260 ENTOMOLOGY 



PLAGUE 



In the ancient history of Europe epidemics of plague occupy a large 

 place. For many years this pestilence has thrived in China and India, 

 and following an outbreak in 1894 in Hong Kong, the plague reached 

 the western hemisphere for the first time, appearing in Brazil, Argentina 

 and other South American countries, in Mexico and San Francisco. 



The cause of plague is Bacillus pestis, an organism abundant in the 

 secretions and excretions of plague-stricken animals. 



Three varieties of the disease are distinguished as follows: 



(1) the bubonic, in which the bacilli cause enlargements of lymphatic 

 glands; 



(2) the septiccemic, characterized by the presence of large numbers 

 of bacilli in the blood and highly virulent; 



(3) the pneumonic, in which the respiratory organs are affected, the 

 sputum showing the bacilli in enormous numbers; this form, relatively 

 rare, is the most fatal. 



Transmission. Plague is primarily a disease of rats, an epidemic 

 of plague in these animals having often been observed to precede as 

 well as accompany an epidemic among human beings. The disease 

 affects also mice, cats, dogs, calves, sheep, pigs, ducks, geese and many 

 other animals. 



Though rats and other of the lower animals may contract the septi- 

 caemic type of the disease from feeding on parts of animals killed by 

 plague or on cultures of Bacillus pestis, the disease is commonly trans- 

 mitted among rats neither by contact nor through the atmosphere, but 

 by means of fleas. Healthy rats in association with diseased rats do 

 not become infected as long as fleas are excluded; but a transfer of fleas 

 from the latter to the former starts the disease. By various experi- 

 ments the Indian Plague Commission demonstrated the important part 

 played by rat-fleas in the transmission of plague. Zirolia found that 

 the bacilli even multiply in the mid-intestine of the flea, retaining their 

 virulence for a week or more. Bacot found that the European rat-flea 

 (Ceratophyllus fasciatus) remained infective, when isolated from a 

 host, for forty-seven days. 



The weight of evidence, both observational and experimental, 

 shows that plague is transmitted from rats to man by several species 

 of fleas and also by bedbugs. Verjbitski, whose experiments on this 

 subject were particularly precise and thorough, found that plague can 

 be conveyed by the bites of these insects and that the opening made 



