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A Single-Dilution Method for the Estimation of LD₅₀ Titers of the Psittacosis-LGV Group of Viruses in Chick Embryos

From Camp Detrick, Frederick, Md.

Abstract

It has been demonstrated with several members of the psittacosis-LGV group of viruses that there exists a linear relationship between the concentration of virus inoculated into eggs by the yolk-sac route and the average day of death of the embryos. By plotting of repeated large scale titrations according to the average day of death at each dilution and the LD₅₀ values obtained, an average line is drawn from which it is possible to estimate directly the LD₅₀ value of a virus suspension from the average day of death of a group of eggs inoculated with one dilution. The linear relationship has been shown to hold with two strains of psittacosis virus (Gleason and 6BC), meningopneumonitis virus (Cal 10), and Louisiana pneumonitis virus (Borg).

It is possible to use either eight or nine day old embryonated eggs, the resulting titer differences being within the limits of error of the test.