LIVE POLIO IRUS VACCINES

Correlation of In Vitro Markers and Monkey Neurovirulence
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Fic. 1. Induction of variants of various marker combinations from d-t- Schaeffer's LSc Type 1
poliovirus.
poliovirus strain is not necessarily synonymous
with virulence for monkeys.
As regards the t marker, however, the finding
by Lwoff and co-workers 16 that selection of t+
variants from an originally t- strain was accompanied
by reappearance of monkey virulence
implied a closer correlation between the t marker
and neurovirulence than is the case for the d
character. Our results support this implication.
It may be, as intimated by Lwoff, that this correlation
is due to the relative multiplication rates
of "cold" or "hot" virus variants within the central
nervous system of the monkey whose normal
body temperature varies between 39 ° C. and 40 °
C. In this regard, it may be interesting to speculate
on what the results might be if Pasteur's
classic experimental method with anthrax in
chickens were applied to an avirulent t- poliovirus
strain in monkeys. Would the behavior of
the virus remain the same in an animal whose
body temperature was lowered to that optimal
for virus multiplication? Be that as it may, as
yet insufficient investigation of field isolates has
been reported to be sure that a t+ character is
invariably associated with neurovirulence, and
some exceptions have already been pointed out.
For example, a mouse-selected derivative of Type
2 MEF 1 virus which lost its neurovirulence for
monkeys was shown by Sabin to be still capable
of growing readily at 39 ° and 40 ° C., perhaps
even better than a fully virulent Type 1 Mahoney
strain.l 3 Also, in at least one instance a d+t+
Type 1 isolate from children fed Sabin's LSc
strain was found by Yoshioka and others '4 to
be still avirulent for monkeys by the intracerebral
route. On the other hand, Melnick et al.," 7
studying viruses excreted by children vaccinated
with Sabin's strain of virus, obtained evidence