BETTER TESTS and an increasing awareness of Johne’s disease has led to a big rise in the number of cases diagnosed in Britain, said SAC Vet Services’ George Caldow.

“But we have no national survey data so we don’t know the prevalence,” he added. “Slaughter tests in 1996 showed 3.5%, although it could be higher due to stock imports as there is a 55% prevalencein Holland.”

Dr Caldow said Johne’s, caused by Mycobacterium avium subspecies paratuberculosis (MAP), takes hold in a herd and is difficult to control. “Infection occurs in the first few months of life followed by a long incubation period. The common time for cows to show clinical signs is when 3-5 years old.”

For every clinical case, there are 7-10 animals excreting the MAP organism and a further 7-10 in the silent period of infection. However, it is unclear whether there is lateral spread between groups of youngstock or adults, added Dr Caldow.

But some herds are proving they are free of the disease. SAC’s Johne’s programme has 405 members, with 106 herds accredited Johne’s-free.