See the TV 13 ABC coverage on Heartworm story http://abclocal.go.com/wtvg/story?section=news/local&id=7016278

Heartworm in dogs is caused by an infestation of the organism Dirofilaria immitis, a parasitic nematode (roundworm) commonly referred to as the heartworm. The severity of this disease is directly dependent upon the number of worms present in the body, the duration of the infestation, and the response of the host (the infested dog is the host).

Heartworms are spread through mosquitos that carry the infective heartworm larvae. These larvae migrate from the bite wound through the dog’s body until they reach the heart and blood vessels of the lungs, a process that takes approximately six months. The larvae mature in the dog’s body -- an adult heartworm can grow to be about 12 inches long. These adults reproduce and release immature heartworms, known as microfilariae, directly into the dog’s blood. When a mosquito bites an already infected dog, it may take in these microfilariae with the dog’s blood, and then pass on the infective heartworm larvae (the microfilariae develop once inside the mosquito) to another dog, thereby continuing the parasite’s life cycle and spreading the disease to the next host.
Risk factors associated with heartworm disease include residence in endemic regions, such as tropical areas, outdoor habitation, and lack of the proper prophylaxis to prevent heartworm infestation.

Heartworm disease is preventable with the administration of a heartworm prophylaxis (preventative) medication, as recommended by a veterinarian. For those dogs that do contract heartworm disease, the prognosis is good for mild to moderate cases, and such conditions can be relatively uneventful. Dogs with more severe cases may suffer from lung complications resulting from extreme medication given to kill serious infestations.