Description

• Worm-like parasites infecting humans include:
  • Nematodes: roundworm (Ascaris lumbricoides), whipworm (Trichuris trichiura), hookworm (Necator americanus, Ancylostoma duodenale), pinworm (Enterobius vermicularis), filaria (such as Wuchereria bancrofti, Brugia malayi, Loa loa, and Onchocerca volvulus), guinea worm (Dracunculus medinensis), and threadworm (Strongyloides stercoralis)
  • Cestodes: tapeworms, such as beef tapeworm (Taenia saginata), pork tapeworm (Taenia solium), dwarf tapeworm (Hymenolepis nana), broad or fish tapeworm (Diphyllobothrium latum), and cystic and alveolar echinococcosis (Echinococcus granulosus and Echinococcus multilocularis)
  • Trematodes: flukes, including Schistosoma (Schistosoma haematobium, Schistosoma mansoni, and Schistosoma japonicum), avian and mammal schistosomes (cercarial dermatitis or swimmer’s itch), and other lung flukes (Paragonimus westermani)
• Helminths are commonly acquired through oral ingestion or direct skin penetration. They may infect the GI system and skin or migrate to other tissues (liver, muscles, lungs, eyes, and brain).
• Cysticercosis (T. solium), guinea worm, echinococcosis, fascioliasis (Fasciola hepatica, a fluke), lymphatic filariasis, river blindness (O. volvulus), schistosomiasis, and the soil-transmitted helminths (STHs, roundworms, hookworms, and whipworms) are neglected tropical diseases (NTDs).
• NTDs disproportionately infect poor communities without adequate water, sanitation, and hygiene; international efforts underway to eradicate NTDs

Epidemiology

>1/4 of the world’s population (almost 2 billion people) infected with a helminth parasite; most common in poor, tropical communities

• Pinworm (E. vermicularis) and swimmer’s itch (multiple causative trematodes) are common in temperate climates.
• Helminth infections account for >6 million disability-adjusted life years lost annually worldwide (1).

Prevalence

• Nematodes
  • A. lumbricoides, T. trichiura, and hookworms N. americanus and A. duodenale are rare in the United States but infect many people worldwide.
  • Global prevalence for Ascaris is between 807 and 1,121 million; T. trichiura infects 604 to 795 million and the hookworms 576 to 740 million.
  • E. vermicularis only infects humans and is the most common parasitic infection in the United States. Up to 20% (mostly children) are infected annually in some populations.
• Cestodes
  • T. saginata and H. nana are the most common human tapeworms infecting an estimated 60 million and 50 to 75 million people, respectively.
• Trematodes
  • Schistosomiasis is most common in Africa, where in some communities, >50% are infected.
  • Endemic Schistosoma (swimmer’s itch) is common worldwide.

Etiology and Pathophysiology

• Helminths progress through egg, larval, and adult stages; larval and adult forms cause illness.
• Most helminths have part of their life cycle outside their definitive hosts in soil or another species (snails for Schistosoma; insects for filariae).
• Clinical illness is caused by
  • Direct effect from the larval or adult parasite: T. trichiura burrows into the intestines; A. lumbricoides larva can migrate through the liver, lungs, and other organs, causing irritation en route; swimmer’s itch (multiple species) causes local skin irritation as humans are an accidental host, the parasite cannot progress beyond the integumentary system.
  • Local and systemic inflammatory responses: IgE production, mast cell degranulation, and eosinophilia; long-term effects such as lymphatic filariasis and bladder cancer from Schistosoma
  • T. solium and the Echinococcus sp. can cause space-occupying tissue–organ lesions: (T. solium) encysted migratory larva in neurocysticercosis (CNS infection) leading to acquired epilepsy; hydatid disease (alveolar and cystic) tapeworms E. granulosus and E. multilocularis
  • Many helminths remain viable for years inside and outside their hosts (e.g., eggs in soil).
• Helminthic parasites are spread through
  • Fecal–oral transmission of eggs (A. lumbricoides, T. trichiura, E. vermicularis, and H. nana)
  • E. vermicularis completes their life cycle in humans and can be transmitted fecal–orally person-to-person and through autoinfection.
  • Eating undercooked beef (T. saginata), pork (T. solium), fish (D. latum), or crustaceans (Paragonimus sp.)
  • Insect bites: filariae mosquitoes; O. volvulus black fly
  • Direct penetration of skin by larva (N. americanus, A. duodenale, S. stercoralis, D. medinensis, Schistosoma, and Trichobilharzia regenti)

Risk Factors

• Low income and poor sanitation
• Increased risk of Schistosoma in close proximity to contaminated fresh water sources
• Malnutrition is associated with higher morbidity and mortality, particularly with the NTDs.
• Immunocompromised individuals are particularly susceptible to parasites that autoinfect and parasites that spread person to person (pinworm and threadworm).

General Prevention

• Personal protective measures: clothing, personal and food hygiene, especially for travelers
• Education and community sanitation
• Global efforts ongoing to eradicate NTDs