AGf~ICULTURAL RESEARCH, PUSA.

258 PRACTICAL BACTERIOLOGY
inoculation, and no erythema when readings of the test
are made. For the reaction to be recorded as positive
the diameter of the erythematous area should be at
least 1 cm.; a typical positive usually shows an area
2--3 em. in diameter. Pseudo-reactions, in which both
toxin and control injections produce an erythematous
reaction, may occasionally occur in adults, but such
reslllts are relatively uncommon in children. A positive
Dick reaction indicates that the subject is sensitive to
the streptococcal toxin, and that there is an absence of
natural or acquired immunity. The reaction is positive
in the early stages of scarlet fever, being visible over
the redness of the rash, but becomes less intense as
the disease progresses, and is usually negative after the
third or fourth week of the disease. Convalescents
from scarlet fevel' generally yield a negative reaction.
It may be stated briefly that a positive Dick l'eaction
shows susceptibility to scarlet fever, and an absence
of immunity to the streptococcus toxin. Up to the
present it has not been found possible to standaTdise
streptocoecus (Dick) toxin aecuratdy as lnboratol'Y
animals do not normally l'cuct to this product.
The toxin is prepared hy growing a strain of Strepto­
COCCIIS scarlatinar: in Hartley's hl'nth for forty-eight
hours at 37° C. The culture is then centrifugcd at
high spced and the supernatant fluid decanted and
passed tllTough a tested earthenware (Berkefeld) or
Seitz iilter. The filtmte constitutes the "toxin."
Phenol (0'5 pel' cent.) is added as a preservative, and
for use the toxin is diluted 1 : 1000 'with normal saline
or preferably the buffer solution used for the reag'cuts
of thc Schick test (vide p. 288). .
It has been shown that seruIfl from a convalescent
scarlet fever patient, whcn injected intradermally in
any early case of scarlet fever, causes a blanching
or extinction of the rash around the site of injection.
This is termed the Schultz-Chadian reaction, and
shows that convalescent serum contains neutralising
substances for the scarlet fever toxin. A similar
result is obtained with antitoxic serum (vide injj·a).