The influence of temperature on the biology of Biomphalaria glabrata (Say), intermediate host of Schistosoma mansoni on St. Lucia, West Indies

Life tables were obtained over 16 fortnights for three cohorts of Biomphalaria glabrata at 20§C, 25§C and 30§C respectively. At 35§C no eggs hatched. The intrinsic rate of natural increase r was greater at 25§C(0.8834) but the mean generation time T was similar to that at 30§C(8-9 fortnights). Egg production and survival were reduced at 30§C but r(0.6866) was higher than at 20§C(0.4725). T was greatest at 20§C(12 fortnights) due to the delay in the onset of egg laying. The growth rate at 30§C was only a little faster than that at 25§C, but both were greater than that at 20§C. Comparison of these results with those of similar studies elsewhere suggests that the St. Lucians strain of B. glabrata is adapted to living in unstable, temporary habitats. B. glabrata, unlike its African relatives, is capable of surviving prolonged exposure to temperatures of 30§C so that, despite reduced fecundity, it can colonize the hot, low lying coastal plains of northeastern South America. (AU)

Observations on the susceptibility to schistosoma mansoni from St. Lucia of several Caribbean strains of snails of the genus Biomphalaria

Planorbid snails were collected from six Caribbean countries, Barbados, Dominica, Dominican Republic, Puerto Rico, St. Vincent and Trinidad, and from French Guiana on the South American mainland. No Biomphalaria ssp., the intermediate host for Schistosoma mansoni, were found on Barbados or St. Vincent, but B. straminea was found on Trinidad and B. glabrata were collected from the remaining countries. Laboratory-bred, first generation offspring of all the collections of Biomphalaria were exposed to S. mansoni miracidia from St. Lucia. The B. straminea failed to become infected, but all the strains of B. glabrata were susceptible, although the infection rates were lower than among St. Lucian B. glabrata exposed simultaneously. Only the B. glabrata from French Guiana shhowed any true sign of resistance to infection. In view of the widespread origin of the snails tested in these experiments, it is probable that any B. glabrata from the Caribbean area will be susceptible to the St. Lucian S. mansoni. Potentially, people infected by S. mansoni in St. Lucia could spread this parasite throughout the region wherever B. glabrata is found. The chance of this happening, though, varies greatly from country to country because of the complexity of the dynamics of transmission. (AU)

Factors affecting laboratory growth curves of biomphalaria glabrata in St. lucia

It is necessary to obtain a normal growth curve, from hatching until death, to interpret field data collected on Bromphalaria glabrata. The curve is used to age field snails whence birth rates, death rates and life span can be calculated together with information on the age at which snails become infected. Field growth curves may be obtained by the following successive generations seen as modes in the size structure of the population, for a period of time. However, laboratory curves are needed to assist in interpreting anomalous cases. Unfortunately, several factors, including temperature, parasitism and crowding, can modify this curve both in the field and the laboratory and the results will be presented at the meeting together with a field growth curve(AU)