A daunting fear: BSE may be endemic in sheep

The possibility of "mad cow" disease infecting the national sheep flock was always considered the nightmare scenario that would deal a knock-out blow to an industry already reeling from one crisis to another.

The possibility of "mad cow" disease infecting the national sheep flock was always considered the nightmare scenario that would deal a knock-out blow to an industry already reeling from one crisis to another.

Preliminary findings of an experiment to see whether bovine spongiform encephalopathy (BSE) unwittingly infected sheep in the early 1990s now appear to suggest that the possibility is real. But questions remain about how strong this evidence is, and what further action has to be taken

When Stephen Dorrell announced as Health Secretary in March 1996 that a dozen people were likely to have been infected with BSE, the Government's Scientific Advisory Committee on Spongiform Encephalopathy (Seac) warned him that the most likely source of the infection was contaminated beef.

However, Seac was soon to consider another possibility: that sheep had also become infected with BSE by eating contaminated feed. It meant that people who had eaten lamb might also be at risk of developing the human form of BSE, the variant form of Creutzfeldt-Jakob disease (vCJD).

Although there was no hard evidence of transmission of BSE from cattle to sheep, it was always talked about as a "theoretical" risk – there was strong circumstantial evidence to point in that direction.

Firstly, it was known that many tons of meat-and-bonemeal feed contaminated with BSE from rendered cattle carcasses had been fed to sheep in the late 1980s.

The theoretical risk became more real following experiments attempting to transmit BSE into sheep under laboratory conditions. These experiments clearly demonstrated that sheep can easily develop the bovine brain disease after eating relatively small amounts of food contaminated with BSE.

If BSE had infected sheep grazing in the countryside, it raised important questions, such as whether the disease was acting any differently to BSE in cattle, how to assess its prevalence and, most importantly, what further measures needed to be taken to protect the human food chain.

Detection has proved difficult because there is no simple blood test for BSE or indeed scrapie, the "natural" brain disease of sheep that has been endemic to the national flock for at least 300 years without any signs that it has affected humans. One fear is that scrapie may be masking genuine cases of BSE in sheep because the two diseases have similar clinical symptoms.

If BSE has infected sheep there are other concerns that could affect existing measures to protect humans. One of the principle barriers protecting people from BSE is the specified bovine offal ban, which ensures that the most infective parts of cattle are removed.

A similar sheep offal ban exists to address the theoretical risk of BSE in sheep but this is less extensive than with cattle. Yet the laboratory experiments show that when sheep become infected with BSE, the infective agent is more widespread in the carcass. Some scientists believe this suggests that the sheep offal ban must be strengthened. Sir John Krebs, the head of the Food Standards Agency, admitted as much when he said that if BSE was ever shown to be present in sheep, then stronger measures would have to be taken to protect human health.

If the early results confirm that BSE has infected sheep, it would also mean there is a possibility that the disease is being transmitted from mother to lamb, much like scrapie. It raises the daunting possibility that BSE, like scrapie, might have become endemic within the national flock.

A truly nightmarish scenario now looks more real than at any time before.