Many species of nematodes, or roundworms, commonly infect humans. Varying in length from less than one-tenth of an inch to more than three feet, roundworms cause an array of human diseases with broad-ranging severity. The Merck Manual for Healthcare Professionals reports that various species of roundworms found throughout the world infect hundreds of millions of people.
Pinworms, or Enterobius vermicularis, remain the most frequent parasitic worm infestation in the United States, according to the Centers for Disease Control and Prevention. Inadvertent ingestion of pinworm eggs serve as the route of infection. Reinfection proves common as pinworm eggs remain infectious for up to three weeks on bedding, pajamas, clothing, towels and inanimate surfaces. Thorough laundering and cleaning of inanimate surfaces significantly reduces the risk of reinfection after successful treatment.
The Centers for Disease Control and Prevention reports that ascariasis prevails as the most common human worm infestation worldwide. The roundworm Ascaris lumbricoides causes the disease. Approximately 1.5 billion people across the globe were infested with Ascaris lumbricoides as of 2002. The Merck Manual for Healthcare Professionals states that an estimated 4 million people in the United States have an Ascaris infestation. The illness most commonly occurs in areas with poor sanitation and crowding, with children disproportionately affected.
Infection with the roundworm Onchocerca volvulus ranks as the second most common cause of infectious blindness across the world, reports The Merck Manual for Healthcare Professionals. Onchocerca volvulus worms invade the eye structures causing visual disturbances ranging from mild impairments to blindness. The World Health Organization estimates that 500,000 people worldwide are blind due to onchocerciasis, more commonly known as "river blindness." People in West and Central Africa remain at greatest risk for this parasitic infection.
Ancylostoma duodenale and Necator americanus are hookworms, a subtype of roundworms. The immature larval hookworm burrows into the skin of the foot, travels via the bloodstream to the lungs and ascends out of the lungs into the throat where it is swallowed. The worms mature within and inhabit the small intestine, feeding on blood extracted from the intestinal wall, notes the Centers for Disease Control and Prevention. Heavy hookworm infestation causes chronic blood loss, potentially resulting in anemia and malnutrition, especially among infected children.
The immature larval forms of the roundworms Anisakis simplex and Pseudoterranova decipiens commonly inhabit the gastrointestinal tract of saltwater fish and squid. Ingestion of raw or undercooked contaminated seafood leads to a human infection known as anisakiasis. The Merck Manual for Healthcare Professionals explains that the larval worms invade the lining of the small intestine and stomach, causing nausea, vomiting and abdominal pain.
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