BSE

I

Introduction

BSE (BSE), degenerative brain disorder of cattle, popularly known as “mad cow disease”, which was first identified in the United Kingdom in November 1986. It is one of a group of diseases known as transmissible spongiform encephalopathies, which at post-mortem examination show holes in the brain tissue that give it a spongy (spongiform) texture. Similar spongiform diseases have been identified in humans (Creutzfeldt-Jakob disease or CJD) since 1920, in sheep (scrapie) for over 250 years, in cats (feline spongiform encephalopathy) since 1990, and also in humans, in variant CJD (vCJD), since 1996.

The causative agent is a small proteinaceous particle called a prion, which is transmissible, but does not contain deoxyribonucleic acids (DNA). Prions are usually resistant to conventional decontamination agents and can survive in the soil for several years. They have not been visualized, and have not been shown to contain a nucleic acid; also, they are assayed for infectivity by mouse inoculation by the intra-cerebral or parenteral route. The PrP scrapie (PrP^sc) is an isoform that is more proteinase-k-resistant than PrP^c, and self aggregates into fibrils that appear as amyloid rods in infected brains. Despite the debate on whether prions contain a nucleic acid being unresolved, different strains of scrapie, for example, exist, with more than 20 strains recognized, which can be passed separately in identical mice as if they have some genomic material. However, infectivity is not destroyed by ultraviolet light or nucleases that destroy nucleic acid.

The first sign in an animal with BSE is often a change in behaviour, with other symptoms including a loss of coordination and a typical staggering gait developing over several weeks. Affected animals also show signs of senility, for example, as well as a lack of interest in their surroundings, the abandonment of routine habits, a lack of interest in feed and water, and unpredictable behaviour including kicking during milking. There is a progressive weakness leading to recumbency. In the UK the youngest animal diagnosed with BSE was 21 months of age, and the oldest over 19 years. The majority of cases are around five to ten years old, with a peak at around five years.

At the beginning of February 2006, about 184,000 cases of BSE had been recorded in the United Kingdom. Worldwide, about 5,500 cases of BSE had been reported in 23 other countries including, in the European Union (EU), Austria, Belgium, Czech Republic, Denmark, Finland, France, Germany, Greece, Ireland, Italy, Luxembourg, the Netherlands, Poland, Portugal, Slovakia, Slovenia, and Spain, as well as Canada, Israel, Japan, Liechtenstein, Switzerland, and the United States.

II

BSE and CJD

Until 1996 many scientists were insistent that BSE could not be transferred to humans through milk or meat. However, on March 20, 1996, the UK government’s independent Spongiform Encephalopathy Advisory Committee (SEAC) reported that the consumption of BSE-infected manufactured beef products (essentially those containing fragments of brain tissue or spinal cord) in the period before 1989 was the most likely scientific explanation at present for the appearance of a new variant of CJD (nvCJD, now known as vCJD) that had appeared to have afflicted ten people, all under the age of 42 years, in the previous three years.

At the same time, the SEAC confirmed that it had no proof of a direct connection between eating beef, BSE, and CJD, and advised that the substance of its statement was cautionary. However, by November 1996, when the number of confirmed cases of nvCJD had risen to 14 in the United Kingdom, with 1 case confirmed in France, SEAC acknowledged that scientific evidence of a direct connection between BSE and nvCJD was mounting—although this still fell short of incontrovertible proof.

BSE has been of national interest in the United Kingdom for some time. It was made a “notifiable disease” under the diseases of animals’ legislation introduced by the government in June 1988 to protect the cattle herd. Later in the same year the government also introduced the compulsory destruction of suspect animals and their carcasses. This was further extended to include a cull of offspring born after August 1, 1996, and to dams in which BSE was confirmed. In other countries, discovery of cattle with BSE has been dealt with by subsequent slaughter of the herd concerned.

III

Causes

Recycled, rendered waste products from portions of infected cow carcasses have been identified as the source of the epidemic. Until July 1988, when such products were banned from cattle feed, these were fed back to the UK dairy herd as a protein supplement. It was initially thought that BSE developed after cows absorbed proteins from scrapie-infected sheep’s brains that had been included in cattle feed, and that its spread was progressively accelerated by the ingestion of recycled brain tissue from cows that had themselves become infected.

However, the Philips Committee of Inquiry rejected the scrapie link. There were, however, changes in the system of rendering down abattoir by-products from 1979 onward, which meant that processed tissue was no longer exposed to either solvent chemicals or temperatures high enough to kill the BSE agent. Essential to this being the reason for the spread is an assumption that very low levels of BSE were already endemic in the UK cattle herd and possibly in many others too. For example, there are said to be many similarities between BSE-affected cows and those with Downer syndrome in the United States.

IV

Measures to Combat BSE

The first measure to combat BSE was a ban in July 1988 on the feeding of ruminant-derived protein to ruminants in the UK. This was followed by the removal at the abattoir, following slaughter, of specified bovine offal (SBO)—certain tissues considered to have high risk of containing the BSE agent. The SBO legislation was extended and now includes tissues from sheep and goats and additional tissues from cattle, and is called specified risk material (SRM). Following the introduction of the first measures there was a subsequent decline in the number of confirmed cases but the rate of decrease was slower than expected. Since then, the controls, including monitoring of compliance, on SRM and of animal feedstuff, has been extended and enhanced. The date when all controls were in place on animal feed is accepted as being August 1, 1996. The reduction in cases is recorded by the then Ministry of Agriculture, Fisheries, and Food (MAFF), now the Department for Environment, Food, and Rural Affairs (DEFRA), and published monthly. Since the peak of cases at 36,680 in 1992, there has been a fall in cases each year. (The numbers of cases also varied between counties within Britain.) For example, in 1995 there were 14,301 cases, with the national incidence having fallen from the 0.92 per cent of all adult cattle in 1992 to 0.33 per cent. This fall has continued, with, in 1996, some 8,013 cases recorded in Britain; by 2000 this had fallen to 1,311 cases, and in 2002 a total of 440 cases were confirmed.

Early predictions that the disease would be eliminated by 1999 were revised because of unsatisfactory handling at some abattoirs of SRM, and the poor controls on animal feed in the early 1990s. The SBO and SRM were stained with a heat-stable dye and conveyed for incineration under very strict rules to prevent the tissue being rendered, along with other offal and abattoir waste used as the protein-supplement meat and bone meal (MBM). The use of MBM was allowed to continue for pigs and poultry—a standard practice in many other livestock-rearing countries over the past century. However, a government survey in 1994 revealed that only half of SRM tissue was being destroyed, and that the balance was still being included in the animal-feed rendering process.

New rules in addition to the existing requirements for the separation and dyeing of all prescribed tissue, backed with strict enforcement measures, were then introduced. This was considered essential, as it was known that the accidental ingestion of as little as one gram of suspect brain tissue could infect a cow or calf with BSE. The feeding of all forms of mammalian protein to ruminants was banned in November 1994. The feeding of any farmed livestock with mammalian MBM was made illegal on April 4, 1996, with a feed recall scheme to be completed by August 1, 1996.

To reinforce these regulations, the possession of MBM on farms or animal feed mills was made illegal from August 1, 1996. As a result, it is expected that animals born after that date cannot have been exposed to the disease through contaminated foodstuffs. Scientists have still to prove that other routes of infection, such as maternal transmission through blood, milk, or amniotic fluid, exist. Some scientists suspect there may be a small degree of maternal transmission, but there is also evidence that animals can inherit a genetic susceptibility to BSE.

If exposure through feedstuffs is the only means of infection it was anticipated that the disease, having been cut off at source, would effectively become extinct by about 2006. This British diagnosis was upheld by the World Health Organization, the European Commission’s Scientific Veterinary Committee, and the world control body, the Fédération Internationale des Épizooties.