Many states now show incidents of heartworms right though the late fall and early winter months.
Yes. I live in Michigan and it starts to snow here in October and will snow through the end of April and my vet advised heartworm through Dec. and then start again in April. I think the reasoning is that the preventative kills the babies already in the dog's system, it doesn't actually prevent them. So if your dog got heartworms in aug/sept. and you stopped in sept/oct b/c you thought it was late enough in the season, the heartworms just keep growing.... I think you want to give the preventative a month or two or three beyond the mosquito season b/c what it does it kills the worms; it doesn't prevent the dog from getting infected.
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