The genus Strongyloides contains 53 species and S. stercoralis is the particular species that causes most human infection. It is often referred to as Threadworm (in Britain it may be known as pinworm), but Strongyloides is a roundworm that causes the disease strongyloidiasis. There are other species that can infect humans, cats, dogs, monkeys and various mammals.
The infectious larvae penetrate the skin when skin comes into contact with the soil. Larvae have been thought to locate their hosts via chemicals in the skin. The predominant one is urocanic acid, a histidine metabolite on the uppermost layer of skin that is removed by sweat or the daily skin-shedding cycle. Urocanic acid concentrations can be up to five times greater in the foot than any other part of the human body.
Some of them enter the superficial veins and ride the blood vessels to the lungs, where they enter the alveoli. They are then coughed up and swallowed into the stomach, where they parasitise the intestinal mucosa (duodenum and jejunum). They can cause both respiratory and gastrointestinal symptoms. In the small intestine, they molt twice and become adult female worms. The females live threaded through the epithelium of the small intestine.
Many people infected are usually asymptomatic at first. Symptoms include dermatitis: swelling, itching, larva currens, and mild hemorrhage at the site where the skin has been penetrated. If the parasite reaches the lungs, the chest may feel as if it is burning, wheezing and coughing may result along with pneumonia-like symptoms (Löffler's syndrome). The intestines could eventually be invaded causing burning pain, tissue damage, sepsis, and ulcers. In severe cases, edema may result in obstruction of the intestinal tract as well as loss of peristaltic contractions.