[Hound Dog Mom]

The heart worm prevevntative needs to be given for at least three months after the last exposure. So, no, the dogs wouldn’t have needed to be started on it prior to the trip – they will need to be given it for three months after the trip if you feel that they have been potentially exposed to heart worm. The preventative kills the immature worms so it wouldn’t matter if the dog had been given it prior to exposure because there would be no worms there to kill.