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Editorial from the Director General - November 2001

Bovine spongiform encephalopathy (BSE) has been experimentally transmitted from cattle infective material to sheep, but so far no cases of naturally BSE infected sheep have been reported. Research has been underway for some time to determine whether BSE has naturally occurred in the UK sheep flock in the early 1990s and has been masked by scrapie. While there is no reported evidence of such infections, the risk of potential BSE in sheep must be considered.

To date, the only reliable method for characterising/differentiating BSE from scrapie in sheep showing clinical signs of scrapie-like disease is through a long laboratory process, of inoculations into mice, which presently takes a minimum of 2 to 3 years to be completed.

Scrapie is a disease affecting sheep and goats which has been known for two centuries. The first report of the existence of scrapie appears in the XIXth century European literature. Although the discovery of BSE in cattle and the new variant of Creutzfeld-Jakob disease (vCJD) in humans has resulted in a resurgence of scrapie research in a number of countries, it is important to state that there is no evidence for a public health risk from scrapie.

The tissue distribution of BSE infectivity in sheep, experimentally infected with the BSE agent, has been shown to be similar to that of scrapie. Although this idea is speculative at the moment, should BSE be present in sheep it may spread laterally, at certain critical points of the infection, between sheep as occurs in scrapie. The detection of scrapie infectivity in the placenta, in combination of failure to detect infectivity in feces, saliva, urine, colostrum or milk has led to a fairly wide acceptance that the placenta and fetal fluids play the most significant role in the spread of scrapie. Hence the transmission would most likely occur from an infected mother to her progeny and other lambs which are in close association mainly at the time of parturition.

It is possible that BSE could be present in small ruminants because there is evidence that BSE-contaminated meat-and-bone meal (MBM) may have been fed to sheep and goats in some countries. Even where grazing is the normal practice, in some cases MBM from European Union member countries has been imported by certain countries as feed supplement for animals, particularly during drought periods.

The recent joint WHO/FAO/OIE Technical Consultation on BSE, hosted at the OIE headquarters in Paris in June 2001, already dealt in depth with this question and stressed the following:

  • Ruminant MBM and greaves should not be fed in any case to ruminant animals. Monitoring of compliance with the feed bans needs further development of reliable certification programmes and screening tests to guarantee the absence of BSE infectivity in ruminant feedstuffs traded internationally.
  • The consultation considered that BSE-contaminated MBM might have been fed to some sheep and goats in certain countries and that these species may have been infected with BSE agent. It is, therefore, recommended that individual countries assess the risk that BSE infection is present in their indigenous sheep/goat population. All countries are encouraged to require notification and surveillance for TSE diseases of sheep/goats and to take steps to mitigate risks identified.
  • In countries where sheep and goat populations have been potentially exposed to BSE infectivity, measures should be taken to minimise the exposure of humans to infectivity from sheep and goats.
  • Efforts to investigate the presence of natural BSE in sheep and goats should be continued.

The Scientific Steering Committee of the European Commission considers that the list of specified risk materials (SRMs), to be removed from the food and feed chain, proposed in its April 2000 opinion to be valid for the time being. If the risk of infection of sheep and goats with the BSE agent exists, the list of SRMs would be more comprehensive than that in cattle.

The list of tissues that possibly pose the highest risk include: skull (including brain, pituitary gland, dura mater, eyes and tonsils) and spinal cord of all small ruminants above 12 months, and spleen of all ages. The potential infectivity of the intestine, lymph nodes and other tissues of sheep experimentally infected with BSE is underway and results are urgently needed to improve the risk assessment.

The World Animal Health Organization (OIE), in its International Animal Health Code, has specific guidelines and recommendations on BSE, and has draft guidelines on scrapie submitted for adoption in May 2002. The OIE has also convened an Ad hoc group of experts to review the recent investigations on BSE in sheep in order to update current recommendations on the prevention of spread of Transmissible pongiform Encephalopathies (TSEs) and the protection of public health. Until additional specific scientific evidence is available, while the presence of BSE in sheep has not been demonstrated, countries are encouraged to follow current OIE guidelines and recommendations on BSE and scrapie.

The OIE will continue, without delay, the updating of its international standards on the subject, as soon as new scientific evidence becomes available.

Bernard Vallat

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