Cats get heartworm when they are bitten by an infected mosquito. The mosquito becomes a carrier by feeding on an infected animal, such as a dog or wild mammals like foxes or coyotes, which have heartworm larvae in their blood. Inside the mosquito, these immature larvae mature over a period of 10 to 14 days to an infective stage. When the infected mosquito bites a cat, the larvae enter the cat's bloodstream through the bite wound and begin migrating toward the heart and lungs. Cats are not natural hosts for heartworms, so many immature larvae die before reaching adulthood, but some mature into adult worms that can cause serious health problems. The process includes larvae going through several developmental stages in the cat's tissues and bloodstream, eventually residing mainly in the pulmonary arteries and right side of the heart. Unlike dogs, cats typically have fewer worms, and these worms live for a shorter time, but even a small number can cause severe disease known as heartworm-associated respiratory disease. Cats can acquire heartworm both indoors and outdoors since mosquitoes can enter homes and cats sometimes go outside. The infection is only transmitted by mosquitoes and not directly from one cat to another or from dogs to cats without the mosquito intermediary.
