The Host-location Mechanisms of the Miracidium and Cercaria of Schistosoma Mansoni

Photographic techniques were used to record the behaviour of schistosome larvae in the presence and absence of host stimuli. A quantitative analysis of the behaviour patterns was performed, not only to provide information about the behaviour of the larvae, but also to enable the techniques of computer simulation modelling to be employed.
In the absence of stimuli, the miracidia swam in relatively straight lines with occasional changes in direction. An unstimulated miracidium, hitting the glass walls of the test cell, usually bounced off at an angle approximately equal to the angle of incidence. A proportion of the miracidia swimming close to a snail showed an increased rate of turning with a possible decrease in- speed. These behaviour patterns were incorporated into a model of the host location process. The model predicted that the increase in turning alone was insufficient to improve the chance of contact with the snail and that sensory adaptation or reaction-time might be important in the location of the host. It was also shown that the length of the relatively straight periods of swimming in between intervals of turning, whether the miracidium was stimulated or unstimulated, greatly affected the chance of contact.
In the absence of host stimuli, the cercariae showed arhythmic pattern of motility comprising of active upward swimming alternating with passive downward sinking. A cercaria reaching the surface usually stopped swimming and started to sink. A cercaria sinking onto the bottom was often stimulated to swim upwards. The cercariae responded to rapid mixing by an increase in the time spent swimming so that an upward migration of the larvae was observed. A shadow also produced a similar upward migration except that, upon contact with the top of the cell, many of the cercariae continued to swim along the surface. In response to skin surface chemicals, the cercariae were found to swim in all directions with occasional reversals in the mode of swimming. The rapid mixing stimulus increased the effect of the chemicals. The response of the cercariae to skin surface chemicals and the unstimulated swimming pattern were incorporated into a model of the host location process. The model predicted that, in likely infection situations, the increased level of activity made the chance of contact with the host more likely than if no stimulation took place.