The prevalence is then read from a standard table or program. The number of animals tested and the herd size are known. In this case the prevalence of the disease, or, if all tests are negative, the maximum level at which the undetected disease could be present, can be read from a standard table or program. This method involves random selection of flocks in which all or a strongly representative sample of animals are tested. This system is valuable where records of the existence of flocks are unreliable.
All methods require some sensitive test of the presence of disease in the individuals chosen.
The nature of scrapie makes the application of these techniques difficult and costly. A variation in genetic susceptibility of sheep has been recognised. The incubation period of the disease is variable however it is usually measured in years. The variation in incubation period can be influenced by a number of factors including host genetics and strain of agent. The lack of definitive information on tissue tropism of the agent and the role of semen and washed embryos and means of spread of the disease.