All monthly heartworm preventions kill the larva in the first 3 stages of their life. All of these stages take a little more than 45 days for the larva to pass through them. The makers of heartworm prevention originally chose to promote them as “monthly” because they felt that it would be easier for people to remember. And before cellphones that was probably true. We got calls all the time from people who had forgotten to give their HW prevention and were in a panic. Now, I just plug into my cellphone a reminder for HW P. HW comes from mosquitos. I live in FL and we have mosquito season all year long, so HW are prevalent and I would never do without, but I have been using it every 45 days since it first came out. I’ve read one person who only uses it every 90 days, but that amounts to being the slowest that the larve can move through the stages and is far too risky for me.
