Manifestations of heartworm infection in cats differ from those in dogs and can be easily overlooked. In cats, the initial signs are often misdiagnosed as asthma or allergic bronchitis when in actuality they are part of a syndrome now known as heartworm-associated respiratory disease (HARD). Approximately 3 to 4 months after infection by a vector mosquito, immature heartworms arrive in the pulmonary arteries and arterioles, causing an arteritis. Most of the immature worms, which are now over two inches long, die and cause a severe inflammatory response, which leads to significant pathology of the arterioles, alveoli, bronchioles, and bronchi. A normal open lumen is shown in the slide on the left, which is from a heartworm challenged cat that received monthly prophylaxis. An occluded lumen is on the right, characteristic of heartworm larval infection. It is important to understand that a cat does not have to have adult worms to develop clinical disease. An occasional heartworm matures into an adult and, when it dies, can cause severe pulmonary inflammation and thromboembolism, leading to fatal acute lung injury.