How do heartworms get into the dog?
The heartworm reproductive cycle is a marvel of ingenuity. Since they reside entirely within the circulatory system, they cannot pass eggs to the outside, as do many of the intestinal parasites. Therefore, the female heartworm (yes, two sexes!)lay microscopic juvenile worms, or larvae. These are called microfilaria. The microfilaria float in the bloodstream. When the dog is bitten by a mosquito, the microfilaria are drawn into the insect. They remain there for a period of time while they undergo development. Then, when the mosquito bites another dog, the microfilaria are transferred to the new host. They eventually migrate back to the heart, grow to maturity, and set up housekeeping again.