Schistosomiasis, a chronic disease, is caused by infection with schistosomes (a trematode parasite) from fresh water infected with specific fresh water snails (Colley et al. 2014). Fecal matter or urine from infected individuals pass eggs into the water; the parasites live in a snail host before releasing the larvae that cause the human infections. Unlike soil-transmitted helminths (STH), schistosomes tend to be focally transmitted because they need a specific freshwater host. The mechanisms by which schistosomiasis causes anemia likely involve a combination of effects, including blood loss, red blood cell destruction associated with sequestration in the spleen, immune mechanisms, iron deficiency as a result of blood loss/destruction, and general inflammation (Friedman, Kanzaria, and McGarvey 2005). Young children, as well as anyone in frequent contact with infested water, have the highest risk of being infected with schistosomiasis (Steketee 2003).
How is schistosomiasis categorized?
Blood in the urine is a morbidity indicator that can signal an intense infection. The number of schistosome eggs can be used to measure the intensity of infection, which is categorized as light, moderate, or heavy, depending on the number of eggs per unit of sample (WHO 2002). Different species and infection types (i.e., bladder or intestinal) determine whether fecal or urine samples should be taken. Table 3 displays the cut-offs for high-, moderate-, and low-risk communities.
Table 3: Schistosomiasis Public Health Risk Based on Prevalence
| Category of Infection | Prevalence in School-Age Children |
|---|---|
| High-risk community | ≥50% by parasitological methods (intestinal and urinary schistosomiasis) or ≥30 by questionnaire for visible haematuria (urinary schistosomiasis) |
| Moderate-risk community | ≥10 but 50% by parasitological methods or 30% by questionnaire for visible haematuria |
| Low-risk community | 10% by parasitological methods |
Source: Crompton and WHO 2006
How is schistosomiasis measured?
To detect the prevalence of intestinal schistosomiasis, an analysis of stool samples is needed. The World Health Organization (WHO) recommends using a microscopic laboratory process to identify parasitic eggs—permanent-stained fecal smears—to detect schistosomiasis. 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, also estimates the intensity of the infection.
Urine samples (analyzed through filtration) can be used to detect urinary schistosomiasis. When interventions have not taken place, questionnaires that ask the target populations about the presence of blood in their urine can be used to identify urinary schistosomiasis, although this type of data is rarely collected. Questionnaires have also been used to identify intestinal schistosomiasis, but with less success (Chitsulo, Lengeler, and Jenkins 1995).
Where can we get this data?
With almost all countries now having mapped neglected tropical diseases, data on schistosomiasis is typically available through the Ministry of Health. As school-age 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 also 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 intestinal schistosomiasis is >10 percent—but use a more sensitive method in settings with a suspected lower prevalence (Speich et al. 2015). Because the Kato-Katz technique, with high infection intensity, identifies many eggs, so the infection is easy to detect. But, with low infection intensity, there will only be 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 schistosomiasis in mind when interpreting results.
- Consider when 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 time points, if one of the time points was collected much closer to the time of deworming campaign.
- Remember that schistosomes are focally transmitted, and the prevalence of schistosomiasis may vary widely across a country, depending on the access to and contamination of freshwater. Most findings are linked to specific geographic areas within a country, or to specific target groups, and cannot be generalized to the whole country. Even so, these data can offer a gauge of the level of schistosomiasis within the country and where to target interventions.