Untreated heartworm infection in pets can be fatal. Heartworm treatment is expensive and lengthy, and only available for dogs (infected cats currently have no options available). The best option for both your pet and you is to prevent an infection.
Mosquitoes carry the heartworm parasite, and one infected mosquito is all it takes. A mosquito can pick up the infection from a lot of different places: cats, dogs, wolves, coyotes, foxes and ferrets are all known hosts to the parasite. If a mosquito bites an infected coyote and then bites your pet, tiny heartworm parasites enter his bloodstream. From there, the worms grow and spread to your pet's heart and the blood vessels around the lungs. These worms can grow to be up to a foot long.