Various parasitic worms can invade the human body and feed off their human hosts. The prevalence of human worm infections in different locations around the world remains largely tied to the presence and sophistication of water and sewage sanitation systems. Many worm infections that occur uncommonly in the United States remain abundantly prevalent in others parts of the world. Several types of worms can live in the human intestine. The skin, eyes and muscles are other common sites of human worm infections.
Intestinal Roundworms
Intestinal roundworms, or nematodes, are the leading cause of human worm infestations globally. Several species of roundworms can invade the small or large intestine. In the United States, intestinal pinworms remain the most common cause of worm infections, reports the Centers for Disease Control and Prevention. Female pinworms, which resemble a small piece of white thread, emerge from the intestine during the night to lay eggs on the skin near the anus. The highly infectious eggs cause intense local itching. Pinworm infections occur most commonly in toddlers and school-aged children. Household members frequently become infected due to inadvertent ingestion of the infectious eggs.
Although uncommon in the United States, ascariasis is the most common worm infection worldwide, notes CDC. Ascaris worms, which range in size from approximately 6 to 12 inches, live in the small intestine. Infection occurs from the inadvertent ingestion of Ascaris eggs, which are passed in the feces of an infected person. Complications of ascariasis include impaired absorption of dietary nutrients, small bowel blockage and obstructed bile flow.
Tissue Roundworms
Tissue roundworms can invade a variety of body sites, including the muscles, skin, lymph glands, eyes and lungs. Infection occurs via ingestion or insect bites. Trichinellosis is an example of an ingested tissue roundworm infection. Ingestion of raw or undercooked meat contaminated with immature Trichinella worms, or larvae, leads to infection. CDC states that pork and the meats of wild animals, such as bears, seals, walruses, foxes and wolves, can be a source of Trichinella infection. The adult worms live in the small intestine and lay eggs, which hatch into larvae. The larvae migrate to muscle tissue, leading to the formation of larvae-containing cysts.
Blackflies transmit the worm Onchocerca volvulus, which lives in the skin and commonly invades the eyes. The World Health Organization reports that onchocerciasis of the eyes remains a leading cause of visual impairment and blindness in certain parts of Africa. Other tissue roundworms transmitted by flies and mosquitoes include Wucheria bancrofti, Brugia malayi and Loa loa.
Tapeworms
Tapeworms, or cestodes, can infect the intestine or migrate elsewhere in the body. Fish, beef and pork tapeworms are large parasites, ranging from approximately 26 to 80 feet in length, reports the medical reference text "Principles and Practice of Infectious Diseases." These tapeworms inhabit the intestines and can survive for decades. Infection occurs from eating raw or undercooked fish, beef or pork contaminated with worm larvae. The larvae of pork, dog and other species of tapeworms can migrate to the brain, heart, lungs and eyes, causing cysts.
References
- Centers for Disease Control and Prevention: Pinworm Infection (Enterobiasis)
- Centers for Disease Control and Prevention: Ascaris Infection (Ascariasis)
- Centers for Disease Control and Prevention: Trichinellosis
- World Health Organization: Onchocerciasis (River Blindness)
- "Principles and Practice of Infectious Diseases, Sixth Edition"; Gerald L. Mandell, M.D., et al., Editors; 2004
