14 June 2002

Rid Johnes and cut culls

By Richard Allison

ERADICATING Johnes disease from cattle herds by following proposed government guidelines will reduce culling rates, offering a £26/cow saving to dairy producers within five years.

Draft guidelines, currently under consultation, aim to provide advice on controlling Johnes disease in dairy herds, says DEFRA. This follows a reported increase in outbreaks over the last 10 years (Livestock, Apr 26).

While there is no proven link between Crohns disease in people and mycobacterium paratuberculosis, which causes Johnes disease, its levels in milk should be reduced, says the report.

Following these guidelines will help producers cut costs of culling animals, says SAC Premium Cattle Health Scheme manager George Caldow. "According to a computer model, Johnes disease is costing a 100-cow dairy herd about £2600/year. But in many cases the cost is higher."

Many infected cows are culled for poor milk yields, an early symptom of Johnes disease. Their owners may not realise they have infected animals in herds, as they are being culled before showing clinical symptoms, such as diarrhoea and weight loss.

One solution is to test herds and cull infected animals on an annual basis. This will reduce infection, but Mr Caldow warns that it will take 5-10 years to eradicate Johnes from a herd where it is well established.

However, Liverpool Vet School researcher Kenton Morgan is sceptical that the disease can be eradicated from dairy and beef herds. "Current tests are not reliable enough to identify all infected animals."

Infected cows dont always shed the bacterium in faeces. "How do you test an animal if the bacteria are not present in every dung pat? In addition, the bug responsible for Johnes disease is difficult to culture, taking about 14 weeks to grow."

Prof Morgan believes one future method is to screen herds by testing milk for the mycobacterium. "A milk-based test presents far fewer problems than testing blood or faeces."

Despite blood tests only picking up half of sub-clinical cases, its better than nothing, says Leics-based vet Peter Orpin. "When infected cows have been identified, a control strategy can be implemented to reduce spread from those carriers not identified. These carrier cows are the greatest source of infection and operating a closed herd is the best strategy to reduce risk.

But Prof Morgan believes Johnes disease is one of the few diseases where having a closed herd can increase spread when infected animals pass it on to replacement animals. "One solution is to sell all heifers and buy in replacements from known Johnes disease-free herds."

One source of cattle is herds within the SAC Premium Cattle Health Scheme, says Mr Caldow. "However, the scheme is facing a shortage of herds."

For producers rearing their own replacements, good calf hygiene is essential as the bacteria can be passed via faeces on teats. Cows should be calved down in a clean area and calves reared in a separate area away from adult stock.

"Pooling colostrum and feeding waste milk to calves is the most efficient way to spread Johnes disease to calves. About 25-30% of infection occurs at this early stage of life," says Mr Caldow.

Having a prevention strategy is important for herds taking in animals from several sources, says Mr Orpin. "Many animals could become infected leading to 3-4% of adult animals being lost each year." While the control measures seem expensive, spending cash to prevent the disease becoming established in a herd is a good investment, he adds. &#42