DEAR DR. WELDY’S: I keep hearing about heartworms. Can you tell me what they are and why I should care about them?
DEAR READER: Heartworms (Dirofilaria immitis) are parasites that live in and damage the large blood vessels of the heart and lungs. They primarily infect dogs, but have been found in many other mammals, including cats, ferrets, wolves, foxes, coyotes and sea lions.
Heartworms are spread when a mosquito carrying heartworm larvae bites a dog. The larvae migrate through tissue and find their way to the large blood vessels of the heart and lungs, where they lodge and mature. It takes 6 to 7 months for larvae to become adults after a dog has been bitten. Once the worms mature, they begin to reproduce and the females release first stage larvae (microfiaria) into the bloodstream. Adult worms can be up to 12 inches long and live 5 to 7 years. A dog can have from 1 to 250 worms in its system. As you can imagine, that many worms living in blood vessels can cause problems — including heart failure and death if not treated.
Canine heartworm infection has been found in dogs native to all 50 states. All dogs, regardless of age, sex or habitat, are susceptible to heartworm infection. The highest infection rates (up to 45 percent) in dogs (not maintained on heartworm preventive) are within 150 miles of the Atlantic and Gulf coasts from the Gulf of Mexico to New Jersey and along the Mississippi River and its major tributaries. Other states have lower infection rates. For the past several years, the infection rate in Indiana has stayed around 1 to 2 percent. While this seems low, consider a few numbers: there are about 1.5 million dogs in Indiana (calculated based on AVMA statistics). An infection rate of 1 percent means that 15,000 infected dogs are diagnosed each year in Indiana.