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An epidemiologic and serologic study of arboviruses was done in 1972 in the basin of Lake Rudolf. The main object of this study was to demarcate the southern limit of the yellow fever epidemic which occurred from 1959 to 1962 in Southern Sudan and Ethiopia. Other purposes were to contribute to the inventory of arboviruses and their distribution in this region. The ethnic groups were studied: the Nyangatom and the Dassanetch in Ethiopia and the Turkana in northern Kenya. The results of tests on sera of the Nyangatom and the Dassanetch are practically identical: a high percentage of positive tests for group B. The Turkana on the other hand show a low percentage of positives. This is attributed to differences in the climatic characteristics of the two regions. The serological results suggest the presence of West Nile virus in the geographical areas inhabited by the three ethnic groups. The presence of Wesselsbron and probably Uganda S virus is suspected. Circulation of these viruses could be explained by the presence of Culex univittatus and other mosquitoes. As for yellow fever, the results suggest that the virus did not reach the areas studied, probably because of absence of efficient vectors.

A serological survey on 623 human sera was conducted in Burundi in 1980-1982, in order to evaluate the frequency of arboviral antibodies in the inhabitants of the three main areas: lowlands, central plateau and mountainous ridge. The results show a rather high activity of arboviruses, mainly in the lowlands (34.2% of inhabitants with antibodies). Chikungunya virus seems to be the most active arbovirus; the activity of Flavivirus is moderate; no trace of activity of yellow fever or West Nile viruses was found; Bunyavirus antibodies (particularly against Ilesha virus) were also detected.

Sera collected in May 1984 from 132 adult residents of Karamoja district, Uganda, were examined by haemagglutination inhibition tests for antibodies against selected arboviruses, namely Chikungunya and Semliki Forest alphaviruses (Togaviridae); dengue type 2, Wesselsbron, West Nile, yellow fever and Zika flaviviruses (Flaviviridae); Bunyamwera, Ilesha and Tahyna bunyaviruses (Bunyaviridae); and Sicilian sandfly fever phlebovirus (Bunyaviridae); and by immunofluorescence tests against certain haemorrhagic fever viruses, Lassa fever arenavirus (Arenaviridae), Ebola-Sudan, Ebola-Zaïre and Marburg filoviruses (Filoviridae), Crimean-Congo haemorrhagic fever nairovirus and Rift Valley fever phlebovirus (Bunyaviridae). Antibodies against Chikungunya virus were the most prevalent (47%), followed by flavivirus antibodies (16%), which were probably due mainly to West Nile virus. No evidence of yellow fever or dengue virus circulation was observed. A few individuals had antibodies against Crimean-Congo haemorrhagic fever, Lassa, Ebola and Marburg viruses, suggesting that these viruses all circulate in the area.

The phenomenon of arbovirus transmission by their vectors is based upon the constitution of virus-vector systems, not only at the species level, but also at an infra-specific level. The study of such systems leads to consider some particular aspects of their ecology which can have important epidemiological consequences (selection of viral clones by the vector, transovarial and sexual transmissions). Finely the authors approach the question of evolution of virus-vector systems, which appear as adaptative, functional and temporary associations.

The authors present here the results of a serological survey on arbovirus infections, which was conducted on 116 out of 505 inhibitants of Wuvulu island, the westernmost island of the Bismarck archipelago, located 260 km north of Wewak, Papua-New Guinea. The islanders are divided in two communities of unequal size: the Wuvulian themselves, who are Micronesians, and the coconut workers who are Melanesians coming from the Sepik district. In addition to the presence of a group A virus, this survey shows a recent infection in the Melanesians by Dengue type 2 virus, probably during the 1971-72 epidemic. The virus did not reach Wuvulu island, as shown by negative results observed in the micronesian sera. A better knowledge of the potential dengue mosquito vectors would be useful in order to define the risk of an eventual introduction of the virus on the remote island.