Antigenic analysis of influenza viruses isolated in Thailand in the rainy season, 1989.

In the rainy season of 1989, 72 strains of influenza viruses were isolated in Thailand by using MDCK cell cultures, which included 36 samples belonging to H1N1, 24 samples belonging to H3N2 subtype of type A and 12 samples of type B1). The identified viruses were inoculated into developing hen's eggs of 10 days' age, and 22 (32.6%) of them were adapted to the egg after one passage, which included 8/36 (22.2%) of H1N1, 7/18 (38.2%) of H3N2 and 7/8 (87.5%) of type B virus. The adapted viruses were analyzed antigenically with the haemagglutination inhibition (HI) test using mouse antisera raised to 16 reference viruses shown in Table 1 and 2. As shown in Table 1, among eight H1N1 strains isolated in Chiang Mai in 1989, four (A/Chiang Mai/28, 71, 73 and 105/89) (50%) were antigenically similar to A/Yamagata/32/89 as well as to A/Chiang Mai/39 or 101/87 (i.e., any differences of HI titers from those of A/Yamagata/32/89 and A/Chiang Mai/39 and 101/89 were within two folds), one (A/Chiang Mai/120/89) (13%) was similar to A/Yamagata/120/86 (i.e., any difference of HI titer was within 2 folds), and the other three (A/Chiang Mai/27, 62 and 108/89) (38%) were antigenically not so different from A/Chiang Mai/39/87 or A/Yamagata/32/89 (i.e., only four folds differences of the HI titers to anti-A/Chiang Mai/39 and/or 101/89 sera were observed). Thus, seven out of eight strains (87.5%) of H1N1 influenza virus isolated in Chiang Mai in August, 1989 were antigenically closely related to what prevailed in Chiang Mai in 1987 and in Japan in 1989, suggesting that the antigenic variation of H1N1 subtype in Thailand and in Japan was minor during 1987 to 1989. Incidentally H1N1 subtype of the virus has not been isolated in Japan from May, 1989 until February, 19912), but 11 strains of the subtype were isolated in Chiang Mai in August 19903). As shown in Table 1, among 10 H3N2 strains, seven (A/Bangkok/235, 238 and 246/89 and A/Chiang Mai/9, 52, 80 and 86/89) (70%) were antigenically similar to A/Akita/4/88 which was equivalent to A/Hokkaido/20/89 (i.e., any difference of HI titer was within two folds), two (A/Chiang Mai/48 and 111/89) (20%) were similar to A/Sichuan/2/87 (i.e., any difference of HI titer was within two folds), and the other one (A/Chiang Mai/51/89) (10%) was antigenically not so different from A/Akita/4/88 (i.e., difference of HI titer was within two folds, except that to anti-A/Fukuoka/c-29/85 serum which was four folds). According to an antigenic analysis of H3N2