444 THE HEMORRHAGIC SEPTICEMIA GROUP 
that a smaller member of the squirrel family, Ammospermophilus 
lecuriis), is also susceptible to infection with B. pestis. The various 
members of the genus ]\Ius-^Ius norwegicus,^ ]\Ius rattus, ]\Ius alex- 
andrinus, and probably INIus musculus, "are the producers of acute 
outbreaks, the conduit for the carriage of the virus from its perpetuat- 
ing reservoir to the body of man." Guinea-pigs are somewhat more 
susceptible to infection with the plague bacillus than other rodents; the 
disease appears to have a seasonal distribution among rodents'- in India, 
and these epidemic periods coincide in time with epidemics in man. 
Immediately following epidemic periods considerable numbers of 
rodents appear to be relatively non-susceptible to infection. Rabbits^ 
and monkeys^ are also susceptible. Dogs and cats are more refractory; 
herbivora appear to be practically immune, at least to natural infection. 
The lesions observed in rats are striking and important because 
plague epidemics usually appear about two weeks earlier among 
these animals than in man. Infection may take place through infected 
fleas from other rats, from ingestion of dead plague-infected animals, 
or by inhalation. The lesions are those of a hemorrhagic septicemia; 
upon laying open the animal,^ the inguinal and axillary glands are 
usually enlarged (buboes), markedly injected and frequently hemor- 
rhagic. The contents may be firm, or, less commonly, purulent. 
The peritoneal and pleural surfaces are red and injected, and there 
is an excess of fluid in both cavities. The spleen is enlarged, engorged 
and moderately soft, and the liver usually presents a mottled appear- 
ance, due to punctiform hemorrhages and areas of necrosis which 
appear yellowish in contrast to the hemorrhagic points. A simple 
inspection suffices as a rule to establish a correct diagnosis, although 
cultures and smears should be prepared. Sometimes, however, rats 
may be infected with plague without visible lesions.*' Occasionally 
rats are submitted for diagnosis which are badly decomposed. The 
rapid overgrowth of adventitious bacteria makes the isolation of B. 
pestis difficult by ordinary methods. Albrecht and Ghon^ have shown 
that the plague bacillus, even in the presence of large numbers of con- 
taminating bacteria, may be obtained in pure culture by rubbing the 
suspected material upon the freshly shaved abdomen of a guinea-pig. 
The plague bacillus readily penetrates the skin and causes a rapidly 
fatal generalized infection with characteristic lesions. It may be 
obtained in pure culture from the internal organs. Fritsche^ has 
found that other bacteria, even of the hemorrhagic septicemia group, 
1 Spencer (Public Health Reports, 1921, 36, 2836) states that M. norwegicus may be 
immune to plague in certain places. 
2 See Jour. Hyg., 1908, 8, 266, for details. 
3 McCoy: Public Health Bull., April, 1911, No. 4.3. 
* Wyssokowitsch and Zabolotny: Ann. Inst. Pasteur, 1897, 11, 663. 
' Which should be done after treatment with an insecticide to kill ecto-parasites. 
« Williams and Kemmerer: Public Health Reports, 1923, 38, 1873. 
" Denkschrift der math-Xaturw. Klasse der kaiserl. Akad d. Wissensch., Wien., 
1898, vol. 66. 
8 Arb. a. d. kais. Gesamte, 1902, 18, 453. 
