32 



rattus by about ten days, and the opinion is expressed by the Indian 

 Plague Commission that the usual course of the infection is from the 

 Miis norvegicus to the Mus rattus, and as the latter rodent is a house 

 dweller in India it is the most frequent source of human infection. 



In San Francisco the Mus rattus population is comparatively 

 small, contributing perhaps 2 per cent of the total rat population of the 

 city; but in the section of the city where the large warehouses are 

 found, especially those where oriental goods are stored, about 15 per 

 cent of the rats taken are Mus rattus. So far as concerns plague in- 

 fection about 5 per cent of the rat cases were in Mus rattus. It may 

 be of interest to note that the last infection found among rats in San 

 Francisco was among the Mus rattus in a large warehouse near the 

 water front. Two plague-infected rats were found in this building, 

 one October 21, 1908, and the other October 23, 1908. A large num- 

 ber of mummified carcasses, all Mus rattus, were found in the building, 

 and it seems not unhkely that a somewhat extensive epizootic had 

 occurred among them. No previous case of rat plague had been 

 found in the city for eighty-five days, though about 25,000 rats had 

 been examined during that period, and none have been found in the 

 six months since, although over 30,000 rats have been examined. 

 Our records show that of 84 infected rats, 79 were Mus norvegicus, 

 and the remainder were Mus rattus. Some of the latter may have 

 been Mus alexandrinus, as the two species (Mus rattus and Mus alex- 

 andrinus) were not clearly differentiated in the earher examinations. 



THE GROSS LESIONS OF NATURAL RAT PLAGUE— ACUTE PLAGUE. 



SUBCUTANEOUS INJECTION. 



This is the sign which usually first attracts attention. Wliite (4), 

 in discussing plague in rats, states that "the most noticeable post- 

 mortem appearance of the plague rat is the engorgement of the sub- 

 cutaneous blood vessels, together with a diffuse pink color of the sub- 

 cutaneous muscles, which have a pecuKar dry, waxy translucency. " 

 It has been our experience frequently to have an attendant who is 

 dissecting rats remark that he had found an infected rat after the 

 first incision was made in reflecting the skin. The injection is dark 

 red, and upon close inspection one sees that the small vessels are 

 uniformly distended with blood. It is usually distributed over the 

 whole surface of the body, but on two occasions we have seen it con- 

 fined to the side of the body on which the primary bubo was found. 

 A bright pink injection is a rather common finding among rats in San 

 Francisco. It is not likely to be mistaken for the injection of plague 

 infection. Subcutaneous oedema, confined to the vicinity of the bubo, 

 is occasionally encountered. 



In our experience in San Francisco an injection identical in ap- 

 pearance with that found in plague infection was found only twice, 



