Johne’s disease is a common infection in cattle herds. It causes signs in adult cattle of weight loss, diarrhoea and they eventually die from the condition. A drop in milk production is a common first sign. The milk yield tends to be lower than expected, weight loss then becomes noticeable and diarrhoea develops. In beef herds it may be noticed that the cow’s calf is not growing as well as expected as a result of her poorer milk production.
Young calves up to about six months old are the most susceptible to infection and the susceptibility reduces as the calf gets older. Adults are considered to be fairly resistant to infection though it can occur if they are exposed to a high enough dose. There is a long incubation period, during which time they will look healthy, before the signs of wasting and diarrhoea occur. They would typically be three to five years old when these signs occur but they can be older. It is now becoming increasingly common for cases to occur in cattle under two years of age.
The bacteria enter the gut wall and an immune reaction to their presence develops over a long period of time resulting in the gut thickening. The gut becomes less able to absorb nutrients from the diet and protein loss also occurs resulting in weight loss and eventually persistent diarrhoea.
Faeces are the main source of infection. Calves born to positive cows are at the highest risk of becoming infected but other calves in the same environment are also at risk. Colostrum and milk from positive cows can contain the agent but it is in much smaller numbers per unit volume than in the faeces. Calves can also become infected in the womb. It is for these reasons that the offspring from positive cows should not be kept as breeding replacements. Her last two calves are deemed to be at highest risk of being infected.
A Johne’s control programme has a focus on herd hygiene and removing the infectious cattle from the herd.
Blood testing is an important part of a Johne’s control programme. All cattle over two years old should be tested. It allows infected cattle to be separated from the herd thereby reducing the risk to the others, particularly the calves. By detecting the infected cows before they start to show signs of disease it allows them to be removed before they are at their most infectious stage, which reduces the contamination of the environment and risk to the young stock.
It is advisable to blood test for Johne’s disease on an annual basis. Herds that carry out annual herd testing usually never experience cases of cattle going down with the signs of Johne’s disease. This is because the infected animals are identified early and can be removed from the herd through planned culling before they succumb to the disease. At this stage they are hopefully still worth a significant amount of money to cull.
In beef herds it can be useful to do the herd blood test a few months before calving as it allows positive cows to be removed from the calving pens. They should be kept separate from the main herd until their calf can be weaned at which point they can be culled from the herd. Their calf should not be kept as a breeding replacement.
In dairy herds it can be useful to blood test the cows at drying off. This allows the infected cows to be identified by a tag or band so that staff know that they are to be kept in a separate calving area thereby reducing contamination of the main calving pen/s. A positive cow’s colostrum and milk should not be fed to calves (or it can be pasteurised first to reduce the risk). The positive cows can be culled from the herd during their next lactation. Quarterly milk testing can be handy for dairy herds but milk testing is not as sensitive at detecting infected cows as blood testing.
Herds can be tested for Johne’s disease and classed with one of five risk levels:
Risk levels:
Risk Level 1: Herds that have had three or more clear consecutive annual herd tests
Risk Level 2: Herds that have had one or two clear consecutive annual herd tests
Risk Level 3: Herds where less than 3% of the eligible animals were positive
Risk Level 4: Herds where more than 3% of the eligible animals were positive
Risk Level 5: Herds that are not complying with the CHeCS rules for the Johne’s programme