If only using Heartgard Plus during mosquito season, the first dose should be given no more than one month after a dog's first exposure to mosquitoes and the final dose should be given no more than one month after a dog's last exposure to mosquitoes. (vetdepot.com)
The final dose must be given within one month after the last exposure to mosquitoes. (medi-vet.com)
Treatment with fewer than three monthly doses after the last exposure to mosquitoes may not provide complete heartworm prevention. (trifexis.com)