OLD feed in a silo is believed to be the most likely cause of a case of classical bovine spongiform encephalopathy (BSE) on a Somerset farm.

The Chief Veterinary Officer and Animal and Plant Health Agency (APHA) confirmed the case of BSE on September 17 this year - the first in the UK since 2018 and the first in England and Wales for six years.

A report published by APHA last Thursday (December 16) says that the animal was euthanised on the small-medium mixed dairy and beef farm.

It was tested for BSE following its death as part of routine controls and it was not destined for the food chain, so there has been no risk to food safety or public health.

A further 70 cows on the farm were also put down, although subsequent tests on them for BSE proved negative.

Investigations have concluded that the most likely source of the BSE was residual material in a pre-1996 silo, which had been in use before the introduction of the reinforced feed ban.

The silo could have contained small amounts of feed produced before that ban that had been caught up in the internal structure, which became dislodged in 2015 or 2016 and entered the cattle rearer ration, and was subsequently fed to the cow.

The case was a homebred dairy cow born in February 2015 that had calved four times.

Clinical signs were first noted on September 1 this year when milk fever (hypocalcaemia) was suspected.

But she was unresponsive to treatment and as a result was euthanised on the farm the following day.

Following a positive result in a BSE test, her youngest two calves, by then part of a different Somerset herd, were also culled and tested for BSE, as per TSE1 legislation. Both tested negative for the disease.

Investigations also identified a total of 68 cohort animals that were still alive and that were born or reared with the infected cow during the relevant risk period of 12 months either side of the date of birth of this case.

All those cohort animals were also culled and all subsequently tested negative for BSE.

The affected farm had previous confirmed BSE cases, all of which were born before the reinforced feed ban.

The report concludes that it is likely that these cases were fed feed stored on-farm in silos.