Reappraisal of modified neutralization test for Inoue-Melnick virus

Ito et al. (1992) reported an improved modification of the neutralization (NT) test for Inoue-Melnick Virus (IMV) by prolonged 90 min incubation of the mixture of the virus and serum. For confirmation, we compared the results of the NT test by 90 and 60 min incubation. Against human sera, the NT test by 90 min incubation was significantly more sensitive than the test by 60 min incubation. By 90 min incubation, the NT titers increased by 3 to 6 times and approximately 6% of antibody-negative adult sera previously determined in the tests by 60 min incubation turned to positive (more than 1:10). However, antibody-negative sera of Japanese children examined previously in the tests by 60 min incubation remained negative after the 90 min tests. On the other hand, against rabbit immune sera, no significant difference of NT titers was observed between the two NT tests for 90 and 60 min incubation. Also we reinvestigated Japanese adult sera for type 2 IMV infection by the NT test for 90 min incubation, but did not find any serum of type 2 or intermediate type infection. Therefore, it is not necessary to change the previous conclusion that in Japan IMV infections are mostly of type 1.