Materials and methods
A surveillance system for influenza and other acute respiratory diseases covering 18 sentinel cities where more than 50 % of the population resides has been established. Information from the policlinics concerning the number of visits to the doctor in connection with ARVI, age, number of hospitalisations and deaths are received daily at the Belarusan Centre for Medical Technology. The statistical data are used by all the institutions under the Belarusan Ministry of Health.
The National Centre for Influenza and Acute Respiratory Infections (NCI) was established in 1996 following a proposal from WHO in collaboration with the Influenza Laboratory of the Institute of Epidemiology and Microbiology, Belarusan Ministry of Health. The NCI relies on information from the virology laboratories of the Centre for Hygiene and Epidemiology in seven administrative regions of the country. Screening, isolation and identification of ARVI agents including influenza virus are performed in these laboratories. Subsequently, the isolates are transferred to the NCI where confirmation and typing is conducted. The isolated strains are sent for thorough investigation at the WHO diagnostics centres in London or Atlanta.
The NCI also analyses laboratory data concerning ARVI investigations from the virology laboratories in the Republics regions, conduct independent investigations addressing the etiology of epidemics, analyse statistical data on ARVI cases and compile forecasts of influenza epidemics in the Republic.
For this study, we used statistical data provided by the Belarusian Centre of Medical Technologies on the incidence of influenza and ARVI in 18 sentinel cities in addition to the number and age distribution of the patients admitted to the hospitals and mortality. Acute respiratory viral infection laboratory diagnostics using an immunofluorescence method were provided by the virology laboratories from the regions in the Republic.
For this study, nasopharyngeal smears, paired sera and pathology specimens provided by hospitals in the regions and in Minsk were investigated in order to determine the etiology of the epidemics. Examinations were performed by immunofluorescence, serological and virological methods. Isolation of influenza viruses was performed on chicken embryos and in MDSK (dog kidney) cell culture. Verification of the isolates and final typing of the influenza viruses were performed at NCI by hemagglutination inhibition method using WHO diagnostic sets for the current epidemic season.
From 1996 till 2003 eight influenza epidemics were registered in Belarus (Table 1). The epidemics were of moderate intensity with a large proportion of children among the patients. In the sentinel cities 20-25% of the children fell ill during the epidemic period. Influenza epidemics during 1996-2003 were mild. The proportion of hospitalised patients varied between 1.0 and 1.5% of the reported cases during the epidemics. Among those hospitalised, 80% were children. The severity of the influenza epidemics may be judged indirectly by looking at the mortality of post-influenza pneumonia that is usually registered 2-3 weeks after the end of the epidemics. As the data in Table 2 indicate, the severity of the influenza epidemics over recent years has decreased.
The 1998-1999 epidemic was the hardest with respect to intensity and length. In the sentinel cities 17.8% of the population fell ill. The epidemic rise was noted in the beginning of December 1998 and was caused predominately by the circulation of parainfluenzaviruses. At the end of January 1999, a new rise in the incidence due to the circulation of influenzaviruses was observed. The proportion of laboratory-confirmed parainfluenzavirus infections during the epidemic rise was 25.7%. Type B and A were revealed in 22.6% and 34.8% of these cases, respectively. Adenoviruses and RS-viruses during the same period constituted 11.5% and 5.4% of the cases, respectively, during the same period.
The majority of the influenza epidemics were caused by type A influenza viruses (Table 1). Type B viruses had a significant influence on the course of the epidemics only twice (in 1995/96 and 2001/02). In addition, the results of immunofluorescence examinations of nasopharyngeal smears results confirmed the circulation of non-influenza respiratory group viruses during the epidemics (Figure 1).
Retrospective analysis of the data over the last 30 years indicates that in more than 70% of the cases epidemics started in January-February. The data also indicate an increase in the incidence of reported respiratory disease in the Belarus Republic during recent years (1). In addition, the intensity of influenza epidemics has decreased during the last years. This observation is in accordance with the decrease in frequency of intense influenza epidemics during the last decades described in the literature (2). The decrease in intensity of the epidemics may be the consequence of prolonged circulation of similar influenza virus strains. Moreover, vaccines have been provided especially to persons at risk in the Belarus Republic during recent years. The observed decrease in mortality from post-influenza pneumonia may be attributed to the factors described above.
It is essential to note that all the influenza virus strains that were isolated during the epidemics are the same as the vaccine strains of corresponding years. This may be associated with the geographic location of Belarus on heavily travelled transit routes that allows the current strains to spread quickly over the territory. All the epidemics were of mixed etiology and were caused by circulation of two subtypes of influenza viruses: A and B. This observation is in accordance with the data from the literature concerning contemporary influenza epidemics (3).
In addition to influenza viruses, non-influenza respiratory viruses contributed to the etiological structure of the epidemics. The intensity of the 1998-99 epidemic can be explained not only by the new influenza virus variant A/Sidney/5/97, but also by activation of parainfluenza-viruses during the epidemic.
The proportion of detection of non-influenza respiratory viruses during the observed epidemic periods has risen and reached up to 70% during the 1999-2000 season. By comparison, up to 90% of isolates were influenza viruses of type A in the 1970s according to the literature (4). The considerable contribution of non-influenza viruses on the incidence of respiratory infections during the epidemic season has also been described by other authors (5, 6).
In conclusion, these data indicate that the evolution of influenza viruses in Belarus Republic is similar to that observed world-wide. Strict monitoring of influenza virus circulation is important with respect to the current pre-pandemic situation concerning avian influenza viruses (7).
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