Heartworms can only be transmitted from animal to animal by mosquitoes.  When a mosquito bites an infected animal, young heartworms called microfilariae enter into that mosquito's system. Within two weeks, the microfilariae develop into infective larvae inside the mosquito; these infective larvae can be transmitted to another animal when this mosquito takes its next blood meal. Unlike dogs, infected cats do not often have microfilariae circulating in their blood, and an infected cat is not likely to transfer the heartworm infection to another mosquito.

 

The infective larvae mature into adult heartworms in approximately six months. During the first three months, the larvae migrate through the animal’s body, eventually reaching the blood vessels of the lungs. During the last three months, the immature worms continue to develop and grow to adults, with females growing to lengths of up to 14 inches. The worms damage the blood vessels, and reduce the heart’s pumping ability, resulting in severe lung and heart disease. When the animal shows signs of illness due to adult heartworm infection, it is called heartworm disease.

 

If adult worms (5-7 months post-infection) of both sexes are present, they will mate and produce new microfilariae. The microfilariae can cause the animal’s immune system to mount a reaction; this immune reaction can actually cause damage to other organs. This life cycle continues when a mosquito bites the infected animal and becomes infected by the microfilariae. After development of the microfilariae to infective larvae within the mosquito (10 days to 2 weeks later) the infective heartworm larvae are capable of infecting another animal. Adult heartworms can survive for 5 to 7 years in dogs and several months to years in cats.

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**Source: American Veterinary Medical Association, 2019