536 SCIENCE IN AFRICA 



African Health Conference in 1935 (League of Nations 1936). 

 It appeared that much of the extension of the veld epizootic, pre- 

 saged at the previous Conference, had taken place. In the Union 

 and countries on its northern and western borders the incidence 

 of human plague is determined by epizootics among wild rodents, 

 infection being for the most part direct from them, and to a 

 less extent from domestic rats infected by fleas from wild rodents. 

 In the more northern territories, such as Kenya, Uganda, and 

 Tanganyika, the increased importance of domestic rats is apparent, 

 but it is certain in some cases that wild rodents have also been 

 infected. The fact that, in several countries, epizootics have 

 occurred among wild rodents for considerable periods before their 

 discovery, points to the necessity for systematic observation and 

 also for co-ordination between adjoining countries. 



The history of plague in South Africa, as summarized by Sir 

 Edward Thornton (League of Nations 1936, p. 100) is as follows: 

 'From 1892 to 1905. plague was introduced at the ports and caused 

 extensive outbreaks amongst domestic rodents and a considerable 

 number of human cases in urban areas. From 1903 to 1905 striped 

 mice [Rhabdomys pumilio) in the bush surrounding one or more of 

 the ports became infected, and the infection spread slowly through 

 the bush until it reached, in 191 4, sandy country inland, where 

 gerbilles were plentiful, when rapid spread all over the inland 

 parts of the Union, with isolated human cases, occurred. During 

 the period 1914-33, human cases were almost entirely infected 

 from veld rodents, and domestic rodents were found infected in 

 exceptional cases only. During 1934-5, with the breeding up of 

 veld rodents owing to favourable seasons, numerous waves of infec- 

 tion occurred, resulting in an increased number of human cases. 

 Further, domestic rodents became infected in a number of 

 places.' 



The fact that epidemics of plague are nearly always associated 

 with increases in the population of rats or wild rodents opens up 

 an important problem in the realm of animal ecology {see Chapter 

 VIII). It is becoming recognized that these animal populations 

 are subject to periodic fluctuations in numbers, first increasing to 

 such a degree that there is food shortage and weakening of stamina, 

 followed by disease epizootics. Meanwhile the flea population 



