The mad cow crisis is a health and socio-economic crisis characterized by the collapse of beef consumption in the 1990s, as consumers became concerned about the transmission of bovine spongiform encephalopathy (BSE) to humans through the ingestion of this type of meat.
Background BSE is a degenerative infection of the central nervous system in cattle. It is a fatal disease, similar to scrapie in sheep and goats, caused by a prion. A major epizootic affected the UK, and to a lesser extent a number of other countries, between 1986 and the 2000s, infecting more than 190,000 animals, not counting those that remained undiagnosed.
The epidemic is thought to have originated in the feeding of cattle with meat and bone meal, obtained from the uneaten parts of cattle and animal cadavers. The epidemic took a particular turn when scientists realized in 1996 that the disease could be transmitted to humans through the consumption of meat products.
As of 24 January 2017, the disease had claimed 223 human victims worldwide (including 177 in the UK and 27 in France) affected by symptoms similar to Creutzfeldt-Jakob disease, a disease of the same nature as BSE.
Communicated to the general public by the media, the crisis erupted in 1996. It involved both ethical aspects, with consumers becoming aware of certain practices that were common in livestock farming but of which they had been unaware, such as the use of meat and bone meal, and economic aspects, with the ensuing fall in beef consumption and the cost of the various measures adopted.
Various measures were taken to curb the epidemic and safeguard human health, including a ban on the use of meat and bone meal in cattle feed, the withdrawal from consumption of products considered to be at risk, and even of certain animals (animals over 30 months of age in the UK), screening for the disease in slaughterhouses, and the systematic slaughter of herds where a sick animal had been observed.
Nowadays, the epidemic is almost completely under control, despite 37 bovine cases still being diagnosed in the UK in 2008.
On 23 March 2016, a new case of mad cow disease was detected in France in the Ardennes department. This is the third isolated case of BSE of this type detected in Europe since 2015.
Epidemiology
The disease was first identified in the United Kingdom in 1986.
Symptoms
BSE affects the brain and spinal cord of cattle. It causes brain lesions characterized by spongy changes visible under the light microscope, corresponding to vacuolated neurons. Neurons are lost to varying degrees, and astrocytes and microglia (brain cells with an immune function) multiply. Pathogens accumulate to form characteristic amyloid plaques, though these are less prevalent than in other transmissible spongiform encephalopathies.
External symptoms generally appear 4 to 5 years after contamination, and always in animals over 2 years of age (generally between 3 and 7 years). Initially, they are manifested by a change in the animal's behavior (nervousness and aggressiveness), which may sometimes kick, show apprehension and hypersensitivity to external stimuli (noise, touch, dazzle), and isolate itself from the rest of the herd. Milk production and weight generally decrease, while appetite remains unchanged. Progression can last from a week to a year, with the different phases of the disease varying in duration from one animal to another. In the final stage of evolution, the animal has real locomotion problems. They frequently lose their balance, sometimes unable to stand up again.
Physiologically, the animal is tachycardic and feverless. However, the appearance of these symptoms is not a sure sign of BSE. In fact, locomotor disorders such as grass tetany are common in cattle, making diagnosis of the disease difficult.
Nature of the pathogen The nature of the infectious agent remains debated. The leading theory is that a protein folds abnormally and becomes a prion, capable of causing other proteins to also fold incorrectly. A resistant form of the prion (PrPSc, also known as PrPRes, PrPD, or PrPBSE) accumulates in the brain, eventually leading to neuronal death and the formation of amyloid plaques.
As a protein, the prion has no metabolism of its own, and is therefore resistant to freezing, desiccation and heat at normal cooking temperatures, even those reached for pasteurization and sterilization. To be destroyed, the prion must be heated to 133 °C for 20 minutes at 3 bars of pressure.
Origins of the epizootic The origins of the BSE pathogen are uncertain. Two hypotheses are widely supported. The first is that the disease originated through interspecific contamination from a closely related disease, scrapie, which affects sheep and goats. The possibility of interspecific transmission of scrapie has been proven experimentally, but the clinical and neuropathological disorders associated with the disease differ from those of bovine spongiform encephalopathy. This observation led to the formulation of a second hypothesis, according to which the disease is endemic to the bovine species, and very thinly spread before it was amplified in the mid-1980s. The description in an 1883 veterinary journal of a case of scrapie in a bovine is an argument used by advocates of this second theory, although this case may correspond to a completely different neurological disease. Numerous other more or less credible theories regularly enter the debate, with none really emerging.
The mad cow disease epidemic has its roots in the recycling of animal cadavers by knackers. Bone and meat parts not used in human food and farm animals that have died from illness or injury are collected for use in cattle feed, and are cooked to separate the fats before being ground into powder.
Before the outbreak of BSE, meat and bone meal was widely used in cattle feed. They are rich in both energy and protein, and are easily digested by ruminants. As a result, they were widely used in cattle, particularly dairy cows. It was the consumption by cattle of meat and bone meal made from calcined tissues from cattle or sheep (depending on the hypothesis adopted), such as brain and spinal cord, and contaminated with the BSE agent, that was responsible for the outbreak of the epidemic.
Initially, the flour manufacturing process used high sterilization temperatures and a fat extraction stage using organic solvents which, without anyone suspecting it, destroyed the prion. But in the mid-1970s, a number of British technicians decided to lower the temperature and drying time of these flours to improve the nutritional quality of the finished product. Due to environmental considerations (hexane discharge into the environment), the solvent fat extraction stage was eliminated in 1981. These changes to the protocol were essentially designed to improve economic profitability, on the one hand by better preserving the proteins contained in the flours, and on the other by reducing the costs of the solvents and energy used, which had risen sharply after the two oil shocks of 1973 and 1979.
In addition, the change in manufacturing process was accelerated by an accident at one of England's main meat and bone meal plants, involving staff handling the solvent: this led to a reinforcement of safety measures, the cost of which was high. This change in practices seems to be at the root of the epidemic. The prion was recycled in meat and bone meal before it was distributed on a large scale in cattle feed, and contaminated animals were, in turn, slaughtered and ground into meal to further amplify the phenomenon.
It is also suspected that there is a mother-calf contamination pathway, which could account for up to 10% of contaminations.
To explain the persistence of the epidemic after drastic measures had been taken, scientists are looking for a possible third route of contamination, which has still not been found. One of the few credible hypotheses is contamination by fodder mites, a phenomenon once observed in scrapie. This hypothesis, like all those involving an external transmission agent, is unlikely, as only the central nervous system is contaminated in cattle, and the prion is not excreted by sick cows. Other hypotheses have suggested contamination through water polluted by knackering plants, or through the soil where meat and bone meal-based fertilizers have been spread, without any tangible evidence.
Epizootic centred on the United Kingdom
Bovine spongiform encephalopathy first appeared in the UK in November 1986, when the British Central Veterinary Laboratory discovered a cow with atypical neurological symptoms on a farm in Surrey. Examination of the cow's nervous tissue revealed vacuolation of certain neurons, forming lesions characteristic of scrapie. Researchers concluded that a new form of Transmissible spongiform encephalopathy (TSE) was at work, infecting cattle. The result was bovine spongiform encephalopathy (BSE), also known as "mad cow" disease.
The number of cases, initially low at the end of 1986, increased rapidly. By mid-1988, there were 50 cases a week, rising to 80 cases a week by October of the same year. The weekly rate continues to rise, reaching 200 cases per week at the end of 1989, 300 in 1990, and peaking in 1992 and 1993 with over 700 new cases per week and 37,280 sick animals in 1992 alone. After 1993, the epidemic began to decline rapidly.
However, 20 years on, the disease has still not completely disappeared from the UK: 67 cases were recorded in 2007 and 37 in 2008. In all, no fewer than 184,588 animals contracted the disease in the UK.
The disease was exported from the United Kingdom from 1989 onwards, when 15 cases occurred in Ireland. Between 1989 and 2004, a total of 4,950 cases were reported outside of the United Kingdom, mainly in continental Europe:
Israel (one case in 2002) Canada (four cases, including one in 1993, two in 2003 and one in 2007) Japan (13 cases between 2002 and 2004) the United States (two cases between 2004 and 2005, and one case in 2006)
Apart from the United Kingdom, the countries most affected are: Ireland (1,488 cases) Portugal (954 cases) France (951 cases) Spain (538 cases) Switzerland (457 cases) Germany (369 cases)
In countries outside the UK, the highest number of cases was recorded in 2001 (1,013 cases) and 2002 (1,005 cases). In total, just over 190,000 animals were affected by the disease in April 2009.
Several hypotheses explain the spread of the disease outside the UK. Firstly, meat and bone meal manufactured in the UK and likely to be contaminated by the prion was exported worldwide. France, Ireland, Belgium, Germany, and later Denmark, Finland, Israel, Indonesia, India and, to a lesser extent, Iceland and Japan, all imported potentially contaminated British meat and bone meal. The export of live animals, possibly healthy carriers of the disease, is also a suspected source of contamination. These animals are then used in the manufacture of local meat and bone meal, generating further contamination.
In Canada, it only took one case in Alberta for the most important customers, the United States and Japan, to take severe boycott measures.
The direct incidence of this disease, despite its spectacular nature and the systematic elimination of any herd where a sick animal is diagnosed, has remained relatively low, even in the United Kingdom it has not exceeded 3% of the herd on an annual basis. The disease mainly affects dairy cows.
Interspecific transmission In humans
A form of transmissible spongiform encephalopathy specific to humans, known as Creutzfeldt-Jakob disease (CJD), is a degeneration of the central nervous system characterized by the accumulation of a prion. The incubation period lasts years, even decades, before balance and sensitivity disorders appear, followed by dementia. The outcome is systematically fatal within a year or so.