The Heartworm “preventative” doesn’t really “prevent” heartworm – it kills existing immature worms. So you start later than you’d think because you would be worming for an already existing infection (I hope that makes sense). Also – keep in mind just because there are mosquitoes there isn’t necessarily hearworm. If the temperature drops below 57 degrees at any point in time the heartworm development cycle (within the mosquito) will have to start again – it has to be over 57 degrees for two consecutive weeks. Right now where I am it’s over 80 degrees during the day and I’ve seen mosquitoes but it still gets into the 40’s at night. The map is just a guideline – monitor the temperature. I usually end up starting at the end of May or very beginning of June.
