Current status of transmissible spongiform encephalopathies in ruminants
Abstract
Transmissible spongiform encephalopathies (TSE) encompass subacute neurological degenerative diseases for which the prototypes are scrapie in sheep and some forms of Creutzfeldt-Jakob disease in man. The emergence of a new form of TSE in cattle in United Kingdom (UK) since 1986, namely bovine spongiform encephalopathy (BSE), sharply increased the interest for these diseases, especially because of the epidemic nature of BSE in UK, its subsequent spread in continental Europe and the later discovery of its zoonotic character. The number of measures of veterinary public health taken to control the disease and to prevent its spread to animals and human beings increased in time and culminated by the total feed ban. Indeed, since the beginning of 2001, feed containing proteins of animal origin is prohibited for the feeding of production animals, including ruminants and monogastric species. The effect of this total ban of mammalian meat and bone meal needs to be evaluated. The incidence of BSE has a trend to decrease in UK and in most of the other European member states. However, as BSE is a rare event distributed in a large bovine population, it is difficult to state unambiguously whether this trend is significant. Furthermore, the evaluation of this measure will be only effective at least five years after its introduction, since this period is the mean incubation time of BSE. The main concern is currently the eradication of BSE in the infected countries. Additionally, the control of scrapie is also carried out due to the possible contamination of sheep with the BSE agent. These actions must take into account several new facts: the recent discovery of BSE cases in countries with a low geographical BSE risk level as Japan, Canada and the United States of America (USA); the growing incidence of chronic wasting disease, a spongiform encephalopathy observed in deer in USA; the characterization of a new pattern of bovine amyloidotic spongiform encephalopathy in Italy, atypical scrapie cases in sheep and atypical BSE cases in cattle in Europe and the efficacy of sheep selection based on scrapie resistant genotypes.