Bovine Spongiform Encephalopathy

Cattle Diseases

Bovine Spongiform Encephalopathy in Cattle

Also known as: BSE, Mad Cow Disease

Bovine Spongiform Encephalopathy (BSE), was first recognised as a separate disease entity in 1986. The disease agent, a prion, 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 crucial event in the pathogenesis of prion diseases is the conversion of the normally occurring cellular prion protein (PrPc) into a pathogenic form, called protease-resistant PrP (PrPres) or scrapie PrP (PrPsc) (Caramelli et al., 2006).

The clinical signs of BSE are highly varied and diagnosis may not always be straightforward (Konold et al., 2004; Saegerman et al., 2004). Most cattle with BSE show a gradual development of symptoms over a period of several weeks or even months, although some can deteriorate very rapidly. Only a small proportion of affected cattle show what would be considered typical “mad cow” signs. Most suspects show several (but not all) of the following if they are observed closely enough:

• apprehensiveness
• nervousness
• reluctance to cross concrete, turn corners, enter yards, go through doorways or permit milking
• occasional aggression directed at other cattle or humans
• manic kicking when milked
• head shyness, with head held low
• over-reaction to external stimuli
• high stepping gait, particularly hind legs
• difficulties in rising
• skin tremors
• loss of condition, weight or milk yield
• bradycardia

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.

Epidemiology of BSE

Eighty-one percent of BSE cases in the UK were detected in dairy herds.

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 are 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). Although the theory of BSE originating from contaminated meat and bone meal is the most widely accepted, other theories about the origin of BSE have been developed. These include suggesting that factors including organophosphorous dipping and an excess of manganese rendered some animals more susceptible to infection than others (Purdey, 1994; Purdey, 2000) or that autoimmune disease may be responsible (Ebringer et al., 2005).

Following first recognition of BSE, the disease epidemic in the UK grew considerably and affected all parts of the country but to different degrees. The majority of cases were diagnosed in the south of England and Wales, which corresponds to a greater number of dairy cows in these areas. It reached its peak in 1992, when 36,680 cases were confirmed, and since then has shown a steady decline. In 2005, the total number of confirmed BSE cases was 164. This includes surveillance which is both passive (suspect cases on farm) and active (testing of fallen stock and animals entering the food chain). Almost 181,000 confirmed cases have been identified in the UK since 1986. The majority of these cases (81%) occurred in dairy herds, whilst only 12% of cases have been reported in suckler herds. (Disease incidence data available at DEFRA web site (BSE Incidence Statistics)).

The UK has been the main geographical focus of the disease epidemic, although in recent years, other countries, mainly in Europe have experienced BSE cases, particularly Ireland, France and Portugal. In 2012 three cases were reported in the UK (OIE BSE Stats, BSE Cases Worldwide). Up to 2012, BSE surveillance has identified 23 cases in North America: 4 BSE cases in the United States and 19 in Canada. Of the 4 cases identified in the United States, one was born in Canada; of the 19 cases identified in Canada, one was imported from the United Kingdom (www.cdc.gov/ncidod/dvrd/bse).

Occurrence of transmissible spongiform encephalopathies (TSEs) in other animal species includes Scrapie in sheep and Chronic Wasting Disease (CWD) in elk in North America (Miller and Williams, 2004). TSEs have also been confirmed in several captive ruminant and feline wild animals as well as domestic cats in the UK (Other TSEs).

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). Therefore, BSE is considered by some as a potential zoonosis. As of December 2005, there have been 183 definite or probable cases of vCJD reported worldwide (Caramelli et al., 2006).

In the UK, the Soil Association banned the feeding of animal proteins to cattle and sheep in 1983, therefore, no cattle which have been managed organically under Soil Association regulations since that time, will have received any meat or bone meal in their feed. There have been no recorded cases of BSE in any herd which has managed fully organically since before 1985 and where no animals have been brought in from outside sources consequently.

There have been cases of BSE in herds which have recently converted to an organic system. These are thought to have occurred when the animals were exposed to contaminated feed before entering an organic feeding regime. However, a fundamental part of organic standards prohibits any animal not born and reared on an organic farm from ever being sold as organic meat. Only the offspring of such animals can eventually gain organic status.

Detection of BSE and Protection of the Food Chain

BSE is a notifiable disease and in the UK any suspicion of the disease on a farm should be reported to the nearest Animal Health Veterinary Laboratories Agency (AHVLA) office immediately (either the Divisional Veterinary Manager of DEFRA or the Department of Agriculture and Rural Development in Northern Ireland). A Departmental Veterinary Officer will visit the farm or other premises to examine the animal. If the animal is suspected of having BSE its movements will be restricted. An order will be served on the owner of the animal prohibiting the movement of the animal and the sale of its milk. If necessary, further visits will be made until a diagnosis can be made based on clinical signs. When the veterinary officer believes the animal is suffering from BSE it is then slaughtered. The carcase is removed and disposed of by incineration following removal of brain tissue for definitive diagnosis. Compensation payable for an animal slaughtered because it is suspected of being affected by BSE is an amount equal to the market value of the animal up to a maximum value.

To minimise the risk of BSE entering the food chain, certain tissues from ruminants slaughtered for human consumption have been banned in the UK, and the EU.

Due to concerns over the link between BSE and new variant CJD, no meat from cattle over 30 months (OTM) of age was allowed to enter the human food chain in the UK between 1996 and 2005. An OTM scheme operated which compensated farmers for the value of the animal when killed, provided it would have been fit for human consumption. However, currently, with a decrease in the incidence of BSE and the availability of BSE testing of OTM cattle born after July 1996, a new system was introduced now which allows OTM animals which are otherwise fit for human consumption to enter the food chain, subject to a negative BSE test result. Cattle born or reared in the UK before 1 August 1996 remain excluded from the food chain.

Cattle born before 1st August 1996 had to be disposed of on the Older Cattle Disposal Scheme (OCDS). The OCDS was an exceptional market support measure providing for disposal of and compensation for cattle born before 1 August 1996. It started on 23 January 2006 and ended on 31 December 2008. Cattle born before 1 August 1996 cannot be slaughtered for human consumption and consignment of these animals to a fresh meat slaughterhouse was an offence under new legislation. The OCDS only accepted animals, including on farm casualties that are born before 1 August 1996 and would otherwise have been eligible for the food chain. Compensation rates were paid on a flat rate per animal for each of the 3 years of operation.

Since the OCDS scheme ended the only means of disposing of animals born before 1st August 1996 is via the Fallen Stock route. These animals are not eligible for the food chain and costs of the disposal rests with the producer. All fallen (dead) stock aged over 48 months must be tested for BSE.

The Soil Association further defines that if an animal has been bought since 31 December 1993 from a herd with an unknown BSE history, it must be removed. The SA standards contain a detailed code of practice for cattle farmers to follow in cases where BSE is suspected or identified. Any offspring from the case are removed from the herd and cannot be sold as organic. This eliminates the possibility of any animal being sold as organic if there is even the remotest likelihood of BSE passing from cow to calf.

Control and Prevention of BSE

Over and above the statutory BSE control measures set out below, the following practices are key:

• Cattle should not be sourced from herds that have had BSE in the last six years;
• Follow good practice with regard to biosecurity, especially the establishment and maintenance of a closed herd policy;
• Feed home-grown feeds, but when necessary source bought-in feed from a reliable and reputable source; and
• Report suspected cases to the authorities immediately.

The control of BSE in the UK is based on statutory control measures that are mainly aimed at preventing further spread of infectious materials in the feed chain. See details at Feed regulations.

The following measures are implemented at the moment:

• Ban on feeding ruminants with all forms of mammalian protein (with specific exceptions, such as calf milk-replacer) since November 1994
• Ban on all other processed animal proteins, including fishmeal to ruminants
• Ban on the use of mammalian meat and bone meal in the feed of any food animal species including fish and horses, since April 1996
• Rendering processes recognised as ineffective in inactivating BSE agent are banned
• Restricted animal protein use only allowed in non-ruminant species
• To prevent the potential effect of maternal transmission in the UK cattle herd, a culling programme of age cohorts of confirmed cases from a certain period was carried out in 1997-1998
• To minimise the possible risk to the human food chain, certain tissues from ruminants slaughtered for human consumption are banned from entry to the food chain. This Specified Risk Material (SRM) includes those tissues of cattle, sheep and goats which are known to, or might potentially, harbour detectable BSE infectivity in infected animals, such as the brain and spinal cord from bovines over six months of age. The tissues which fall within the current definition of SRM are listed on the Food Standards Agency website.

Treating BSE

There is no treatment for BSE. Suspected cases should be reported to the AHVLA immediately.

BSE and Welfare

As BSE is an extremely distressing condition for cattle, suspected cases should be reported to the authorities immediately.