Serological studies on human plague in Southern Africa. Part I. Plague antibody levels in a population during a quiescent and a subsequent active period in an endemic region.

Yersinia pestis fraction l passive haemagglutination studies on human sera obtained randomly during a plague epidemic showed a 10-fold increase in occurrence of antibodies compared with the results obtained on sera from the same population 11 months earlier during a quiescent phase. The pre-epidemic population antibody rate was lower than that in a comparable population tested simultaneously in another endemic region, where a recrudescence of plague was not experienced during the following year. The human plague epidemic was preceded and accompanied by a rodent die-off. A serological survey of the small mammals 2 years prior to the human survey also showed that Y. pestis was present in the environment. Otomys unisulcatus was indicated as being an important rodent in the plague cycle in the area.