9io THE ANIMAL CARRIERS OF DISEASES 



round, for it appears to be profoundly altered by the temperature, 

 being diminished in the hot weather of the tropics and the cold 

 weather of the Temperate Zone. Thus in Bombay the Plague 

 Commission found 1,766 plague- infected rats in one week in the 

 season December to May, and only 20 to 30 in the season June to 

 November. The cause of this variation has already been given. 



But all rats are not equally infected, for it was found in Bombay 

 that there were two principal species, Epimys norvegicus and E. 

 rattus, and that during the epizootic period no less than 1,334 of the 

 1,766 belonged to the former species, while in the non-epizootic 

 period it alone carried on the disease. The reason assigned for this 

 difference was that the numbers of the flea population of the two 

 rats were very different, E. norvegicus possessing about double the 

 number of E. rattus. Further, it was noticed that the curve of 

 E. norvegicus infection began to rise about ten days before that 

 of E. rattus, which points to the origin of the infection of the latter 

 from the former in the first instance. 



E. norvegicus, which is not nearly so numerous in Bombay as 

 E. rattus, lives outside houses, for the most part in sewers, drains, 

 and stables, and has a great facility for burrowing, and is a good 

 climber. It, however, has never been found above the third floor 

 of a house. It forms its nest in one of its burrows, and breeds all 

 the year round, but has two special seasons, one in March and one 

 from June to October, the average family being eight. 



E. rattus is more common in Bombay than E. norvegicus, especially 

 in houses, where it increases, relatively to the other, up to the third 

 floor, but above that level it alone infects the house. It is not so 

 common in gullies, compounds, stables, go-downs, and food and tea 

 shops as E. norvegicus. The common meeting-ground of the two 

 species appears to be the lower floors of houses, gullies, and go- 

 downs. Though a domesticated rat, it can climb and burrow. It 

 forms its nest in cupboards, heaps of firewood, etc., and breeds 

 all the year round, but especially from June to October, the average 

 family being five. 



The spread of the plague from E. norvegicus to E. rattus, accord- 

 ing to the experiments of the Commission, is neither by direct con- 

 tact nor by air, soil, or food, but solely by the flea. Contact was 

 excluded by placing healthy rats in the same room with plague- 

 infected animals from which the fleas had been removed, when it 

 was found that none of them developed plague. The experiment 

 was kept up for a long time, replacing dead infected rats with 

 freshly infected rats, and, further, the room was never cleaned out, 

 so that the healthy animals lived in contact with the infected 

 urine and faeces, and even ate food polluted therewith, and yet not 

 one contracted plague, thus excluding transmission by contact, soil, 

 and food. 



Again, when healthy animals were suspended in cages 2 feet 

 from the ground, so that the fleas could not get to them, or placed 

 on the ground, and surrounded by 6 inches of tangle-foot, over 



