BSE was first recognised as a separate disease entity in the UK in 1986. The disease agent is believed to be a scrapie-like agent (an infectious protein) that causes destruction of the grey matter in brain, leading to a progressive disease. The main clinical symptoms are hypersensitivity to external stimulus, lack of coordination, wasting and death. The disease has a long incubation period, from 2 to 8 years, and is mostly seen in adult cattle (youngest diagnosed animal 20 months). Stress appears to induce the symptoms in cattle, and many cases are diagnosed around calving and after transport.
It is currently believed that BSE originated from either sheep or cattle infected with a scrapie-like agent. Meat and bone meal made from infected animals was recycled back to susceptible cattle in feed, and infection followed. Changes in the processing of meat and bone meal in the early 1980s is believed to have allowed an increased infectivity build-up in the meal (Bradley, 2000). There is some evidence of maternal transmission of BSE to offspring, but it is suggested that this form of disease spread is not significant (Wilesmith et al., 1997). Other theories about the origin of BSE have been developed, suggesting that factors like organophosphorous dipping rendered some animals more susceptible to infection than others (Purdey, 1994).
The UK has been the main geographical focus of the disease epidemic, which is declining rapidly. Currently, the number of suspected cases per week is 55, whereas in 1993, at the peak of the epidemic, the weekly number of suspected cases was 1000. Almost 180,000 confirmed cases have been identified in the UK since 1986. The majority of these cases (81%) have occurred in dairy herds, whilst only 11% of cases have been reported in suckler herds. There is also a clear geographical distribution within the UK, where the majority of cases have been diagnosed in the south of England and Wales. (Data from a MAFF web site, 28/04/00).
In recent years, some other European countries have experienced an increase in BSE cases, with Switzerland and Ireland leading in the number of identified cases (323 and 424, respectively).
Natural occurrence of BSE in other animal species has been confirmed in several captive ruminant and feline wild animals. A closely similar agent has been found causing spongiform encephalopathy in domestic cats in the UK. The same biological strain type of BSE agents as found in cattle has been identified in three human patients suffering from the new variant CJD in the UK (Bruce et al., 1997).
BSE is a notifiable disease and any suspicion of the disease on the farm should be reported to the nearest State Veterinary Services office immediately. Suspected animals are either put under restriction orders and observed or destroyed immediately. Due to concerns over the link between BSE and new variant CJD, no meat from cattle over 30 months of age enters the human food chain in the UK at the moment. These animals are slaughtered through a specific slaughter scheme and the meat, bones and offal are processed into meat and bone meal and destroyed. Only cattle from the UK beef assurance scheme can be slaughtered for human consumption at over 30 months of age (up to 40 months).
The UKROFS standards (UKROFS) prohibit the purchase of cattle into an organic herd from herds that have had BSE within the past six years. All cohort and offspring of BSE cases on organic units must be removed. The Soil Association further defines that if an animal has been bought since 31 December 1993 from a herd with an unknown BSE histroy, it must be removed.