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.

Nippostrongylus brasiliensis: A review of immunity and the host/parasite relationship in the rat

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.

Publisher Summary This chapter reviews the role of hypobiosis in the biology of nematodes. Successful transfer from an infected host to a susceptible one represents a most important achievement in the parasitic way of life. Any developmental adaptation that serves to facilitate this process is, therefore, extremely important in the epidemiology of these infections. Hypobiosis or facultative arrested development represents such an adaptation which, by facilitating the persistence of the larval forms for prolonged periods in the host, enables the parasite to capitalize on optimal opportunities for transfer. The phenomenon appears to be widespread among parasitic organisms. The chapter considers several aspects of this phenomenon as the species involved, factors associated with its induction as well as those influencing a resumption of development, implications of the phenomenon for understanding the epidemiology of these conditions, and its adaptive and population regulatory significance. Host resistance causes the arrested development and that resumption of development is a result of the relaxation of immunity. It is considered to be a host-regulated phenomenon that served to limit the size and pathogenicity of the population of worms, which is accumulating during the season of increasing larval availability. The chapter describes the types of hypobiosis, which include immune-mediated arrest and seasonally induced arrest.

T. Kassai

Papers

Induction of immunological tolerance in rats to Nippostrongylus brasiliensis infection.