A yellow fever epizootic in Zika Forest, Uganda, during 1972: Part 2: Monkey serology.

During the 1972 yellow fever epizootic in Zika Forest, Uganda, sera from 21 monkeys shot in a number of forests around the Entebbe area were tested for the presence of a number of arbovirus antibodies. All sera were tested for antibodies against Chikungunya (CHIK), O'nyong-nyong (ONN), Zika, yellow fever (YF) West Nile (WN) and Wesselsbron (WESS) by the haemagglutination-inhibition (HI) test. Because of the crossreaction within the flaviviruses (group B arboviruses) mouse protection test (PT) was also carried out on the sera against YF, WESS and Zika viruses. Serological studies carried out on monkey sera from different parts of Uganda, including the Entebbe area, during 1968 gave results which reflected a surprisingly low rate of YF immune monkeys (3%) throughout the country compared with the rate of over 40% immune monkeys obtained by Haddow et al. in 1951. 40% of the monkey sera collected during 1972 were immune to YF by the PT. Since no YF virus had been isolated between 1968 and 1972 the results indicate strongly that the monkeys in the Entebbe area were involved in the epizootic of 1972. No sick or dead monkeys were found in all the forests checked around Entebbe area during the epizootic. This indicates that the animal-to-animal cycle of the equatorial African forests involved the mild endemic infection characteristic of a virus in its natural habitat and infecting its natural host.