1422 



PLAGUE 



flea solely with the faeces. It was proved, however, that the bacilli 

 multiplied in the body of the flea by allowing infected fleas to feed 

 solely on uninfected rats, a fresh one being supplied each day, when 

 abundant bacilli were found up to the twelfth, and once to the 

 twentieth day, thus proving that multiplication must have taken 

 place, otherwise the original number of bacilli would have become 

 much diluted by the feeds with fresh blood. Further, it was dis- 

 covered that the proportion of fleas in whose stomach multiplication 

 took place was six times greater in the epidemic than in the non- 

 epidemic season. In the former season the bacilli could be found 

 easily up to the fourth and even to the twelfth day, while in the 

 latter never after the seventh day. Infected fleas were found to 

 transmit the disease for seven to fifteen days. 



The method of infection probably is in one of two ways — either 

 fcccal pollution of the proboscis, or else faecal pollution of the wound 

 made by the proboscis, which was found quite large enough for the 

 purpose of introducing the bacilli into the skin. Martin's experi- 

 ments tend to show that regurgitation often takes place, due to 

 plugs of bacilli in the oesophagus (see Chapter XXXV. ) . Both males 

 and females can transmit the disease, but it was found that one in- 

 fected flea alone was unlikely to do so. The flea most commonly 

 found on rats, and the one by which the infection in these experi- 

 ments was usually spread, was Xenopsylla cheopis Rothschild, but 

 others — e.g., Ceratophyllus fasciatus and Pulex irritans — were found 

 also capable of causing the disease. 



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 days, 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-flea 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. 



In Ceylon, as observed by Hirst, the commonest rat-flea is Xeno- 

 psylla astia Rothschild, which seems to bite man with great reluct- 

 ance at temperatures over 80° F. 



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 



