New Study Finds Evidence That Bse Cases Were Missed

Researchers from Imperial College London have published new results that suggest that over half of BSE cases went unrecognised or unreported during the epidemic in Great Britain. The new figures, to be published in a forthcoming Proceedings B, a learned journal published by the Royal Society, estimate that the total number of cattle infected during the epidemic was over two million.However the paper highlights the need for additional research to reduce the uncertainties in some key biological factors underlying this estimate.

European infection

Estimates are also made of the per-head incidence of infection in cattle born between 1993 and 1997 in other European countries. Comparative data for cattle born after mid 1996 indicates relatively high infection in Greece, Italy and Belgium. Spain and the Netherlands show intermediate levels, whilst the incidence in Great Britain, Germany and France is comparably low.

European data from EU-wide testing of apparently healthy cattle over 30 months old cattle and slaughtered for consumption were analysed to place the British results in context. Although infection rates in other countries are low relative to the peak of the epidemic in Britain, there are significant variations.

What does this mean for vCJD?
The researchers estimate that the new results will not have an effect on the predicted size of the new variant Creutzfeldt-Jakob Disease (vCJD) epidemic in humans as the results indicate that the probability of human infection from eating BSE-infected meat from an individual infected animal is lower than previously thought.

“Our results demonstrate the effectiveness of the additional control measures implemented in Great Britain in 1996 after the identification of vCJD,” concludes Prof. Donnelly. “It also provides a standardised framework forcomparing BSE infection levels in differentEU countries. Furthermore it suggests that some countries with few confirmed cases but relatively high recent infection risks, such as Greece, Italy and Belgium, may warrant additional control and enforcement measures. This will reduce future case numbers in these countries and avoid prolonging the course of these epidemics.”

Back calculation

The study used backcalculation methods adapted for analysis of BSE from those originally developed in the early years of the HIV/AIDS epidemic.Previous workwas extendedto allow integrated analysis of available data on clinical case incidence and the results from screening of apparently healthy cattle to obtain estimates of case rates. “Two possible mechanisms for underreporting were examined,” says Prof. Christl Donnelly of Imperial College London. “Underreporting of clinical cases and differential slaughtering of BSE-infected animals prior to clinical manifestation of the disease due to, for example, possible productivity effects of the disease – for example reduced milk yield.”

In both modelsnew estimates for the extent of infection in Great Britainare markedly abovepreviousfigures.”The best fit to thescreening datais obtainedwith the differential slaughter model, and shows 1.9 millioncattle were infected during the epidemic.1.6 millionof these were slaughtered for consumption beforeonset of clinical signs of disease,” says Prof. Donnelly. “This compares with estimates of 1.05 million infections and 0.87million infected cattle slaughtered for consumption if the possibility of differential survivorship or ongoing under-reporting is not allowed for.”

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