Density-dependence in parasite transmission dynamics.

The transmission of vector-borne parasites is complex, yet to a large extent this complexity can be unravelled through the insights gained from simple mathematical models of the transmission system. The principle is simple because the key question is merely "what is the rate of increase in numbers of hosts affected?" Clearly, if this rate of increase is greater than unity then the infection can spread, while if it is less than unity it will decline. Ronald Ross in 1911 was the first to formulate this idea for malarial(1) and malaria transmission has since attracted most attention from modellers of parasitic diseases(2-4). But although it is implicitly recognized that nothing- not even parasitic transmission - can increase indefinitely, the importance of some degree of density-dependence in regulating the system tends to be neglected (see Box 1). In this article, Klaus Dietz explores some classical ideas of modelling parasitic disease transmission, emphasizing not only the importance of density dependence but also the importance of knowing exactly where such effects operate in the system.