Why Don't Humans Get Heartworm?

According to the American Heartworm Society, The Western Journal of Medicine and Surgery of 1847 makes first mention of heartworm in dogs of the United States. Heartworm was discovered in cats later in the 1920s. This disease is a subtle but deadly threat to dogs, cats, other mammals such as foxes, wolves and ferrets. On very rare occasions, a human can be infected with heartworm, but it so seldom occurs that it is not considered a public health threat.

The type of heartworm found in America’s dogs and cats is called Dirofilaria immitus and is known by its characteristic long, threadlike body. Once an animal is infected, the heartworm larvae travel through into the bloodstream to the arteries of the lungs where they begin to grow in size and number, thus impairing heart and lung function.

Transmission of heartworm begins when the adult female worm, living inside a host animal, releases her larvae into the bloodstream. As the larvae circulate through the bloodstream, mosquitoes that feed on the infected host ingest the infected blood and spread the disease as they go about feeding on other animals.

A mosquito does not instantly become a carrier of heartworm after feeding from an infected animal. It takes about two weeks for heartworm larvae to mature before the mosquito can transmit this disease.

Though heartworm has been documented in humans, it is a very rare occasion. Humans appear to be ineffective hosts for the heartworm to carry out its full life cycle. According to Dr. Ron Hines, DVM Ph.D., the heartworm’s inability to reproduce in humans because of the human immune system's ability to identify the foreign organism and destroy it.

In rare cases, the larvae migrate to the lungs and die, setting off the immune response. In time a lesion will develop, which is often later identified as a tumor, cyst or lesion before being fully diagnosed. This condition is called human pulmonary dirofilariasis or HPD.

Wayne J. Crans, associate research professor in entomology at Rutgers Center for Vector Biology, points to the theory that heartworms that infest non-canine mammals are somehow rendered sterile, thus becoming unable to reproduce.

Heartworm larvae take about six months to mature to the point where they can produce offspring that infests the bloodstream of a host animal. Adult worms reside in the vessels of the lungs and can live up to seven years. Their presence, left untreated, eventually weakens the cardiopulmonary system, causing severe damage and eventual death.

Animals in early stages of heartworm infection show few, if any, signs or symptoms. Without treatment, however, this disease eventually manifests with symptoms including a cough, lethargy, fatigue, poor appetite, weight loss and decreased desire for exercise. Cats with heartworm often exhibit signs that mimic other feline diseases, making diagnoses trickier. Diagnosing heartworm in any animal is confirmed through a blood test. Once a diagnosis is made, your veterinarian will choose an appropriate treatment plan.

Preventative medications come in the forms of monthly tablets, topical liquids or injections that last up to six months for dogs. When administered properly, these medications are highly affective in preventing heartworm and the severe complications it can cause.