First recorded outbreak of yellow fever in Kenya, 1992-1993. II. Entomologic investigations.

The first recorded outbreak of yellow fever in Kenya occurred from mid-1992 through March 1993 in the south Kerio Valley, Rift Valley Province. We conducted entomologic studies in February-March 1993 to identify the likely vectors and determine the potential for transmission in the surrounding rural and urban areas. Mosquitoes were collected by landing capture and processed for virus isolation. Container surveys were conducted around human habitation. Transmission was mainly in woodland of varying density, at altitudes of 1,300-1,800 m. The abundance of Aedes africanus in this biotope, and two isolations of virus from pools of this species, suggest that it was the principal vector in the main period of the outbreak. A third isolate was made from a pool of Ae. keniensis, a little-known species that was collected in the same biotope. Other known yellow fever vectors that were collected in the arid parts of the valley may have been involved at an earlier stage of the epidemic. Vervet monkeys and baboons were present in the outbreak area. Peridomestic mosquito species were absent but abundant at urban sites outside the outbreak area. The entomologic and epidemiologic evidence indicate that this was a sylvatic outbreak in which human cases were directly linked to the epizootic and were independent of other human cases. The region of the Kerio Valley is probably subject to recurrent wandering epizootics of yellow fever, although previous episodes of scattered human infection have gone unrecorded. The risk that the disease could emerge as an urban problem in Kenya should not be ignored.

[Yellow fever virus, dengue 2 and other arboviruses isolated from mosquitos, in Burkina Faso, from 1983 to 1986. Entomological and epidemiological considerations]. / Virus amaril, dengue 2 et autres arbovirus isolés de moustiques, au Burkina Faso, de 1983 a 1986. Considérations entomologiques et épidémiologiques.

An arbovirus surveillance was carried out in Burkina Faso from 1983 to 1986. It was based on crepuscular catches of mosquitoes on human bait in some wooded areas and in one town. The total collection was 228 catches with an average of 8 men per catch. The total number of mosquitoes caught was 44,956 among which 32,010 potential vector of yellow fever; all these mosquitoes were analysed for arbovirology. In the south-western part of the country (region of Bobo-Dioulasso), surveillance was conducted each year from August to November, whilst the circulation of Aedes-borne arboviruses is well known to be favoured. In 1983, 1984 and 1986, seven strains of yellow fever virus were isolated in circumstances remarkably similar. They came from selvatic areas and never from the town. They concerned only Aedes (Stegomyia) luteocephalus which is the very predominant potential vector of yellow fever in the region. They were obtained in low figure, between 1 and 4 per year. They occurred from 27th of October to 21th of November. These observations confirm that the southern portion of the Sudan savanna zone of West Africa is the setting of a customary circulation of yellow fever virus and therefore belongs to the endemic emergence zone. In 1986, two strains of dengue 2 virus were isolated. One concerned Ae. luteocephalus from the selvatic area, the other Ae. (St.) aegypti from the heart of town. These data suggest two distinct cycles for dengue 2 virus, one urban and one selvatic, which could coexist simultaneously in the same region. In the south-eastern part of the country (region of Fada-N'Gourma) a yellow fever epidemic occurred between September and December 1983; its study has enable to precise their entomological aspects. The entomological inoculation rate of yellow fever virus has been evaluated to 22 infected bites per man during the month of october, for a man living close to forest gallery. 25 strains of yellow fever virus strains was isolated from Ae. (Diceromyia) furcifer which is the potential vector the most abundant in this region: the main role of this species in an epidemic was confirmed. An investigation in September 1984 had not permitted isolation of the virus therefore it is suspected that the large epizootic circulation of virus in 1983 has not been renewed the year after. In total 59 viral strains belonging to 10 different viruses were isolated from 9 species of mosquitoes.(ABSTRACT TRUNCATED AT 400 WORDS)

[The epidemiology of yellow fever in Western Africa]. / L'épidémiologie de la fièvre jaune en Afrique de l'Ouest.

Observations made during the epidemics in Côte d'Ivoire (1982), Burkina Faso (1983), Nigeria (1986 and 1987) and Mali (1987), together with studies conducted in the last 10 years, particularly in Côte d'Ivoire, now make it possible, without calling into question the dynamics of yellow fever virus circulation in space and time, to redefine some features of the pattern suggested in 1977 and refined on a number of occasions up to 1983. The endemicity area is still the region of epizootic and enzootic sylvatic circulation, and contains the natural focus and the endemic emergence zone. --The natural focus is no longer confined to the forest alone, now that transovarial transmission has been demonstrated. --The endemic emergence zone is tending to become conterminous with the endemicity area on account of increasing deforestation. Emergence in forest regions, due to Aedes africanus, is still few and isolated, unlike that observed in savanna regions where A. furcifer is the major vector. The different behaviour of these two vectors and their population dynamics determine the quality of the man-vector contact and are responsible for these two patterns of emergence. --The emergence front limits the endemicity area in the north. Its position varies and depends on annual rainfall patterns. The epidemicity area, where the virus does not circulate before an epidemic and where the immunity status of unvaccinated populations is low, is geographically heterogeneous. It consists of regions to the north of the emergence front and of towns anywhere. It is characterized by high potentials for the development of A. aegypti populations. Only man can introduce the virus into this area. Three types of epidemic are distinguished, depending on the vectors: --Urban epidemics resulting from transmission by a domestic vector. These epidemics always occur within the epidemicity area, either in dry savanna (rural subtype) or in towns (urban subtype). The virus is introduced into the ecosystem by man. Transmission is always strictly interhuman. --Intermediate epidemics consist of two successive phases: first of all there is a series of endemic emergences, followed by interhuman transmission involving A. aegypti. These epidemics can only occur in the endemicity area. --Sylvatic epidemics occur in villages, but only involve the sylvatic vectors. They result from a conjunction of a very large number of emergences for which A. furcifer is almost always mainly responsible, and occur in the endemicity area, usually close to the emergence front. Transmission is never strictly interhuman, as the same vector populations are responsible for epizootic and epidemic transmission.(ABSTRACT TRUNCATED AT 400 WORDS)

[Yellow fever in Western Africa, 1973-1987. Observed facts--studies realized, campaign, prevention and forecast]. / La fièvre jaune en Afrique occidentale, 1973-1987. Faits observés--etudes réalisées, lutte, prévention et prévision.

This global analysis of the situation is based on a review of notifications, observations and studies concerning yellow fever in 16 of 17 countries of the West African subregion (Algeria is not affected for the years 1973-1987). In view of this analysis and the epidemiological picture, the author proposes a plan of concerted action to confine yellow fever to its monkey-to-monkey cycle in the wild. Official notifications vary greatly from one country to the next. Any of five major causes could explain this: ecological and ethological conditions that favour circulation of the virus in the wild and man-to-man transmission to different extents; the immune status of the populations; the difficulty of diagnosing especially isolated cases; lack of means for investigation; and negligence. The quantity and gravity of human cases are systematically underestimated, sometimes to a great extent. Lack of resources and difficulty of diagnosis, but also in many instances the attitude of the population, can account for this. Modern means of investigation, faster intervention by specialists, and better knowledge of how the virus is transmitted, have shown recently an increasing gap between notifications and the actual situation. Research and monitoring programmes are particularly important. The programmes under way in Senegal and Côte d'Ivoire have already resulted in considerable improvement in the action against epidemics. Because of these programmes, our knowledge of the very complex pattern of viral circulation is improving, thereby helping us develop systems for prevention and enabling us to forecast epidemics. Priority areas for study and research are: (i) Basic programmes for detailed study of all the topotypes of the virus, and identification of the viral amplification cycles that recur over several years. Such studies are under way in Senegal and Côte d'Ivoire. They would be particularly useful in Ghana and in Nigeria, where the taxonomy and bioecology of A. africanus s.l. should also be studied. (ii) Surveys of sylvatic vectors should show, for the endemic zones of each country, the type of contact between sylvatic vectors and man in both rural and wild biotopes. (iii) A complete map of the Stegomyia foci with an assessment of their potential epidemic risk (an analysis of the productivity of the sites depending on their type). (iv) Assessment of the immune status of the populations of the various ecosystems of each country, taking account of past or present vaccination strategies. There are several prevention strategies to choose from. The author advocates preparation of a scientifically based, subregional plan for optimum cost effectiveness.(ABSTRACT TRUNCATED AT 400 WORDS)

Urban yellow fever epidemic in western Nigeria, 1987.

A large epidemic of urban yellow fever occurred in April and May 1987 in Oyo State, western Nigeria. The principal vector was Aedes aegypti, breeding in domestic water containers. The 1987 outbreak followed an epidemic of sylvatic yellow fever in eastern Nigeria the previous year, and probably resulted from introduction of the virus by viraemic travellers. The outbreak in Oyo State ended in early July, by which time 805 cases and 416 deaths had been officially notified. However, surveys of 3 villages in the epicentre, a region with over 4 million inhabitants, indicated an infection rate of approximately 20%, a clinical attack rate of 2.9% and a mortality rate of 0.6%, suggesting that the true incidence of cases and deaths far exceeded the official reports. Yellow fever virus was isolated from persons with fully developed yellow fever as well as mild febrile illness. One virus isolate was made from blood of an individual with mild illness, who had received 17D vaccine 5 d earlier; monoclonal antibody analysis showed that the isolate was a wild-type virus. Larval indices of Ae. aegypti were very high; however, low vector competence of the Ae aegypti population may have provided a constraint on spread of the epidemic. In late 1987 a third epidemic appeared in Niger State, northern Nigeria, with 644 reported cases and 149 deaths. The vector(s) involved is (are) unknown.

[A rural epidemic of Igbo Ora virus (with interhuman transmission) in the Ivory Coast 1984-1985]. / Epidémie rurale a virus "Igbo Ora" (avec transmission inter-humaine) en Cote-d'Ivoire en 1984-1985.

The first outbreak caused by Igbo Ora virus is described. Virus circulation has been demonstrated up on thirty-three sick persons in Yamoussoukro area (Ivory Coast).

[Sylvatic yellow fever in Africa recent advances and present approach (author's transl)]. / La fièvre jaune selvatique en Afrique: donnees récentes et conceptions actuelles.

Recent works carried out in west and central Africa resulted in numerous yellow fever virus isolations from sylvatic mosquitoes in the forest-savanna transitional zone. This virus was also obtained from monkeys, whereas studies on sequence and duration of the observed epizootics permitted a previsional approach of the yellow fever mechanisms in the same belt, the epidemiological importance of which was stressed ("emergence zone"). In the same course of research and publications, the prevalent part of the involved mosquitoes as virus-reservoirs was constantly emphasized ("reservoir-vector"). Recent investigations on transovarial transmission and yellow fever isolations from male mosquitoes caught in the field, provide decisive support to such a conception. It can explain that epizootics may be locally observed several years in succession, despite the fact that yellow fever virus circulation seems to be fundamentally of a dynamic character. Yellow fever virus was recently obtained from ticks and tick-eggs.

[A strain of amaril virus isolated from Aedes africanus in the Ivory Coast]. / Une souche de virus amaril isolée d'Aedes africanus en Côte-d'Ivoire

The yellow fever virus has been isolated from a pool of Aedes africanus caught on men in a region situated between forest and savannah near Touba, in the Ivory Coast. This isolation, occurring at an interepidemic period, proves for the first time in West Africa, the part played by this mosquito in the transmission of the yellow fever virus.