We investigate whether a calf born to a dam that develops bovine spongiform encephalopathy (BSE) (prior or subsequent to the birth) is itself at an enhanced risk of developing BSE. Analyses utilize the main database on reported BSE cases in the British cattle herd maintained by the Central Veterinary Laboratory in Weybridge to trace the dams of BSE-a¡ected animals born following the ruminant feed ban in July 1988. The data reveal a signi¢cantly enhanced risk of disease in calves born to BSE-a¡ected dams, with the risk being greatest when birth occurs after the onset of clinical signs of disease in the dam. The dependence of the maternally enhanced risk on the maternal incubation stage at birth argues for a signi¢cant component of direct maternal transmission of the aetiological agent of BSE, and o¡ers little support for the hypothesis of genetic predisposition. Using a statistical likelihood model, we obtain estimates of the rate of direct maternal transmission by maternal incubation stage; however, biases in the available data make these values minimum estimates.
The basic reproduction number, R 0 , of an infectious agent is a key factor determining the rate of spread and the proportion of the host population a¡ected. We formulate a general mathematical framework to describe the transmission dynamics of long incubation period diseases with complex pathogenesis. This is used to derive expressions for R 0 of bovine spongiform encephalopathy (BSE) in British cattle, and backcalculation methods are used to estimate R 0 throughout the time-course of the BSE epidemic. We show that the 1988 meat and bonemeal ban was e¡ective in rapidly reducing R 0 below 1, and demonstrate that this indicates that BSE will be unable to become endemic in the UK cattle population even when case clustering is taken into account. The analysis provides some insight into absolute infectiousness for bovine-to-bovine transmission, indicating maximally infectious animals may have infected up to 400 animals each. The relationship between R 0 and the early stages of the BSE epidemic and the requirements for additional research are also discussed.
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