§12 



THE ANIMAL CARRIERS OF DISEASES 



Bacot and Martin have proved the inoculative method of infection 

 in Ceratophyllus fasciatus, which, when blood- sucking, injects the 

 bacilli, when they are present in its stomach in such numbers as 

 to cause temporary obstruction at the entrance to that organ. 



With regard to the spread of the disease to man, the Commission 

 believes that the infection generally comes from Epimys rattus, 

 because the habits of that rodent bring it into close relationship 

 with man, and because the curve of its epizootic begins to rise ten 

 to fourteen days before that of the epidemic. This period is calcu- 

 lated to be made up of three days, during which the flea leaves 

 the dead rat, to which is added another three ^a^ys, which is the 

 incubation period of plague in man, and five and a half days, which 

 is the average duration of the fatal illness in man. 



Xenopsylla cheopis appears to be the flea by which plague is 

 spread from Epimys rattus to man. This rat-fiea will not merely bite 

 man when it cannot get rat's blood, but is capable of living for 

 three to four weeks on man's blood, and is often found on human 

 beings after inspection of plague-stricken houses. 



Further, it is believed that the spread of plague is due, not to 

 migration of rats, but to the carriage of infected rats on ships, and 

 of fleas in merchandise or on human beings. The Commission 

 apparently consider the last to be the most important method. 



Pneumonic plague, which occurs only in 2-5 per cent, of cases 

 during bubonic epidemics, spreads from man to man by bacilli 

 carried by the air, for Strong and Teague demonstrated that the 

 sputum in invisible droplets containing viable plague bacilli was 

 frequently to be found in the air near a patient. Teague and Barber 

 have shown that the fine droplets of sputum disappear very quickly 

 unless there is a considerable amount of aqueous vapour in the 

 atmosphere, as is found in very cold climates, and hence the ten- 

 dency for pneumonic plague to spread in those rather than in warm 

 climates. On the other hand, the bubonic or septicsemic is not 

 spread from man to man, but from rats to man. The epizootic 

 is the real disease, and the epidemic is only an offshoot. 



The above aetiology explains fully the predisposing causes of sex, 

 women staying more in the house than men; of house, of season, of 

 climate, and also the carriage of the disease from one place to 

 another by people, fodder, grain, bales of cotton and clothing, 

 rags, etc. 



Verjbitski in 1908 showed that bugs could act as carriers of the 

 bacilli, and this has been confirmed by Jordansky and Kladnitsky, 

 while Walker considersClinocoris rotundatus to be one of the carriers 

 of plague in India, having found 22 per cent, infected with 5. pestis 

 when collected from infected native huts. Moreover, he successfully 

 transmitted the disease from man to the rat by means of C. rotun- 

 datus. 



In California, Wherry, McCay, and others have shown that the 

 ground-squirrel [Citellus beecheyi) is subject to plague, and that its 

 commonest flea, Ceratophyllus acutus Baker, is the vector from 



