Rift Valley fever in Egypt 1986. Surveillance of sheep flocks grazing in the northeast Nile Delta

From October 1985 through November 1986, 1714 presumably unvaccinated sheep in 13 nomadic flocks located in four provinces in Dakahliya Governorate, in the northeast Nile Delta, were ear tagged and monitored for acquisition of Rift Valley fever virus (RVFV) antibodies. Sheep were bled at approximately 3 month intervals and sera were tested for haemagglutination inhibition (HI) antibodies to RVFV. HI reactors were tested for RVFV specific IgM antibody by enzyme-linked immunosorbent assay (ELISA) and neutralizing antibody to RVFV by plaque reduction neutralization (PRN) tests. Base line results showed 1.2% prevalence of HI antibody to RVFV with titres from 1:20 to 1:320. All HI positive sera were PRN positive though PRN titres were generally higher than HI titres. No RVFV specific IgM antibody was detected in the HI and PRN positive sera. Throughout the study, no initially seronegative sheep became positive and no HI positive sheep showed an appreciable increase above initial antibody titre. These data indicate absence of RVFV transmission to sheep in Dakahliya Governorate during the period of the study.