Heartworms are a type of parasite that migrate to the heart, where they mature into adult heartworms. The presence of heartworms causes extensive damage to the heart, the lungs, and the surrounding major blood vessels. Untreated heartworm disease is always fatal, although a dog may live for months or even years harboring heartworms, depending on the number of heartworms present. Even if treated, the damage already done at the time of the treatment remains. Therefore, the quality and quantity of life for dogs treated for heartworms depends on the number of worms and amount of damage present at the time of treatment. Dogs with low numbers of heartworms and minimal damage present can live a normal life span after treatment. Dogs with existing damage may need medication life-long to keep them comfortable. Heartworm disease in dogs is a tragedy because it is so easily prevented.

Heartworms are transmitted by mosquitoes. Mosquitoes carrying heartworm larvae bite dogs and inject the infective larvae into the dog under their skin, or into tiny blood vessels as the mosquito takes its blood meal. These larvae mature through several stages, as they also migrate towards the heart, where the adult heartworms take up residence. The physical presence of the heartworms irritates the vessel and heart walls, and, depending on the number present, occlude vessels, causing the heart to pump harder against increased resistance to blood flow. Eventually, the heart will fail.

Heartworms also damage the lungs. They can cause pneumonitis, inflammation of lung tissue that interferes with the dog’s ability to breathe and with the normal exchange of oxygen. Eventually, the dog will begin to cough as a result of heart failure, pneumonitis, or both. Sometimes a large number of heartworms will almost completely obstruct blood flow to the heart, leading to caval syndrome. These dogs will collapse and die unless treatment is initiated very quickly to relieve the clog. Treatment for caval syndrome involves emergency surgery to remove the worms from the heart and major vessels. Treatment of this end-stage of heartworm disease is frequently unsuccessful.

The good news is that heartworms are extremely easy to prevent. Preventative Care consists of using either a monthly heartworm preventative medication or a newer six-month injectable heartworm preventative medication. These medications kill the larval forms of heartworms before they have a chance to develop into adult heartworms. All of these methods are safe and effective under most circumstances. Year-round prevention is strongly recommended in this area due to our year-round mosquito population. Some heartworm preventative medications also offer protections against other types of parasites as well.

Cats are not a natural host for heartworms, but can become infected. Cats become infected the same way as dogs, by being bitten by infected mosquitoes. Because cats are not a natural host, many heartworms never mature in cats. Some do, however, and when they do tend to find their way not only into the heart, but into the lungs as well. Cats rarely show the same signs as dogs when infected. Diagnosis in cats is difficult because the signs are not always definitive. Infected cats can suffer from chronic vomiting, coughing, or asthma-like signs. Unfortunately, one of the more common signs in cats is sudden death. Even if heartworm infection is diagnosed in cats, there are no known treatments that will kill the worms without a high risk of killing the cat as well. Therefore treatment consists of symptomatic relief of whatever symptoms occur. Heartworm prevention is available for cats.