Middle East DVEP - Armed Forces Pest Management Board

Khorramshahr, Mian Ab, Sardasht, and Shustar. Snails generally occur in the samehabitats as Bi. arabica.Snails prefer still waters and thrive in irrigation canals. Expansion of the number ofirrigation projects throughout the Middle East has generally increased the availablehabitat for snails. Most of these projects are located in lowland areas. Highland areasabove 500 m or streams in steep terrain are unlikely snail habitats. Concrete-lined,covered canals are also usually poor habitats, but soil-lined canals that allow reeds ormarshy vegetation to grow provide excellent snail habitats. As salinity in irrigated areasdecreases because of leaching, many previously saline agricultural areas become suitablesnail habitats.Uncovered concrete canals or tanks (the “magid”) may be suitable for snails and must betreated with molluscicides. Wadis, oases, and other domestic water storage tanks mayprovide snail habitats. In Yemen, ablution basins in mosques have become snail habitats.Tidal areas and saline waters are not suitable habitat for vector snails.Adult snails lay eggs in masses of 50 or more below the water line. Snails can survive intemporary pools for considerable periods of time following flooding or heavy rains.Snails survive dry seasons by burrowing beneath riverbeds. Bulinus truncatus is moreresistant to desiccation than Bi. arabica because the former is better able to burrow inmud. Snails may be transported by man, and sometimes by birds, to uninfested areas.Vector Surveillance and Suppression. The most important preventive measure inreducing the incidence of schistosomiasis is avoidance of fresh water with infectivecercariae. Assume that all fresh water in endemic areas is infested unless provenotherwise. The absence of snails in an area does not preclude infection, since cercariaecan be transported considerable distances by water currents. Combat commanders andtroops must be instructed in the risk of infection and measures for schistosomiasisprevention. No topical repellent is currently available that provides long-term protectionagainst cercarial penetration. Experimental studies have shown the insect repellentDEET to provide a significant level of protection. Cercariae penetrate the skin rapidly, soefforts to remove cercariae after exposure by applying alcohol or other disinfectants tothe skin have limited value. Standard issue BDUs offer substantial protection againstpenetration, especially when trousers are tucked into boots. Rubber boots and gloves canprovide additional protection for personnel whose duties require prolonged contact withwater.Cercarial emergence from snails is periodic, and the numbers found in natural watersvary with the time of day. Light stimulates cercarial release for S. mansoni and S.haematobium, and in the Middle East peak numbers of cercariae are found from aboutmid-day to 1400 hours. Minimal numbers of cercariae are present at night and early inthe morning. Restricting water contact during peak cercarial density may reduce risk ofinfection. However, stepping on and crushing an infected snail will release thousands ofcercariae, regardless of the time of day.120