Excreted load and infective dose

The number of organisms excreted can vary considerably due to the type of infection or the stage of the disease. In diseases such as cholera, there may be vast numbers of organisms excreted (106-1012 vibrios/g of faeces), whereas in hookworm infection, the number of eggs may be comparatively few. In Schistosoma mansoni, asymptomatic children excrete the largest number of eggs, whereas the adult exhibiting severe manifestations may be almost non-infectious. In the otherwise harmless typhoid carrier, about of diarrhoea can cause the passage of a sufficient number of organisms to initiate an epidemic.

For each infectious agent, a minimum number of organisms - the infective dose - is required to overcome the defences of the host and cause the disease. A large dose of organisms may be required, such as with Vibrio cholerae or very few, as with E. histo-lytica. In most infections, once this number is surpassed, the severity of the disease is the same whether a few or large number of organisms are introduced, while in others, there is a correlation between dose and severity of illness. Estimates of doses have been attempted in cholera and typhoid using healthy volunteers, but variables such as host susceptibility prevent any degree of precision. An example is food poisoning where the severity of the illness is determined by the quantity of the infected food item that is consumed. On the beneficial side, a low dose of organisms may produce no symptoms of disease, but may be sufficient to induce immunity. Poliomyelitis is one of the many examples.

Infections with a low infective dose (e.g. enteric viruses and E. histolytica) can spread by person-to-person contact. This means that the provision of a safe water supply or sanitation will have little or no effect. At the other extreme are organisms like typhoid and cholera, when a high infective dose (of the order of 106 organisms/ml of water) is required to produce the disease. Improving water quality and the reduction of pathogens in the sewage will be beneficial to the community.

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