Heartworm is an insidious disease that has spread to virtually all parts of the US and many parts of Canada since the early 1970s. It is spread only by mosquitoes; thus, areas heavily populated by these insects tend to have a greater incidence of heartworm disease. Heartworm can strike both dogs and cats, although it is much more commonly seen in dogs. As its name implies, heartworm lives in the blood of a dog's or cat's heart and adjacent blood vessels. The adult heartworms living in the heart produce offspring, called microfilariae, which circulate in the infected animal's blood. When a mosquito "bites" an infected pet, it sucks out blood containing the microfilariae. After about two weeks in the mosquito, the microfilariae become infective larvae. This step is necessary for the transmission of heartworm. When the mosquito bites another pet, the infective larvae are transmitted.
