The past incidence and distribution of yellow fever in west Africa as indicated by protection test surveys

Abstract

1. 1. The results of a yellow fever protection test survey covering the examination of 7,580 sera collected in 181 towns in eight colonies of West Africa are presented. 2. 2. The findings have demonstrated that yellow fever is far more prevalent in West Africa than was previously supposed ; great numbers of cases occur both in the coastal regions and in the interior, and approximately 25 per cent. of all specimens examined gave positive results. Few towns have escaped the disease during the life of the present generation. 3. 3. Practically the entire area covered by the survey has been demonstrated to be infectible, and positive sera have been obtained even from natives on the Jos Plateau at an elevation of over 4,000 feet. 4. 4. Work carried out in French colonies north of Nigeria indicates that epidemics occur in that semi-arid region but that the Sahara desert forms an effective barrier against the infection. 5. 5. Yellow fever is rarely recognized in the native, and the relatively small number of cases reported in Europeans gives no indication of the incidence or distribution of the disease. 6. 6. The findings have confirmed the opinion previously held that an endemic area of yellow fever exists in south-western Nigeria. They have not, however, made it possible to delimit this area, which may extend westward into neighbouring colonies. Furthermore, it has not been possible definitely to exclude endemicity in some other regions. Meteorological conditions in northern Nigeria would, however, seem to be unfavourable for the permanent existence of yellow fever infection, and this probably applies to the far interior of West Africa in general. 7. 7. Age-group studies in cities in the endemic area have brought out facts which are not in accord with opinions previously held by epidemiologists in the Western Hemisphere and based on less exact methods of investigation than are available at the present time. They have shown that the disease in endemic cities is not confined to children but that the percentage of immunes increases gradually up to old age and that some persons escape throughout life. It has also been shown that extensive epidemics occur in endemic areas. 8. 8. All these findings are of importance in connection with the formulation of quarantine regulations, especially for air services. 9. 9. The great value of piped water supplies and effective sanitary services in reducing or eliminating yellow fever infection was brought out in several of the more important places studied and especially in Freetown, Sierra Leone, and in some of the coastal cities of the Gold Coast and Nigeria.