Some 350 snail species are estimated to be of possible medical or veterinary importance. Most intermediate hosts of human Schistosoma parasites belong to three genera, Biomphalaria, Bulinus and Oncomelania. The species involved can be identified by the shape of the outer shell. Simple regional keys are available for the determination of most species. The snails can be divided into two main groups: aquatic snails that live under water and cannot usually survive elsewhere (Biomphalaria, Bulinus), and amphibious snails adapted for living in and out of water (Oncomelania). In Africa and the Americas, snails of the genus Biomphalaria serve as intermediate hosts of S. mansoni (Fig. 8.1). Snails of the genus Bulinus serve as the intermediate hosts of S. haematobium in Africa and the Eastern Mediterranean, as well as of S. intercalatum in Africa. In south-east Asia, Oncomelania serves as the intermediate host of S. japonicum, and Tricula as the intermediate host of S. mekongi. Among the snail intermediate hosts of trematodes, the species belonging to the genus Lymnaea are of importance in the transmission of liver flukes. Lymnaea species may be either aquatic or amphibious (Fig. 8.2).

Life cycle

All species of Biomphalaria and Bulinus are hermaphrodite, possessing both male and female organs and being capable of self- or cross-fertilization. A single specimen can invade and populate a new habitat. The eggs are laid at intervals in batches of 5 - 40, each batch being enclosed in a mass of jelly-like material. The young snails hatch after 6 - 8 days and reach maturity in 4 - 7 weeks, depending on the species and environmental conditions. Temperature and food availability are among the most important limiting factors. A snail lays up to 1000 eggs during its life, which may last more than a year.

The amphibious Oncomelania snails, which may live for several years, have separate sexes. The female lays its eggs singly near the water margin.

Ecology

Snail habitats include almost all types of freshwater bodies ranging from small temporary ponds and streams to large lakes and rivers. Within each habitat, snail distribution may be patchy and detection requires examination of different sites. Moreover, snail densities vary significantly with the season. In general, the aquatic snail hosts of schistosomes occur in shallow water near the shores of lakes, ponds, marshes, streams and irrigation channels. They live on water plants and mud that is rich in decaying organic matter. They can also be found on rocks, stones or concrete covered with algae or on various types of debris. They are most common in waters where water plants are abundant and in water moderately polluted with organic matter, such as faeces and urine, as is often the case near human habitations. Plants serve as substrates for feeding and oviposition as well as providing protection from high water velocities and predators such as fish and birds.

Most aquatic snail species die when stranded on dry land in the dry season. However, a proportion of some snail species are able to withstand desiccation for months while buried in the mud bottom by sealing their shell opening with a layer of mucus. Most species can survive outside water for short periods.

Fig. 8.2 Shells of the medically important snail genera:

(a) Biomphalaria; (© WHO)

(b) Bulinus; (© WHO)

(c) Oncomelania; (© WHO)

(d) Lymnaea (© WHO)

For reproduction, temperatures between 22 °C and 26 °C are usually optimal, but Bulinus snails in Ghana and other hot places have a wider temperature range. The snails can easily survive between 10 °C and 35 °C. They are not found in salty or acidic water.

In most areas, seasonal changes in rainfall, water level and temperature cause marked fluctuations in snail population densities and transmission rates. Reservoirs that contain water for several months of the year in Sahelian Africa can be intensive transmission sites of urinary schistosomiasis during a very limited period, because surviving Bulinus species rapidly recolonize the reservoirs after the rains start.

Oncomelania snails can survive periods of drought because they possess an operculum capable of closing the shell opening. In the temperate zone they can survive for 2 - 4 months, in the tropics much less. They live both in and out of water in humid areas such as poorly tilled rice fields, sluggish streams, secondary and tertiary canals of irrigation systems, swamps and roadside ditches. The vegetation in these sites is important in maintaining a suitable temperature and humidity. Their food is similar to that of aquatic snails but they also feed on plant surfaces above water