What are the effects of worms?
Effects of soil-transmitted helminths
Only rarely does infection with STH result in death. Instead, the course of infection is generally over the long term and goes unnoticed. Chronic infection can delay the growth, learning and education of children. Effects are typically worse in those with heavy (high-intensity) infection.
Chronic infection with Ascaris and Trichuris during childhood can lead to malnutrition and growth retardation. Heavy infection with hookworms can cause anaemia, especially among children and pregnant women.
Effects of schistosomiasis
Infection with schistosomes can result in serious clinical disease. Much of this occurs 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 the formation of inflammatory granulomas and to fibrosis, an excess of fibrous connective tissue.
Infection with S. haematobium causes bladder wall pathology, leading to ulcer formation, blood in urine and pain when urinating. Granulomatous changes 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 (due to S. mansoni and S. japonicum), 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.
Effects of lymphatic filariasis
LF infections can cause disfigurement but are not directly fatal. Infection starts to appear in children of around five years and infection levels increase throughout childhood and early adulthood, levelling out at around 30 years.
Although many infections are asymptomatic and cause no clinical disease, virtually all have 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 infection can cause hydrocele (accumulation of fluid in the sac surrounding the testicles) and lymphedema (elephantiasis), swelling of the legs and occasionally the genitals and female breasts.
Intensity of infection
Worms do not replicate within humans, and the amount of disease they cause is roughly proportional to their numbers or the intensity of infection. Although worms can infect anyone, some groups are at greater risk of illness than others and are more vulnerable to the harmful effects of chronic infections.
The control of disease due to worms therefore relies on identifying the populations at risk of acquiring moderate and heavy helminth burdens, i.e. high intensity infections. Field studies show that school-aged children harbour the highest burden of Ascaris and Trichuris, while burdens of hookworms and schistosomes are highest in both adolescents and adults.