Heartworms (Dirofilaria immitis) are a blood parasite transmitted to dogs by many species of mosquitos. A female mosquito picks up larvae while feeding on an infected dog. The larvae develops in the mosquito and is then transmitted to another dog when the mosquito feeds again. The larvae then develops further and migrates to the heart of the dog to live as an adult. Signs include exercise intolerance, coughing, and difficulty breathing. Diagnosis requires a blood test. Treatment is a delicate procedure and involves killing the adult worms in the heart and killing the larvae in the bloodstream. However, a dead adult worm can drift through the arteries and become lodged in the lungs causing complications and even death. Thus, the treatment regimen must be closely monitored by a veterinarian. Prevention involves monthly medication designed to kill the larva, but should only be used after a negative blood test. If adult worms are present, the preventative could kill them also, resulting in unexpected and adverse side effects. Consult your veterinarian for accurate diagnosis, prevention, and treatment.