The prepatent period and cercarial production of Schistosoma haematobium in Bulinus truncatus (Egyptian field strains) at different constant temperatures.

The developmental time of Schistosoma haematobium in Bulinus truncatus snails (field strains) was determined in the laboratory at different constant temperatures between 18 and 32 degrees C. The basic relationship between the length of the minimum prepatent period (y, in days) and the temperature (x, in degree C) is given by the hyperbolic formula y = 295/(x-15.3), 15.3 being the theoretical "developmental null point" and 295 the constant time-temperature product. The shortest prepatency was 17-19 days at 30, 31 and 32 degrees C; at 18 degrees C, cercarial development required at least 106-113 days. The maturation time frequently exceeded the possible minimum by several weeks. No schistosome matured in our experiments at 17 degrees or 33 degrees C. The cercarial release per snail at weekly exposures showed a maximum at 25 degrees C with a geometric mean of 109 cercariae (95% confidence limits 79-149), decreasing to 8 (2-30) at 18 degrees C and 62 (38-100) at 32 degrees C. The absolute maximum of cercariae shed by one snail during 5 h "stimulation" was 2,150 in a 25 degrees C batch, 48 at 18 degrees C and 529 at 32 degrees C. The epidemiological application, the prognosis of the transmission period and the estimation of the transmission potential in relation to climatic conditions are discussed.

Helisoma duryi: its present range of distribution and implications with schistosomiasis snails in Egypt.

The range of distribution of Helisoma duryi, a potential competitor to schistosomiasis snail vectors in Egypt, was determined in the south of Nile Delta. Results show that H. duryi was found in the River Nile, Geziret El Kerateyeen banks, Damietta and Rosetta Branches, Tawfiky and Menoufy Canals and their branches and Bassoussiya, Bahadiya and Sheshaa canals and their branches. Helisoma was found in all categories of canals and drains co-existing in various degrees with other snail species including schistosomiasis vectors (Bulinus truncatus and Biomphalaria alexandrina). The population density of H. duryi and associated B. truncatus, B. alexandrina and Physa acuta were studied for one year in three channels. Statistical analysis of results show that Helisoma has a significantly negative correlation with schistosomiasis vectors in these channels. Helisoma population showed a major peak in the warmest months of the year (July-August) thus showing recovery in population after "Winter Closure" later than Bulinus, Biomphalaria and Physa. Helisoma proved to be free of any natural trematode infection.

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

Moderate freezing, desiccation, and pH levels of 5 and 6 killed the Echinostoma liei egg immediately or after only partial development, and increasing salinity levels above 3.79 0/00 and temperatures of 33 degrees C and 35 degrees C reduced the developmental potential. Temperatures of 6 degrees C and 8 degrees C and maintenance of eggs in mouse faeces arrested the development of the egg, and increasing temperatures decreaseds the time of development from 40 days at 18 degrees C to nine days at 35 degrees C. Maintenance of unembryonated eggs for 14 weeks in faeces at 12 degrees C and 22 degrees C and for 20 weeks in filtered pond water at 4 degrees C allowed a subsequent normal development, while embryonated eggs maintained at 4 degrees 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 of E. liei was shown to be restricted to the genus Biomphalaria, 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), and B. pfeifferi (Malumfashi) were found to be susceptible, while B. camerunensis (Kinshasa) was almost refractory. Increasing size of B. 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 in B. 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 fom the host snail occurred during the day. Five species of the genus Bulinus and Physa acuta (Egypt) were found to be highly susceptible as second intermediate host snails, whereas three species of the genus Biomphalaria, Planorbarius corneus (Rabat), Lymnaea natalensis (Egypt), and Helisoma 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 patient 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 living B. glabrata (Puerto Rico) or when maintained in freshwater at 4 degrees C. Metacercariae in dead decaying B...

Effect of X-ray on the snails of schistosomiasis in Egypt.

Biomphalaria alexandrina and Bulinus truncatus snails were exposed to sublethal doses 0.2, 3, 5, 10 and 20 rad of X-ray. The survival and reproductive rates of these snails were highly affected by these doses. The maximum survival periods of laboratory populations of Biomphalaria snails were less than those of field ones which means a high sensitivity of laboratory snails to X-ray. The reproductive capacity of irradiated Biomphalaria and Bulinus snails was highly suppressed and this will interrupt Schistosomiasis transmission. A deleterious effect of gametogenesis of irradiated Biomphalaria was histologically proved. After 3 weeks of snail irradiation with high dose (40 rad) the hermaphrodite gland became completely evacuated.