1917] The Ecology of Bubonic Plague 205 



Gauthier and Raybaud (1903) find that the Indian Plague 

 flea, X. cheopis, constituted 25% of the flea population upon 

 ship rats at Marseilles, and that the numbers rapidly became 

 fewer as the distance of a locality from the docks increases. 

 Jennings (1910) found that 97.9% of the fleas on rats examined 

 by him at Panama were the Indian plague flea. We have no 

 plague endemic in the Canal Zone (thanks to efficient quaran- 

 tine), but we have everything favorable to plague epidemics — 

 the right fleas, plenty of rats, and a wet and dry season. 



D. The Plague Association. 



The severity of an epidemic of bubonic plague was shown 

 to depend upon flea abundance and upon humidity. Fleas 

 are abundant if rats are abundant, and humidity is the critical 

 factor determining at what time of the year fleas are most 

 abundant. The reports of the plague commission show that 

 at Bombay City rat breeding was at its minimum w^hen 

 humidity was lowest, and vice versa, it was most vigorous 

 when humidity was highest. Plague was highest when humidity 

 was lowest, and large numbers of rats were killed off, leaving 

 only a few immune ones with which to start the next progeny. 

 As plague dropped off, and to readjust the loss of equilibrium 

 in the rat world, there followed a vigorous breeding spell. 

 This is with humidity high. A new colony of non-immune 

 rats resulted. The rat epizootic began in January and declined 

 in April. During this period fleas reached their maximum-. 



Referring to the chart for a moment : Fleas on all rats were 

 at a maximum in March and April. Plague mortality in rats 

 reached its culmination in March. The fleas which left their 

 dead hosts increased plague among human beings from about 

 plus 20 to plus 360 within one month! From May on, plague 

 recedes; this is the period of the S-W monsoons, the rain winds. 



The chart shows more fleas on the black rat, M. rattus, 

 than on the-brown rat. This was found true on the Canal Zone 

 by Jennings. It places the black rat into greater importance 

 with respect to the transmission of human plague. This rat 

 is the common Canal Zone rat; so is X. cheopis the common 

 flea, whose natural host is the black rat. 



