Testing for Johne's disease is only part of the overall strategy when aiming to reduce Johne's prevalence on farm. The most important aspect of Johne's reduction is implementing control strategies on farm. Breaking the cycle of infection will reduce the overall infection pressure over time. Your farm may have implemented strategies to prevent high risk transmission routes and may be looking to reduce lower risk transmission routes.
The most important aspect of Johne's reduction is implementing control strategies on farm
Established Transmission Routes
When aiming to control Johne's disease on your farm, it helps to firstly eliminate 'one to many' transmission routes, and then look at 'one to one' transmission routes.
One to many transmission:
Via colostrum - if colostrum is pooled (not advised)
Faeco-oral route - dirty environment/cow udders etc
Feeding waste milk
Calf to calf transmission
An infected calf can infect ~3 other calves
Red tag high risk calves and keep away from green calves
One to one transmission:
Via the uterus - especially in heavily shedding/clinically infected cows
A calf is x3 more likely to be johnes positive if her dam is johnes positive, even if the cow is not currently testing positive.
Via colostrum - if colostrum is not pooled
Controlling 'one to many' transmission
One to many transmission involves keeping calving areas clean and implementing breeding protocols to cull Johne's positive cows to keep them away from the calving area. If Johnes positive cows do calve they should, where possible, not enter the main calving area. Calving pens should be cleaned out as regularly as possible, i.e. every 3 weeks to remove sources of infection for new born calves.
Feeding calves pooled colostrum or waste milk is not advised. This opens a window of opportunity for many calves to be infected by contaminated milk.
Calves born from Johne's positive cows are x3 more likely to be infected, and can affect approximately 3 other calves in the same pen. Calves born from high risk cows should be identified (i.e. red tagged) and reared separately from low risk calves where possible. For example, a calf born from a Johne's positive cow could be reared in a single hutch until weaning to reduce the risk of transmission.
Controlling 'one to one' transmission
Johne's positive cows should be culled from the herd as soon as possible. Whilst many control strategies around calving are crucial, and Johne's is not a disease you can simply cull your way out of, reducing the number of high risk cows from calving will reduce the overall infection pressure.
When managing colostrum it is advised calves are fed good quality, clean colostrum from their own dam, unless this cow is a Johne's positive cow. Records should be kept of the colostrum calves receive in case cows later test positive for Johnes. These calves can then be identified as high risk animals.
Summary
Implementing Johne's control strategies must be targeted and tailored to each farm. Farms should firstly control higher risk transmission routes (one to many routes) as these will have the greatest impact, and then look at reducing lower risk transmission routes (one to one routes).