Johne's Disease

Johne’s disease is on the increase. This is caused by a bacterium that is very similar to TB. As with TB it is not practically treatable in cattle and causes weight loss and scour. Cows can also present with oedema, particularly under the chin. The normal pattern of infection is that carrier animals infect calves either via faeces or milk (calves are the most susceptible animals, particularly less than a month old). 1g of faeces from an infected cow can contain 5 million organisms and less than 1000 of these are needed to infect a young calf. The incubation period is from 1 to 15 years, but more typically around 3-6 years. This means that after infection the animal looks perfectly normal for the early years of its life and indeed may never develop the disease. For a few months before it comes down with clinical disease it is possible to detect carriers by blood sampling and this is used to aid eradication. Unfortunately carriers will test negative until this point when blood sampled. Once they start to scour it is often possible to find the bacteria by looking at a stained smear of their faeces. At all times culture of the faeces can be attempted. This takes at least 6 weeks and costs apx £25 so is not often undertaken.
The increase in Johne’s is probably down to a number of factors. Stocking densities have increased on farms and multiple calving yards are more common. Allied to this many farmers stopped feeding milk powder choosing instead to feed cows’ milk back to calves. Both of these practices increase the potential for a carrier to infect a number of calves/replacements. For the beef calves this is not really a problem as they are not destined to live long enough to come down with the disease. Replacement heifers are the problem as they commonly come down with the disease in their first or second lactation, resulting in huge losses. The bug can be passed across the placenta which means the offspring of infected cattle are extremely likely to be infected (also via the colostrums).
In America, where there is currently a big push on controlling Johne’s disease, they go as far as pasteurising the milk before feeding it to calves. Most control programs involve removing the daughters of affected cows as they are normally quite likely to be carriers themselves. Control measures that are practical include removing replacement calves from cows at birth and allowing them access to only their own mother’s colostrum. Also screen incoming stock, particularly beef bulls, for Johne’s. Maintain good hygiene with anything involved with feeding or water administration to cows and calves so that carrier animals do not infect other animals via their faeces. Cull infected animals as soon as possible and consider culling their offspring.
Johne’s monitored herds do exist and this is becoming particularly common in the beef industry as the other control methods are difficult to apply to beef systems. Monitoring involves annual testing of the older portion of the herd to identify carriers and remove them. The SAC monitoring scheme also involves culling of the offspring of carriers. When sourcing animals, particularly bulls you should enquire about the Johne’s status of the herd and consider testing. Often you will be met by a blank look or a denial of any knowledge of the disease but this is normally ignorance rather than awareness. If in doubt consult your incoming stock policy in your Health Plan or contact the vets.
Vaccination is possible but is quite expensive and involves injecting replacement heifer calves less than 1 month old. This is only possible after agreement from DEFRA.