TRANSMISSION OF DISEASES BY INSECTS 245 



one of the simplest methods of treating stable manure being to spread it 

 out to dry, since the maggots cannot develop without moisture. 



For detailed information on everything of importance relating to the 

 house fly, and particularly on the mitigation of the fly-nuisance by con- 

 certed action in communities, Dr. Howard's admirable book on the house 

 fly should be consulted. 



PLAGUE. 



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

 place. In recent 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 scpticcemic, 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 enoimous 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 experiments 

 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. 



The weight of evidence, both observational and experimental, shows 



