Influence of temperature on penetration of snail hosts by Schistosoma mansoni miracidia

Abstract

Studies were conducted to ascertain the influence of environmental temperature on the ability of S. mansoni miracidia to penetrate the snail host as judged by later development of the infection under known favorable conditions. Five geographic strains of A. glabratus obtained from Dominican Republic, Puerto Rico, Brazil, and two from Venezuela were employed. The strain of S. mansoni came from Puerto Rico. All snails were exposed individually to five miracidia each for 2 hours. Temperatures of 25, 35, and 40 °C were tested on all five strains, the lowest temperature 10 °C on four of the five and an intermediate temperature, 15 °C on one strain. Subsequent to exposure the snails were held at 26 to 28 °C. Infection rates ranging from 0 to 90% were obtained. Exposure at 10 °C did not result in a single snail infection. Except for the Brazilian strain, a known resistant strain, the rate of infection increased when the exposure temperature was elevated. A large percentage of the snails exposed at 40 °C died during the exposure period, indicating that this temperature exceeds the normal heat tolerance of A. glabratus. Motility of freshly hatched miracidia was found to be directly related to temperature. At 10 °C they were observed to be almost motionless, and gradual increase in temperature to 40 °C resulted in hyperactivity. The implications of the findings presented in this paper were discussed in relation to the biology of the parasite and to the epidemiology and control of the disease caused by it. It is interesting to note that A. glabratus imported from the same general area of Venezuela in 1946 as was A. glabratus in 1952 gave strikingly different results (Fig. 1). Among the possible interpretations of this finding is (1) changes have taken place in the colony imported in 1946 due to breeding under laboratory conditions over an extended period, or (2) changes have taken place in the wild stock in its natural habitat. If the former is true, then one must be mindful of the possibility that snail colonies maintained in an artificial environment for an extended period may give rise to progeny that possess different characteristics than the original wild stock, and experimental data obtained from them must be so interpreted.