Contrary to the European Union, which is a net importer of agro-alimentary products, Belgium has a positive balance. In order to maintain and improve this position, the country needs to guarantee the safety of, among others, products of bovine origin. Epidemiosurveillance of rare events is particularly important in this respect.

An event is classified as rare, when its relative frequency is below 1% or when it occurs only sporadically (Toma, 1998). It must be appreciated that ‘rare event’ as such is a relative term and a precise definition remains difficult. A rare event can be in three different epidemiological states in function of the change in its incidence: emerging, persisting or disappearing.

An emerging rare event was considered in the case of the appearance of bovine spongiform encephalopathy (BSE) in Belgium. A detailed classification of neurologically expressed disorders (NED) in western Europe was carried out, the population-level incidence rate of NED in domesticated and wild ruminants was determined in Belgium and an innovative decisionsupporting tool, independent of sample size and disease prevalence, was developed, all with the explicit goal to improve the clinical detection of BSE.

A persisting rare event was studied in the context of isolated contaminations of the food chain by PCBs/dioxins, which add to the environmental pollution of the bovine herd. The aim of this study was essentially to estimate the potential exposure of the consumer. This was translated into (i) an effort to improve the detection of this type of contamination and (ii) to work out a methodology to evaluate the potential exposure of the consumer issuing from meat and milk products.

The acquisition and maintenance of the officially brucellosis-free status of Belgium was used to exemplify a disappearing rare event. The specific problem here was to deal with the increasing importance of false positive serology test results. The development and evaluation of a highly specific diagnostic test (based on a delayed hypersensitivity) and three highly sensitive indirect ELISAs, targeting IgG1 and/or IgG2, addressed this problem.

Lastly, an indicator of the epidemiological state of a rare event was developed, using BSE as an example. The evolution of the age distribution is proposed as a reliable indicator of the epidemic curve.

As a result of this work, a generalised model of the epidemiosurveillance of rare events is proposed. The improvement of the detection and monitoring of rare events and the evaluation of their impact on the public health are two key elements, which allow the formulation of modes of action to limit the emergence and re-emergence of diseases and disorders.

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Saegerman C., Claes L., Dewaele A., Desmecht D., Rollin F., Hamoir J., Gustin P., Czaplicki G., Bughin J., Wullepit J., Laureyns J., Roels S., Berkvens D., Vanopdenbosch E., Thiry E. Diagnostic différentiel des troubles à expression nerveuse dans l’espèce bovine en Europe occidentale. In Risk analysis of prion diseases in animals (thematic issue). Rev. Sc. Tech. O.I.E., 2003, 22 (1), 61-82.

Saegerman C., Claes L., Dewaele A., Desmecht D., Rollin F., Hamoir J., Gustin P., Czaplicki G., Bughin J., Wullepit J., Laureyns J., Roels S., Berkvens D., Vanopdenbosch E., Thiry E. Differential diagnosis of neurologically expressed disorders in Western European cattle. In Risk analysis of prion diseases in animals (thematic issue). Rev. Sc. Tech. O.I.E., 2003, 22 (1), 83-102.

Saegerman C., Berkvens D., Claes L., Dewaele A., Coignoul F., Ducatelle R., Cassart D., Brochier B., Costy F., Roels S., Deluyker H., Vanopdenbosch E., Thiry E. Population-level retrospective study of neurologically expressed disorders in ruminants before the onset of bovine spongiform encephalopathy (BSE) in Belgium, a geographic BSE risk III country. J. Clin. Microbiol., 2005, 43(2), 862-869.

Saegerman C., Speybroeck N., Roels S., Vanopdenbosch E., Thiry E., Berkvens D. Decision support tools in clinical diagnosis in cows with suspected bovine spongiform encephalopathy. J. Clin. Microbiol., 2004, 42, 172-178.

Saegerman C., Berkvens D., Boelaert F., Speybroeck N., Van Vlanderen I., Lomba M., Ermens A., Biront P., Broeckaert F., De Cock A., Mohimont L., Demont S., De Poorter G., Torfs B., Robijns J-M., Monfort V., Vermeersch J-P., Lengele L., Bernard A. Detection of polychlorinated biphenyls and dioxins in Belgian cattle and estimate of the maximal potential exposure in humans through diets of bovine origin. J. Toxicol. Env. Health, 2002, 65(18), 1289-1305.

Saegerman C., Vo T. K O., De Waele L., Gilson D., Bastin A., Dubray G., Flanagan P., Limet J.N., Letesson J J, Godfroid J. Bovine brucellosis diagnosis by skin test: conditions for its use and evaluation of its performance. Vet. Rec., 1999, 145, 214-218.

Saegerman C., De Waele L., Gilson D., Godfroid J., Thiange P., Limbourg B., Vo T. K O., Limet J.N., Letesson J J, Berkvens D. Field evaluation of three serum ELISA using monoclonal antibodies or protein G as peroxidase conjugate for the diagnosis of bovine brucellosis. Vet. Microbiol., 2004, 100, 91-105.

Saegerman C., Speybroeck N., Vanopdenbosch E., Wilesmith J., Berkvens D. Trends in age-at-detection in Bovine Spongiform Encephalopathy cases: a useful indicator of the epidemic curve. Vet. Rec., 2006, 159, 583-587.

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