The accuracy of the bovine TB skin test has been questioned after a cow that tested negative for five years was found to be riddled with the disease.

The cow was part of the Gelli Aur College Farm in Carmarthenshire, which has been under TB restrictions since 2007 and lost 300 cattle to the disease.

Farm manager John Owen said the level of infection in the fifth-lactation animal only came to light when it was slaughtered.

“We culled her at the end of her productive life and at slaughter she was found to be riddled with TB,” said Mr Owen. “The level of infection in that cow suggests she had been infected for years.”

He is concerned that the homebred cow may have been at the root of successive test failures.

“The cow kept passing the test, there was nothing to suggest that she was infected,” he said. “It is a concern that she may have been spreading the disease to other animals within the herd.”

The farm has just recorded its first clear test and will have its next test later this month. “We are keeping our fingers crossed that we will get the all-clear,” said Mr Owen.

He admitted that six years of movement restrictions had affected the farm business significantly and had pushed the stocking rate to its upper limit.

Heifers will in future be contract reared off farm to reduce both the 2.8lu/ha stocking rate and reliance on purchased feed, he said.

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