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Volume 25-21 |
1998-1999 INFLUENZA SEASON: CANADIAN LABORATORY DIAGNOSES AND STRAIN CHARACTERIZATIONIntroduction In collaboration with the World Health Organization (WHO) international collaborating laboratories, provincial laboratories, and other Canadian hospital and university-based laboratories, the Laboratory Centre for Disease Control (LCDC) conducts national surveillance on human influenza viruses. This surveillance monitors influenza activity, detects and describes antigenic changes in the circulating strains of influenza virus in Canada, and estimates, through periodic serosurveys, susceptibility to currently circulating and emerging strains. Canadian influenza surveillance information and actual representative strains are then shared with the WHO's collaborating centres for influenza to contribute to global influenza monitoring. This report summarizes influenza surveillance in Canada for the 1998-1999 influenza season. Influenza Activity Figure 1 shows the number and month of laboratory-confirmed virus isolation, detections, and serodiagnoses reported from laboratories that contribute to the Canadian Virus Reporting (CVR) program, a surveillance program covering all laboratory-diagnosed viral infections. In general, the influenza season in Canada began late in 1998 and continued into June 1999. During this period, there were 5,365 cases of laboratory-confirmed influenza infection. Of the 5,365 reported cases, 4,500 (84%) were influenza type A with the largest number (1,613) occurring in February. There were 865 (14%) laboratory reports of influenza B in the same period; the peak was reached in March (Figure 1). By comparison, in the 1997-1998 season, there were 4,441 (99%) reports of influenza type A and only 29 (< 1%) reports of type B(1). These data indicate that influenza A viruses remained predominant in the 1998-1999 season, and influenza type B virus activity also increased significantly. Figure 1 Laboratory evidence of human influenza virus infections in Canada, 1998-1999 season
Strain Characterization During the 1998-1999 influenza season, 378 isolates were collected from October 1998 through June 1999, and antigenically characterized at LCDC; 267 (71%) were similar to the 1998-1999 influenza type A(H3N2) vaccine strain, A/Sydney/5/97, and 110 (29%) were similar to the recommended type B vaccine strain, B/Beijing/184/93 (Figure 2and Table 1). One influenza A(H1N1) was characterized as A/Bayern/7/95-like virus, which is antigenically distinct from the 1998-1999 type A(H1N1) vaccine strain, A/Beijing/262/95. A large number of influenza A isolates arrived at LCDC in February, and most of the type B isolates were received in February and April (Figure 2). Table 1 indicates the provincial source and identity of submitted isolates. Figure 2 LCDC antigenic characterization completed on influenza virus isolates, 1998-1999 season, by month of submission
Table 1 LCDC strain characterization completed on influenza isolates in Canada, submitted from October 1998 to 30 June 1999
In late June, the United States Centers for Disease Control and Prevention(CDC) and Health Canada first reported an influenza outbreak among tourists visiting Alaska and the Yukon Territory. Four culture-confirmed cases of influenza A infection were reported from the Yukon Territory (three of them in tourists) and the virus isolates were submitted to LCDC for strain characterization. Hemagglutination (HA) inhibition testing and HA gene sequencing identified all four influenza isolates as A/Sydney/5/97(H3N2)-like virus, which is similar to the predominant influenza strain that circulated in Canada during the 1998-1999 influenza season. Recent increased use of antiviral drugs for the prophylaxis and treatment of infections caused by influenza A virus has made testing of isolates for drug resistance more important than in the past. A rapid assay has been established at LCDC to test influenza viruses for resistance to amantadine, an antiviral drug which is currently available in Canada for the prevention and therapy of influenza A infection. Preliminary testing of the assay on influenza isolates received during the 1998-1999 season and the isolates obtained from individuals who received amantadine has shown that this assay is rapid, sensitive, and specific. This assay will be available to all provincial laboratories in the next influenza season. Discussion During the 1998-1999 influenza season, both influenza A(H3N2) and influenza B viruses circulated in Canada, and the predominant virus was influenza A(H3N2). However, there was also a significant increase in influenza B activity toward the end of the season. Only one influenza A(H1N1) was isolated. All of the characterized influenza isolates were homogenous and antigenically similar to the vaccine component strains, A/Sydney/5/97-like(H3N2) and B/Beijing/184/93-like, indicating that the 1998-1999 influenza vaccine strains were well matched with the circulating virus strains(2). Globally, both influenza types A and B circulated worldwide. Influenza A(H3N2) viruses predominated in some countries and influenza B viruses predominated in others. In several countries, both viruses co-circulated. The majority of influenza A(H3N2) isolates from Africa, the Americas, Asia, Europe, and Oceania were antigentically closely related to A/Sydney/5/97-like viruses. A small portion of viruses were antigenically distinguishable from A/Sydney/5/97; however, these viruses were heterogeneous, and antigenic and genetic analysis did not reveal the emergence of a representative variant(2,3). Influenza type B isolates from all continents except Asia were antigenically related to B/Beijing/184/93. Influenza B viruses similar to either B/Beijing/184/93 or B/Shangdong/7/97 (belonging to B/Victoria/2/87 antigenic/genetic group) continued to co-circulate in Asia (China, Japan, Singapore, and Thailand). However, B/Victoria/2/87-like viruses were not isolated outside of Asia(2). The few laboratory-confirmed cases of influenza A(H1N1) identified and reported in Europe and North America were influenza A/Bayern/7/95-like. The majority of A(H1N1) viruses from Asia were related to A/Beijing/262/95(2,3). Persons who were vaccinated in an experimental vaccine trial with A/Beijing/262/95 in 1998 developed equivalent antibody levels against A/Bayern/7/95 and A/Beijing/262/95. The antigenic characteristics of the current and emerging influenza virus strains provide the basis for selecting the strain included in each year's vaccine. Therefore, WHO recommended the following strains as vaccine components for the 1999-2000 season:
or
Basically, the H3N2 and H1N1 vaccine components remained unchanged for the 1999-2000 vaccines. However, it should be pointed out that, due to the evolution of influenza B virus, recent influenza B strains have become antigenically different from the previous B/Beijing/184/93-like vaccine strain - B/Harbin/7/94. Therefore, the actual B strain used by North America manufacturers is B/Yamanashi/166/98 because of its growth properties and its antigenic similarity to circulating B/Beijing/184/93-like viruses(2). The B/Shangdong/7/97-like strain was recommended for use in the vaccines to be used in southern Asia countries. Acknowledgements The collaboration of laboratories in the CVR program and of provincial and hospital laboratories who forwarded early and representative isolates of influenza virus is a vital part of influenza surveillance in Canada. Influenza virus isolates were submitted from the following centres:
Carol Stansfield, Susan Normand, and Donalda Bowness of LCDC conducted influenza strain typing, genetic characterization, and amantadine susceptibility testing. References
Source: Y Li, PhD, Respiratory Viruses Section, Bureau of Microbiology, LCDC, Canadian Science Centre for Human and Animal Health, Winnipeg, Man.
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