Heartworm disease is a very serious disease that most commonly affects dogs. Worms that can be up to a foot long live inside the infected animal's heart, causing problems in the heart as well as the lungs. If left untreated, heartworms can lead to death because of heart failure.
Heartworm larvae are carried by mosquitoes. When an infected mosquito bites a dog, it transmits the larvae to the dog. The larvae then develop into adults that live in the heart and produce more larvae. Treatment is very expensive and is also risky for the animal. It requires a series of three injections that are administered deep in the muscle next to the spine. It also requires strict exercise restriction for the duration of the treatment period. It may even require surgical removal of worms from the heart. It is much easier to prevent heartworms than it is to treat them!
Prevention of heartworms is accomplished by giving your dog a monthly chewable. The prevention works retroactively; it kills larvae that have been transmitted to the dog within the last month. This is why it is very important to continue giving heartworm prevention even after it starts to get cold out. The American Heartworm Society recommends giving heartworm prevention year-round, to be safe from heartworms and also because it will prevent several common gastrointestinal parasites. Year-round use also prevents owners from forgetting to start using heartworm preventatives again in the spring.
The American Heartworm Society's guidelines for heartworm prevention also include yearly testing. Heartworm tests check for an antigen that is only produced by adult female heartworms. It takes six months after the mosquito bite for the larvae to develop into adults, so dogs that were recently infected with heartworms will not test positive until six months after the mosquito bite.
Testing should be done before an animal is started on preventatives or if a dose has been missed, because if the dog has a lot of larvae, giving prevention can result in the dog going into shock. Puppies can be started on preventatives but should be watched closely and should be tested at around seven months of age. They should be tested because heartworm preventatives don't kill certain developmental stages of the larvae. Any larvae that have already reached the non-susceptible stages when the puppy is started on preventative will still develop into adults and will then cause health problems. The test that we have at the clinic also checks for three common tick-borne diseases: Lyme disease, Ehrlichia, and Anaplasmosis.
Check out this site from the American Heartworm Society for more information: https://www.heartwormsociety.org/pet-owner-resources