A mosquito with the infective form of heartworm then feeds upon an uninfected pet, transmitting the larvae to the pet. These larvae migrate throughout the pet's body until they make it to the heart, where they mature into adults 12 inches long and begin to produce microfilaria. This entire cycle takes about six months. The parasite is most commonly found in the Southeast and Midwest United States and is very prevalent in Illinois, where mosquito season includes late spring, summer, and early fall. In cats the signs of heartworms can be vague and can mimic other feline diseases, making diagnosis difficult.
Many cats are able to clear the infection before the worms reach the heart.
Other cats can appear healthy and then suddenly die from heartworms. Unfortunately, heartworms can cause a lot of damage before the pet shows any clinical signs. Several drugs prevent heartworm infection. Heartworm medication for cats has recently been approved. The drugs kill the immature form of the heartworm before it reaches the heart.