They undergo changes in the mosquito that prepares them to infect the dog, and they transfer back to the original host species the next time the mosquito bites. The process of change in the mosquito takes about 10 days in warm climates, but can take six weeks in colder temperatures. The worms grow and multiply, infesting the chambers on the right side of the heart and the arteries in the lungs. They can also lodge in the veins of the liver and the veins entering the heart. The first sign of heartworm infestation may not manifest for a year after infection, and even then the soft cough that increases with exercise may be dismissed as unimportant by the owner.
Breathing becomes more difficult as the disease progresses.
Congestive heart failure ensues, and the once-active, outgoing pet is in grave danger. If a blood test or the onset of symptoms alert owner and veterinarian to the presence of this devastating parasite, treatment is possible and successful if the disease has not progressed too far. The first step is to evaluate the dog and treat any secondary problems of heart failure or liver or kidney insufficiency so that he can withstand the treatment. The drug is administered in two doses each day for two days, followed by several weeks of inactivity to give the dog's system a chance to absorb the dead worms. Exertion can cause the dead worms to dislodge, travel to the lungs, and cause death.