Arthropod-borne virus zoonosis surveillance in the Cape Province: 1. Prospective serological investigations for virus activity in the Beaufort West and Middelburg Districts during 1981.

In addition to the routine sero-epidemiological surveillance for arthropod-borne viral zoonoses in the Cape Province carried out by the Department of Medical Microbiology and State Health Department of Virology laboratory, we conducted a prospective serological investigation for virus activity during 1981 in two districts of the Province, namely the Beaufort West and Middelburg districts, which experienced heavy rainfall during the first two months of that year. The approach used was to obtain paired serum samples from identified domestic stock representative of several species from 2-5 months apart and to test them for haemagglutination inhibition antibodies to Rift Valley fever, Wesselsbron and Middelburg virus antigen preparations in order to ascertain, as an indication of viral activity, whether changes in antibody levels occurred between the collection dates. The results indicated that there was probable activity of Rift Valley fever virus and activity of Wesselsbron virus (or related flaviviruses) and Middelburg virus (or related alphaviruses) in the Karoo between the middle of February and the end of July 1981. Despite this activity and heavy rainfall registered at meteorological stations in both the Beaufort West and Middelburg districts as well as general reports of heavy rainfall and considerable mosquito activity over widespread areas of the Karoo, Eastern and S.W. Cape Province, there were no epizootics or epidemics of overt arthropod-borne zoonotic viral disease in the province during 1981.