PUBLIC HEALTH 33 



in the younger stages and as adults on domesticated animals, 

 such as cattle, and from these become transferred to man. 

 It has been experimentally shown that certain rodents are 

 susceptible to the disease, and a tick thus infected in the 

 nymphal stage can retain the disease organism till it becomes 

 adult. It may then reach its human host through the medium 

 of domesticated animals such as cattle. It appears that this 

 is the ordinary way in which human cases have their origin, 

 i.e., through the bites of adult ticks, although the newly 

 hatched "seed ticks" derived from eggs laid by infected 

 mother ticks are known to contain the organism also. 



Although Rocky Mountain fever is of minor importance at 

 present, it is feared that it may increase its range at any time, 

 since other ticks of wider distribution are apparently capable 

 of acting as carriers. 



Whether this may happen is by no means certain, however, 

 and the vigorous measures already undertaken to reduce the 

 abundance of ticks on domesticated animals will undoubtedly 

 bear fruit in the gradual reduction of this locally much- 

 dreaded disease. 



The flea is another domestic insect which was looked upon 

 only as a nuisance until it was shown that certain kinds of 

 fleas are agents in spreading bubonic plague. The most 

 terrible epidemics of which we have any historical record 

 have been those of plague, or "black death." One swept 

 from Egypt in the sixth century before the Christian era and 

 invaded Europe and Asia, where it raged for sixty years. A 

 similar one spread through the whole known world in the 

 fourteenth century and is thought to have caused over 

 twenty-five million deaths before it subsided. In 1898 Si- 

 mond suspected fleas as agents in the spread of plague and 

 his suspicions were later proved to be true through the re- 

 searches of Verjbitski and the Indian Plague Commission. 

 Plague is primarily a disease of rats and certain other ro- 

 dents, and is usually carried to man by the bites of fleas which 

 have become infected from plague-stricken rats. From 300,- 

 000 to 400,000 cases of this disease are reported from India 



