JOHNES – A SECRET RISK
By John Burns
TRUE incidence of Johnes disease in UK sheep is not known. Records at MAFF Veterinary Investiga-tion Centres suggest it is 10 times more common in cattle than in sheep.
But no one puts much weight on those statistics, because it is accepted few sheep farmers would spend the minimum of £25 to have a post-mortem examination done on the occasional ewe which goes thin for no apparent reason.
Mike Cranwell, based at Devons Starcross branch of the Veterinary Laboratory Agency (formerly VICs), has a special interest in this disease. He says typical symptoms would be ill thrift, weight loss, open fleece and occasional diarrhoea. (In cattle, bubbly, smelly, thin dung is a characteristic symptom.)
His "gut feeling" is that many farms have an occasional clinical case of Johnes in sheep. But there are documented instances where farms have had significant losses of up to 9% a year through it.
An example described in detail by Mr Cranwell in the Veterinary Record (Aug 28, 1993) concerned a Devon flock which he began investigating in 1984.
The farmer had become concerned about the increasing number of ewes, mainly three- and four-year olds, which lost condition over a period of weeks or months and did not respond to worming or trace elements. Most of the cases occurred close to lambing (April) or weaning (July).
Diarrhoea was noted in many animals, though it was sometimes intermittent, and the farmer had noticed that a typical sign was the inside of the ewes hocks caked with dung. Often the fleeces were broken and the wool came away in the hand.
He took two live ewes (three-year-olds) showing typical symptoms to the then VIC at Starcross. Blood tests showed low protein, low albumen and slight anaemia. There was no evidence of deficiency of selenium, copper or cobalt.
Detailed post-mortem examinations confirmed Johnes. In the year this investigation began, 1994, 9% of that flock was culled prematurely. In the three years since 1981 the disease had cost them over 50 ewes.
Mr Cranwell introduced an experimental vaccination programme at that farm. It involved giving half a cattle-dose of Weybridge vaccine against Johnes to each ewe lamb likely to be kept for breeding.
All suspect ewes and their latest offspring were culled. In that flock clinical cases of Johnes had almost ceased three years after the vaccination programme started in 1985, though vaccinations continued until 1990.
The farmer reported that four vaccinated ewes (out of 580 lambs vaccinated) had died of Johnes, though they were not examined by the VIC. It seems likely vaccinations were effective, though Mr Cranwell points out the farmer had, at the same time, stopped keeping purebred replacements. Instead, he kept Suffolk-crosses from them, which, he had observed, did not appear so susceptible to the disease. Thus it is possible that a breed effect helped the vaccine.
An alternative way of clearing Johnes is to slaughter the farms sheep population and leave the whole farm clear of sheep or cattle for at least a year.
Mr Cranwell suggests farmers who suspect cases of Johnes in their flocks should first have dung samples examined by the VLA. The dung sample test is about 50% accurate. It would also be checked for worms, fluke and salmonella. Blood tests are currently being assessed. At present they are only considered "helpful", says Mr Cranwell. "A positive blood test result is a good indication."
Test costs at VLAs: Post-mortem £25 (plus cost of transporting sheep to VLA). Blood tests and dung samples about £8 each (plus costs of own vet taking samples and transport to VLA). Vaccine £6 a lamb. *