The nature and action of host signals.

It is clear that excystations in vitro of the coccidia so far examined involves two steps, in the first of which CO2 is important, and the second, in which an external source of chymotrypsin and surface-active agents are required. However, the details of the mechanism of excystment are not clear. We do not know how the presence of CO2 changes the permeability of the oocyst wall. We do not know whether CO2 does anything to the sporozoite or sporocyst; the circumstance that mechanically-released sporocysts readily excyst under appropriate conditions without the necessity for high concentrations of, or perhaps any, CO2 suggests it does not. Circumstantial evidence suggests that the substrate in which chymotrypsin acts is the Stieda body, but whether the enzyme has other roles we do not know. Similarly, the role of bile is ill-defined, although it does seem that the induction of activity is important--but how is this brought about? The techniques available to excyst oocysts are, for many species, very efficient. If CO2 is, as it seems to be, a fundamental stimulus, then efficiency might be enhanced if more attention was given, not so much to increasing the time of exposure and amount of CO2 in the gas phase, but rather to the pH of the medium, which is rarely stated or apparently, controlled. The pH determines the proportion of the different carbonate species in solution, which may be of greater significance than the partial pressure of CO2 in the gas phase (see also Section V A). Although high numbers of excysted sporocysts can be obtained with a particular technique, this does not necessarily mean that all the signals supplied by the host are reproduced in vitro. Jackson (1962) found it necessary to wash oocysts in water or dilute buffers between the primary phase and the secondary phase, a step which implies a deficiency in the methods he used. Commonly, oocysts are exposed to a strong solution of L-cysteine. Does this reflect a general deficiency in the technique, or a counterpart of strongly reducing conditions in ruminant and non-ruminant alike? It seems that we have only a very general outline of excystment, and that we do not understand the details. Yet the problem seems to have been put aside; the most recent relevant reference we have found is dated 1983.