Bovine spongiform encephalopathy is being maintained by vertical and horizontal transmissionBMJ 1996; 312 doi: https://doi.org/10.1136/bmj.312.7024.180b (Published 20 January 1996) Cite this as: BMJ 1996;312:180
- R W Lacey
- Professor of clinical microbiology Department of Microbiology, University of Leeds, Chapel Allerton Hospital, Leeds LS7 4SA
EDITOR,—In their article looking at the possibility of maternal transmission of spongiform encephalopathy R M Ridley and H F Baker analyse previously published information and conclude that natural sheep scrapie has a genetic basis.1 Of course this is so. Because of the absence of nucleic acid within the infective particle, host genome is required for the pathogenesis of the disease,2 and it follows that such genes are inherited in various ways. This in no way negates the well established observations that the transmissible spongiform encephalopathies are due to infectious agents which are acquired by vulnerable hosts in a variety of ways, including maternal transmission.3 Maternal transmission cannot sustain the disease in a species in the long term, but it will occur in association with horizontal transmission due, for example, to several lambs or ewes eating or having contact with an infected placenta.4
Ridley and Baker omit to provide any of the evidence supporting the occurrence of vertical and horizontal transmission of the infectious agent for bovine spongiform encephalopathy under farm conditions. This evidence includes the following.
By early 1989 bovine spongiform encephalopathy had been carefully studied for three years, and it was predicted that the total number of cases would be 17000-20000, on the assumption that neither vertical nor horizontal transmission occurred.5 By 26 October 1995 the number of cases of bovine spongiform encephalopathy that had been confirmed in the United Kingdom was 154150.6
Because of the belief that recycled animal remains was the cause of bovine spongiform encephalopathy5 this practice was made illegal from 18 July 1988 after extensive consultation. Indeed, in early 1989 this ban was considered to have been implemented effectively.5 From September 1988 to April 1989, newborn calves were removed from 600 dams by the Ministry of Agriculture, Fisheries and Food and were reared on fresh grass so that the extent, if any, of vertical transfer could be quantified.5 By January 1995 more than 30 of these animals had died of bovine spongiform encephalopathy,7 although the details of the experiment have not been published.
Furthermore, since the ban on cannibalistic feeding was imposed in 1988 over 22000 animals have been born that have subsequently developed bovine spongiform encephalopathy; one was born as recently as June 1993. Interestingly, the range of ages at which the disease was diagnosed in these animals is virtually the same as that in animals born before the ban (table).
These figures show that bovine spongiform encephalopathy is now endemic and is being maintained by vertical and horizontal transmission. However, the Ministry of Agriculture, Fisheries and Food maintains that all the affected animals that were born after the ban were exposed to remnants of contaminated feed (data given in letter from A Browning (minister for food) to D Hinchcliffe (member of parliament), Nov 1995),6 7 with the implication that those involved in the production of feed and also farmers have been breaking the law on a massive scale. But if traces of infectivity from the feed were responsible for the disease in these animals, the animals would be expected to be older, as the incubation period of transmissible spongiform encephalopathies is inversely related to the infecting dose.3