FLEAS 



The Plague. — The aetiology of plague has been placed on a sure 

 footing by the labours of Kitasato, Yersin, Cantlie, Simpson, 

 Thompson, Kolle, Martini, and the Special Committee already 

 mentioned, together with the Second Indian Commission. 



It is caused by the Bacillus pestis of Kitasato and Yersin, which 

 is found in the fluid of the initial cutaneous vesicle, the buboes, the 

 spleen, the blood, and the sputum in cases of pneumonia. When 

 inoculated into monkeys, cats, rats, guinea-pigs, squirrels, mon- 

 gooses, bats, jerboas, etc., it causes the typical disease, while in 

 bovines and equines it only causes local reactions. 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 



Fig. 473. — Rat Flea. 



must be looked upon as an offshoot. In rats it produces either as 

 acute or a chronic attack, the post-mortem appearances of which 

 are different. 



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 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 



