Updated BSE Situation Worldwide and Risk Analysis
- Dr D. Heim, Swiss Federal Veterinary Services
Current situation of BSE worldwide
Initially, BSE was seen as a purely British problem. Greater attention was paid to the problem after the occurrence of BSE in exported cases in 1989. Not until the end of 1989 were the first indigenous cases reported outside the United Kingdom (UK): in Ireland, France, Portugal and Switzerland. In the mid-90s, other countries reported cases of BSE: The Netherlands, Luxembourg, Belgium, and Liechtenstein. With the introduction of targeted surveillance in 2000 in risk populations a large number of countries in Europe and also the first countries outside Europe detected the first BSE cases.
Fig. 1: First occurrence of indigenous BSE cases
Passive and active surveillance
The reported number of BSE cases in a country has to be seen in the light of the quality of its surveillance system and the measures taken. Until 1999, surveillance systems for bovine spongiform encephalopathy (BSE) worldwide were primarily passive, i.e. relying on the examination of diseased adult cattle showing clinical signs reported to the veterinary authorities. Brains were examined by histopathology and immunhistochemistry. Histology and immunhistochemistry are reliable to confirm BSE in cattle, but the procedure is cumbersome, time consuming and therefore not suited for mass testing of animals.
The number of reported cases in a passive surveillance system is influenced by the degree of disease awareness of farmers and veterinary practitioners, and by the willingness to report a suspect case. The motivation to report suspect cases is depending on the measures taken and the compensation paid. Furthermore, it depends on the stage of the disease, because early clinical signs of BSE are not always typical. In such a system the reported number of clinical cases underestimates the true number of clinical disease in the population. The passively acquired surveillance data for BSE therefore did not provide a reliable or comparable estimate of the BSE status of a country, other than indicating that BSE was present when cases were detected.
With the availability of a rapid test for BSE, Switzerland has initiated in January 1999 an active surveillance scheme to enhance the detection of BSE cases in the adult cattle population. In addition to the mandatory reporting of all suspect clinical cases, all fallen or killed bovines with at least four permanent incisors are investigated. Since March 1999, also all emergency slaughtered cows are examined. These two groups were chosen, because in these risk populations seemed the probability highest to find not detected BSE cases. Additionally, a random sample of cows is examined during routine slaughtering.
Based on the results of the Swiss targeted surveillance system, many countries have now been implemented an active surveillance system; tests for BSE have been carried out in populations at risk since 1999/2000 in some countries and 2001 in others.
The results indicate clearly that active surveillance is a more objective approach and can help to assess the real BSE situation in a country. Consequently, some countries that for years have been considered BSE free are now shown to have BSE.
Risk assessment for BSE
To estimate the extent of the BSE problem in countries, a serious risk assessment must be carried out.
On the basis of criteria defined in the International Animal Health Code of the World Organisation for Animal Health (OIE), the Scientific Steering Committee of the European Union (EU) has carried out a geographical BSE risk assessment (GBR) in a number of countries. The GBR is a qualitative indicator of the likelihood of the presence of one or more cattle being infected with BSE, at a given point in time, in a country An important component of a BSE risk assessment is to evaluate whether potentially infected material was imported and, if so, whether the conditions in the country were/are sufficient to cope with this material, i.e. to prevent the amplification of the agent.
| GBR level | Countries |
|---|---|
| I: Highly unlikely | Argentina, Australia, Botswana, Brazil, Chile, Namibia, New Zealand, Nicaragua, Norway, Paraguay, Singapore, Swaziland, Uruguay, El Salvador, Panama, Costa Rica |
| II: Unlikely but not excluded | Colombia, India, Kenya, Mauritius, Nigeria, Pakistan, Sweden, Canada, USA |
| III: Likely but not confirmed or confirmed at a lower level | Albania*, Belgium, Denmark**, Cyprus*, Czech Republic**, Estonia*, France, Germany**, Hungary*, Ireland, Italy**, Lithuania*, Luxembourg, Poland**, The Netherlands, Romania, Slovak Republic**, Spain**, Switzerland, Austria***, Slovenia***, Finland***, Gerece****, Japan****, Israel |
| IV: Comfirmed at a higher level | United Kingdam, Portugal |
- * countries with no BSE reported
- ** countries with no BSE reported after finalisation of the GBR, but cases reported later
- *** country first assessed to be in GBR II
- **** risk assessment not yet published
Measures
The current measures for BSE control and reduction of human exposure are aimed at eradicating BSE in livestock and reducing the risk of human exposure through contaminated food and other products. Measures related to animal health include ban on feeding meat and bone meal (MBM) to ruminants, Specified Risk Material (SRM) ban in feed, elimination of BSE cases, processing of animal wastes and restrictions on imports.
Minimising risks for human health
Measures for minimising risks for human health require identification and elimination of infected animals before entering the slaughterhouse. A broad information on BSE symptoms and intensive ante-mortem inspection of cows is necessary. Sanctions for non-reporting of clear clinical suspects should be considered.
Until today infectivity has never been detected in muscle or milk. In field cases of BSE in cattle tested so far, infectivity has not been recorded outside the brain, spinal cord and eyes in mouse tests. In experimental studies, BSE infectivity has been found in the distal ileum at intervals during the incubation period starting six months after exposure. Furthermore, central nervous tissues and dorsal root and trigeminal ganglia were found to be infective shortly before the onset of clinical signs. Besides the SRM, mechanically recovered meat (MRM) originating in particular from vertebral columns with dorsal root ganglia or residues of spinal cord is regarded as a risk factor. Therefore, the systematic ban of SRM and MRM from the food chain is and will remain crucial for the protection of the consumer. The list of tissues included in the SRM varies in the different countries. Nevertheless, the most important point is that tissue, known to harbour potential infectivity should be removed from the food chain.
Measures to eradicate BSE
The ban on feeding MBM to ruminants is considered to be the most important measure. Although this measure is quite effective, it is not enough to bring the rate of new infections among cattle down to zero. This is mainly due to the problem of cross-contamination and cross-feeding. In order to be effective, it should include measures to avoid cross contamination like separated feed lines.
Another important issue is the removal and incineration of specified risk material (SRM).
Known infectious tissues in BSE affected cattle comprise brain, spinal cord, eye, trigeminal ganglia, dorsal root ganglia and ileum. Removal and incineration of SRM, such as brain and spinal cord, reduce the risk of recycling of the BSE agent. Further processing of such material amplify the risk.
Processing raw material in a batch process at 133oC with 3 bars of pressure for 20 minutes has proved effective although this does not completely inactivate the agent if the initial infective load is high.
Conclusions
- To estimate the extent of the BSE problem in countries, a serious risk assessment must be carried out. Active surveillance in populations at risk plus passive surveillance can help to reveal a better estimate of the incidence.
- In the event of uncertainty as to the BSE status of a country, the most important preventive measures for the protection of cattle are the implementation of an effective MBM feed ban to ruminants and the prevention of cross-contamination in feed.
- The most important measure for protection of the consumer is a ban on SRM and MRM.
- The utmost attention should be paid to enforcing the implementation of all the stipulated measures. The slightest gaps in the system of disease control can undermine all the efforts to eradicate BSE.
- Although the first discovery of a BSE-case is seen in the short term as a grave problem, the failure to detect the disease even though it is present would have more severe consequences - and prove far more costly - both in the medium and in the long term. In the event of uncertainty, prophylactic measures are cheaper than late action - which may ultimately prove to be too late. A transparent, proactive information policy will not only help consumer confidence, but is also important for credible trading relations.