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Morbidity registration by a network of sentinel general practitioners (SGPs) in Belgium raises a number of problems related to possible biases in the network procedure, such as unequal geographical distribution, non-participation of a segment of the target population of practitioners and difficulties in the estimation of the denominator population at risk for the health problems under study. Through the application of two hierarchical clustering procedures, the initial number of 43 districts in the country has been reduced to 15 homogeneous district clusters. These represent the new geographical framework from which the geographical spread of the network is checked. This network is subsequently corrected for such socio-demographic parameters as age, sex and occupation in order to match more closely the total population of Belgian general practitioners (GPs). The population covered by the network is estimated on the basis of the annual number of patient contacts. Application of the described procedures should result in a network allowing valid estimations for a number of health issues as seen by Belgian GPs.

For the last five years, the Brussels Institute of Hygiene and Epidemiology has been involved in the surveillance of acute respiratory infections (ARI). The four indicators used (number of encounters of ARI by GP's/100 encounters, virus isolations, absenteeism and mortality) are discussed. A regression procedure is applied to the data collected by a sentinel network of general practitioners (GP's). This procedure permits the baseline to be visualized and an epidemic threshold to be determined in order to recognize early an influenza outbreak. The traditional use of flu-like illnesses as an indicator might be improved by the addition of non-specific ARI which are more precocious, especially in children. The criteria for an accurate definition of an influenza epidemic are discussed. The same mathematical model can be used for the analysis of mortality linked with an outbreak. It shows that the last epidemic in the winter 1989-1990 was responsible for about 4900 deaths directly or indirectly related to influenza.

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Viral hepatitis is a serious health problem throughout the world. No recent prevalence data on hepatitis A, B and C were available for the population in Flanders, Belgium. For this reason, a sero-epidemiological study was undertaken in 1993-1994 in a sample of the general population. The purpose of this study was to obtain a clear picture of the prevalence of hepatitis A, B and C. Between April 1993 and February 1994, 4,058 blood samples were drawn and collected in 10 hospitals in Flanders. The study group was representative for the Flemish population. For hepatitis A a seroprevalence of 55.1% was found. In the non-Belgian residents the HAV prevalence was significantly higher than in Belgians (62% versus 52%; chi2 = 8.05; p = 0.005). For hepatitis B. 9.9% of the study group showed serological evidence of hepatitis B markers: 6.9% of the participants was positive for anti-HBs/anti-HBc, 0.7% appeared to be HBsAg positive and 3.5% was solely anti-HBs positive. The prevalence of HBV markers in Belgians was 6.9%, significantly lower compared to the 13.4% among non-Belgians (chi 2 = 14.05; p = 0.00018). 4055 serum samples were analysed for hepatitis C serology by second generation anti-HCV tests. Anti-HCV was detected in 0.87% of the serum samples. No statistically significant difference was found in HCV prevalnece between Belgians and non-Belgians. Results of this study should help policy makers in their decisions on the most appropriate hepatitis A and B vaccination strategy and on the most effective prevention strategy for hepatitis C.

SUMMARYIn the development of a surveillance programme for infectious diseases in Belgium, a national network of microbiological laboratories has been responsible, since February 1983, for the weekly registration of certain pathogenic agents. Thus, the main epidemiological features of a selected number of infections in Belgium can be characterized.

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