Heartworm disease is carried by mosquitoes and the first stage of the disease occurs when an affected mosquito bites a dog and transmits microfilaria into the blood. This stage itself does not pose a health threat; heart disease occurs when these microfilaria develop into worms that lodge in the heart about 50-60 days after infection.
Heartworm medication given monthly prevents heartworm larvae that may have infected the dog in the previous 30-60 days from becoming worms embedded in the heart. This is why we say that it "protects for the month before it is given."
This is also why in the Chicago-area climate, unless there is an unseasonably warm March, we give the medication from June (to protect for May and some of April) through December (to protect for November and some of October) during any possible mosquito exposure.
There is no need to give the heartworm medication if there are no mosquitoes.
If you travel with your pet where there are mosquitoes, it is important to give the heartworm medication within 30 days after any exposure.
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