The Number of Elementary Bodies Per 50% Lethal Dose of Meningo-Pneumonitis Virus as Determined by Electron Microscopic Counting

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Abstract
Summary: Counting of elementary bodies of meningo-pneumonitis virus in dialysed allantoic fluid by electron microscopy is described. Dialysis against distilled water for 18 to 24 hours at 5°C did not alter the lethal potency of the fluid for chick embryos inoculated via the yolk sac. Morphologic features of the elementary bodies in allantoic fluid, although altered slightly by dialysis, remained distinctive. The number of elementary bodies per LD50 in 4 experiments was found to be 200 to 1000, and the number per ID50 in a single experiment was 43. Factors influencing the numbers of elementary bodies per 50% lethal or infectious unit are discussed.