The influence of environmental conditions and parasite-intermediate host-related factors on the transmission ofEchinostoma liei

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Abstract
Moderate freezing, desiccation, and pH levels of 5 and 6 killed theEchinostoma liei egg immediately or after only partial development, and increasing salinity levels above 3.79‰ and temperatures of 33°C and 35°C reduced the developmental potential. Temperatures of 6°C and 8°C and maintenance of eggs in mouse faeces arrested the development of the egg, and increasing temperatures decreased the time of development from 40 days at 18°C to nine days at 35°C. Maintenance of unembryonated eggs for 14 weeks in faeces at 12°C and 22°C and for 20 weeks in filtered pond water at 4°C allowed a subsequent normal development, while embryonated eggs maintained at 4°C only retained an unchanged hatchability for three weeks. A miracidium/snail density of 10∶10 in 18 litres of water gave rise to an average level of parasitisation of 21%. Increasing miracidium/snail densities gave rise to increasing levels of parasitisation but to a level less than expected. The first intermediate host spectrum ofE. liei was shown to be restricted to the genusBiomphalaria, but species variability in susceptibility within the genus and differences in survival of infected susceptible species was also demonstrated to exist.B. glabrata (Puerto Rico and St. Lucia),B. alexandrina (Qalyub), andB. pfeifferi (Malumfashi) were found to be susceptible, whileB. camerunensis (Kinshasa) was almost refractory. Increasing size ofB. glabrata (Puerto Rico) resulted in increasing daily cercarial production, but resistance to infection with increasing snail size was also demonstrated to exist.E. liei cercarial infectivity to the second intermediate host snail and the subsequent metacercarial infectivity to the mouse was found to be independent of the species of the first intermediate host and of the age of the infection inB. glabrata (Puerto Rico) for up to at least 6 1/2 weeks after the end of the prepatent period. No obvious peak in cercarial shedding from the host snail occurred during the day. Five species of the genusBulinus andPhysa acuta (Egypt) were found to be highly susceptible as second intermediate host snails, whereas three species of the genusBiomphalaria, Planorbarius corneus (Rabat),Lymnaea natalensis (Egypt), andHelisoma duryi (Florida) all had a lower degree of susceptibility. On the other hand,E. liei metacercarial infectivity to the mouse was independent of the species and size of the second intermediate host snail. Also, the infectivity of metacercariae encysted on snail mucus, in snails harbouring patent redial infections, and in clean second intermediate host snails was comparable. Metacercarial infectivity remained unchanged for at least 12 and 18 weeks, respectively, when encysted in livingB. glabrata (Puerto Rico) or when maintained in freshwater at 4°C. Metacercariae in dead decayingB. glabrata remained unharmed for one week, followed by a loss of infectivity after 4 1/2 weeks, while the infectivity of metacercariae maintained in freshwater with snail debris at 22°C declined for four weeks, followed by a loss in infectivity after five weeks.