Factors which may determine trematode–snail interactions
were assessed in the present study. Compatibility was examined
using a bacterial clearance assay to detect the modulatory effects of
both compatible and incompatible trematode infections
on the activity of haemocytes from Lymnaea stagnalis, during the
early stages of infection. Exposure to and injection with
Trichobilharzia ocellata, a compatible trematode, or the incompatible
Schistosoma mansoni, resulted in modulation of
haemocyte activity. However, T. ocellata activated haemocytes
1·5 h post-infection (p.i.) and then suppressed activity
24–72 h p.i. whereas with S. mansoni no suppression, only
activation of haemocytes was observed throughout the test
period (1·5–72 h p.i.). In previous studies, modulation of
the
haemocyte clearance activity by T. ocellata was found to be
mediated by 2 E–S fractions, an activating fraction and a suppressing
one.
Investigations to assess whether the lack of
suppression of haemocyte activity, observed in the S. mansoni–L.
stagnalis incompatible trematode–snail interaction
studied, was due to either the absence or ineffectiveness of the suppressing
E–S fraction, were performed on a second
incompatible combination, T. ocellata–Planorbis corneus.
Using
this combination it was revealed that only the activating
E–S fraction had modulatory effects on P. corneus haemocytes,
indicating that the suppressing E–S fraction, which actively
interferes with the clearance activity of haemocytes from L.
stagnalis, appears to act in a host-specific manner. In
conclusion, the suppressing E–S fraction determines, at least in
part,
compatibility in the trematode–snail association
studied. This is also probably likely in other trematode–snail
combinations.