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ESTIMATION OF TITER OF VENEZUELAN EQUINE ENCEPHALOMYELITIS VIRUS PREPARATIONS FROM A SINGLE-DILUTION ASSAY¹

^a United States Army Biological Laboratories, Fort Detrick, Frederick, Maryland

ABSTRACT

RILEY, JEAN M. (U.S. Army Biological Laboratories, Fort Detrick, Frederick, Md.), WILLIAM C. PATRICK, III, AND WILLIAM E. CAMPBELL, JR. Estimation of titer of Venezuelan equine encephalomyelitis virus preparations from a single-dilution assay. J. Bacteriol. 85:1256–1260. 1963.—When suspensions of Venezuelan equine encephalomyelitis (VEE) virus were injected intracerebrally into groups of mice, a nearly linear relationship was observed between the concentration of the virus injected and the mean reciprocal time-to-death of the mice. A total of 91 VEE preparations were assayed in duplicate, and, by plotting the relationship between the reciprocal time-to-death for mice given the 10^–6 dilution of virus and the MICLD₅₀ (mouse intracerebral challenge, LD₅₀ response) values for the virus preparations, a reference curve was established. Using this reference curve, it was possible to estimate directly the LD₅₀ values of virus suspensions of unknown concentration from the mean reciprocal time-to-death of a group of mice injected with a single dilution. In this work, the number of mice used was reduced by 62.5%, the titrations were complete in 3 to 5 days compared with the usual 10 to 14 days, three to four times as many assays could be done in a day, and no assays had to be repeated since end points were not missed. The precision of the single-dilution assay compared favorably with that of the LD₅₀ titration.

¹ In conducting the research reported herein, the investigators adhered to the Principles of Laboratory Animal Care as established by the National Society for Medical Research.