Heartworm is transmitted by infected mosquitoes. The immature larval stage of the worm enters the dog's skin and then the bloodstream through the bite wound of the mosquito. In a few months, the juvenile worms migrate through the bloodstream to the right side of the heart, to grow into adult worms. Female worms produce offspring called "microfilariae," which grow and lodge in the heart and large blood vessels going from the heart to the lungs. If untreated, a heartworm-infected dog will usually die of heart failure.
In dogs, the first outward signs of heartworm disease may not be apparent until a year after infection.
Many dog owners give their dogs a heartworm preventive so they don't have to worry about their dogs acquiring heartworm disease. Monthly heartworm preventives interfere with the development of the immature heartworm and prevent it from maturing into an adult. The American Heartworm Society recommends giving the preventive year round. Some pet owners, either do not give a preventive or forget to give it in a timely manner.
Sometimes owners, during the course of their pets' exam, forget to tell the veterinarian relevant information about their pets. Here is a useful checklist to take with you to your next veterinary appointment if you are concerned about the possibility of heartworm disease in your dog.