Why am I giving heartworm prevention to my dog in winter if it is carried by mosquitoes?

This is a great question many people ask here during our cold Wisconsin winter. The answer isn't because there are mosquitoes out there now. The answer is in how the medication works.

Everyone is familiar with applying or giving flea and tick prevention monthly to protect their pets. For example, when a flea and tick preventative is applied June 1st, the medication stays active in their system for approximately 30 days.

Heartworm preventatives are only active for a short time and work in reverse. Heartgard has the active ingredient ivermectin in it. Ivermectin has a half-life of 2 days in the dog. That means 1/2 of the drug is eliminated from the body in 2 days. So at most it is active for 4 days per month. For example, when Heartgard is given June 1st, it is active for at most 4 days, eliminating a specific heartworm larva stage that the dog was infected with in May.

Because it works to eliminate a specific stage in a short time if we miss a dose or are late giving the next dose there can be break in the protection. This break allows the larva to age past the point that the next single dose would prevent. By giving the medication monthly year round we improve the likelihood that we will kill or sterilize worms, thus preventing or limiting the disease.

As much as we try, I think we can all agree that we may miss a dose, or forget to give the medication on time with our busy schedules. By giving this medication monthly year round we can greatly improve our chances of preventing heartworm disease.