Heartworm is a parasitic infection that mainly infects dogs and related species. As the name implies, the parasite itself is a worm that survives in the heart and blood vessels leading from the heart to the lungs in infected individuals. Living worms cause illness by obstructing normal blood flow. Heartworm is transmitted from an infected individual by mosquitoes, who will pick up the microscopic worm larvae in a bloodmeal and then transfer these larvae when they bite.
Because Heartworm is transmitted by mosquitoes, in Ontario we only have to worry about prevention during the warmer months. Typically we start heartworm prevention in June and continue monthly until November.