Experiments were undertaken to investigate the factors which influence the transmission dynamics of Gyrodactylus bullatarudis within populations of laboratory guppies. The parasites possess a number of biological attributes which are almost unique amongst helminth parasites of vertebrates. These include the ability to reproduce viviparously and directly on the surface of the host, the ability to survive death of the host (for a short period of time) and the ability to transfer between hosts (despite the absence of a specialized transmission stage in the parasite's life-cycle). Long-term laboratory experiments demonstrated the inability of the parasite to persist within populations of the host in the absence of the continual introduction of naive susceptible fish. With regular addition of susceptible fish, the parasite population exhibited recurrent epidemic behaviour. The magnitudes of the epidemics and the time interval between them, were dependent on the rate at which fish were added to the populations. The parasite was over-dispersed in its distribution within the experimental fish populations and was a significant cause of host mortality (in a manner related to parasite burden). The experimental results suggest that acquired resistance to infection is an important factor determining epidemic behaviour.

When mice were infected with female cercariae of Schistosoma mansoni and male cercariae of Schistosomatium douthitti, many mixed pairs formed. The paired females were approximately the same size as those in unisexual infections, far smaller than females paired with S. mansoni males. Although the Sch. douthitti males possessed well-developed testes, sperm were not found in their female partners, which developed scanty vitelline glands and produced laterally spined eggs typical of S. mansoni. Such eggs yielded swimming miracidia infective to the snail host of S. mansoni, Biomphalaria glabrata, but not to the lymnaeid snail host of Sch. douthitti. Sporocysts arising from these miracidia were haploid and produced cercariae infective to mice. Parthenogenetically derived female cercariae in mice co-infected with either parthenogenetically derived male or normal diploid male S. mansoni developed to large adults of normal appearance, whose eggs yielded diploid miracidia and subsequent generations of normal diploid schistosomes. Parthenogenetically derived females co-infected with Sch. douthitti males also paired and produced some eggs containing viable miracidia, which gave rise once again to haploid sporocysts. These observations confirm previous suggestions that the stimulus for maturation in female S. mansoni is distinct from that for growth, and is independent of insemination and fertilization. It is concluded that both diploid and haploid S. mansoni females are capable either of parthenogenesis or of bisexual reproduction when appropriately stimulated.

Simple mathematical models are developed to examine the influence of variability in host susceptibility to infection, on the dynamics of host–parasite population interactions. When hosts differ in their innate susceptibility (at birth), to infection by a specific parasite, the average susceptibility of the host population as a whole may show systematic changes through time. Such patterns may arise as a result of demographic factors associated with the interaction between host and parasite populations, in the absence of inheritance mechanisms (a genetic component) or acquired resistance (an immunological component). The general significance of this observation is discussed in terms of the coevolution of host–parasite associations.

During autumn and winter, separate pasture plots were seeded with Ostertagia ostertagi eggs produced by worms which had persisted in the host over summer as inhibited larvae, or which had developed directly from larvae acquired from pasture in late summer. Successive groups of parasite-free calves grazed the plots in spring for 14 days at 4-week intervals and were killed for worm counts 14 days after removal from pasture. The proportion of inhibited early 4th-stage larvae was substantially greater in calves which grazed the plot carrying the progeny of previously inhibited worms. This finding is more easily reconciled with differential survival on pasture over spring and summer of inhibiting and non-inhibiting morphs, than with the concept of a physiological switch from inhibiting to non-inhibiting behaviour in individual larvae.