Aims and objectives

This exercise is designed to demonstrate:

1. The importance of various factors in the activation and excystation of the cysticercoids.

2. Changes in the appearance of the cysticercoids during excystation

3. The behaviour of freshly excysted larvae.

Introduction

Many parasites use more than a single host species during the course of their life cycle. Often the larval stages develop in one host and the adults in another. Thus, the parasite must be transmitted from one host to the next in order to complete development. The most common relationship between hosts exploited by parasites for transmission, particularly those living in the intestine as adults, is that involved in the predator-prey food chain. For example, some tapeworms develop as larvae in small flour beetles, Tribolium confusum, but parasitise rodents such as mice and rats as adults. Rodents are essentially grain feeders, but will consume insects also and therefore by depending on two host species, both of which have habitats closely associated with cereals and grain, the tapeworms ensure that their transmission cycle is completed successfully.

This practical is based on two species of tapeworm, Hymenolepis diminuta and Rodentolepis microstoma (formerly known as H. microstoma; see Exercise 1.14). The former develops successfully to the adult stage in the rat only (Fig. 3.2.1), the latter matures in mice. Both utilise the flour beetle, Tribolium confusum, as the intermediate host.

The larval (metacestode) stage developing in beetles is called a cysticercoid.