Heartworm disease is a serious and potentially fatal disease in pets. It is caused by worms that grow up to a foot long and live in your pet’s heart and lungs, as well as blood vessels in the lungs. The disease is spread when mosquitoes bite infected dogs (as well as coyotes and foxes), then—once infected—bite other dogs and cats. An indoor lifestyle does not protect a pet from heartworm infection, because mosquitoes carrying heartworm infection can easily come indoors. Heartworm prevention is essential. Although dogs can be treated for heartworm infection, heartworms can permanently damage their blood vessels and lungs. Meanwhile, there is no approved treatment for cats, and heartworm can lead to chronic respiratory disease and even sudden death.