Period of communicability years

Occurrence and distribution S. haemato-bium and S. mansoni were originally diseases of Africa, where they are widely distributed, but with the massive exodus of slaves that took place in the 17th and 18th centuries, this legacy was carried with them. The East African slave trade carried S. mansoni to the Arabian peninsula and S. haematobium to the Yemen and Iraq. The Western trade was solely in S. mansoni, which found a suitable snail host in South America and the Caribbean. S. japonicum probably originated in China, where it has been found in mummified bodies, but is also found in the Philippines, Taiwan and Sulawesi in Indonesia (Fig. 11.2). No cases have been found in Japan since 1978. A separate species S. intercalatum, pathogenically similar to S. mansoni, is found in Congo, Cameroon, Central African Republic, Chad, Gabon and Sao Tome. S. mekongi is restricted to the Mekong River basin in Laos,

Fig. 11.2. Distribution of schistosomiasis. ¡1 S. mansoni, H| S. haematobium, ^ S.

Thailand and Cambodia. Other localized species are S. malayensis in peninsular Malaysia and S. mattheei found in southern Africa.

Control and prevention There are two approaches to the control of schistosomia-sis:

• reduce the transmission of the parasite;

• reduce the level of infection in individuals.

The first attempts to control the parasite, while the second aims at minimizing the pathological effects. The various methods of control are as follows.

reduction of contamination of the environment

Humans pollute the environment by urinating or defecating into bodies of water. This can be minimized by encouraging the use of latrines. Unfortunately, it is very difficult to get everybody in a family or community to always use a latrine and the few nonusers will be sufficient to maintain a level of pollution (see Section 2.4.2). There is also the longevity of the adult worms, meaning that prevalence rates will remain static in the community for a considerable period of time.

reduction of the snail intermediate host The snail is a vulnerable link in the life cycle of the parasite and can be attacked in an effort to break transmission. The various methods that can be used are:

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