Mass control of soil-borne helminth infections: swings and roundabouts?
It is estimated that a quarter of the world’s population, predominantly those living in tropical and sub-tropical areas with inadequate sanitation, are infected by soil-borne nematode worms, the eggs of which are passed in human feces. The species of most concern are Ascaris lumbricoides, Trichuris trichiura and the hookworms Ancylostoma duodenale and Necator americanus. Their impact is insidious, with chronic infections resulting in increasingly debilitating micronutrient deficiencies that affect physical growth and mental development in children. Heavy hookworm infections are also associated with maternal morbidity and even mortality due to severe iron deficiency anemia.
The current WHO control strategy is first to examine fecal samples of older schoolchildren to establish the prevalence of infection in a community. If this exceeds 50%, all children from age one to fifteen, and ideally all women of child-bearing age except those in the first trimester of pregnancy, as well as workers in occupations with a high risk of infection, are treated with a benzimidazole antihelmintic twice a year. If the prevalence falls between 20% and 50%, treatment is annual. In areas with a prevalence lower than 20%, mass drug administration (MDA) is not recommended. However, several recently published articles have suggested that a more effective strategy would be community-wide MDA to eliminate helminth transmission entirely. Many such studies emphasize that, quite apart from the benefit for men and older women as well as those in communities with a lower prevalence, this approach would actually be a more efficient use of the limited resources available in the longer term. So what are the problems?
Firstly similar drugs have been used to control nematode worm infections in lifestock, and after several years of continual use high levels of resistance to the drugs developed; the same could occur in human populations. But there could be another problem. These antihelmintic drugs also kill ubiquitous and essentially innocuous parasites, and during the quarter of a century since Strachan first proposed his ‘Hygiene hypothesis’, suggesting that a lower exposure to microorganisms was linked to the noted rise in allergic conditions, there has been a plethora of publications supporting it. Many of these are based on very robust studies demonstrating and explaining the inverse relationship between parasitic infections and allergies and autoimmune disorders. Although it is admirable to alleviate the suffering caused by severe helminth infections, is it really prudent to eliminate all parasites and risk replacing the micronutrient deficiencies of less developed areas by the allergies and autoimmune diseases so common in the West?