Animals are usually infected as calves with approximately 80% of infections occurring within the first month of life.Infection is mainly, but not only, caused by ingesting faeces often from contaminated bedding, udders, teats or buckets or from colostrum or milk. Much less commonly the disease can be acquired in the womb or later in life.
Infected animals will test negative for a variable period of time, usually several years before the disease progresses and they become infectious to others at which time they will usually test positive by one or more of the available diagnostic tests.
Johne’s infections are almost always introduced to a herd by purchasing infected replacement breeding stock – although there are other risks of introducing the disease including importing slurry from other farms and swapping colostrum between herds.
Johne’s disease can adversely affect the physical and economic performance of a dairy herd. Johne’s test positive cows are twice as likely to have a cell count > 200,000 cells/ml and are twice as likely to have milk yields 25% below their adjusted herd average. Johne’s disease costs can rise to excess of 1-2p/ litre with higher disease incidences and these costs remain for a number of years until the disease is brought under control.
With Johne’s, the major costs come from increased susceptibility to other conditions and increased forced culling and the retention of cows that should otherwise be culled. Clinical Johne’s cases are just the tip of the iceberg.
International experience has shown that if a rigorous control program is implemented and applied robustly Johne’s disease can be brought under control. In Denmark the Johne’s test prevalence (annual percentage of cows within the herd testing positive) for herds adopting the national control program has reduced from 10% to 2% over 6 years.