Epidemiology

Geographical Distribution

Schistosomiasis is one of the world's major health problems. In 1993, the World Health Organization estimated that at least 200 million people in 74 countries were infected, and at least 600 million more are at risk (World Health Organization, 1994, 1996). S. mansoni is endemic throughout Africa and the Middle East. It was brought in the fifteenth and sixteenth centuries to

Transmission

Transmission of schistosomiasis depends on human contact with fresh water, the presence of a specific snail species capable of completing the schistome life-cycle, and contamination of fresh water with human waste. In endemic areas, the highest prevalence and intensity of infection occurs in adolescents, 10-16 years of age (Davis, 1985). Males generally have a much higher prevalence and intensity than females, presumably through higher water contact. It is common to have marked variations in prevalence rates, even in nearby communities. It is not uncommon, for example, to have a village with a community prevalence of 30-40% located within a few miles of another village with little or no infection. In communities with a population of more than 1000, it is not uncommon to have one or two 'hot spots' where prevalence may be high only for a cluster of a few families. This is due to the microtransmission dynamics created by the overlap of contributing factors (snails, contamination and human contact with water). High-prevalence areas have a greater frequency of patients with heavy infections. In S. mansoni and S. haematobium endemic communities, there is often a sharp drop-off in the prevalence and intensity in adults over 25 years of age (Jordan and Webbe, 1993). This is partially explained by decreased water contact. Many investigators believe that this epidemiologic pattern also results from the slow development of acquired resistance to reinfection over time (Macdonald, 1965; Butterworth, 1998). This age distribution (peak in adolescence) is not seen, for example, in some populations who relocate to schistosomia-sis endemic areas (Stelma et al., 1993; Butterworth, 1998).

Fig. 16.4 A world map with the current distribution of various schistosomes

Table 16.2 Geographic location of different Schistosoma species*

Asia, Orient Europe, Middle-East

Americas

Africa

Zimbabwe

Data from World Health Organization, 1993.

In S. japonicum, water contact is often continuous, due to the important linkage between rice farming and exposure. In these populations, a high prevalence of infection can be seen in all age groups above age 6 years (World Health Organization, 1985). The intensity of infection, however, often reaches its peak in the same 1216 year age range and then declines. This has similar implications for the probable development of at least partial immunity to reinfection. In one large longitudinal study from The Philippines, individuals previously infected and cured of a schistosome infection appeared to acquire a second infection slower than age- and sex-matched controls living in the same village (Olveda et al., 1996). These observations again

Laos

Zimbabwe suggest that prior immunologic experience with the parasite induces some degree of resistance to newly invading cercariae (Butterworth, 1998).

Occupational Risk

Some specific occupations are strongly associated with schistosome infections in endemic regions. These include farming, fishing (fresh water) and working in irrigation canals. Performing laundry or other domestic activities in open bodies of water are also considered high-risk. The presence of piped water significantly reduces the risk of

S. japonicum

S. mansoni

S. haematobium

S. intercalatum

S. mekongi

China, Indonesia, The Philippines, Thailand Not reported

Not reported

Not reported

Not reported

India

Not reported

Egypt, Libya, Morocco, Oman, Saudi Arabia, Somalia, Sudan, Yemen

Brazil, Dominican Republic, Puerto Rico, Suriname, Venezuela

Angola, Botswana, Burundi, Cameroon, Central Africa, Chad, Congo, Ethiopia, Gambia, Ghana, Guinea, Ivory Coast, Kenya, Liberia, Madagascar, Malawi, Mozambique, Niger, Nigeria, Rwanda, Senegal, Sierra Leone, South Africa, Uganda, Democratic Republic of Congo (formerly Zaire), Zambia, Zanzibar,

Egypt, Iran, Iraq, Jordan, Lebanon, Libya, Morocco, Saudi Arabia, Somalia, Syria, Turkey

Not reported

Not reported

Not reported

Algeria, Angola, Botswana, Cameroon, Central Africa, Chad, Congo, Ethiopia, Gambia, Ghana, Guinea, Ivory Coast, Kenya, Liberia, Madagascar, Malawi, Mali, Mauritius, Mozambique, Namibia, Niger, Nigeria, Rwanda, Senegal, Sierra Leone, South Africa, Uganda, Democratic Republic of Congo (formerly Zaire), Zambia, Zanzibar,

Cameroon, Central Africa, Chad, Congo, Gabon, Nigeria, Democratic Republic of Congo (formerly Zaire

Cambodia,

Not reporte

Not reporte

Not reporte infection, as does the availability of pit latrines (Hairston, 1973; Wilkins, 1987b).

In addition to the marked micro-ecologic characteristics of the infection, the degree of morbidity induced by infection is also highly variable. Communities in which the prevalence, intensity and duration of exposure appear similar often have marked differences in morbidity. In one study from Kenya, for example, the village prevalence of hepatosplenic enlargement differed dramatically between villages despite similar exposure to the parasite (Fulford et al., 1991). Regional variation in morbidity has also been reported in China with S. japonicum (Warren et al., 1983; Wiest et al., 1992; World Health Organization, 1994). Whether this is due primarily to other co-morbid conditions or genetic variability in either the parasite or the host is currently unknown. Some experimental evidence exists for all three hypotheses.

The implementation of large national control programs has also changed the epidemiology of schistosomiasis. The most successful control programs have occurred in the Americas, the Middle East and Asia, so that today over 80% of cases of schistosomiasis are now found in sub-Saharan Africa. In these countries, the historic epidemiology of infection and disease is found. In countries with active control programs, access to health care, compliance and participation in primary schools now greatly influence the persistence of infection within a population. In these countries, schistosomiasis is increasingly characterized by recurrent acute infections.

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