Heartworms are a very common parasite affecting our dogs, in fact in many parts of the country the disease has reached endemic proportions. The disease itself, if left undetected and untreated can often be fatal, and the sad truth is that screening and prevention is so easy and inexpensive. The disease is caused by a worm which migrates to and invades the heart and the vessels around the heart and lungs. The parasite is introduced into the dog's bloodstream via the bite of an infected mosquito which injects the infant form of the heartworm, called microfilaria. It is because of this mosquito transmission that the disease is so prevalent in parts of the world where mosquitoes are such a problem