Volume 22-22
15 November 1996

[Table of Contents]

1995-1996 INFLUENZA SEASON: CANADIAN LABORATORY DIAGNOSES AND STRAIN CHARACTERIZATION

Introduction

In collaboration with the World Health Organization (WHO), international collaborating laboratories, provincial laboratories, and other Canadian hospital and university-based virus 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 circulating strains of influenza virus in Canada and, through periodic serosurveys, estimates 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.

Influenza Activity

Figure 1 shows the number and month of laboratory-confirmed influenza virus isolations, detections, and serodiagnoses reported from 32 of the 39 laboratories that contribute to the Canadian Virus Reporting (CVR) program, a surveillance program covering all laboratory-diagnosed viral diseases. The influenza season in Canada began in late November 1995 and continued to late June 1996. In addition, at least one influenza isolate was reported each month from July to October 1995.There were 281 reports of influenza B from November 1995 through June 1996 with the largest number (126) occurring in April. One thousand, one hundred and ninety laboratory reports of influenza A and influenza A subtypes were received in the same period. The data from the Respiratory Virus Surveillance Program established by the Bureau of Infectious Diseases, LCDC, are in good agreement with what are shown in figure 1

Strain Characterization

Table 1 indicates the provincial source and identity of submitted isolates that were strain-typed at LCDC, while Figure 2 shows the change in influenza virus strains by month of submission to LCDC as the season progressed. Most influenza A strains received early in the season were A/Texas/36/91 (H1N1)-like viruses whereas A/Johannesburg/33/94(H3N2)-like strains accounted for most of the late-season influenza A isolates. A/Taiwan/1/86(H1N1)-like strains also were identified early in the season. Isolates resembling antigenic variant A/Wuhan/359/95 (H3N2) appeared later in April and May 1996. Two of the A/Wuhan/ 359/95-like isolates were later characterized at the Centers for Diseases Control and Prevention as similar to A/Alaska/ 10/95, a strain related to A/Wuhan/359/95. All the influenza B isolates received were B/Beijing/184/93- like strains, the majority of which arrived in April 1996.

Table 1 LCDC strain characterization completed on influenza isolates in Canada (submitted from 22 November 1995 to 28 June 1996)

* A/Texas/36/91 is the (H1N1) component of the 1995-1996 vaccine and A/Taiwan/1/86 is very closely related to A/Texas/36/91.

** A/Johannesburg/33/94-like virus was the influenza A(H3N2) vaccine component in the 1995-1996 influenza vaccine.

*** A/Wuhan/359/95-like virus is the WHO recommended influenza A(H3N2) component of the 1996-1997 influenza vaccine.

**** B/Beijing/184/93-like virus is the WHO recommended influenza B component of the 1995-1996 influenza vaccine and is antigenically indistinguishable from B/Harbin/7/94.

(Figure 3) shows the distribution of influenza A virus strains received at LCDC by month from different regions of Canada as the season progressed. Isolates of all four strains were received first from the Prairie provinces. In addition, A/Wuhan/ 359/95(H3N2)-like strains appeared in all regions except the Atlantic provinces in April 1996, suggesting that either the virus entered different regions in Canada simultaneously or it spread very rapidly once introduced into the country.

Figure 1 Laboratory evidence of human influenza virus infections in Canada, 1995-1996 season

Figure 2 LCDC antigenic characterization completed on influenza virus isolates in the 1995-1996 season by month of submission

Figure 3 Influenza A virus isolates characterized at LCDC by month of submission and region

Discussion

In Canada, the 1995-1996 influenza season came earlier and was over sooner than the previous one (1) . The predominating virus strains were antigenitically similar to vaccine component strains (2) . A/Taiwan/1/86-like strains had been the predominant H1N1 isolates in Canada from the 1987-1988 season to the 1991-1992 season and reappeared along with A/Texas/36/91-like strains in the 1995-1996 season (Table 1); however, antigenically A/Taiwan/1/86-like strains are closely related to the A/Texas/ 36/91-like vaccine component. B/Beijing/184/93 was the influenza B component of the 1995-1996 influenza vaccine, as recommended by WHO, and is antigenically indistinguishable from B/Harbin/7/94.

Influenza A(H3N2), A(H1N1), and B viruses continued to circulate worldwide (3) . Although most influenza A isolates were similar to A/Johannesburg/33/94, the influenza A(H3N2) vaccine component in the 1995-1996 influenza vaccine, an increasing number of recently isolated A(H3N2) strains from Asia, Europe, and North America are similar to the antigenic variant A/Wuhan/359/95. Virtually all influenza A(H1N1) viruses (98%) that have been antigenically characterized were similar to the reference strains A/Taiwan/1/86 and A/Texas/36/91 (3) . Antigeni-cally characterized influenza B viruses isolated recently in Asia, Europe, and the United States were similar to the reference strains B/Beijing/184/93 and B/Harbin/7/94 which are antigenically indistinguishable (3) . The 1995-1996 season experience in Canada was similar to the global picture. The National Advisory Committee on Immunization has recommended that the trivalent vaccines for the 1996-1997 season contain an A/Wuhan/359/95(H3N2)-like strain,an A/Texas/ 36/91(H1N1)-like strain, and a B/Beijing/184/93-like strain (4) .

Acknowledgements

The collaboration of laboratories in the CVR program and of provincial and hospital laboratories that 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:

• British Columbia Centre for Disease Control, Virology Services, Vancouver, BC;
• Provincial Laboratory of Public Health for Southern Alberta, Calgary, Alberta;
• Saskatchewan Public Health Laboratory, Laboratory and Disease Control Services Branch, Regina, Saskatchewan;
• Cadham Provincial Laboratory, Winnipeg, Manitoba;
• Regional Public Health Laboratory, Laboratory Services Branch, Virus Laboratory, Toronto, Ontario;
• Laboratoire de santé publique du Québec, Sainte-Anne-de-Bellevue, Québec;
• Hôpital G.L. Dumont, Moncton, New Brunswick;
• Victoria General Hospital, Halifax, Nova Scotia;
• Provincial Public Health Laboratory, St. John's, Newfoundland.
• Carol Stansfield of LCDC conducted the influenza strain typing.

References

1. Weber JM. 1994-1995 Influenza season: canadian laboratory diagnoses and strain characterization. CCDR 1995;21:181-85.

2. National Advisory Committee on Immunization. Statement on influenza vaccination for the 1995-96 season. CCDR 1995;21:105-13.

3. CDC. Update: Influenza activity - United States and Worldwide, 1995-96 season, and composition of the 1996-97 influenza vaccine. MMWR 1996;45:326-29.

4. National Advisory Committee on Immunization. Statement on influenza vaccination for the 1996-97 season. CCDR 1996;22:89-97.

Source:

S Zou, PhD, J Weber, PhD, Surveillance, Influenza and Viral Exanthemata, National Laboratory for Special Pathogens, Bureau of Microbiology, LCDC, Ottawa, Ontario.

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