PLAUGE AND BACILLUS PESTIS 817 



ground squirrel, 19 and for various species of ground moles such as 

 the Manchurian tarbagan (Arctomys bobac). 20 



The spread of plague by rats has long been recognized and even 

 in ancient times mortality in rats has been associated with large 

 epidemic outbreaks. The most important recent experimental work 

 on this matter was done by the British Indian Plague Commission 

 at Bombay. This commission demonstrated the relationship between 

 rats and plague infection in carefully conducted experiments in 

 which considerable numbers of rats were used. According to the 

 Commission, the most important species of rats are the Epimys nor- 

 vegicus and Epimys rattus. Over thirteen hundred of some seventeen 

 hundred rats found infected, belonged to these two species. Other 

 rats can also be infected and the danger of plague exists wherever 

 rats are found. 



The rat problem is a very important one, not only in connection 

 with plague, but in connection with economic loss as well. Creel of 

 the United States Public Health Service 21 has called attention to the 

 necessity of rat extermination for economic reasons alone. The dis- 

 tribution and number of rats in the world is much greater than 

 anyone ordinarily supposes. Creel states that in the cane producing 

 tropical and semi-tropical countries, Porto Rico, the West Indies, 

 the Hawaiian Islands and the Philippines, there is an enormous rat 

 population. He states that on one cane plantation in Porto Rico 

 where there were less than 500 people, 25,000 rats were killed in six 

 months. He estimates that in the United States the rat population 

 is probably as great as the human population, and the annual economic 

 cost per rodent is higher than $1.00 a piece. Computing the upkeep 

 of rats as one-half cent per day and estimating their number as above, 

 Creel says that a sum of $167,000,000 is lost annually to the country 

 by rat depredations. 



According to the British Plague Commission, the usual way by 

 which rats are infected from others is by means of fleas, and this, 

 as first suggested in 1898 by Simond, is the method by which the 

 disease is carried to man. In the British Indian Plague Commission 

 experiments, when healthy and infected rats, entirely free from fleas 

 were placed together, no plague developed, even when these rats were in 

 contact with the urine and feces of the infected ones and with polluted 

 food. But when fleas were introduced, infection occurred. The most 



19 McCoy, Jour. Infec. Dis., 5, 1909. 



20 Wu Lien Teh., Jour. Hygiene, 13, 1913. 



n Creel, Rep. No. 135, TT. S. Pub. Health Serv., Vol. 28, No. 27, 1913. 



