Influenza surveillance in Pune, India, 1978-90.

Influenza surveillance in Pune, India, 1978-90* B. L. Rao1 & K. Banerjee2 Continuous surveillance of influenza was carried out in Pune between 1978 and 1990. Most of the cases were identified during investigation of 16 outbreaks of influenza in Pune over this period. The majority of cases were children. Ten of the outbreaks occurred during rainy seasons. A total of 290 iso- lates consisting of several antigenic variants of influenza type A (H3N2), type A (HlNl), and type B viruses were isolated from throat/nasal swabs that were processed in chick embryos and MDCK cell culture and identified using the haemagglutination inhibition test. These variants circulated every year or in alternate years. Nearly two-thirds of the influenza virus isolates (181 out of 290) were from children aged <10 years. Seasonal analysis indicated that the highest number of isolates (174) were collected during the rainy months of July, August and September, with the maximum number (93) in July. Introduction Acute respiratory diseases (ARD) cause enormous morbidity and mortality, particularly in infants and children in developing countries. It is estimated that about 15 million infants and young children die each year in the world from diarrhoea, ARD, and other diseases. Of these deaths, 14 million occur in developing countries, and ARD is responsible for about 4 million deaths and 500-900 million episodes of infection per annum in children (1, 2). In India estimates indicate that 600 000 children under 5 years of age die every year from ARD (3). Over 300 antigenic types of bacteria and viruses are known to cause ARD, with viruses outnumbering bacteria (4). In particular, influenza viruses cause frequent epidemics and periodic pandemics that affect large segments of the global population, owing to antigenic variation of the viral surface antigens. In view of the public health importance of influenza, WHO initiated a global network of surveillance centres in 1948. Influenza surveillance is carried out throughout the year to study the morbidity and mor- tality pattems and to detect as early as possible new antigenic variant strains for use in vaccines (5). In Pune influenza surveillance was initiated by the National Institute of Virology in 1976-77. The present article reports the results of the influenza sur- veillance conducted on humans in Pune between 1978 and 1990 as an ongoing programme. * From the National Institute of Virology, 20-A, Dr Ambedkar Road, Post Box No. 11, Pune - 411 001, India. I Assistant Director. Requests for reprints should be sent to Dr Rao. 2 Director. Reprint No. 5371 Materials and methods Selection of dispensaries/hospitals When influenza surveillance was initiated in Pune, dispensaries and small hospitals were chosen rather than large hospitals because the majority of patients with ARD seek medical aid from these dispensaries rather than from large hospitals. Subsequently, the paediatric departments of large hospitals were also included in the surveillance. The dispensaries/hospi- tals selected are located in congested localities of the city where the population is of low socioeconomic status. Attendance at the outpatient departments is high. New dispensaries were selected as and when necessary. Frequency of visits to the dispensaries! hospitals Regular weekly visits were made to two or three selected dispensaries/hospitals, irrespective of the incidence of ARD in the community. Whenever high incidence was noticed, frequent or daily visits were made to investigate the cases and to collect a suffi- cient number of specimens. This approach permitted outbreaks to be detected and investigated as early as possible. Influenza outbreaks characteristically arise and wane within a month; it is essential that transient outbreaks are detected as soon as they occur. Our approach also facilitated investigation of sporadic cases during the interepidemic periods. Thus, it was possible to make an overall assessment of the inci- dence of ARD in the community throughout the year. Case definition An individual with one or more of the following conditions, with or without systemic manifestations Bulletin of the World Health Organization, 71 (2): 177-181 (1993) © World Health Organization 1993 177 B. L. Rao & K. Banerjee (fever, bodyache, headache, malaise, weakness) was taken to be a case of ARD: common cold, pharyngi- tis, laryngitis, tracheitis, bronchitis, bronchiolitis, pneumonia, or bronchopneumonia. Clinico-epidemiology During the course of surveillance over the period 1978-90 (13 years), more than 10 000 cases of ARD among various age groups were investigated in Pune. The majority of cases were children, including infants. Most of the patients were seen during inves- tigations of 16 outbreaks of influenza. Generally, the cases presented with two or three manifestations of respiratory diseases and one or two systemic mani- festations. Some cases had only respiratory manifes- tations. Collection of specimens Throat and nasal swabs were collected from ARD cases during the acute phase of their illness (1-4 days) since viruses could be isolated equally easily from either. Because they are easy to collect, nasal swabs were generally taken from infants and young children. Throat/nasal swabs were taken from over 10 000 ARD cases. About 80% of these specimens were cultivated for influenza virus in embryonated chicken eggs (9-11 days' old) and about 30% in Madin-Darby canine kidney cell culture (MDCK) with crystalline trypsin. By and large, it was not possible to collect pair- ed blood samples from outpatient cases since they frequently refused to give blood. The method described by Kendal et al. was followed with some modifications for isolation of virus, conducting haemagglutination and haemagglutination inhibition tests using guinea-pig red blood cells to identify influenza isolates (6). Results Outbreaks investigated During the period 1978-90, several variants of influenza virus types A and B were isolated during the 16 outbreaks investigated in Pune, including the following important variant strains: A/USSR/77 (HlNI) in 1978 (7); A/Singapore/6/86 (HlNI) in 1986 (8); and B/Yamagata/16/88-like in 1990 (9). Virus isolation A total of 290 influenza virus isolates comprising several variants of influenza type A (H3N2) and A (HINI) and type B were isolated and identified (Table 1). MDCK cell culture, which was used from 1983, was superior to embryonated chicken eggs for influenza virus isolation (10). However, both methods of cultivation were employed to obtain opti- mum results. The strains circulated singly or in dif- ferent combinations depending on the particular outbreak. The variant strains of influenza type A (HINI), type A (H3N2), and type B circulated regu- larly either every year or in altemate years. Nearly two-thirds of the influenza isolates (181 out of 290) were from children aged <10 years. There were some isolates from infants also. Analysis of the isolates, by month, showed the greatest num- ber (174) were from the rainy months (July, August, September) with the maximum number (93) occur- ring in July (Fig. 1). Seasonality Pune has a tropical monsoon climate with a hot sea- son from March to May, a rainy season from June to September (with October as a transitional hot month) and a cool season from November to February. The annual average rainfall is 714.7 mm. Average climate data for Pune are shown in Fig. 1. Of the 16 outbreaks of influenza, 10 occurred in the rainy season, three in the hot season, one in the cool sea- son, and two in February and March. Correlation coefficients (r) were calculated between the number of isolates and each of the fol- lowing variables: rainfall (mm), relative humidity at noon and in the morning, maximum temperature, and minimum temperature (Table 2). To construct an equation for the statistically significant independent variables that gives the best Table 1: Influenza virus Isolates from human cases in Pune, 1978-90 No. of isolatesVirus strain 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 Total HlNl 6 29 10 6 1 9 - - 10 2 1 2 14 90 H3N2 1 - 26 35 1 20 5 6 - - 2 20 - 116 B - 2 1 5 - 6 - 23 1 11 6 6 23 84 Total 7 31 37 46 2 35 5 29 11 13 9 28 37 290 WHO Bulletin OMS. Vol 71 1993178 Influenza surveillance in Pune, 1978-90 Fig. 1. Plots showing correlation between Influenza virus isolates from Pune 1978-90, and various seasonal factors. J F M A M J J A S O N D Months Cool 0 Hot Rainy season season season prediction of the number of isolates, a multiple regression was carried out for rainfall (XI), tempera- ture difference (X2) and the number of isolates (Y). The following expression was obtained: Yestimate = 0.5512X1 + 3.1738X2-52.564 The analysis indicated that in addition to XI and X2 some other unidentified variables that we had not included were responsible for more than half of the variation in the values of Y (11). Collaboration with WHO Details on the occurrence of influenza outbreaks and on the strains isolated during the period of sur- veillance were communicated regularly to WHO. A representative number of isolates from each outbreak Table 2: Correlation between influenza virus isolates and the variables examined in the study Correlation between the number of Correlation Degrees of Statistical isolates and: coefficient (r) freedom evaluation Rainfall 0.697 10 Significant, P <0.05 Relative 0.572 10 Borderline humidity (noon) significance, 0.1 >P>0.05 Relative 0.257 10 Not significant, humidity (morning) P>0.05 Maximum -0.354 10 Not significant, temperature (Tmax) P>0.05 Minimum 0.452 10 Not significant, temperature (Tmin) P>005 Temperature -0.608 10 Significant, difference P<0.05 (Tmax-Tmin) was also sent to the WHO Collaborating Centre on Influenza Reference and Research, Atlanta, GA, USA, for further characterization of the strains. Most of these isolates were found to be similar to the various strains that were prevalent in the world when they were collected (Table 3). One of the strains (A/India/6263/80 (HINI)) that was isolated during the July 1980 monsoon outbreak, however, was iden- tified as a minor antigenic variant of the HINI strain, and is being used as a reference strain (12). Table 3: Influenza virus Isolates from Pune 1978-90 that were identified to be similar to the reference strains shown Type A (H3N2) Type A (H1Nl) Type B strains strains strains A/Victoria/3/75 A/USSR/90/77 B/Hongkong/5/72 A/Texas/1/77 A/Brazil/i 1/78 B/Singapore/222/79 A/Taiwan/i /79 A/lndia/6263/80 B/USSR/100/83 A/Bangkok/2/79 A/England/333/80 B/Hong Kong/8/83 A/Oregon/4/80 A/New Caledonia/4/83 B/Kanagawa/2/84 A/Shanghai/31/80 A/Singapore/6/86 B/Texas/1 /84 A/Arizona/2/80 A/S. Carolina/6/88 B/Ann Arbor/i/86 A/Philippines/2/82 B/Victoria/2/87 A/Taiwan/16/83 B/USSR/2/87 A/Mississippi/l/85 B/Yamagata/16/88 A/Sichuan/2/87 A/Sichuan/68/89 A/Beijing/353/89 WHO Bulletin OMS. Vol 71 1993 179 B. L. Rao & K. Banerjee Discussion The outbreaks caused by variant HINI strains in 1978 and 1986 and the B/Yamagata/16/88-like strain in 1990 in Pune were important epidemiologically. The A/USSR/77 (HINI) strain was first isolated in the USSR in November 1977 and then spread to other countries. This strain was responsible for out- breaks of influenza predominantly among persons aged <25 years; it was found to be similar to the influenza A (H lNl) strains that were prevalent in the world in the period 1946-57. The A/USSR/77 strain therefore infected mainly the non-immune younger age groups who were born after 1950, and it may have appeared because of the recycling of earlier influenza A (HINI) strains (13). In 1986 a new variant of influenza A (HlNl) strain (A/Singapore/ 6/86 (HINI)) was isolated and reported subsequently from a number of locations, including Pune. This strain was readily distinguishable from all the refer- ence A (HINI) virus variants that had been preva- lent between 1978 and 1985. The pattern of circula- tion of the various influenza virus strains did not vary from area to area of the city (14). Our continuous surveillance in Pune, which has a tropical monsoon climate, shows that influenza outbreaks occurred predominantly during the rainy months. Only few reports have appeared on the sea- sonality of influenza outbreaks in the tropics; for example, in Panama and Thailand (15, 16) during the rainy season (15, 16) and in Nigeria and Singapore during the hot and rainy seasons (17, 18). In countries with a temperate climate, influenza outbreaks occur in winter. This may be related to the improved transmission of airborne viruses at that time of the year and to the impairment of the host's respiratory tract defences due to the seasonal fall in temperature (19). A study of five successive winters of different severity in the United Kingdom indicated that the incidence of influenza was less in mild win- ters than in severe (20). Studies conducted under carefully controlled conditions of humidity and temperature in winter and summer have found that the transmission rates of influenza are higher in winter, suggesting that either relative humidity before contact may be a contributo- ry factor or unknown factors associated with winter may be operative (21). Our study has demonstrated that rainfall, relative humidity, and small differences between minimum and maximum temperatures influence the occurrence of influenza outbreaks in the rainy season but that other unidentified factors may also be involved. A serological survey employing sera from blood donors from Pune was carried out in 1980-81 (22). The findings indicated a broadening of antibody res- ponse to a greater number of influenza virus strains as the age of the subjects increased, an overall higher prevalence of antibodies to the pandemic strains, and a low-to-moderate prevalence of antibodies to the epidemic strains. Acknowledgements We are grateful to the Director and staff of the WHO Col- laborating Centre on Influenza Reference and Research, Atlanta, GA, USA, the medical officers of the Corporation Dispensaries and Paediatrics Department, Sassoon Gen- eral Hospital, Pune, and the staff of the Influenza Sec- tion, National Institute of Virology, Pune, for their help and cooperation during this investigation. We gratefully acknowledge the help of Ms N.K. Athalye, National Institute of Virology, Pune, in the statistical analysis of the data. The Meteorological Office, Pune, is thanked for kindly providing the average weather chart for Pune. Resume Surveillance de la grippe A Poona, Inde, 1978-1990 Une surveillance continue de la grippe a ete r6ali- s6e a Poona entre 1978 et 1990. La plupart des cas ont ete identifies au cours des investigations sur les flambees grippales survenues a Poona pendant cette periode. 11 s'agissait pour la plupart de cas p6diatriques. Dix de ces flambees ont eu lieu au cours de la saison des pluies. Au total, 290 isolements, comprenant plusieurs variants antig6niques des virus grippaux de type A (H3N2), de type A (HiN1) et de type B, ont ete obtenus a partir d'ecouvillonnages de gorge et d'6couvillon- nages nasaux cultives sur embryons de poulet et sur cellules MDCK, et identifi6s par inhibition de l'hemagglutination. Ces variants circulaient soit chaque annee, soit une ann6e sur deux. Pres des deux tiers des isolements grippaux (181 sur 290) provenaient d'enfants de moins de dix ans. L'analyse saisonniere a montre que le plus grand nombre d'isolements (174) ont ete recueillis au cours des mois pluvieux (juillet, aout et septembre), avec le maximum (93) en juillet. Les r6sultats de cette surveillance continue demontrent que la grippe survient essentiellement au cours de la saison de pluies a Poona, ou regne un climat de mousson. Ces observations donnent des renseignements pr6cieux sur les aspects 6pidemiologiques des maladies respira- toires aigues et de la grippe, qui seront utiles lors de la planification des programmes de prevention et de lutte. WHO Bulletin OMS. Vol 71 1993180 Influenza surveillance in Pune, 1978-90 References 1. Grant, J.P. The state of the world's children, 1990. Oxford, Oxford University Press, 1990. 2. Pio, A. WHO Programme on Acute Respiratory Infections. Indian journal of paediatrics, 55: 197-205 (1988). 3. Kumar, V. Need for a national control programme for acute respiratory infections. Indian journal of paediatrics, 54: 145-148 (1987). 4. A programme for controlling acute respiratory infec- tions in children: Memorandum from a WHO meet- ing. Bulletin of the World Health Organization, 62: 47-58 (1984). 5. Ghendon, Y. Influenza surveillance - update. Bul- letin of the World Health Organization, 69: 509-515 (1991). 6. Kendal, A.P. et al. 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