What are they?

Parasitic worms (helminths) can be found in the human intestinal tract, urinary tract or bloodstream.

There are many different types of worms, but the most common worldwide are the intestinal nematodes or soil-transmitted helminths (STH), the schistosomes (the parasites of schistosomiasis) and filarial worms (which cause LF and onchocerciasis). You can learn more about onchocerciasis and its control from WHO.

Infection is typically determined by presence of eggs or worms in biological samples using microscopy or other modern techniques. 

  • Ascaris lumbricoides

    STH

    The most common soil-transmitted helminths (STH) are Ascaris lumbricoides (roundworm), Trichuris trichiura (whipworm) and two species of hookworms: Necator americanus and Ancylostoma duodenale.

    Chronic STH infection can delay the growth, learning and education of children. It is very rare for STH infection to result in death, but because it can go unnoticed for a long time, people may develop high intensities of infection over the long term. The higher the intensity of infection, the worse its effects.

    Chronic infection with Ascaris and Trichuris during childhood can lead to malnutrition and growth retardation. Heavy infection with hookworms causes anaemia, especially among children and pregnant women.

  • Schistosomiasis

    Schistosomiasis is caused by several parasitic schistosome worms, leading to two major forms of clinical disease: urinary schistosomiasis, caused by Schistosoma haematobium, and intestinal schistosomiasis, caused by Schistosoma mansoni in Africa, Middle East, the Caribbean and Latin America or Schistosoma japonicum in Asia.

    Infection with schistosomes can result in serious clinical disease when parasite eggs become trapped in the wall of the intestine or bladder, or in the liver. The immune response to the trapped eggs can lead to inflammation and fibrosis, an excess of fibrous connective tissue.

    Urinary schistosomiasiscauses bladder wall disease, leading to ulcer formation, blood in urine and pain when urinating. Inflammations and ulcers of the bladder wall and urethra can lead to bladder obstruction, renal failure, lesions of the genital tracts, and an increased risk of bladder cancer.

    In intestinal schistosomiasis, there is progressive enlargement of the liver and spleen and intestinal damage, which can have severe and sometimes fatal consequences. Schistosomiasis also causes chronic growth faltering and can contribute to anaemia, especially among children.

  • Elephantiasis

    LF

    Lymphatic filariasis (LF) is caused by a species of filarial round worms, or nematodes, transmitted by mosquitoes. Up to 90% of all infections are caused by Wuchereria bancrofti. In some parts of Asia, LF is caused by Brugia malayi and Brugia timori. Various mosquito species transmit these worms: Culex mosquitoesin urban and semi-urban areas, Aedes mosquitoes throughout the Pacific, and Anopheles mosquitoes in rural environments, especially in Africa. Anopheles mosquitoes also carry malaria.

    Although many infections are asymptomatic and cause no clinical disease, virtually all cause subclinical lymphatic damage and around 40% of infections present kidney damage. Early infections can cause filarial fever, characterised by periodic attacks of malaise, fever and chills, as well as enlarged painful lymph nodes.

    Chronic LF infection results in severe disfigurement and disability. It can cause hydrocele, or accumulation of fluid in the sac surrounding the testicles, and lymphedema, or elephantiasis, swelling of the legs and occasionally the genitals and female breasts.