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


Myth: ‘Indoor’ dogs can’t get heartworm

Reality: A single mosquito bite can spread the disease

[From Tufts July 2011 Issue]

 A myth persists that dogs who live indoors with only brief trips outside don’t need protection against heartworm. The reality about the disease — which is spread by mosquitoes — comes down to simple mathematics. “Dogs bitten by more mosquitoes are at increased risk of developing heartworm disease when compared with dogs bitten by fewer mosquitoes,” says Michael Stone, DVM, a specialist in internal medicine at Cummings School of Veterinary Medicine at Tufts University.

“However, less risk does not mean no risk. We have all been bothered by a mosquito buzzing in the bedroom after the lights have been turned out. So the risk of getting heartworm disease for a strictly indoor dog is lower than the risk to an outdoor hunting dog. But the risk is not zero.” A bite from a single mosquito can spread the disease. And while owners and some veterinarians once believed dogs were at risk only in areas of high humidity, heartworm is now seen in all 50 states.

Year-round protection
The likelihood of a dog contracting heartworm disease varies by state and mosquito population, Dr. Stone says. “Generally, heartworm preventives need to be administered during mosquito season. For dogs in Florida, year-round preventive medication is recommended. In Massachusetts we need to worry only about a six-month mosquito season.”

But owners have compelling reasons to give the preventive year-round, Dr. Stone says. “One is convenience in that the dog doesn’t have to come in every spring for a blood test for heartworm. Another is that owners get into the habit of administering medication on the first of the month. Many heartworm pills also prevent fleas and include a de-wormer for intestinal parasites.”

If that’s not enough evidence to support year-round prevention, consider that advanced heartworm cases can result in fatal complications. The cost of treatment is considerable — $1,500 to $2,000 — and symptoms may not be readily visible.

“In general, signs of the disease are seen no sooner than five months after infection because the tiny worm passed by the mosquito must develop into a large adult worm,” Dr. Stone says. Severe cases can cause a swollen abdomen due to fluid retention, weakness, collapse and death. Meanwhile, the cost of a monthly pill that prevents heartworm in a 25- to 50-pound dog is a mere $8.

Comments (2)

My vet (WA state) and Merck Animal Health had some kind of deal going where you get a "free" heartworm test with a 12 month supply of Triheart Plus. This was the cheapest option they gave me for a year of protection. 12 tabs of Ivermectin/pyrantel. A different vet in Canada had my dog on 1 tab of Ivermectin every three months.

Posted by: Mr_Liss | September 18, 2017 9:37 PM    Report this comment

My dogs are on year round heartworm prevention, but my vet (and the heartworm preventative manufacturer) still insist on an annual blood test to check for heartworms. Why is this? Your article suggests that it is not necessary.

Posted by: AnneV | September 18, 2017 8:58 AM    Report this comment

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