_Plague in the Rat._—With acute plague in the rat one finds marked injection of the subcuticular surface of an exposed abdominal flap; certain glands, especially the neck ones, show marked congestion, haemorrhagic necrosis and periglandular infiltration; the pleural cavity contains much fluid; the liver has a yellow mottled appearance liberally sprinkled with discrete, yellowish-white granules about the size of a pin’s head, while the spleen is enlarged. Smears from the spleen or affected glands, as a rule, show the bacilli in great numbers. The San Francisco findings in plague rat autopsies varied somewhat from those noted above, which held for Indian plague rats. _Confusing Organisms._—Other organisms which may be obtained from plague-suspected material are those of the proteus or colon group, which may show bipolar staining, but culturally are quite different. Klein has noted that a nonmotile rod, which gives a striking bipolar staining and named _Bacillus bristolensis_, may be mistaken for the plague bacillus. Its growth in bouillon is similar to that of _B. coli_ and it coagulates milk. An organism, _B. pseudotuberculosis rodentium_, resembles the plague bacillus in a striking manner but is without virulence for rats. It is virulent however for guinea pigs and these animals can be immunized against plague with this closely related organism. Litmus milk cultures of plague show a very slight acidity while with _B. pseudotuberculosis rodentium_ there is a high degree of alkalinity produced. Wherry has reported two cases of ulcerative conjunctivitis with lymphadenitis of cervical glands, fever and marked prostration, due to infection with _B. tularense_, occurring in persons who had handled rabbits which had died of this plague-like infection. The organism was first noted by McCoy in squirrels in California. The symptoms and lesions in these animals are those of plague. Guinea pigs succumb after the cutaneous inoculation of material and show lesions markedly resembling plague. The organism, however, will not grow on ordinary simple media as does the plague bacillus. As will be noted in the chapter on Tularaemia the disease has a very limited geographical distribution. The _crucial test_ for any plague material is the power of the plague bacillus to infect a rat or guinea pig, when the material is rubbed on the shaven skin of the animal. _B. tularense_ will also pass through intact shaven skin and it produces lesions in the guinea pig similar to those of plague. Other organisms, however which might infect through intact skin produce lesions unlike those of plague. As a practical point it may be stated that cases showing a profusion of oval, bipolarly staining bacilli, in smears from glands or sputum, and with clinical manifestations of plague, are not likely to be other than plague; still, to be conservative, one should always inoculate animals cutaneously or subcutaneously.