Publisher Summary This chapter discusses the epidemiology and control of nematode infections of grazing animals. The disease resulting from nematode infections is the consequence of exposing insufficiently resistant animals to grazing that is too heavily infected so that they pick up infective larvae at an excessive rate. The disease producing infection is acquired over a relatively short space of time, in some cases because the animals quickly become refractory to infection, in others because worm numbers are regulated in the host to maintain a level proportional to the rate at which new infection is acquired. The number of generations of the parasites involved in the process of auto-infestation is always small. This is either because of the rapid development by the host of a resistance to reinfection or because of the long time taken for the free living phase of the life cycle to be completed. In some cases both factors operate. Disease because of the simple transmission of infections can be avoided by withholding groups of susceptible animals from pasture contaminated by other groups. The choice of control measures where disease is because of auto-infestation depends on how the number of generations involved is normally limited.

The epidemiology and control of some nematode infections of grazing animals.

Publisher Summary This chapter discusses arrested development in nematode parasites. It explains that many parasitic nematodes have a resting stage or stages, further development depending on the reception of some stimulus or stimuli. This pause in development at some precise point in the life cycle of the parasite occurs only in certain hosts or circumstances or at certain times of the year, and it affects only some of the roundworms. The chapter describes parasitic genera and species within nine families, more than 30 genera and many more species helping to build up as complete a general picture as is possible at the present time. The similarities and differences are discussed. Evidence for a direct effect of immunity or of size of infection on arrested development is still not convincing, although host resistance may be crucial in some systems. Another view is that arrested development serves to synchronize the life cycle of the parasite either with that of the host or else with seasonal changes in the external environment, and it implies a response to signals to halt development, to cause it to be resumed, or to serve both functions.

Arrested development of nematodes and some related phenomena.

The epidemiology of helminth disease in farm animals.

Studies on the arrested development of Haemonchus contortus in sheep. I. The induction of arrested development.

This review considers in a selective way the literature on diapause in parasitic nematodes, concentrating on four species of animal parasites and three species of plant parasites. We define diapause as a developmental arrest which is temporarily irreversible, so development will not resume, even under favourable conditions, until some intrinsic changes have been completed. Our analysis recognises four stages in diapause. The first is induction, typically brought about by environmental signals (although diapause may be genetically programmed independently of the environment). These environmental signals typically do not have an immediate effect on development, but we recognise a second phase, which we call the diapause pathway, in which worms have been induced to enter diapause at a later developmental stage. Surprisingly, entry into the diapause pathway may under some circumstances be reversible. The third stage is diapause development, a period during which development is suspended, but some ill-understo...

Diapause in parasitic nematodes: a review.

A group of thirty-nine Merino-Border Leicester cross-bred lambs were given six sensitizing doses each of 3000 infective Haemonchus contortus larvae. Previous studies showed that this treatment produced a state of immunological exhaustion (Dineen & Wagland, 1966a). In the present study the regeneration of the immunological response was followed by challenging the sensitized animals and previously uninfected control animals with 3000 infective larvae at 2, 4, 8 and 16 weeks after termination of the sensitizing infections with an anthelmintic. Faecal egg counts were carried out on infected animals twice weekly throughout the course of the experiment, and differential worm counts were performed on groups of animals slaughtered 48 days after challenge.The results show that maximum regeneration of the immune response occurred when periods of 4 and 8 weeks were permitted to elapse between removal of the sensitizing infections and challenge. At 16 weeks the response of the sensitized animals to challenge infection was not significantly different from that of the control animals.In contrast to results recorded in the previous communication, a high mortality (49%) occurred among the sheep during sensitization. In the earlier experiment the sensitizing infections were given at fortnightly intervals whereas they were given at weekly intervals in the present experiment.Interpretation of the study is based upon the threshold behaviour of immunological responsiveness in parasite immunity, the occurrence of an immunological latent period and the relationship of this period to the developmental rate of the parasite, the susceptibility of the 4th larval stage to immunological attack, and the immunological privilege of the adult worm. These phenomena have been described in previous communications in the present series.We are greatly indebted to Mr Emil Teleki and Misses Helen Giller and Lindy Stothart for their excellent technical assistance.

J. K. Dineen

Papers

The dynamics of the host–parasite relationship: VI. Regeneration of the immune response in sheep infected with Haemonchus contortus