A cohort study to examine maternally‐associated risk factors for bovine spongiform encephalopathy

Abstract

This long-term cohort study, initiated in July 1989, was designed to examine maternally-associated risk factors for bovine spongiform encephalopathy (BSE), forming part of the epidemiological research programme to assess the risks of non-feedborne transmission of BSE. In this study, the incidence of BSE in offspring of cows which developed clinical signs of BSE is compared with that in offspring, born in the same calving season and herd, of cows which had reached at least six years of age and had not developed BSE. All offspring were allowed to live to seven years of age. The results indicate a statistically significant risk difference between the two cohorts of 9.7 per cent and a relative risk of 3.2 for offspring of cows which developed clinical BSE. However, there is some evidence that this enhanced risk for offspring of BSE cases declined the later the offspring was born, but was increased the later the offspring was born in relation to the stage of the incubation period of the dam. The results presented cannot distinguish between a genetic component and true maternal transmission or a combination of both risks, but they do not indicate either that the BSE epidemic will be unduly prolonged or that the future incidence of BSE in Great Britain will increase significantly.

The epidemiology of BSE in cattle herds in Great Britain. I. Epidemiological processes, demography of cattle and approaches to control by culling

“…Full results are reported in Wilesmith et al (1997), Donnelly et al (1997a), Gore et al (1997) and Curnow et al (1997). In brief, the study reveals an enhanced risk of disease in maternally 'exposed' animals of 9.6% (with 95% confidence limits of 5.1-14.2%).…”

Experimentally induced bovine spongiform encephalopathy did not transmit via goat embryos.

“…Natural scrapie also appears to be transmissible between sheep and goats from infected to uninfected animals, if they are reared in close proximity (Greig, 1950 ;Brotherston et al, 1968). Evidence for the transmission of BSE from an infected cow to her calf is not conclusive, and if it does occur it only does so at a frequency of about 10 % (Wilesmith et al, 1997). The findings from the present study have shown that the transmission of BSE from experimentally infected embryo-donor goats did not occur in any of their embryo-derived progeny, with 22 of the 37 goat progeny surviving until culling at over 2000 days of age.…”

Case-control study on feed risk factors for BSE cases born after the feed ban in France

“…Among them, maternal transmission was demonstrated to explain only a very restricted number of cases [15,22]. So, the feeding of cattle has always been assumed to be the main way of infection.…”