1420 



PLAGUE 



guinea-pigs, squirrels, mongooses, bats, jerboas, etc., it causes the 

 typical disease, while in bovines and equines it only causes local 

 reactions, while camels are insusceptible. Canines, birds, and 

 reptiles appear to be immune. It causes a natural epizootic in rats, 

 which apparently is the true disease, from which that of man must 

 be looked upon as an offshoot. In typical outbreaks there should 

 be, firstly, the enzootic; secondly, the epizootic; then the endemic, fol- 

 lowed by the epidemic in the afiected country; and imdcViy pandemic. 

 In rats it produces either an acute or a chronic attack, the post- 

 mortem appearances of which are different [vide Chapter XXXV.). 



In the acute type a bubo is found in 85 per cent, of infected rats, 

 being present usually in the neck or axilla. If the bubo. is absent, 

 a plague rat can often be recognized by the subcutaneous congestion, 

 the purplish-red appearance of the muscles, the waxy, mottled, or 

 finely granular appearance of the liver, the haemorrhages, and the 

 pleural effusions. The diagnosis can be confirmed by finding the 

 bacilli in the bubo, the spleen, or the blood. 



The chronic type of the disease consists of encapsuled caseous 

 foci, or abscesses containing bacilli, and found in the spleen and 

 other organs. This form plays no part in the spread of the disease. 



The epizootic does not continue with equal virulence all the year 

 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, -E^m^ys norvegicus a.nd 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 differ- 

 ence 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. 



In Ceylon also the percentage of infected rats was higher in 

 E. norvegicus than in E. rufescens, the local representative of 

 E. rattus, but the septicsemic type was more frequent in the latter. 



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, 



