The evaluation of exposure risks for natural transmission of scrapie within an infected flock

In VRQ/VRQ lambs exposed to infected sheep at pasture or during lambing, and exposed to the buildings in which lambing took place, the attack rate was high and survival times were short. Where exposure was to pasture alone the number of sheep affected in each experimental group was reduced, and survival times were longer and related to length of exposure.At the flock level, eradication and control strategies for scrapie must take into account the need to decontaminate buildings used for lambing, and to reduce (or prevent) the exposure of lambs to infected sheep, especially in the later stages of incubation, and at lambing. The potential for environmental contamination from pasture should also be considered. Genotype selection may still prove to be the only viable tool to prevent infection from contaminated pasture, reduce environmental contamination and limit direct transmission from sheep to sheep.Attempts to unravel the epidemiology of scrapie have been frustrated by many factors including long incubation periods, lack of host immune response, difficulties associated with isolation and characterisation of the infectious agent, and the challenge of proving that animals which are exposed to natural or experimental infection are truly naive.The association of disease with family lines at one time led to suggestions that the disease was entirely genetic [1], but subsequent studies in several breeds highlighted the fact that disease results from an interaction between host genotype and the infectious agent [2-7]. In recent years research into the relationship between genotype and susceptibility to infection or disease has expanded considerably [8] and highlighted the influence of three codons of the prion protein gene (136, 154, 171). Such findings have resulted in their use, on a precautionary basis, in structuring voluntary and compulsory breeding programmes intended to reduce the risk to consumers from the potential presence of BSE in sheep [9-11].Reviews [12-14] re