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Bovine spongiform encephalopathy (BSE)

 KEY FACTS

  • Bovine spongiform encephalopathy (BSE) is a progressive, fatal disease of the nervous system of cattle.
  • BSE is caused by the accumulation of an abnormal protein called “prion” in nervous tissue.
  • Two forms, or strains, can be distinguished: classical BSE occurs in cattle after ingesting prion contaminated feed; atypical BSE is believed to occur spontaneously in all cattle populations.
  • First detected in 1986, the implementation of appropriate control measures resulted in the decline of classical BSE cases worldwide. To date, the incidence of both forms is negligible and estimated to approach zero cases per million cattle.
  • BSE is considered zoonotic due to its assumed link with the emergence of variant Creutzfeldt-Jakob disease (vCJD) in humans.
  • BSE is an OIE-listed disease. The OIE has established official recognition of sanitary risk status for its classical form.

 

What is BSE?

BSE is a disease of the nervous system of cattle, which has a long incubation period of between two and eight years, and occasionally longer. There is currently no treatment or vaccine against it.

It is one of a group of diseases known as transmissible spongiform encephalopathies (TSE), or prion diseases, characterised by the accumulation in nervous tissue of an abnormal infectious protein called a prion. This group notably includes scrapie in sheep and goats, chronic wasting disease (CWD) in cervids, and variant Creutzfeldt-Jakob disease (vCJD) in humans. A neurological disease in domestic and zoo felids has also been linked to BSE.

The hypothesis that BSE prions have been passed to humans, causing vCJD, is strongly supported by epidemiological and clinicopathological studies.

Classical versus atypical BSE
There is a distinction to be made between these two forms, or strains:

  • Classical BSE occurs through the consumption of contaminated feed (see section ‘transmission and spread’). Whilst classical BSE was identified as a significant threat in the 90s, its occurrence has markedly decreased over the past years, as a result of the successful implementation of effective control measures and is now estimated to be extremely low (close to 0 cases).
  • Atypical BSE refers to naturally and sporadically occurring forms, which are believed to occur in all cattle populations at a very low rate, and which have only been identified in older cattle when conducting intensive surveillance.
    In the early 2000s, atypical prions causing atypical BSE were identified as the result of enhanced surveillance for transmissible spongiform encephalopathies. The number of cases of atypical BSE is negligible.
    Indeed, whilst to date there is no evidence that atypical BSE is transmissible, recycling of the atypical BSE agent has not been ruled out, and therefore measures to manage exposure risk in the feed chain continue to be recommended as a precautionary measure.

For more details, see BSE situation in the world and annual incidence rate.

 

BSE is an OIE-listed disease and must be reported to the OIE, as indicated in the OIE Terrestrial Animal Health Code. The occurrence of atypical BSE is not considered for the purpose of official BSE risk status recognition as it can spontaneously occur in any cattle population.

 

Transmission and spread

The clear understanding of the origin and development of the disease in animals is still subject to scientific research. Nevertheless, it has been proven that certain tissues of infected animals, so-called specified risk materials (SRMs), are most likely to contain and therefore transmit the BSE prion. According to the OIE Terrestrial Animal Health Code, these tissues include brain, eyes, spinal cord, skull, vertebral column, tonsils and distal ileum.

Scientists believe that cattle are usually infected through the dietary intake of prion contaminated feed during their first year of life. The risk of contamination occurs if the feed contains products derived from ruminants, such as meat-and-bone meal (MBM), which is the protein product obtained by rendering certain parts of animal carcasses, including those of farmed small ruminants and cattle, that are not used for human consumption.

The infectious prion is resistant to commercial inactivation procedures such as heat, which means that it may not be destroyed in the rendering process. The incidence of BSE was much greater for dairy than beef cattle, since generally, dairy herds are fed concentrate rations that, before the introduction of stricter controls, contained MBM.

Meanwhile, there is no evidence of direct transmission between animals (horizontal transmission) and little data support that BSE is transmitted from mother to offspring (vertical transmission).

Classical BSE was first diagnosed in cattle in the United Kingdom (UK) in 1986, but had probably been present in the country’s cattle population since the 1970s or earlier. It has then been reported in 25 countries other than the UK, mainly in Europe, Asia, the Middle East and North America.

Nowadays, as a result of the successful implementation of effective control measures, the prevalence of classical BSE is extremely low, as well as its global sanitary impact and public health risk.

For more details, see BSE situation in the world and annual incidence rate

 

Public health risk

The likely transmission of BSE to humans, assumed to cause in vCJD, along with the incapability to predict the size of the vCJD epidemic triggered a public health crisis during the 90s. To date, the number of vCJD clinical cases identified is extremely low.

There are compelling indications that vCJD could be acquired through the consumption of contaminated beef products (as defined below), or contact with medical devices contaminated with BSE prions. It is to be noted that dietary exposure to red meat (i.e., deboned skeletal muscle) and milk and milk products is considered safe.

To prevent human and animal infection, and the recycling and amplification of prions, many countries have enforced the systematic removal of tissues that might contain a significant amount of BSE infectivity, named specified risk materials (SRMs), from bovine carcasses. This measure, together with the ban on the use of processed animal proteins in feed (i.e., ruminant-to-ruminant feed ban), have been demonstrated to be strongly efficient in controlling exposure to BSE agents.

The production of human and veterinary pharmaceutical products, and medical devices or cosmetics, should adhere to strict requirements and ideally avoid the use of bovine materials or materials from other animal species in which prion diseases naturally occur.

For more information on vCJD, consult the WHO's website

 

Clinical signs

The time between the moment an animal gets infected with the BSE agent and the onset of clinical signs is between two to eight years. Therefore clinical signs of BSE are found in adult animals, which may demonstrate some of the following clinical signs:

  • nervous or aggressive behaviour;
  • depression;
  • hypersensitivity to sound and touch, twitching, tremors;
  • abnormal posture;
  • lack of coordination and difficulty in rising from a lying position;
  • weight loss, or;
  • decreased milk production.

The course of disease is usually subacute to chronic, and affected animals display progressive neurological signs.

There is no effective treatment and affected animals will inevitably die if the disease is left to run its course.

 

Diagnostic

BSE may be suspected based on clinical signs.

To date, there is no method that allows to confirm the presence of the BSE agent in live animals.

As indicated in the OIE Manual of Diagnostic Tests and Vaccines for Terrestrial Animals, diagnosis can be achieved by histopathology (i.e. microscopic examination) of the medulla oblongata (a portion of the brain). However, histopathology as the primary method would not be appropriate for any of the BSE surveillance streams indicated in the Terrestrial Code. The confirmation of the diagnosis is reached with immunochemical methods including immunohistochemical (IHC) techniques and Western immunoblot.

 

Prevention and control

In accordance with the OIE Terrestrial Animal Health Code, an effective strategy for preventing the introduction or dealing with occurrences of BSE includes:

  • targeted surveillance of occurrences of clinical neurological disease;
  • transparency in reporting findings of BSE;
  • safeguards on importation of live ruminant species and their products;
  • removal of specified risk materials (SRMs) (brain, spinal column, etc.) during slaughter and processing of carcasses and from the human food and the animal feed chains;
  • prohibition of the inclusion of SRMs in animal feeds, thus removing potentially contaminated material from the food chain;
  • humane destruction of all suspected animals exposed to prion contaminated feed;
  • banning the use of MBM in ruminant feed (ruminant-to-ruminant feed ban, further reinforced by a mammalian-to-ruminant feed ban);
  • appropriate disposal of carcasses and all animal products; and
  • livestock identification to enable effective surveillance and tracing of suspected livestock.

 

BSE-risk Status

BSE is a disease for which the OIE has established official recognition of sanitary risk status in countries in their entirety or in defined zones and compartments, through a transparent, science-based and impartial procedure.

The categorisation of BSE risk status only applies to classical BSE.

‘Atypical BSE’ forms are excluded from the scope of the categorisation, because they are believed to occur spontaneously in all cattle populations at a very low rate.

The OIE officially recognises two categories of BSE risk status. The current requirements for the recognition and maintenance of these categories can be summarised as follows:

A country, zone or compartment not recognised as having a ‘controlled’ or ‘negligible’ BSE risk status falls into a third category: ‘undetermined’ BSE risk status.

View the list of OIE Member Countries with a BSE risk status here

 

Last update : 06/08/2018

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