Soil Transmitted Helminthiasis


Geohelminthiasis or soil-transmitted helminthiasis (commonly known as intestinal worms) are the most common infections worldwide and affect the poorest communities. The causative agents are the nematodes Ascaris lumbricoides, Trichuris trichiura, and hookworms (Necator americanus and Ancylostoma duodenale). Infection occurs by ingestion of infective eggs from soil contaminated with human feces, ingestion of raw agricultural products contaminated with soil containing infective eggs (A. lumbricoides and T. trichiura), or by penetration of larvae from the soil through skin (hookworms).

Signs and symptoms

The main manifestations of STH are related to malabsorption of nutrients. These clinical manifestations include the reduction of food intake capacity and blockage of the gastrointestinal and bile tracts due to ascariasis, dysentery and rectal prolapse due to trichuriasis and iron-deficiency anemia due to hookworms. However, the burden of disease is primarily associated with its chronic and insidious impact on health and quality of life of those infected and those living in endemic communities rather than the risk of death. The prevalence and intensity of infection with A. lumbricoides and T. trichiura commonly reaches peak levels among children aged 5-14 years, while the maximum level of hookworm infection occurs in adults over 20 years of age. Serious helminth infection affects physical growth and cognitive development of children. It causes iron deficiency anemia leading to poor school performance and absenteeism in children. They also produce reduction in labor productivity of adults. The costs attributable to helminth infections in endemic communities and countries in terms of nutrient loss and reduced productivity are quite important.

Distribution and extent of disease

The STH are widely distributed in tropical and subtropical areas, and are linked to poor sanitation and affect poor populations. In the Region of the Americas, it is estimated that about 46 million children of preschool and school age children at risk for helminth infection. In 2012, 6.4 million pre-school and 19.2 million school-age children were administered anti-parasitic medicines in 13 countries (Belize, Bolivia, Colombia, Dominican Republic, El Salvador, Guatemala, Guyana, Haiti, Honduras, Mexico, Nicaragua, Uruguay, and Venezuela), of which seven reported coverage rates of 75% or higher in school age children (Belize, El Salvador, Guyana, Haiti, Dominican Republic, Mexico, and Nicaragua), and five in pre-school age children (Belize, Guyana, Haiti, Mexico and Nicaragua).  

Prevention and control

The main risk factors for the occurrence of helminth infection are related to the lack of access to water and basic sanitation, and poor hygiene and living conditions. Proper disposal of human waste is especially critical, since a gram of feces can contain up to 100 eggs of parasites. Therefore, contaminated water supplies can infect and reinfect people of a village or an entire community. Agriculture irrigation based on the use of treated waste water, waste stabilization ponds and other methods can reduce the transfer of parasite eggs to food crops. Once a community has access to proper sanitation, education on the proper use of latrines and the importance of washing hands can keep families safe and healthy. 

To combat geohelminthiasis, PAHO/WHO recommends mass administration of anti-parasitic medicines (albendazole or mebendazole) for pre-school and school-aged children living in areas of high risk for infection (once per year in areas of low risk -prevalence between 20 and 50%, and twice per year in areas of high risk -prevalence >50%), as well as, the promotion of access to safe water, basic sanitation and sanitary education through intersectoral coordinated work.  Currently, these safe and effective anthelmintic drugs are available for less than 5 cents per dose, which means that mass drug administration in school children, is probably the most economically efficient public health intervention that can be implemented in low-income countries where STH are endemic.

PAHO/WHO's response 

  • In 2001, the World Health Assembly approved Resolution WHA54.19, whose objectives are to reduce death and disability and improve the health and development of communities affected by soil-transmitted helminthiasis and schistosomiasis; and  to guarantee access to essential medicines against these infections, with the goal of treating between 75 and 100% of all  school-age children at risk for geohelminthiasis by the year 2010, in conjunction with plans to increase access to basic  sanitation and safe water. At the global level, this goal has not been achieved but it has increased the number of children receiving anti-parasitic medicines. 
  • In 2009 the PAHO's Directing Council approved Resolution CD49.R19 on the elimination of neglected infectious diseases and other poverty-related diseases, urging Member States to adopt necessary measures to control geohelminthiasis (among other neglected diseases).The measures were to reduce the burden of disease as a public health problem by 2015, and reduce the prevalence among school-age children in high-risk populations from above 50% to below 20% by 2015. 
  • In 2013, the Organization of American States-OAS approved Resolution AG/RES.2810(XLIII-O/13) in support of PAHO's Resolution  CD49.R19,  and the World Health Assembly approved Resolution WHA62.12 urging Member States to implement interventions against neglected tropical diseases (including STH), to reach the targets as set out in WHO's roadmap for these diseases.