**2. Epidemiology and burden of schistosomiasis and STH**

Schistosomiasis and STH transmission are intimately associated with poverty and poor sanitation. For schistosomiasis, infecion is caused by penetration of the skin by larvae in water; whereas for STH, infection is caused by the ingestion of parasite eggs from contaminated food or dirty hands – in the case of *A. lumbricoides* and *T. trichiura* – or by active penetration of the skin by larvae in the soil – in the case of hookworms. People who get infected carry parasite eggs in the feces or in the urine (in the case of urinary schistosomiasis), and in areas where there is no latrine systems the soil and water around the villages and communities are contaminated with feces or urine containing worm eggs. Although schistosomiasis and STH infections occur predominantly in rural areas, the social and environmental conditions in many unplanned slums and squatter settlements of developing countries are ideal for their persistence (Crompton & Savioli, 1993). In endemic populations, infections are aggregated: most infected individuals in a community will have infections of a light or moderate intensity, while a few will be heavily infected. Heavily infected individuals suffer most of the clinical consequences of the infections and are the major source of infection for the rest of the community (WHO, 2002).

countries, the most affected part of the world. In the 1990s, interest in the control of these diseases in Africa waned. Therefore, as with other neglected tropical diseases (NTDs), schistosomiasis and STH control has been overshadowed by other health priorities. The highest priority of the international health community was given to the 'big three', i.e. HIV/AIDS, tuberculosis and malaria, with less attention to other infections related to

Recent years have witnessed an increased interest in the control of NTDs, and today there exists a global momentum for the control of these diseases. The control of NTDs has become a priority on the agenda of many governments, donors and international agencies. The World Health Organization (WHO) has played a major role in this prospect. Indeed, under the aegis of WHO, all member states of WHO (over 200 countries) have endorsed in May 2001 the World Health Assembly resolution WHA 54.19, with as a major objective the regular treatment of at least 75% of all school-aged children at risk of morbidity by 2010. The renewed impetus for schistosomiasis and STH control has generated a greater political commitment, as well as an unprecedented opportunity for cost-effective action (Molyneux et al., 2005). This momentum has encouraged many countries to establish national action plans and programmes to control schistosomiasis, STHs and other NTDs (Hotez et al., 2009; Tchuem Tchuenté & N'Goran, 2009). Within the past decade, significant progress has been made on large scale treatments through integrated control of schistosomiasis, STH and other NTDs, thanks to a number of international organizations, donor foundations, bilateral institutions and non-governmental organizations that responded to the 2001 WHO's call for action (Savioli et al., 2009). Today, treatment is cost-effective and the 'preventive chemotherapy' is currently the strategy of choice (WHO, 2006). With a support from the American (USAID) and British (DFID) governments, as well as the Bill and Melinda Gates Foundation, the pharmaceutical industry, and many not-for profit organizations, millions of children are regularly treated for schistosomiasis, STH and other NTDs. However, the control of these diseases is a long-term undertaking which involves several challenges. This paper highlights the progress made and also focuses on some main challenges that are

poverty (Molyneux et al., 2005).

reviewed and discussed.

**2. Epidemiology and burden of schistosomiasis and STH** 

major source of infection for the rest of the community (WHO, 2002).

Schistosomiasis and STH transmission are intimately associated with poverty and poor sanitation. For schistosomiasis, infecion is caused by penetration of the skin by larvae in water; whereas for STH, infection is caused by the ingestion of parasite eggs from contaminated food or dirty hands – in the case of *A. lumbricoides* and *T. trichiura* – or by active penetration of the skin by larvae in the soil – in the case of hookworms. People who get infected carry parasite eggs in the feces or in the urine (in the case of urinary schistosomiasis), and in areas where there is no latrine systems the soil and water around the villages and communities are contaminated with feces or urine containing worm eggs. Although schistosomiasis and STH infections occur predominantly in rural areas, the social and environmental conditions in many unplanned slums and squatter settlements of developing countries are ideal for their persistence (Crompton & Savioli, 1993). In endemic populations, infections are aggregated: most infected individuals in a community will have infections of a light or moderate intensity, while a few will be heavily infected. Heavily infected individuals suffer most of the clinical consequences of the infections and are the The epidemiology of helminth infections is influenced by several key determinants, including environment, population heterogeneity, age, household clustering, genetics and polyparasitism (Hotez et al., 2008). In recent years, considerable progress has been made in the use of geographical information system (GIS) and remote sensing (RS) to better understand helminth ecology and epidemiology, and to develop low-cost ways to identify target populations for treatment. GIS and RS were used to describe the global distribution of schistosomiasis and STH infections and to estimate the number of infections in school-age children in sub-Saharan Africa.

There is considerable geographical variation in the occurrence of infections (Brooker et al., 2009). In general, changes with age in the average intensity of helminth infections tend to be convex, rising in childhood and declining in adulthood. For schistosomiasis, *A. lumbicoides* and *T. trichiura*, the heaviest and most frequent infections are in children aged 5–15 years, with a decline in intensity and frequency in adulthood (Gilles, 1996). In contrast, hookworm frequently exhibits a steady rise of intensity of infection with age, peaking in adulthood (Bethony et al., 2002). Household clustering of infected individuals has been demonstrated for STH (Forrester et al., 1988), and this can persist through time, as shown by familial predisposition to heavy infection with *A. lumbricoides* and *T. trichiura* (Forrester et al., 1990).

Because morbidity is associated with worm burden rather than the absence or presence of infection, prevalence is commonly combined with worm burden (intensity of infection) to assess the epidemiological situation for schistosomiasis and STH infections. Worm burden is commonly measured by the number of eggs per gram (EPG) of feces or eggs per 10ml of urine (Anderson, 1982; Montresor et al., 1998). Prevalence and intensity of infections are used to classify communities into transmission categories, which enable to determine the appropriate type of mass treatment a community should receive (WHO, 2006). Both should be assessed in monitoring the impact of deworming campaigns.
