HUMAN DISEASES 537 



increases accordingly, and on the death of the rodent hosts, the 

 infected fleas transfer themselves to man or other animals. 



In South Africa the recent work of Harvey Pirie and Murray 

 at the Institute for Medical Research has shown that amonsr at 

 least some of these wild rodents this periodicity exists; therefore it 

 seems probable that the small South African rodents fluctuate in 

 numbers like the voles and lemmings in the northern hemisphere; 

 that they normally die off every few years from diseases harmless 

 to man; and that bubonic plague has spread among them and in 

 some instances replaced these natural diseases, though in other 

 instances the latter have reasserted their importance. Although 

 human plague has not so far become very serious or widespread in 

 Southern Africa, except in Angola, the area in which rodents have 

 become endemJcally infected has steadily increased since 1921, 

 which gives a sinister aspect to the situation. 



In South Africa the Witwatersrand plague committee was estab- 

 lished early in 1935. An assistant health officer and a senior 

 rodent inspector were detailed to carry out inspection of the reef 

 area, in which an epizootic had been notified. Their findings are 

 published in the report of the department of public health (South 

 Africa 1936, D.R.). 



The possibility that plague in other parts of the continent has a 

 periodicity, dependent on fluctuations in numbers of rodents and 

 their fleas, has been considered by C. B. Symes (1930), who re- 

 viewed the outbreaks in East and West Africa, and concluded that 

 there is at least an indication of periodicity, the main epidemics 

 having been in 1912-13, 1916-17, 1920-1, and 1923-4. 



In East Africa the centre of plague infection is Lake Victoria, 

 where its spread since the establishment of shipping in the lake 

 ports has paralleled the medieval epidemic which spread around 

 the Mediterranean. In Uganda the seriousness of the disease can 

 be judged from the fact that in the twenty years up to 1932 some 

 52,000 deaths are estimated to have taken place from plague, the 

 climax being in 1929 during which year there were over 5,000 

 deaths. In 1930 Sir Edward Thornton visited the Protectorate to 

 advise on control measures, and since 1932 there has been a con- 

 siderable decrease, followed by a slight rise in 1935. It is feared 

 that the decrease after 1932 may have been due not so much to the 



