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This paper describes a simple mathematical model to investigate the impact of herd immunity on the epidemiology of waterborne and non-waterborne outbreaks of cryptosporidiosis. The model was run on a standard spreadsheet package. Essentially the model divided a population into four groups each with increasing water consumption behavior. Within each group there were susceptibles and immunes and for each cycle, susceptibles converted to immunes in relation to their water consumption and "water infectivity". Each cycle a proportion of the immunes were replaced by susceptibles to reflect deaths and replacement by new births. Each model was run to equilibrium between immunes and susceptibles with the water infectivity varied to make different proportions of the overall population immune. Increasing population immunity (caused by prior waterborne infection) has a marked negative impact on relative risk of infection associated with water consumption during any subsequent outbreak. The affect is most marked as the percentage immune increases from 0 to 20%. Prior exposure due to nonwater consumption had no effect on calculated relative risks due to water in outbreaks. Includes 9 references, figures.

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Edition: Vol. - No. Published: 08/29/1999File Size: 1 file , 62 KB