The American Heartworm Society (AHS) recently issued a release emphasizing that "Contrary to what owners may think, heartworm disease is a year-round threat" and encouraging people to use heartworm preventive medications year round.

I’m in Canada, not the US, but some US regions have a similar climate and similar issues to us. Also, some people try to directly apply US recommendations to Canada, so I’ve critiqued their reasoning below, from an Ontario context.

The AHS based the release on 3 "facts":

Fact #1: Pesky pests pay no attention to the calendar.

• Mosquitos and heartworm don’t pay attention to the calendar, but they do pay attention to the weather. The picture to the right is the view of our deck (prior to the last couple rounds of snow). I don’t think there are many mosquitoes hanging around out there. Yes, they are somewhere, since they don’t become extinct over the winter, but is mosquito exposure a reasonable concern now? No.
• Furthermore, development of Dirofilaria immitis (the heartworm parasite) in mosquitoes ceases at temperatures below 57F, a level we haven’t seen in a while (and likely won’t for a couple of more months). So, even if there are mosquitoes hanging around at the beginning and end of the "heartworm season" in temperate areas, if the parasite can’t develop, it doesn’t matter much.

Fact #2: Mosquitoes know when to come in from the cold.  When weather changes prompt pets to spend more time inside, mosquitoes follow, keeping the possibility of heartworm transmission alive. This means that so-called “indoor” pets are as much at risk as their more outdoorsy counterparts.

• I’d like to see some data backing that up. I haven’t had a mosquito bite in a while.

Fact #3: Staying on schedule with heartworm prevention keeps pets safe.

• Potentially, but assuming every pet owner is forgetful and unable to figure out how to treat their animal once a month over a prescribed part of the year isn’t really a reasonable justification for a treatment regimen that uses more drug and costs more money.
• There is no evidence that 12-months-a-year treatment results in any better compliance than targeted seasonal treatment. (If I can’t remember to give it 6 months of the year, does adding 6 more months really help?)
• Around here, the vast majority of heartworm cases are in dogs that are not on heartworm prevention medications, not in dogs in which treatment failed, potentially because of compliance problems. Is it possible that some people would be more reliable with monthly treatment? Sure. It’s also possible that some people would be as bad (or worse) with year round treatment. It’s also possible that some dogs would go untreated altogether if their owners balked at the cost of year-round treatment.

The bottom line for me:

Heartworm’s a nasty disease and one that we need to prevent.

There are effective preventive medications.

Ontario isn’t Louisiana. In some areas of North America, year round treatment is absolutely needed. In others, the risk period is much shorter.

Decisions about the approach to prevention need to be based on the risk in the area, the duration of likely mosquito exposure, the outdoor temperature and the owner’s ability to comply with treatment. This is a discussion that needs to take place between pet owner and veterinarian on an individual basis.

There is no one-size-fits-all approach to heartworm prevention.

Merlin and I will each get hundreds of mosquito bites this year (surrounded by wetland as we are). He’ll get his monthly heartworm prophylaxis during what I feel to be the "at-risk" time of year. That doesn’t include today, when it was -21C this morning!