Studies of host resistance to parasite infection are usually based on experimental designs involving a primary infection and subsequent challenge exposure, resistance being recorded as the percentage reduction in parasite establishment in challenged hosts when compared with that in uninfected animals. Few studies have focused on the dynamic nature of helminth establishment and mortality (and their presumed dependency on the rate of current exposure and past experiences of infection) in hosts repeatedly exposed to low levels of infection. Here, we report the results of population studies on the dynamics of resistance to Schistosoma mansoni infection (a helminth parasite) in mice repeatedly exposed to cercarial invasion. Parasite burdens created by different levels and durations of exposure to infection reflect a dynamic interplay between rates of helminth establishment and mortality. Depending on the intensity of exposure, changes in worm load with duration of host infection vary from monotonic growth to a stable average parasite burden to convex curves in which the average load attains a maximum value before decaying in old animals. These trends are similar to observed patterns of S. mansoni infection in human communities.

Age Factors↗

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Herd immunity to helminth infection and implications for parasite control.

Despite much research on immunological responses to helminth parasites, knowledge of the dynamic interplay between levels of herd immunity in humans and the rates of exposure, establishment and mortality of parasites remains limited. We describe here a simple mathematical model for the population dynamics of helminth infections which mirrors the development of a degree of acquired immunity within populations which are genetically heterogeneous with respect to immunological responsiveness. We interpret observed patterns in the age-specific intensity of infection and attempt to understand the possible effects of control measures based on chemotherapy and vaccination. Mass chemotherapy can, in some circumstances, reduce the level of herd immunity such that average worm burdens in the adult age classes rise above their precontrol levels. When certain individuals or groups are predisposed to heavy infection, selective or targeted drug treatment can have significantly greater impact than mass or random application. Conversely, model predictions suggest that effective parasite control by vaccination (if and when vaccines become available) is difficult to achieve in communities that are genetically heterogeneous in their ability to mount protective responses to infection.

Age Factors↗

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Vaccination and herd immunity to infectious diseases.

An understanding of the relationship between the transmission dynamics of infectious agents and herd immunity provides a template for the design of effective control programmes based on mass immunization. Mathematical models of the spread and persistence of infection provide important insights into the problem of how best to protect the community against disease.

Age Factors↗

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Rabies control. Vaccination of wildlife reservoirs.

Explore the source record for details and available documents.

Animal Population Groups↗

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Transmission dynamics of HIV infection.

Simple mathematical models of the transmission dynamics of human immunodeficiency virus help to clarify some of the essential relations between epidemiological factors, such as distributed incubation periods and heterogeneity in sexual activity, and the overall pattern of the AIDS epidemic. They also help to identify what kinds of epidemiological data are needed to make predictions of future trends.

Acquired Immunodeficiency Syndrome↗