Early spring and heartworm questions

We've had a really bizarre spring around here. At a time of year when we're usually bouncing back and forth between snow and rain, we seem to have skipped over spring and moved right into summer. It was 25 C (77F) today, the snow's long gone, frogs are deafening at night and, on the bad side, mosquitoes are swarming at dusk.

This has led to a lot of questions about the implications of the weather on the regional approach to heartworm prevention, since the typical approach is to start treatment in early June. Specifically, does the early spring mean that there's increased risk from this mosquito-borne parasite or that we need to start treating dogs earlier?

Dr. Andrew Peregrine, a veterinary parasitologist at the University of Guelph, has taken a break from fielding calls about this subject to provide some insight.

A big question is whether over-wintering infectious mosquitoes are a concern.

If over-wintering mosquitoes aren't a concern, which is a leading consensus, then the mosquitoes we're seeing now are newly hatched.

  • If that's the case, they have to feed on an infected animal and the parasite has to mature inside the mosquito before the mosquito poses a threat to dogs (and less commonly cats, and very rare humans).
  • With the current temperatures, it should take at least three weeks for this to happen. Therefore, if a mosquito hatches in March and happens to feed on an infected animal, it would be into April before it's infectious. Heartworm preventive medication has 'reach-back' activity of at least 7-8 weeks, meaning that the medication will effectively prevent disease from exposure at least 7-8 weeks prior to treatment.
  • So, if a mosquito promptly feeds on and infects a dog after it becomes infectious, heartworm medication given at the beginning of June would still be effective at preventing disease because of this reach-back period.
  • In this scenario, there's no need to start treatment any earlier around here than the traditional June 1 start date.

There are some people who think that overwintering infectious mosquitoes are an issue, although evidence supporting that seems to be lacking. If overwintering infected mosquitoes are an issue, the mosquitoes that are around now might be infectious.

  • In that case, June 1 might be too late in the event of an early exposure, and May 1 would be a more appropriate start.

Bottom line, there is no answer that's 100% certain since we don't have all the information that we need. June 1 is most likely perfectly fine. Anyone who's's ultra-concerned could start treatment on May 1. There's no real downside (apart from a little extra cost) of this extra treatment, but it's probably not necessary.

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