by Joanna Hay

Created on: July 11, 2009   Last Updated: July 12, 2009

Heartworm disease is a widespread and devastating plague that kills hundreds of dogs and cats each year. This invasion of the heart and lungs by voracious worms can be treated, but not always successfully. Luckily, the disease can be easily prevented when owners become informed and take responsibility for their animal companions.

Heartworm disease is caused by infection with a worm called Dirofilaria immitis. Infection is transmitted between dogs by mosquitoes. When an infected mosquito bites a dog it injects tiny worms called microfilaria into the bloodstream. These worms swim through the blood to the heart, where they make a permanent home. The worms feed off the nutrient rich blood in the heart and begin to grow. They can reach a length of over a foot long and fill multiple chambers of the heart. As the worms get longer, they overflow the heart and reach into the blood vessels of the lungs as well. Adult worms give birth to more tiny microfilaria that are released into the bloodstream. Some of the microfilaria that are produced will remain in the heart and grow, while others are drawn into mosquitoes that are feeding on the infected animal and transmitted to a new host to begin the cycle again.

Dogs are by far the most susceptible to this horrific disease, but cats, ferrets, raccoons, and even seals and sea lions can also be infected. Although the worm can be passed to humans by infected mosquitoes, it does not cause disease in people. The microfilaria can reach the heart and lungs of humans, but cannot survive and grow there. There is no risk of the disease passing to humans from infected dogs and cats.

The most common sign that owners notice in infected animals is coughing. The coughing usually starts out very mild and becomes progressively worse. Owners may also notice that the dog is becoming less active or becomes tired very quickly when exercising. As the disease progresses the dog will lose weight and can have a rough hair coat. Once the worms have grown and multiplied, the sheer volume of them begins to interfere with the functioning of the heart. At this point the dog will begin to show signs of heart failure. These signs include difficulty breathing, nosebleeds, a distended abdomen from fluid pooling, fainting, and eventually even death. Cats show similar signs, although the progression is often slower.

Diagnosis of heartworm disease in dogs is made using a simple blood test. Most veterinary clinics can do this test in the clinic in about ten

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