FAILURE OF SCHISTOSOMA-MANSONI TO REINFECT BIOMPHALARIA-GLABRATA SNAILS - ACQUIRED HUMORAL RESISTANCE OR INTRA-SPECIFIC LARVAL ANTAGONISM

Failure of snail reinfection by Schistosorna mansoni has been demonstr ated in susceptible Biomphalaria glabrata infected with 1 miracidium a nd subsequently re-exposed to 1 or 5 homologous parasite larvae. The a cquisition of 'resistance' to secondary parasite infection was time de pendent since complete inhibition was observed at 2 weeks and longer f ollowing monomiracidial exposure. This phenomenon was still observed i n snails challenged 8 weeks after primary infection. Histological obse rvations revealed that sporocysts from the challenge infection were fr ee of encapsulation, their development was stopped and they degenerate d slowly in the absence of haemocytic reaction of the host. Under the hypothesis of an acquired homologous resistance mechanism, this strong ly suggests that 1 or several unidentified humoral factors are respons ible for the non-development of the sporocysts from the challenge infe ction. However, considering the time-dependent nature of the phenomeno n, an intraspecific larval antagonism process between sporocysts resul ting from the primary infection and those from the challenge infection may be involved.