Once inside the mosquito, the microfilaria take several weeks to develop into the larval form that is infective to pets. A mosquito with the infective form of the heartworm then feeds upon an uninfected pet, transmitting the larvae. These larvae migrate throughout the pet's body to the heart, where they mature into the adults and begin to produce microfilaria. The entire life cycle of the parasite takes about six months. Unfortunately, much of the damage by the heartworm can be caused before the pet shows any clinical signs.
Several drugs can prevent heartworm infection.
Some are given monthly, others daily. Both drugs kill the immature form of the heartworm before it reaches the heart, and both must be administered throughout the mosquito season. Consult your veterinarian for recommendations that fit your particular area. Before giving any preventive medication, however, a veterinarian should perform a quick and simple blood test to determine if the pet is already infected. Even pets that have consistently been on a preventative program should be rechecked every spring.