CHLAMYDIA LAYS FALSE TEST TRAIL
CHLAMYDIA LAYS FALSE TEST TRAIL
STRAINS of chlamydia which can infect eyes, joints, and the gut of sheep are causing false positive results in routine tests for enzootic abortion.
The difficulty affects only about 1% of flocks in the Scottish sheep health scheme run by the Scottish Agricultural College. But the loss of reputation, as well as cash, can be serious, and this years round of ewe lamb sales has given clear evidence that non-accredited sheep are not wanted.
At the biggest of the Scotch Mule sales, at Castle Douglas in August, it was not so much a case of a premium for accredited stock as a discount of at least £15 a head for those without that status.
That was exactly the penalty Ayrshire producer Allan Grierson suffered last year after a false positive result meant he had to sell in the non-accredited section of the Ayr Mule sale.
"We have never had abortion on this farm, but after our so-called breakdown in 1996 our prices at Ayr fell by £15 a head. We had every ewe on the place tested and butchered the handful that were in any way doubtful.
"The college people knew we had never had abortion in the ewes, we regained our accredited status, and our prices were up £16 at the Ayr sale this year," says Mr Grierson, Laight Farm, Dalmellington.
His case – one of four – is the only one which is still puzzling scientists at SAC and the Moredun Research Institute, Edinburgh, explains Gareth Jones, who heads the chlamydia research group at Moredun.
"We have carried out an in-depth study of the four cases where false positive results were suspected. In two cases we found the abortion agent, in the third, the positive reaction to the test came from a form of chlamydia which causes conjunctivitis but not abortion. The fourth case was the one in Ayrshire where we have not yet found the cause of the false test result," says Dr Jones.
Dr Jones recently completed an EU and Scottish Office-funded project which compared various tests for Chlamydia psittaci – the agent causing enzootic abortion in sheep. He is confident that the testing procedures available can give accurate diagnosis of Chlamydia pecorum – the strains which may have caused false positives.
"We are confident that the strain which infects the gut of sheep is not clouding enzootic abortion testing. But the pecorum strain, which causes both conjunctivitis and a form of arthritis, can give a positive result when testing for abortion. This may be largely responsible for the 1% of false positives the scheme has been facing," says Dr Jones.
However, he admits that he is still puzzled by the Laight Farm case, and says it is to be investigated in-depth again from tupping time right through to lambing. *
False positive results in routine enzootic abortion tests on Allan Griersons Laight Farm cost £15/lamb in reduced sale prices last year, and are still being investigated by SAC and Edinburghs Moredun Research Institute.
FALSE TEST RESULTS
• One strain of chlamydia giving false positives.
• Current tests sufficient.
• One case for further investigation.