my arms when her blood count got so low she couldn't catch her breath after I carried her outside to go to the bathroom. Abigail was 7 years old.
After all my research, I knew the vaccine killed her. Every vet, for every vaccine, should warn the owner and advise a full blood panel before any vaccine or preventative treatment, including heartworm.
Still fighting the rabies vaccine. My new dog Piper has been titered and still is protected. With heartworm, I do give it to her year round, every 45 days, not 30. Piper also gets biannual bloodwork to make certain she is benefiting from the heartworm medication, not hurting her.
Please do your homework..if not, have biannual bloodwork done on your animal companion before giving them anything. I miss Abigail everyday...

Posted by: Kpohorylo | March 25, 2019 6:58 PM    Report this comment

I would like to know what double blind studies or at least, long term studies have been done and peer reviewed to prove administering poision year round is necessary. I can see it MAY be necessary in areas that have mosquitoes year round, or for pups who are "snowbirds". I've never found mosquitoes in the winter in the far north. And, last summer was so dry there were none for at least two months.

Posted by: skeptic | March 25, 2019 3:03 PM    Report this comment

I am distressed that you don't address the toxicity of heartworm medication, only compounded if given year-round and amplified in dogs with compromised immune systems

Posted by: keisenstadt | March 25, 2019 2:44 PM    Report this comment

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