IMMUNITY AND IMMUNIZATION 447 
found in diseased rats, producing lesions superficiall\' not unlike 
plague, will immunize rats against B. pestis and rice rcr.sa. This 
bacillus must be sharply dift'erentiated from the plague bacillus in the 
microscopic diagnosis of plague in rodents. It fails to infect guinea-pigs 
by the cutaneous method, however, and is less virulent for laboratory 
animals. 
Active immunization of man against plague has been attempted 
by Haffkine;^ using broth cultures of plague bacilli grown in shallow 
layers of bouillon containing droplets of cocoanut or other neutral oil 
on the surface to increase the development of the organisms by stalac- 
tite formation. After about six weeks' incubation, during which time 
several crops of bacilli develop and sink to the bottom, the culture 
is heated to 60° to 65° C. for an hour, and 0.5 per cent phenol is 
added. Two to 3.5 cc. of the killed culture are injected subcutaneously 
into adults, proportionately smaller amounts into children.- Usually 
a second injection is given, somewdiat larger in amount, after ten 
days. Rowland^ has some evidence in favor of using vaccines prepared 
from bacilli obtained in the same geographical region, rather than 
heterologous cultures. The German Plague Commission^ used forty- 
eight-hour agar cultures of virulent plague bacilli emulsified in salt 
solution and sterilized at 65° C; 0.5 per cent phenol was added as a 
p^eser^'ative. The amount for injection into an adult was the equiva- 
lent of one agar culture of the organism. Available evidence indicates 
that prophylactic inoculation against plague reduces materially both 
the morbidity and mortality of the disease. The protection, as the 
statistics show, is by no means absolute, and it appears that the 
duration of resistance to infection is indeterminate, probably on the 
average several months. A serum^ has been prepared from horses 
immunized against plague bacilli, but its use in man has on the whole 
been irregular and disappointing; the chief practical use appears to be 
in those cases where exposure to infection is reasonably certain, as for 
example, in those attending plague patients.^ The excessive cost of 
the serum is prohibitive for general use. Seeman/ however, has 
reported good results with an antiplague serum in an outbreak of 
plague in 1914. Massive doses (200 cc.) were given, and repeated if 
necessary. Baxter-Tyrie has also found the Yersin serum to have been 
distinctly valuable in the Queensland epidemic.^ 
Active destruction of rodents appears to be the best prophylactic 
measure against plague. 
' Report of Indian Plague Commission, 1901, vol. 5; Bull. Pasteur Inst., 1906, 4, 825. 
- For discussion of results see Bannermann (Centralbl. f. BakterioL, 1901, 29, 87.3). 
^ .Jour. Hyg., Plague Supplement, 1915, No. 4, p. 759. 
■* Gaffky, Pfeiffer, Sticker and Dieudonne: Bericht u. d. Thatigkeit der zur Erfor- 
schung der Pest im Jahre, 1897, etc., Berlin, 1899. 
5 Yersin: Ann. Inst. Pasteur, 1897, 11, 81. 
8 See Utility of Antiplague Vaccines and Serums. McCoy and Chapin: Public 
Health Reports, 1920, 35, 1647. 
• Am. Jour. Trop. Dis., 1915, 3, 281. 
» Jour. Hyg., 1905, 5, 311. 
