Factors Affecting Protozoal Populations In vivo

Abstract

Young ruminants or ciliate-free adult animals can only become infected with a given rumen ciliate if this species is already present in a normal adult animal with which the young animal or ciliate-free adult has very close contact. There is no resistant phase in the life cycle of rumen ciliate protozoa (Strelkow et al., 1933). In the authors′ experience some of the less common large ciliates can disappear from individual sheep, and presumably cattle, for no obvious reason. If this occurs, and if, during the normal management of a flock or herd, animals containing such species are culled, then this protozoal species is lost until a new animal containing this species is reintroduced. The absence of any protozoal species from a group of animals may mean that conditions, such as the nature of the host species or its food, are unsuitable for the growth of the protozoon or that the protozoon is unavailable for infection. The absence of Polyplastron multivesiculatum from New Zealand sheep (Clarke, 1964) may well be the result of its lack of introduction into that country. When sheep were first introduced, they would have had to endure the trauma of a long sea voyage with far-from-ideal feeding conditions and large protozoa could have been lost due to, for example, an acidosis. Those working with experimental animals that have been used for a variety of nutritional experiments should also be careful that, in a previous experiment, the large ciliates have not all been killed by some treatment.