gii 



which a flea cannot pass, as it is said to be incapable of jumping 

 more than 4 inches, or surrounded by a curtain of wire gauze so 

 fine that a flea could not penetrate it, and exposed to infection, 

 they escaped, though others not so protected became infected, thus 

 disproving aerial infection. Further, the transmission by the rat- 

 flea was proved by constructing a glass box^, inside which two wire 

 cages were placed at a little distance, but side by side, each standing 

 in a tray filled with sand. Each cage had a lid, through which rats, 

 food, water, etc., could be introduced, and the whole apparatus 

 was covered in with fine muslin to prevent the escape of the fleas. 

 A plague-infected rat and a number of rat-fleas were placed in one 

 cage. When this rat died, a healthy rat was placed in the other 

 cage, and after some time the dead body of the infected rat was 

 removed, when it was found that the new rat became infected with 

 plague, and fleas containing plague bacilli were found upon it. 

 This experiment was repeated many times, 45 per cent, of the 

 exposed rats taking the disease. Further, fleas infected by biting 

 plague rats, when placed upon healthy rats, produced the disease in 

 55 per cent, of the experiments. 



The Commission calculated that the blood of an ordinary plague 

 rat in two-thirds of the cases contains more than 100,000,000 bacilli 

 per cubic centimetre, and that a flea's stomach could hold 0-5 cubic 

 millimetre of blood. Therefore, when the flea gorged itself on the 

 average plague-stricken rat it received at least 5,000 bacilli. These 

 bacilli are found only in the stomach and in the alimentary canal 

 posterior to that viscus, especially the rectum, and escape from the 

 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 dfluted 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 

 faecal 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. Both males and 

 females can transmit the disease, but it was found that one infected 

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

 on rats, and the one by which the infection in these experiments 

 was usually spread, wsiS Xenopsylla cheopis, but others — e.g.,Cerato- 

 phyllus fasciatus and Pulex irritans — were found also capable of 

 causing the disease. 



