Cause
Dogs get heartworms from a mosquito that transmits the infective larvae from another animal with the disease; the mosquito obtains this microfilaria through biting. While in the mosquito, the heartworm larvae mature into the infective stage of their lifecycle. Next, the mosquito bites another dog, and in the process, passes on the infective heartworm larvae to that dog. In the newly infected dog, the heartworm larvae will continue to develop over the course of five to seven months. The larvae will gain access to the vascular system, develop into adult heartworms, and migrate to the heart and pulmonary arteries to live. It is here that the female heartworms produce many microfilaria, which circulate in the dog's bloodstream and can be picked up by another mosquito.