Despite measures taken in France since 1990 to control the spread of bovine spongiform encephalopathy (BSE), the incidence of BSE cases notified in France by the BSE Epidemiological Surveillance Network has been rising in recent years. By 1 July 2000, 103 cases of BSE had been detected, including 74 ‘born after the ban’ (BAB) cases. This increase expresses the changes over time in both the real incidence of the disease and in disease surveillance.

In 1988, the role of meat and bone meal (MBM) in the transmission of bovine spongiform encephalopathy (BSE) was shown (1). These animal foodstuffs are prepared with animal by-products from the slaugtherhouse and rendering plants. Changes to the rendering process (lowering of the temperature and change of the fat extraction process) of MBM led to a failure of an infectious agent inactivation and had been therefore associated with BSE. The European Union (EU) had become aware of the risk that the British epidemic might spread to other European countries as a result of British exports of MBM and decided to add BSE to the list of notifiable diseases (Decision 90/134/EEC). It then made it a requirement for member states to investigate clinical cases of BSE (Decision 90/200/EEC). Under French law, this resulted in the setting up of a BSE epidemiological surveillance network, which became operational at the end of 1990 (2). The network detected the first case of BSE in France on 28 February 1991. In August 1989, French regulations were introduced that prohibited imports of MBM from the United Kingdom as cattle feed and, in July 1990, the use of mammalian meal in cattle feeds. Despite those measures to prevent the disease from spreading, cattle were contaminated – as of May 1996, the first ‘born after the ban’ (BAB) cases were detected. Taking into account the time taken to deplete feed stocks, those animals were born after 1 January 1991. This article reviews the frequency of BAB cases in France to date and the hypotheses concerning the origin of these cases.

The BSE epidemiological surveillance network is based on the systematic surveillance of cattle aged over 2 years presenting with neurological disorders, by using a defined list of criteria (age, symptoms, course of disease) (3)). In addition, BSE surveillance has been extended to all animals notified with suspected rabies in the context of the national surveillance scheme for rabies. Clinical suspicions are raised either on farms, by veterinary practitioners , or in slaughterhouses, by veterinarian inspectors, and then confirmed by a county coordinating veterinarian specially trained for that purpose. After slaughter, the diagnosis is confirmed by the reference laboratory of the French Agency for the Food Sanitary Safety (Agence Française de Sécurité Sanitaire des Aliments, AFSSA) in Lyon by histology, or Western blot,when the sample is of poor quality. Subsequently, and particularly in May 1999, surveillance was intensified by introducing in depth control of animals subjected to emergency slaughtering and imported animals born in Portugal or Switzerland, and, in November 1999, by control of a quota of certain culled animals, i.e. animals too old to be used for breeding purposes and sent to be slaughtered (2).

Since 1996, the National Brigade for Veterinary and Health Inquiries (Brigade Nationale d'Enquêtes Vétérinaires et Sanitaires, BNEVS) has conducted an in depth epidemiological investigation in each farm affected by BSE (4). The investigation is based on an interview with the farmer and an analysis of the farm's registers and accounts, and addresses the status of the infected animal’s mother, its diet since birth, the suppliers of the commercially available feeds distributed, the presence of other animal husbandry units on the farm, the use of organic fertilisers from outside the farm, and medications used. Further investigations are carried out on the premises of the cattle feed manufacturers supplying the farm, to evaluate the potential contamination of the feeds by products of rendering plants.

Control of the spread of BSE has become stricter over time. The prohibition of mammalian meals for cattle feeds was extended to feeds for all ruminants (July 1994) and, since July 1996, has covered all proteins of animal origin (with the exception of milk protein). In addition, beef offal associated with a specified risk and carcasses from dead animals were prohibited in the manufacture of animal meals in April and June 1996, respectively. Lastly, heat treatment of MBM, in compliance with recommendations by the European Community (133°C, 3 bars, 20 min) became mandatory in February 1998 (2).

By 1 July 2000, 103 cases of BSE had been detected in France (figure 1), 74 (72%) of which were BAB cases. Since 1997, almost all of the cases observed have been BAB cases (figure 2) The number of cases detected per year has been rising since 1997 and reached 31 in 1999. This means an annual incidence of 2.7 per million cattle aged over 2 years (5). Most of the BAB cases were born during the second half of 1993 and in 1994 (figure 3).

The investigations conducted by BNEVS have yielded information about the BAB cases. Vertical transmission from mother to offspring seems to be excluded in most cases since the infected animal’s mother was still alive more than one year after its birth in 85% of the cases emerging in 1999. In contrast, at the current stage of the inquiries, in no case can all risk of feed related contamination be excluded with certainty. Complementary feeds that are not produced on the farm, made from various ingredients, and given additionally to foodstuffs produced in the farm, are commonly used, at least during the first year of an animal's life. In all cases, the plausibility that a feed probably consumed by the animal had been cross contaminated by products likely to be entirely or partly derived from high risk ruminant byproducts has been demonstrated. Contamination may have occurred during feed manufacture, transport, or distribution.

The increased incidence of BSE cases in recent years is to be interpreted as a function of two variables whose effects are linked: firstly, the changes over time in the real incidence of the disease and, secondly, the changes over time of disease surveillance. Surveillance has become increasingly effective over time, which has led to a rise in the number of clinically suspected cases each year and hence an increase in the additional controls imposed on emergency slaughtering and culled animals. As a result, it is impossible to estimate the rise in the real incidence of BSE: the rise in the observed incidence may in part be related to an increase in the effectiveness of surveillance.

Retrospective case studies argue in favour of one of the hypotheses concerning the sources of contamination of BAB cattle formulated by the French expert committee on transmissible subacute spongiform encephalopathies (TSSEs) and prions in the committee's opinions dated 17 December 1998 and 27 February 1999 (6) – namely, a dietary source related to cross contamination between feeds for non-ruminants and feeds for ruminants. In 1993 and 1994, when most of the cases were born, the animal tissues associated with the highest risk (the brain and spinal cord) and the carcasses from dead animals were not excluded from MBM manufacture. During those years, animal meal was potentially contaminated by tissues from French cattle dying at the end of their incubation period, or for which the disease was not detected, and in addition MBM was imported from various EU countries. Besides, a high tonnage of cattle carcasses was imported from the United Kingdom. This may also have caused contamination of animal meals used in products for non-ruminants. The hypotheses on the origin of the BAB cases could be investigated by means of the case control study of farmers' practices with regard to cattle feeds and supply sources, whose project was validated by the French expert committee.

Given the stringent measures taken since 1996 to improve control of BSE and the four to five year interval required to observe the effects of those measures (mean incubation period), a marked decrease in the real incidence of BSE is expected as of 2002. The decrease could be masked by a qualitative change in BSE surveillance, with the introduction of a screening programme in July 2000 for cattle that die or are humanely slaughtered on farms, and cattle subject to emergency slaughter after an accident.

1. Wilesmith JW, Wells GAH, Cranwell MP, Ryan JBM. Bovine spongiform encephalopathy: epidemiological studies. Vet Rec 1988; 123: 638-44.
2. DGAL. Mesures réglementaires prises au regard de l'ESB depuis 1990. Paris: Direction Générale de l'Alimentation, publication pending.
3. Savey M, Belli P, Coudert M. Le réseau d'épidémiosurveillance de la BSE en France: principes – premiers résultats. Epidémiologie et Santé Animale. 1991; 19: 49-61
4. DGAL. Enquête épidémiologique relative aux cas d'ESB survenus en France en 1999. Paris: Direction Générale de l'Alimentation, 2000: 7.
5. DGAL. Nombre de cas d'encéphalopathie spongiforme bovine. Paris: Direction Générale de l'Alimentation, 2000: 4.
6. Comité d'Experts sur les Encéphalopathies Subaiguës Spongiformes Transmissibles et les Prions. Hypothèses explicatives de l'apparition de cas d'ESB chez des animaux nés après l'interdiction des farines de viande et d'os dans l'alimentation des ruminants. Paris: Comité d'Experts sur les Encéphalopathies Subaiguës Spongiformes Transmissibles et les Prions, 1999: 7.