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JOHNE'S disease is a chronic wasting disease affecting wild and domestic ruminants, of which the causative agent is Mycobacterium avium subspecies paratuberculosis (MAP) (Collins 2003). The disease is of considerable economic importance in cattle, causing losses as a result of reduced production, premature culling of infected animals, reduced slaughter value and the cost of testing procedures and control measures (Benedictus and others 1987, Jones 2001).
Effective control can be achieved by timely detection and culling of infectious animals and by removing or reducing transmission from these animals (Kudahl and others 2008, Nielsen and Toft 2008). However, diagnosis, and therefore prevention and control, of infections with MAP can prove difficult, due to the long incubation period and a lack of tests that can accurately predict the future status of animals, and this difficulty should not be underestimated (Kennedy and Benedictus 2001, Nielsen 2008).
The Orkney Johne's Disease Eradication Scheme (OJDES), which provides a voluntary screening service for cattle, was launched in November 2008 and aims to reduce and potentially eliminate the disease on Orkney. The scheme, which is inclusive to all Orkney farmers, is administered by the Orkney Livestock Association (OLA), a farmer-led organisation formed to promote animal health. OLA provides a Cattle Health Certification Standards (CHeCS)-accredited cattle health scheme via the company HI Health. Aid provided under this scheme is granted by the Orkney Islands Council, which pays the costs associated with laboratory testing for the disease for a three-year period. Participating farms have to test all breeding cattle two years of age and over for Johne's disease by milk or blood ELISA on an annual …