What Causes Heartworm Disease?
Mosquitoes spread heartworm disease. Animals infected by heartworms have millions of baby heartworms (microfilaria) in their blood. When a mosquito bites an infected animal, microfilaria enters its mouth. While living in the mosquito the heartworm becomes a larva, which is passed to an animal when bitten by the mosquito. The larvae are then deposited under the skin of the animal.