Heartworms are a parasite that are carried by mosquitoes. The larvae (baby worms) are passed into the dog's bloodstream when the mosquito bites, travel to the heart, and grow into adult worms. The heart will fill with these worms until it can no longer pump blood. Death by heartworm infestation is slow, but deadly no less, and difficult as the dog basically suffocates slowly since oxygen can not be carried to the tissues as it should be.
Heartworm prevention medication kills the larvae in the bloodstream so they can not become adult worms in the heart. Most medication is active in the body for 4-6 weeks, no longer. In parts of the world where mosquitoes are rampant, heartworm prevention is a year round must. It is the best habit to give it year round regardless of geographical location.
Heartworm prevention can be purchased from a veterinarian or from an online pharmacy, with a veterinary prescription.
Further information on heartworm infestation may be obtained from the American Heartworm Society.
One pill, once a month, will prevent your dog's death from heartworms. That is why it is so important.