An epidemic of urban yellow fever occurred in Senegal in 1965, in a region of dry savanna some 3,000 km2 in area around the town of Diourbel. The human population of this area, about 150,000, is largely divided among villages of from 200 to 1,000 inhabitants. The population at risk consisted almost exclusively of the 50,000 children under 12 yr of age. Mosquito-eradication and mass vaccination campaigns had wiped out yellow fever in French-speaking Africa by 1953. The Dakar vaccine used caused a certain proportion of encephalitic complications among children; it was therefore decided in 1960 to cease systematic vaccination of children under 10 yr of age, and to build up a stock of 17D vaccine for use on children in an emergency. Routine serological tests had indicated the need for revaccination of the child population in Senegal at about the time that the epidemic broke out. The first confirmed case of yellow fever was announced on 12 November 1965; the first fatal case revealed by subsequent inquiries was on 7 October, but it is probable that the start of the epidemic dated back to the beginning of the rainy season in July of 1965. Prompt antimosquito, vaccination and sanitary measures terminated the epidemic by 6 December 1965, although isolated cases were recorded up to 15 January 1966. The morbidity has been variously estimated as between 2,000 and 20,000 and the case-fatality rate may have been of the order of 15%; about 90% of the deaths were among children less than 10 yr of age. Virological studies on sera from 106 children with yellow fever aged from 4 mth to 13 yr from the focus of the epidemic allowed the isolation of 16 strains of virus, antigenically identical with the French neurotropic strain. Serological studies of 38 pairs of sera from patients aged up to 10 yr from the village of N'Goye in the epidemic zone gave evidence of the development of infection in 19 cases; in 9 of these cases, the virus had also been isolated. Further serological studies on more than 1,000 sera from outside the focus of the epidemic showed the presence of immunity in about 5% of the cases, although in some areas the immunity level was considerably higher. These studies also confirmed the activity of West Nile and Zika virus in the region. A relatively thorough clinical study of 67 patients showed that many of the classical clinical signs of yellow fever were missing in most patients, largely owing to their youth. The situation was also complicated by a simultaneous epidemic of acute infective hepatitis. It may be concluded that there is a risk that an epidemic of yellow fever might be overlooked for some considerable time where only children are susceptible, as in the present case, unless special attention is paid to this possibility. The measures taken to stamp out the epidemic were 3-fold: mosquito eradication, vaccination and sanitary measures. The DDT spraying campaign was so effective that within 5 days of the declaration of the epidemic, only 159 adult mosquitoes could be captured in the epidemic zone - insufficient to allow the isolation of yellow fever virus. Nearly 2,000,000 vaccinations were carried out between mid-November 1965 and mid-January 1966, about 120,000 with 17D vaccine and the rest with Dakar vaccine. In view of the urgency of the situation, it was decided to lower the minimum age for administration of the Dakar vaccine from 10 yr to 2 yr in the epidemic zone and to 5 yr in the rest of the region involved, including the town of Dakar. The use of the Dakar vaccine led to 240 cases of meningo-encephalitic complications, nearly all in Dakar itself; in 25 of these cases, the vaccinee died. No meningo-encephalitic complications were observed in the epidemic zone, although the minimum age for vaccination was much lower there. A careful sanitary check on all routes out of the epidemic zone allowed only 3 persons suffering from yellow fever to leave this region; 2 of these died in Dakar, and one at the border between Senegal and Mauritania.
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