Soil-Transmitted Helminths

Individuals are usually infected with soil-transmitted helminths (STH)—Ascaris, Trichuris, or hookworm—after eating or drinking food or liquids contaminated with parasitic worms or eggs. They can also become infected with hookworms if their skin comes in contact with soil that contains infective larvae of hookworm. Anemia from STH infections is caused by blood loss in the gastrointestinal system by interfering with the absorption of nutrients, suppressing appetite, and general inflammation (see Inflammation and Micronutrient Deficiencies sections). Hookworm, due to the high levels of intestinal blood loss, is likely the main STH contributing to anemia. As a result of moderate to severe STH infections, children have an estimated loss of iron equal to twice the amount of their daily iron requirement (see Iron Deficiency section) (Stoltzfus et al. 1996; Smith and Brooker 2010). Young children, including school-age children, bear most of the infection burden (Albonjco et al. 1998), but pregnant women are also vulnerable to infection (Steketee 2003).

How is STH infection categorized?

A community’s risk for STH infection is categorized as high-risk or low-risk, depending on the prevalence. Table 2 displays the cut-offs for high- and low-risk communities.

Table 2: STH Public Health Risk Based on Prevalence

Categories of Infection Prevalence of Any STH Infection Among School-Aged Children
High-risk community ≥50%
Low-risk community ≥20% and <50%

Source: Crompton and WHO 2006

How is STH infection measured?

An analysis of stool samples is needed to detect the prevalence of STH. The World Health Organization (WHO) recommends identifying parasitic eggs by microscopic laboratory process—permanent-stained fecal smears—to detect STH infection. Laboratory technicians can use many methods to prepare and examine samples, with varying levels of sensitivity, specificity, and cost (Nikolay, Brooker, and Pullan 2014). The Kato-Katz technique, useful for field surveys, estimates the intensity of the infection.

Where can we get these data?

Now that almost all countries have mapped neglected tropical diseases, data on STH is typically available through the Ministry of Health. Because school-aged children are most at risk, and for logistical purposes, surveys are often done in schools. Some National Micronutrient Surveys also include this data for different populations at risk. In the coming years, data will be available through the WHO Regional Office for Africa portal (WHO 2016).

Methodological issues

  • WHO recommends the Kato-Katz technique in areas where the percentage of infected individuals is >20 percent, but use a more sensitive method in settings with a suspected lower prevalence (Speich et al. 2015). This is because when using the Kato-Katz technique with high infection intensity, there will be many eggs, so the infection will be easy to detect. But, with low infection intensity, there will be just a few eggs, so the infection may be missed. In general, the Kato-Katz technique will result in light infections not being diagnosed; keep this potential for underestimating STH infection in mind when interpreting data results.
  • To determine hookworm levels, stool samples must be examined shortly after specimen collection, either on the spot or at a field laboratory, because trophozoites (active stage) disintegrate rapidly (Crompton and WHO 2006). You may not have adequate information on the extent to which samples were examined at the appropriate time unless you were directly involved in the data collection or obtained this information from someone directly involved in the survey. If you have this information, include it in your report.
  • Consider the timing the data on STH prevalence was collected in relation to recent deworming campaigns. Data collected immediately after a mass deworming campaign will temporarily show a lower than normal rate of infection. Therefore, if the data were collected shortly after a deworming campaign, note this as a limitation. This will be especially problematic when comparing prevalence levels collected at two points in time, if one of the time points was collected much closer to the time of deworming campaign.

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