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The Heartworm Story The first published description of heartworm in dogs appeared more than 100 years ago in an issue of "The Western Journal of Medicine and Surgery."1 Heartworm in cats was first described in the early 1920's.2, 3 Since then, naturally acquired heartworm infection in cats and dogs is identified as a worldwide clinical problem. Despite improved diagnostic methods, effective preventives and increasing awareness among veterinary professionals and pet owners, cases of heartworm infection continue to appear in pet dogs around the world. The diagnosis of the disease is still complex and elusive in cats. 1 Osborne, TC. Worms found in the Heart and Bloodvessels of a Dog; Symptoms of Hydrophobia. The Western Journal of Medicine and Surgery, 1847. 2 Riley, WA. Dirofilaria immitis in the heart of a cat. J Parasitol 1922;9:48 3 Travassos, LP. Notas Helminthologicas. Brazil-Med. An. 1921;35 2(6):67 What do we know about it? Heartworm disease is a serious and potentially fatal condition caused by parasitic worms living in the arteries of the lungs and in the right side of the heart of dogs, cats and other species of mammals, including wolves, foxes, ferrets, sea lions and (in rare instances) humans. Heartworms are classified as nematodes (roundworms) and are but one of many species of roundworms. The specific roundworm causing heartworm in dogs and cats is known as Dirofilaria immitis. |
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Adult female heartworms, that may grow to be 14-inches long, living within the infected animal's body release their young, called microfilariae (pronounced: micro fil ar ee), into their host's bloodstream. The mosquitoes become infected with these microscopic worms after biting into an infected adult animal for a blood meal. During the next 10 to 14 days, the microfilariae mature to the infective larval stage within the mosquito. When the mosquito bites another host (dog, cat or other susceptible animal), the infective larvae enters a new host through the bite wound. It then takes a little over six months for the infective larvae to mature into adult worms that may live for five to seven years (in a dog). The microfilaria cannot mature into adult heartworms without first passing through a mosquito. Dogs or other animals harboring adult worms are the recognized reservoir of infection. Adult worms produce the offspring that circulate in the blood, and are then transmitted to mosquitoes once they bite the infected animal. These offspring (microfilariae) undergo development to an infective larval stage within 14 days in the mosquito, and can then be transmitted to another host (such as a cat) or back to another dog, when the infected mosquito bites again. The infective heartworm larvae travel through a tubular organ within the mosquito's head and are injected into the skin of a new host animal through the mosquito bite wound. In the dog, the larvae progress in their development to an adult form of the worm, and live in the heart and pulmonary vessels, where they continue the life cycle and cause extensive injury. In the cat, the larvae molt as well, but fewer worms survive to adulthood. While dogs suffer severe heart and lung damage from heartworm infection, cats typically exhibit minimal changes in the heart. The cat's primary response to the presence of heartworms occurs in the lungs. Within the dog, the time frame between initial infection and growth to adult worms is approximately six to seven months, eventually arriving in the heart and pulmonary vessels where they begin to produce new offspring. This period is referred to as patency. In cats, it takes seven to eight months before adult worms arrive in the heart and pulmonary vessels, and this is referred to as transient patency. In most cases the life cycle of the heartworm ends here since microfilaria are produced in less than 20% of the cats. |
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For a more in-depth look at heartworms visit The American Heartworm Society. |
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