This is because, in many cases, the test will fail to detect infection in an individual animal and so an animal that tests negative, especially if it is less than two years old, may still be infected.
Spread of infection within the herd
The younger an animal is, the more susceptible it will be to acquiring infection. Therefore most controls for the spread of infection within a herd should focus on the management practices around calving and calf rearing.
At calving it is important to minimise the contact newborn calves are likely to have with dung that might contain Map. The following are measures that should be considered in any control programme.
1. Ensure no test positive or test inconclusive cows are permitted to calve in the same environment as test negative cows. The dung of positive or inconclusive cows could contaminate the calving area with Map. As far as possible, these high-risk categories of animals should calve down individually and the calving areas should be cleaned thoroughly before any other cattle, particularly cattle less than six months of age, are permitted access.
2. The offspring of infected cows have a higher risk of themselves being infected. Therefore, they should be treated as high-risk animals and, as far as practicable, calved separately from where test negative cows calve down.
3. Ensure dry cows are clean as they enter the calving area i.e. teats and flanks are free from dung.
4. Remove all calves as soon as possible from their dam ('snatch calving') to minimise calf contact with adult dung.
5. Avoid group calving as far as possible as this allows new-born calves to be exposed to multiple cows and their dung.
6. Ensure calving areas are regularly cleaned and, as a minimum, that additional bedding is added so that cows are calving down on a clean and dry surface.
7. Calves contaminated at or around birth can potentially carry the Map bacteria to other calves on their coat or in their gut.
Therefore, the size of calf groups during the first week of life should be minimised. Wherever possible, keep calves penned individually during the first few days of life.
Dr Sam Strain is a programme manager for Johne's Disease at Animal Health and Welfare, NI