Continue Heartworm Preventative in Winter?

Here in Virginia it’s 22 degrees out. -4 wind chill today. About two weeks ago, we had 18 inches of snow – that’s a lot for our area. Since fall, there have been several cold snaps prior to this one.

The kitchen window isn’t very well insulated, and that’s the farthest room from the forced-air gas heat. It’s so cold in there, I suggested we close the door, seal it off, and retreat into the few remaining tolerable rooms to await the end.

As we huddle together, I realize it’s also a new month. Time for the heartworm preventative medication (in this case, Interceptor). We take heartworms seriously, since one of our dogs, Sadie, was infected with full-blown adult worms in her heart before being rescued.

But…surely no adult mosquito is biting today. Wikipedia says they hibernate during temperate winters.

I’m not worried about saving a few doses of the medication, but I do like to avoid medicating animals when it’s completely unnecessary. We don’t use topical flea preventatives such as Frontline any more, because the dogs became listless for a few days after each application, and they’ve never had fleas. If they do somehow get fleas, we would attack the problem with baths and vacuuming first, but they haven’t been exposed in…gosh, at least a year and a half.

I haven’t noticed any particular reaction to the Interceptor, but I’d hate to be altering their metabolisms or stressing their systems for nothing.

Some brief searching around the Internet does not clarify this question for me. More sites seem to recommend heartworm preventative year round. But I have seen some posts from DVMs [see the comments] saying “have your veterinarian help you decide if seasonal prophylaxis is an option, and if so, when you should stop and restart preventative.”

I know the manufacturers say the heartworm medications won’t hurt the dogs, but they say that about the Frontline stuff too, and someone no doubt said Chinese pet food was safe, for that matter, so I don’t necessarily entirely trust the assertions of vested interests.

Is it necessary to medicate them in this weather? Recommended? What are the risks, if they are quantifiable?

Thank you for your time and attention.


I do it because if I don’t, my vet charges me for an office visit and another heartworm test before supplying the medication in the spring. The difference between that and just continuing the medication for three months is minimal.

Heartworm disease is very uncommon in western PA, but I use it (year-round) for the other effects, namely protection from roundworms, hookworms, etc.

Well, I’m no vet, but I can tell you that here in the Frozen North the doggies on only on heartworm meds 6 months a year, starting in June. But I can just about guarantee there are no mosquitoes around during those 6 months. Skeeters tend to stay away from 3 feet of snow.

I’m with Kayaker. Interceptor protects against more than just heartworms. Plus, like Fear Itself pointed out, if you let it go, the vet will charge you to retest. I have 4 dogs, so that is a chunk of change. I’d rather just keep them on it and be done with it.