Heartworm
A parasitic worm that lives in the heart and major blood vessels of dogs (and rarely cats), transmitted by mosquitos. Preventable. Treatment is risky and expensive.
Heartworm (Dirofilaria immitis) is a roughly 30cm worm that lives in the right heart and pulmonary arteries. A single mosquito bite from an infected mosquito can transmit the disease; over months the worm matures and adult worms cause coughing, exercise intolerance, weight loss, and ultimately heart failure.
Geography matters. Heartworm is endemic in northern and tropical Australia, much of the southern US, parts of Europe, and across South-East Asia. Prevention is straightforward: monthly chewable tablets, topical drops, or annual injection. Year-round prevention is recommended in endemic areas regardless of season.
Treatment of established heartworm is melarsomine injections plus strict rest for months — risky, expensive, and uncomfortable. Prevention is 100x easier than treatment.
In cats, heartworm is less common but still occurs, and there is no safe adult-worm treatment in cats — prevention is the only option.
If your dog has lapsed on prevention, your vet will heartworm-test before restarting, because giving prevention to a heartworm-positive dog can be dangerous.