The following is based largely on the authors' summary and conclusions. In the course of mainly routine surveillance work in the vicinity of Entebbe, Uganda, in 1968, 15 strains of chikungunya virus were isolated from man or mosquitos. These comprised nine strains from Aedes africanus (Theo.) collected from the Zika forest in March-May, one strain in March from a mosquito collector who had been working at the Zika forest, four strains in April-June. from patients whose homes were scattered over the Entebbe peninsula, and one strain from Mansonia fuscopennata (Theo.) collected at the Zika forest in June. Intensified mosquito collections in the immediate vicinity of Entebbe township from April to July failed to yield further isolations. Haemagglutination inhibition tests on the sera of a small sample of redtail monkeys (Cercopithecus ascanius schmidtï) collected in November 1968 and January 1969 and a larger sample collected in August-October 1969 revealed a high incidence of immunity to chikungunya, indicating their recent involvement with the virus. The evidence indicates that there was an intensive epizootic in forest monkeys transmitted by A. africanus with incidental leakage to man by inefficient vector systems, which could not maintain an outbreak by themselves.

There is no evidence that chikungunya virus is maintained within the Entebbe area, even in a latent form, during the intervals of 5 to 8 or 10 years between epizootics. The similar but not coincident periodicity of 5 to 8 years exhibited by Zika virus supports the view that this is an intrinsic perodicity brought about by the dynamics of immunity in forest monkey populations. The possible explanation of such periodicity is that the virus approaches an over-all balanced enzootic status within a larger maintenance area, despite the evidence of travelling epizootics at more limited sites.