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Progress Report on Japanese B Encephalitis OCT-541 Attenuated Virus Strain^*

W. WcD. Hammon AND Johng S. Rhim

Last year we presented at these meetings a very brief report on the derivation and characteristics of an attenuated strain of Japanese B encephalitis (JBE) virus. This will be briefly summarized before presenting new work, including human trials.

From Okayama Culex tritaeniorhynchus pool 541 (abbreviated OCT-541), known to contain JBE virus, we reisolated the virus directly in hamster kidney tissue culture (HKC), passed it serially in heavy inoculum by decreasing temperature steps, finally at 24°C. Then three serial plaque selections were made for particles of lowest mouse neurovirulence. Several pools grown at 24°C from this line proved to have exceedingly low neurovirulence for all laboratory animals by essentially all routes of inoculation, including more limited trials in monkeys and Mexican burros. Failure to develop antibody or immunity by almost all animals, except those inoculated intracerebrally with tremendous amounts of virus, suggested that this was too "cold" a strain and was essentially incapable of multiplying at the normal temperature of these animals.

^* Presented at Symposium on Immunization Against Arbor Virus Infections, American Society of Tropical Medicine and Hygiene, Atlanta, Georgia, 2 November 1962.

This work was carried out under the sponsorship of the Commission on Viral Infections, Armed Forces Epidemiological Board, and was supported by the U. S. Army Medical Research and Development Command, Department of the Army, under Contract No. DA-49-194-MD-2042.

Dr. Rhim was a post-doctoral fellow of this department for one year under a training grant in Microbiology, National Institutes of Health, and is now Research Associate, Louisiana State University International Center for Medical Research and Training, Apartado 688, San José, Costa Rica.