What are they: Parasitic worms that reside in the heart and cause death by congestive heart failure.
Who gets them: Usually dogs, but cats get them on rare occasions.
How do they get them: Mosquito bites.
How do you prevent them: Start with a heartworm test. If negative, give heartworm preventative once a month at least April 1 to December 1. The preventative treats other parasites as well so there are benefits to treating year around. Keep in mind that humans, especially children who do not wash their hands as often, can be infected by some of the intestinal parasites that the preventative treats.
Can you treat an animal infected by them: Yes, but treatment can be very expensive and life threatening for some animals.