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 upon 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 upon the number of worms present and the amount of damage already present at the time of treatment. Dogs with low numbers of heartworms with 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 the 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 vessel and heart walls and, depending upon the number present, occlude vessels, causing the heart to pump harder against increased resistence 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 breath 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. Prevention 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.

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