524 SCIENCE IN AFRICA 



(i) Urban, which is carried by Aedes, is generally recognizable 

 clinically, is normally a house infection, is often epidemic and has 

 a high mortality; (2) Rural or jungle, which may or may not be 

 carried by Aedes, is often not recognized clinically, is associated 

 with forest occupations and not with houses, is sporadic and has 

 a low mortality. 



Study of its geographical distribution have been largely carried 

 out by the International Health Division of the Rockefeller Foun- 

 dation, which maintained until 1934 a yellow fever laboratory at 

 Yaba near Lagos. This laboratory has been taken over as part of 

 the Medical Research Institute of the Nigerian Government, 

 which maintains, with the assistance of a grant from the Rocke- 

 feller Foundation, a yellow fever unit under Dr. Merrett. The 

 chief method in studying distribution consists in examining speci- 

 mens of blood from persons selected at random in order to ascer- 

 tain what proportion of them have been infected with the virus. 

 Persons who have had yellow fever, or who have been infected 

 with the virus without having the disease, become immune. Their 

 blood, when injected into white mice, protects these animals 

 against a dose of virus which is ordinarily fatal. This is called the 

 'mouse protection test'. The survey work already completed has 

 resulted in maps and descriptions indicating the endemic distribu- 

 tion of the disease. Roughly this covers a wedge-shaped area 

 extending on the west from Nigeria, the Gold Coast, Sierra 

 Leone, the Gambia, Liberia, and French West Africa, through 

 French Equatorial Africa, the northern and central part of the 

 Belgian Congo to the south-western part of the Anglo-Egyptian 

 Sudan, and Uganda. Many of the areas where the mouse protec- 

 tion test has proved positive, but no clinical cases have been 

 found, are termed 'silent'. They are possibly infected with the 

 rural type of yellow fever mentioned above. Recent work in South 

 America has shown, however, that cases of yellow fever do occur 

 in such silent areas, so these areas may be sources of infection 

 spreading to towns and villages where the disease appears in its 

 dangerous urban form. Accordingly much importance has been 

 attached to fuller investigation of the affected areas, especially 

 in Uganda, where the dense population presents the greatest 

 dangers, and to this end the Rockefeller Foundation in 1937 



