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Increase in Virulence of the Lansing Strain of Poliomyelitis Virus with Passage in Mice¹

From the Department of Epidemiology and Virus Laboratory, School of Public Health, University of Michigan, Ann Arbor, Michigan

Abstract

There was a progressive increase in the titer of the line of the Lansing strain of poliomyelitis virus maintained in this laboratory between the 52nd and 84th consecutive passages in mice, from 10^–2.5 to 10^–4.6. The greatest single increase between pools was 5-fold, and occurred after the 71st passage. The daily morbidity data, and the number of days before 50% of the mice were paralyzed demonstrate that with increasing numbers of passages there was a progressive shift toward earlier involvement.

There were no detectable immunologic differences between the line of the Lansing strain of poliomyelitis virus maintained in this laboratory and that maintained in another laboratory through approximately 200 passages in mice and 3 recent passages in monkeys. The line maintained in this laboratory was apparently unrelated to, and consequently not contaminated with, 2 strains of virus isolated from normal stock mice which developed spontaneous paralysis during the course of these experiments.

Footnotes

¹ Aided by a grant from the National Foundation for Infantile Paralysis, Inc.