While some strains of ECHO 9 virus were found to be completely incapable of multiplying in newborn mice or even of being adsorbed by their tissues (e.g., the prototype Hill strain), other naturally occurring strains readily multiplied even after inoculation of as little as 3 TCD50 of virus. With the multiplying strains, the infection remained clinically inapparent except after inoculation of very large doses, usually in the range of 105 to 107.5 TCD50. Investigation of the question why such large doses were required to produce paralysis indicated that for paralysis to occur virus multiplication had to reach a level of 108 TCD50 or more within 4 days after inoculation of mice less than 1 day old. The reason for this was found in the fact that at 5 to 6 days of age the mice lost their susceptibility to paralysis even when multiplication was capable of progressing to the indicated high level. Thus, speed of multiplication and extent of muscle involvement before the 5th day of life were the determining factors. Passage in tissue culture had no effect except to yield a larger dose for inoculation, while serial propagation in mice resulted in a gradual enrichment of virus particles capable of more rapid multiplication in mice and in a concurrent greater paralytogenic activity of smaller doses.

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