Immunologic Studies of Japanese Encephalitis Virus in Japan

Summary Viremia and clinically inapparent infection by JE virus were readily induced in birds of several species by subcutaneous inoculation of six different strains in low mouse passage. Virus titers usually reached 102.5–3.0 LD50/0.03 ml of blood. Incidence of viremia was related directly to size of virus inoculum and inversely to mouse passage level of virus and age of chickens (but probably not black-crowned night herons). Contrariwise, the time of appearance of viremia was related inversely to size of virus inoculum and directly to mouse passage level of virus. Maximal titer of virus and duration of viremia were unaffected by size of virus inoculum, and duration was inversely related to mouse passage level and age of chickens. Neutralizing and HI, but not CF antibodies, were found in plasma of previously viremic birds when tested for 8–12 weeks after infection. Antibodies formed in some birds that failed to circulate virus. HI antibody appeared promptly within 2–3 days of cessation of viremia and reached maximal titers within 14–21 days following infection. Neutralizing antibody did not appear until the 3rd–12th postinfection weeks and an occasional bird never produced significant levels of neutralizing antibody during the 3 months after development of viremia and HI antibody. The titer of HI antibody usually decreased during the second and third postinfection months but neutralizing antibody persisted longer. Occasionally both antibodies became undetectable between 3 and 9 months after infection.