Soil-Transmitted Helminthiases: Ascariasis, Hookworm Infection
Soil-transmitted helminth (STH) infections are usually asymptomatic.
All soil-transmitted helminths compete with the host for nutrients, causing mal- absorption of fats, proteins, carbohydrates and vitamins, and contribute directly to malnutrition. Those most at risk include preschool and school-age children, women of child- bearing age, women in the second or third trimester of pregnancy, lactating mothers and adults in certain occupations (e.g. tea-pickers and miners).
Overt infection may produce a wide range of symptoms, including intestinal manifes- tations (diarrhoea and abdominal pain), general malaise and weakness. These symptoms may affect working and learning capacities and impair physical growth. Among infants and children, they can cause growth retardation.
Ascaris infection exacerbates vitamin A deficiency (which leads to night blindness), and elimination of ascarids may result in rapid clinical improvement in night blindness and dryness around the eye.
Hookworm infections may cause an itchy maculopapular rash at the site of larval penetration. Transient pneumonitis, epigastric pain, anorexia, diarrhoea and eosinophilia may occur, complicated by iron-deficiency anaemia due to chronic intestinal blood loss.
Heavy Trichuris infection may cause diarrhoea and severe malabsorption of nutrients. In endemic regions, typically only 10% of those infected have heavy worm burdens and suffer most from the disease due to a friable intestinal mucosa leading to tenesmus and bloody, mucoid stools. Recurrent rectal prolapse, iron-deficiency anaemia, malnutrition and growth retardation may be seen.
The four major soil-transmitted helminths are:
- Ascaris lumbricoides or roundworm (white or pinkish adult worm, 15–30 cm long)
- hookworms – Ancyclostoma duodenale and Necator americanus (small, cylin- drical, greyish white nematodes, 7–13 mm long)
- Trichuris trichiura or whipworm (pinkish grey adult worm, 4 cm long).
Mode of transmission
Ascariasis and trichuriasis are transmitted by ingestion of their eggs, mainly as contaminants of food. Hookworm lavae in the soil are transmitted by active pen- etration of the skin.
Adequate sanitation is the key to prevention. However, even if there is no improve- ment in sanitation, regular treatment of school-age children and other high-risk groups will help to avoid the worst effects of infection. Implementing and sustain- ing universal treatment twice a year is recommended for high-risk communities with a prevalence of STH above 50%. Once-a-year treatment is sufficient in low- risk communities with a prevalence of STH of 20–50%.
Preventive Chemotherapy and Transmission Control Databank. Geneva, World Health Organization, 2010 (Preventive chemotherapy (who.int)).