Parasitic worms (helminths) can be found in the human intestinal tract, urinary tract or bloodstream. There are many different types of worm, but the most common worldwide are the intestinal nematodes or soil-transmitted helminths (STH), the schistosomes (the parasites of schistosomiasis) and filarial worms (some species of which cause lymphatic filariasis, LF).
The most important STH are Ascaris lumbricoides (roundworm), Trichuris trichiura (whipworm) and two species of hookworms, Necator americanus and Ancylostoma duodenale.
Schistosomiasis is caused by several parasitic schistosome worms, leading to two major forms of clinical disease. Urinary schistosomiasis is caused by Schistosoma haematobium, whilst intestinal schistosomiasis is caused by Schistosoma mansoni (in Africa, Middle East, the Caribbean and Latin America) or Schistosoma japonicum (in Asia).
LF is caused by nematode filarial worm species, with most infections caused by Wuchereria bancrofti and, in some parts of Asia, by Brugia malayi and Brugia timori.
The ways in which these worm species are spread are outlined below.
How are soil-transmitted helminths spread?
Ascaris and Trichuris have a faecal–oral transmission route. Infection occurs when eggs present in contaminated food or on dirty fingers are ingested. For hookworms, infection usually occurs when infective larvae, present in soil contaminated with faeces, enter the skin (commonly feet, legs or buttocks).
Infection with STH is associated with poverty, inadequate sanitation and hygiene, and certain behaviours such as defaecating in the open.
To start the animation showing the transmission of STH, click the Play (>) icon.
How is schistosomiasis spread?
People become infected with schistosomes when they come into contact with bodies of fresh water which harbour infective parasite stages. These parasites are released from the freshwater snails which act as intermediate hosts for schistosomes.
Spread of schistosomiasis is linked to contamination of freshwater sources from infected people urinating in water bodies and defaecating in the open. As with STH, the underlying factors are poverty, lack of access to clean water and sanitation, and poor hygiene.
To start the animation showing the transmission of schistosomiasis, click the Play (>) icon.
How is lymphatic filariasis (LF) spread?
Humans are infected with LF when they are bitten by a mosquito carrying infective stage larvae, which develop into adult worms going on to live in the lymphatics. After mating, each female worm produces thousands of offspring that circulate in peripheral blood which can then be picked up by biting mosquitoes.
The distribution of LF is strongly related to environmental factors that influence the distribution of mosquito species, and is particularly prevalent in areas with hot and humid climates.
To start the animation showing the transmission of lymphatic filariasis, click the Play (>) icon.