Bovine Spongiform Encephalopathy (BSE) is a transmissible, neuro-degenerative, fatal brain disease of cattle. The disease has a long incubation period of four to five years, but ultimately is fatal for cattle within weeks to months of its onset. BSE first came to the attention of the scientific community in November 1986 with the appearance in cattle of a newly-recognized form of neurological disease in the United Kingdom (UK).
Epidemiological studies conducted in the UK suggest that the source of BSE was cattle feed prepared from carcasses of ruminants. Speculation as to the cause of the appearance of the agent causing the disease has ranged from spontaneous occurrence in cattle, the carcasses of which then entered the cattle food chain, to entry into the cattle food chain from the carcasses of sheep with a similar disease, scrapie.
BSE is associated with a transmissible agent. The agent is highly stable, resisting freezing, drying and heating at normal cooking temperatures, even those used for pasteurization and sterilization. The nature of the BSE agent is still a matter of debate. According to the prion theory, the agent is composed largely, if not entirely, of a self-replicating protein, referred to as a prion. Another theory argues that the agent is virus-like and possesses nucleic acids which carry genetic information. Strong evidence collected over the past decade supports the prion theory, but the ability of the BSE agent to form multiple strains is more easily explained by a virus-like agent.
Between November 1986 and April 2001 approximately 180,900 cases of BSE were confirmed in the UK.
In July 1988, the UK banned the use of ruminant proteins in the preparation of animal feed. The use in the food chain of bovine offals considered to pose a potential risk to humans was also banned in the UK in 1989. The list of banned bovine offals was revised and expanded on several occasions as new information became available. In other countries, including those in Europe, measures taken, the date of implementation and the extent of enforcement vary from country to country.
Starting in 1996, bans prevented the sale of food and food products containing beef from the UK to other countries. Other products (e.g. tallow, gelatin) derived from bovine tissues were also prohibited from sale from the UK to other countries. Originally, no infectivity was detected in skeletal muscle tissue. Recent developments have thrown doubt on this and the approach that reassurance can be provided by removal of visible nervous and lymphatic tissue from meat (skeletal muscle).
Milk and milk products are considered safe. Tallow and gelatin are considered safe if prepared by a manufacturing process which has been shown experimentally to inactivate the transmissible agent.
However, in 1999 the European Union (EU) lifted the ban for meat fulfilling specific requirements; for example, de-boned beef from animals from farms where there have been no cases of BSE and where the animals are less than 30 months of age at slaughter.
Cattle are continuously monitored for BSE and BSE is decreasing in the UK. The number of reports of BSE in the UK began to decline in 1992 and has continuously declined year by year since then. New monitoring programmes using newly developed tests for the diagnosis of BSE in dead and slaughtered cattle have been introduced throughout the EU.
A newly recognized form of CJD, variant Creutzfeldt-Jakob disease (vCJD) was first reported in March 1996 in the UK (cf. WHO Fact Sheet N° 180 on variant Creutzfeldt-Jakob Disease). In contrast to the classical forms of CJD, vCJD has affected younger patients (average age 29 years, as opposed to 65 years), has a relatively longer duration of illness (median of 14 months as opposed to 4.5 months) and is strongly linked to exposure, probably through food, to BSE. Recent studies have confirmed that vCJD is distinct from sporadic and acquired CJD.
From October 1996 to early June 2001, 101 cases of vCJD have been reported in the United Kingdom (UK), three in France and a single case in the Republic of Ireland. Insufficient information is available at present to make any precise prediction about the future number of vCJD cases. Since few countries have surveillance systems, the geographical distribution of the incidence of vCJD needs to be better defined.
Similarities observed between the strain of the agent responsible for vCJD and those of BSE and closely related agents transmitted naturally and experimentally to different animal species, are consistent with the hypothesis discussed during two 1996 WHO consultations: that the cluster of vCJD cases is due to the same agent that caused BSE in cattle.
With the exception of brain, spinal cord and the retina, all other tissues and secretions from confirmed BSE cattle have failed to transmit the disease when either injected into or fed to mice. In particular, milk and beef meat have not caused the disease in experimentally infected animals. Brain and nervous tissue represent 0.1% by wt. of edible cow, 1% of offal.
Prior to the 1989 SBO ban in Great Britain, some 446,000 BSE-infected animals likely entered the food chain and were consumed. Between 1989 and 1995 when there was about a 50% compliance with the SBO ban, another 283,000 BSE-infected animals entered the food chain. It has been estimated that 3-5% of the animals that entered the food chain were BSE infected.
Experiments with mice being fed approximately 10 g of BSE-affected cow brain killed the majority of exposed mice (Barlow & Middleton 1990). If the bovine-to-human species barrier were similar to that for mice, it would suggest an oral LD50 in humans of an order of magnitude also similar to that for mice (approximately 10 g).
The worst case scenario would have occurred in Britain before 1995. In the worst case, where low-quality meat products were made entirely with offal, it would require the eating of 20Kg of such a meat product in order to actually eat 10g of BSE infected nervous tissue.
1% of offal contains nervous tissue, of which a maximum of 5% would be at risk of coming from a BSE infected cow. That is for every gram of offal, it might be expected to have 5 ten-thousands of a gram of BSE infectious nervous tissue. To achieve the 10g level would therefore require 20 Kg of such meat product. Since the abnormal prions are not, as far as is known, removed from the body by any biological process this is a cumulative figure.
The average burger size in the UK is 57g and contains 80% meat. That means a worst case burger would have 46g of meat or meat product. To consume the hypothetical 20Kg of contaminated meat product would mean eating 435 such burgers. At a rate of 2 each week, 104 each year that would be 4 years worth.
If we consider that only 10% of the market may suffer from such unscrupulous practices, things look a good deal better. That 4 years becomes 40 years.
Furthermore, genetic influences mean that, for a Caucasian population, only 37% of people would have the genetic propensity to contract vCJD.
So, even in Britain, which has had the highest exposure to BSE, the problem does not look significant. Official figures seem to bear this out.
One possible means of transmission remains a concern, namely that of transmission by blood. Bearing in mind that the most plausible means of encephalopathy transmission is from digestion to brain, then, the path from digestive tract to brain would be via the blood supply. Consequently, it would be expected to find prions in the blood supply, Furthermore, blood transfusions would by pass one barrier to transmission, namely the gut barrier.
As a consequence many countries have banned people who have visited the United Kingdom from being blood donors. Within the United Kingdom, people who have received a blood transfusion are themselves banned from donating blood.
In countries where solitary or very small numbers of cattle have developed BSE public health would not seem to be an issue. This is providing the steps introduced by the British to remove brain and spinal cords are carried out.
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