Definition of Heartworm
Heartworm disease refers to infection by Dirofilaria immitis , which are parasites that live in the right heart, chambers, the pulmonary arteries and, in severe cases, the vena caval and liver veins. Immature heartworms are transferred from infected to non-infected dogs in the saliva of adult female mosquitoes. The parasites are most prevalent along the southeastern Atlantic and Gulf coasts, although they have been found in all parts of the United States. Adult heartworms can approach 15 inches in length. A single mature female can produce up to 5,000 offspring in one day, each of which can survive in a dog’s bloodstream for about 8 years, producing more offspring. These parasites can infect people, although this is rare. Heartworms in dogs can be life-threatening if not treated properly. When a dog receives drugs that kill heartworms, the dead and dying parasites can clog blood vessels, wrap around heart valves and mechanically interfere with heart function. Fortunately, heartworm disease is usually preventable or medically manageable.
Heartworms are transferred from infected to non-infected dogs by adult mosquitoes. Infection begins when a female mosquito feeds on a dog whose blood contains the immature offspring of adult heartworms, called stage 1 larva, or “L1 microfilariae.” The microfilariae go through several stages of development inside the female mosquito, molting from larval stage L1 to L2 and then L3. It is only during the brief L3 stage, which occurs between 1 and 2½ weeks after
Heartworms irritate the sensitive lining of blood vessels and heart chambers by direct physical contact. They obstruct blood flow through the pulmonary arteries and cause allergic reactions, vascular inflammation and fluid accumulation (edema). Up to one-half of dogs infected with heartworms eventually develop right-sided congestive heart failure. Kidney, lung and liver damage are also common. Heartworms that block critical large vessels can cause “caval syndrome,” a serious condition involving liver failure, anemia, jaundice, internal bleeding,
Many dogs with heartworm disease show no outward signs of illness, especially early in the course of their illness. The parasites usually are diagnosed during routine blood work that is done for some other reason. The initial diagnostic protocol for dogs presenting with a deep, soft cough and signs of weakness or exercise intolerance includes a thorough history and physical examination. Most veterinarians will recommend drawing blood for a complete blood count and serum chemistry
Treating heartworm disease can be difficult and dangerous. The therapeutic goals are to kill all adult worms and microfilariae that are present in the dog’s bloodstream. Other goals are to resolve any associated complications that the dog is suffering from and to prevent future reinfection. In considering treatment options, owners and veterinarians must pay special attention to potential adverse drug reactions. They also must consider the possibility that dead and dying adult worms may plug