Parasitic worms are a significant cause of infectious disease worldwide, especially in developing nations without adequate sanitation facilities. Many of these worms inhabit the intestines, feeding off the nutrients and blood supply found there. The three types of human intestinal worms are nematodes, cestodes and trematodes. Several different species within each group affect humans. Some intestinal worms cause few if any symptoms; others may cause significant illness, especially in children.
Nematodes
Nematodes are also known as roundworms. Different roundworm species can infest the small intestine, large intestine and rectum. The Centers for Disease Control and Prevention reports that pinworms are the most common worm infestation in the United States. Pinworms live in the rectum but migrate to the skin surrounding the anus to lay eggs. The presence of the eggs causes intense itching. Pinworm infestations most commonly occur in young children.
The roundworms Ascaris lumbricoides, Ancylostoma duodenale, Necator americanus and Strongyloides stercoralis inhabit the small intestine. "The Merck Manual for Healthcare Professionals" notes that these roundworms may cause abdominal cramps and diarrhea. Heavy infestations may lead to malnutrition and weight loss, especially in young children. Anemia may develop with Ancylostoma duodenale and Necator americanus, commonly known as hookworms. These worms feed on blood flowing in the intestinal wall.
Cestodes
Cestodes are tapeworms. Three species of tapeworms most commonly affect humans. Taenia saginata and Taenia solium are beef and pork tapeworms, respectively. Diphyllobothrium latum is a fish tapeworm. "The Merck Manual for Healthcare Professionals" warns that these worms are contracted by eating raw or undercooked beef, pork or freshwater fish contaminated with an immature form of the worms. Tapeworms are the largest of the human intestinal worms. According to the medical reference text "Principles and Practice of Infectious Diseases," mature tapeworms may reach 30 feet or more in length. Notably, people with tapeworm infestations often have no intestinal symptoms. However, "The Merck Manual for Healthcare Professionals" advises that protracted infestation with fish tapeworms can lead to a deficiency of vitamin B12. The deficiency typically causes anemia. The nerves may also be affected by the vitamin B12 deficiency, leading to numbness or tingling in the extremities.
Trematodes
Trematodes are commonly known as flukes. Only one species of fluke inhabits the human intestine, Fasciolopsis buski. "The Merck Manual for Healthcare Professionals" notes that these worms are contracted through ingestion of an immature form of the worm that inhabits aquatic plants. The worms live in the small intestine. Only heavy infestations are likely to cause symptoms, including diarrhea and abdominal pain. Most people with intestinal flukes are asymptomatic.
References
- "Principles and Practice of Infectious Diseases, Sixth Edition"; Gerald L. Mandell, M.D., et al, Editors; 2004
- Merck Manual for Healthcare Professionals: Pinworm Infestation
- Centers for Disease Control and Prevention: Pinworm Infection
- Merck Manual for Healthcare Professionals: Ascariasis
- Centers for Disease Control and Prevention: Hookworm Infection
