A parasitic roundworm that infects the hearts and lungs of dogs, cats, and other animals, transmitted by mosquitoes.
From 'heart' + 'worm' (Old English 'wyrm'), named for its location in the host animal's heart. Modern identification occurred in the 19th century as microscopy improved.
Heartworm is genuinely one of the most significant parasites in veterinary medicine—a single infected mosquito can start an infestation that damages the heart's ability to pump blood. Pet owners now give monthly preventative medication because these worms are so deadly and widespread.
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