Relative Resistance of Newborn Mice to Poliomyelitis Virus. ∗

Summary

The Lansing strain of poliomyelitis virus, unlike other neurotropic viruses in mice, produced the disease more readily and rapidly in mature mice than in newborn animals. The relative resistance of the newborn manifested itself by prolonged incubation periods and survival times after onset of paralysis in those inoculated with larger amounts of virus (170 LD₅₀), and by a failure to develop the disease following the intracerebral inoculation of smaller amounts (2-17 LD₅₀). The prolongation in incubation period was most marked in mice inoculated at 1, 2 and 3 days of age, was still definite but progressively less marked in those tested at 4 to 7 days of age, while suckling mice inoculated at 15 days of age exhibited incubation periods almost identical in their distribution with those of weaned, 21 to 35-day-old mice. The amount of intracerebrally inoculated virus required to initiate infection in newborn mice 1 to 3 days of age was 10 to 30 times greater than in 21- to 35-day-old mice. Although the incubation period was markedly prolonged in the newborn mice, the level of virus multiplication attained at the onset of paralysis was the same in the suckling as in the older mice. Mice becoming paralyzed during the first 14 days of life survived much longer (61% for 3 to 9 days) than the litter mates whose incubation periods were prolonged beyond 15 days of age (12% for 3 to 5 days). There was no evidence of in-apparent infection in mice inoculated during the first week of life or subsequently, since the majority of surviving mice succumbed to a second inoculation of virus 35 to 80 days later.