Description

• Nonsegmented, elongated helminthic parasites (nematodes). The adult stage infects the human intestinal tract. Larval stages can exist elsewhere in the body. Eggs and/or larvae of roundworms can be isolated from stool of infected persons (except for Trichinella spiralis, which encysts in muscle). Roundworms are the most common human helminth. Some have a requisite soil cycle; Strongyloides and Enterobius can complete their life cycle within the human body.
• Nematodes that infect humans include:
  • Enterobius vermicularis (pinworm)
  • Trichuris trichiura (whipworm)
  • Ascaris lumbricoides (large roundworm)
  • Necator americanus (hookworm)
  • Ancylostoma duodenale (hookworm)
  • Strongyloides stercoralis
  • T. spiralis (trichinosis)
• System(s) potentially affected: cardiovascular; gastrointestinal (GI); musculoskeletal; nervous; pulmonary

Pediatric Considerations
Children often have a larger parasite load, which can adversely impact physical and cognitive development.

Epidemiology

Incidence

• Predominant age
  • All ages; children have a heavier worm burden.
  • Pinworm infections are more common in children.
• Predominant sex: male = female

Prevalence

• E. vermicularis: most common helminth infection in the United States; no association with socioeconomic status
• T. trichiura: 25% of world population infected; more common in tropics and subtropics; associated with poor sanitation
• A. lumbricoides: most common worldwide human helminth; 1.3 billion infected worldwide; 2% of the U.S. population may be infected.
• N. americanus/A. duodenale: 10% of world’s population infected; requires warm climate with abundant rainfall
• S. stercoralis: up to 25% infected in subtropical or tropical areas
• Mass deworming campaigns are decreasing the prevalence in many parts of the world.

Etiology and Pathophysiology

Life cycles

• E. vermicularis: Eggs can survive in a moist environment for several weeks. The entire life cycle takes place in the GI tract. Gravid female worms in the colon migrate to the perianal area at night to lay eggs that are autoingested and hatch in duodenum.
• T. trichiura: Larvae emerge from ingested eggs in the small intestine and mature. Adults embed in colonic epithelium. Eggs shed in feces embryonate in soil. Ingestion completes the cycle.
• A. lumbricoides: After ingestion, eggs hatch and the larval form penetrates the intestinal wall, traveling via the bloodstream to the lungs and trachea where they are swallowed to reach the small intestine and mature to adult worms. Female adults then lay eggs to complete the cycle.
• N. americanus, A. duodenale, and S. stercoralis: Larvae penetrate skin, migrate to lungs via the bloodstream, ascend to the trachea, are swallowed, and reach the small intestine where they mature to adult worms. Eggs shed in stool hatch in soil in a suitable climate. Strongyloides can complete life cycle in human host; autoinfection can occur.
• T. spiralis: Larvae ingested in raw/undercooked meat penetrate the bowel mucosa and travel via lymphatics and the bloodstream to encyst in striated muscle.
• Humans can acquire nematodes through ingestion of mature eggs in contaminated food or drink or uncooked liver from a chicken or pig (Ascaris larvae).
• Larval penetration of skin (hookworm, Strongyloides)
• Person-to-person transmission (Strongyloides, pinworm)

Genetics
Genetic predisposition to susceptibility to some nematode infections (i.e., Ascaris and Trichuriasis)

Risk Factors

• Substandard personal hygiene
• Poor sanitation
• Use of human feces to fertilize crops
• T. spiralis (trichinosis): eating raw or undercooked swine, bear, deer, horse, or cougar meat

Commonly Associated Conditions

Dientamoeba fragilis might be transmitted through pinworm eggs (controversial).