Deworming to combat the health and nutritional impact of soil-transmitted helminths
Biological, behavioural and contextual rationale
Soil-transmitted helminths – which include roundworms (Ascaris lumbricoides), whipworms (Trichuris trichiura) and hookworms (Necator americanus and Ancylostoma duodenale) – are among the most common causes of infection in people who live in the developing world. WHO estimates that over 270 million preschool children and over 600 million school-age children are living in areas where these parasites are intensively transmitted, and are in need of treatment and preventive interventions (1).
Soil-transmitted helminths are transmitted by eggs excreted in human faeces, which contaminate the soil in areas that lack adequate sanitation. People are infected through ingestion of infective eggs or larvae that contaminate food, hands or utensils, or by penetration of the skin by infective larvae that contaminate the soil. Since these parasites do not multiply in the human host, reinfection occurs only as a result of contact with infective stages in the environment.
Soil-transmitted helminths impair the nutritional status of the individuals they infect in multiple ways including: feeding on host tissues, including blood, which leads to a loss of iron and protein (2, 3); and increasing malabsorption of nutrients. In addition, roundworm may possibly compete for vitamin A in the intestine (4). Some soil-transmitted helminths also cause loss of appetite and therefore a reduction of nutrition intake and physical fitness (5). In particular, T. trichiura is responsible for diarrhoea and dysentery (6).
The nutritional impairment caused by soil-transmitted helminths is recognized to have a significant impact on growth and physical development (7). In addition to their nutritional effects, soil-transmitted helminth infections are also reported to impair cognitive development (8, 9), limit educational advancement and hinder economic development (10).
Activities that are recommended for the control of morbidity due to soil-transmitted helminth infections are simple and should be administered to children without previous individual diagnosis (11).
- Periodic drug treatment (deworming) to all children living in endemic areas (once a year when the prevalence of soil-transmitted helminth infections in the community is over 20%, and twice a year when the prevalence of soil-transmitted helminth infections in the community is over 50%). This intervention reduces morbidity by reducing the worm burden (12).
- Health and hygiene education reduces transmission and reinfection by encouraging healthy behaviours (13).
- Provision of adequate sanitation is also important but not always possible in resource-poor settings.
The recommended drugs (albendazole 400 mg and mebendazole 500 mg) are effective, inexpensive and easy to administer by non-medical personnel (e.g. teachers) (14). They have been through extensive safety testing and have been used in millions of people with few and minor side-effects.
A recent meta-analysis indicates that if the prevalence of soil-transmitted helminths is 50% or more, deworming leads to significant extra gains in weight, height, mid-upper arm circumference and skinfold thickness in comparison with untreated controls; there was no evidence of an immediate effect on haemoglobin concentration (15). However, it is important to note that improvements in anthropometry do not occur as a result of deworming alone, and if the extra nutrients required for catch-up growth are not available, growth rates are likely to remain unchanged (15). Deworming has also been linked to improvements in appetite (16), which may contribute to increased growth.
Periodic deworming can be easily integrated with child health days or supplementation programmes for preschool children, or integrated with school health programmes. In 2009, over 300 million preschool and school-age children were dewormed in endemic countries, corresponding to 35% of the children at risk (17). The global target is to cover at least 75% of these at-risk children (18). Schools provide a particularly good entry point for deworming activities, as they allow easy provision of the health and hygiene education component such as the promotion of hand washing and improved sanitation.
1. PCT Databank. Geneva, World Health Organization, 2010 (http://www.who.int/neglected_diseases/preventive_chemotherapy/databank/en/, accessed 24 January 2012).
2. Solomons NW. Pathways to the impairment of human nutritional status by gastrointestinal pathogens. Parasitology, 1993, 107(Suppl):S19–S35.
3. Crompton DWT, Nesheim MC. Nutritional impact of intestinal helminthiasis during the human life cycle.Annual Review of Nutrition, 2002, 22:35–59.
4. Curtale F et al. Intestinal helminths and xerophthalmia in Nepal. A case-control study. Journal of Tropical Pediatrics, 1995, 41(6):334–337.
5. Stephenson LS et al. Physical fitness, growth and appetite of Kenyan school boys with hookworm, Trichuris trichiura and Ascaris lumbricoides infections are improved four months after a single dose of albendazole. Journal of Nutrition, 1993, 123(6):1036–1046.
6. Callender JE et al. Growth and development four years after treatment for the Trichuris dysentery syndrome. Acta Paediatrica, 1998, 87(12):1247–1249.
7. Stephenson LS, Latham MC, Ottesen EA. Malnutrition and parasitic helminth infections. Parasitology, 2000, 121(Suppl):S23–S38.
8. Nokes C et al. Parasitic helminth infection and cognitive function in school children. Proceedings of Biological Sciences, 1992, 247(1319):77–81.
9. Kvalsvig JD, Cooppan RM, Connolly KJ. The effects of parasite infections on cognitive processes in children. Annals of Tropical Medicine and Parasitology, 1991, 85(5):551–568.
10. Miguel E, Kremer M. Identifying impacts on education and health in the presence of treatment externalities. National Bureau of Economic Research Working Paper, 2001, 8481.
11. Preventive chemotherapy in human helminthiasis. Coordinated use of anthelminthic drugs in control interventions: a manual for health professionals and programme managers. Geneva, World Health Organization, 2006.
12. Albonico M et al. Intervention for the control of soil-transmitted helminthiasis in the community. Advances in Parasitology, 2006, 61:311–348.
13. Strengthening interventions to reduce helminth infections as an entry point for the development of health promoting schools. Geneva, World Health Organization, 1996.
14. Montresor A et al. Helminth control in school-age children. Geneva, World Health Organization, 2002.
15. Hall A et al. A review and meta-analysis of the impact of intestinal worms on child growth and nutrition. Maternal and Child Nutrition, 2008, 4:118–236.
16. Latham MC et al. Metrifonate or praziquantel treatment improves physical fitness and appetite of Kenyan schoolboys with Schistosoma haematobium and hookworm infections. American Journal of Tropical Medicine and Hygiene, 43:170.
17. World Health Organization. Soil-transmitted helminthiases: estimates of the number of children needing preventive chemotherapy and number treated, 2009. Weekly Epidemiological Record, 2011, 25(86):257–268.
18. Resolution WHA54.19. Schistosomiasis and soil-transmitted helminth infection. In: Fifty-fourth World Health Assembly, Geneva, 22 May 2001. Geneva, World Health Organization, 2001.