Heartworm has a complicated life cycle. Infected dogs have microfilaria, an immature form of heartworm, circulating in their bloodstream. Microfilariae are sucked up by mosquitoes when feeding on the blood of infected dogs. The immature parasite develops into a heartworm larva inside the mosquito, then a single bite from a carrier mosquito can infect your pet (dog or cat). As the worms mature in the heart they can cause a physical blockage as well as thickening of the heart and associated blood vessels. In the earlystages of infection there may be no visible signs, however, infection may eventually lead to signs of heart failure (reluctance to exercise, lethargy, coughing) and even death. Heartworm is present throughout most of Australia (except Tasmania and arid areas).