Heartworm is contracted by a mosquito bite and as long as there is not a chance of your puppy being bitten by a mosquito, you can start right back on the preventative.
Heartworm preventatives actually work backwards, you are not preventing heartworm for the future 30 days, you are preventing heartworm for the previous 30 days.
Heartworm preventatives actually work backwards, you are not preventing heartworm for the future 30 days, you are preventing heartworm for the previous 30 days.