Bovine Spongiform Encephalopathy in Cattle

Cattle Diseases

Bovine Spongiform Encephalopathy in Cattle

Also known as: BSE, Mad Cow Disease

Bovine Spongiform Encephalopathy (BSE), was first recognized 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 pathenogenesis 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). BSE is USDA reportable disease.

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
• overreaction 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. As a result there have been no recorded cases of BSE in any herd which has been managed fully organically since before 1985 and where no animals have been brought in from outside sources.

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.

Control and Prevention of BSE

• 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.

Treating BSE

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

BSE and Welfare

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