140 



Zetek (J.). The Ecology of Bubonic Plague.— ^nw. Enlom. Soc. 

 America, Columbus, Ohio, x. no. 2, June 1917, pp. 198-206. 



The excellent results obtained by the sanitary officers of the 

 Isthmian Canal Commission wdth regard to the control of malaria in the 

 Canal Zone, where it was found that maximum malaria and maximum 

 numbers of transmitters of malaria coincided with conditions of 

 maximum humidity, suggested to the author the desirability of 

 learning if similar ecological relations could be traced in the case of 

 any of the other diseases directly transmitted by Arthropods, especially 

 bubonic plague. The study of reports on plague investigations in 

 India lead to the conclusion that, while temperature has a certain 

 bearing on the incidence of plague, the most important factor is 

 humidity, ^^^len plague mortality and humidity are placed on the 

 same chart, it becomes at once evident that there is a direct relation 

 between the two. 



Nearly all reports on plague show that its maximum coincides with 

 the period of maximum numbers of fleas. Fleas are abundant if rats 

 are abundant and humidity is the critical factor determining at what 

 time of the year fleas are most numerous. Investigation has sho\\Ti 

 that a temperature above 80° F. affects the conditions to which the 

 bacillus is subjected in the flea's stomach, the bacillus disappearing 

 from the stomach more quickly than at lower temperatures, during 

 which fleas remain infective for longer periods. High temperatures 

 retard both oviposition and development, and low temperatures 

 prolong the life-cycle. High temperatures in India are always asso- 

 ciated with high humidity. Humidity is therefore inimical to the flea. 



Rat breeding was found to be at its minimum when humidity was 

 lowest, and, at its maximum when humidity was highest. Plague is 

 highest when humidity is lowest, and large numbers of rats are killed 

 oft', leaving only a few immune individuals to produce the next 

 generation. As the plague incidence decreases, rat-breeding becomes 

 more vigorous under conditions of high humidity, and a new colony 

 of non-immune rats results. The rat epizootic began in January and 

 declined in April ; during this period fleas reached their maximum. 

 A chart shows the corresponding prevalence of fleas on rats and of 

 plague in both rats and man. Fleas on all species of 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 caused a 

 marked increase of plague in man. From May onwards, plague 

 decreases, this being the period of rains. 



The black rat, M^is rattus, was foimd to harbour more fleas than the 

 brown rat, M. norvegieus, and is thus of greater importance with 

 respect to the transmission of human plague. The reason is that the 

 brown rat burrows wherever it can, preferably in moist situations, 

 whereas Mus rattus builds its nest above ground, in a drier habitat, 

 such as the walls of buildings. Moisture is inimical to fleas, and 

 therefore the greater number are found on the black rat, which is the 

 common Canal Zone rat, while Xerwpsylla cheo'pis is the common flea, 

 its natural host being the black rat. 



In India, then, the severity of an epidemic of bubonic plague bears 

 a direct ratio to flea abundance and to humidity, and this probably 

 holds true for other places where bubonic plague is endemic. The 



