In 2008, a suspected outbreak of Rift Valley fever (RVF) was reported on a farm in the
Bela-Bela area, Limpopo Province, South Africa. Seven calves died on the affected dairy
farm, where no RVF vaccination programme was practised. No apparent clinical disease was
reported in the other 300 cattle (33 calves included) or 200 sheep on the farm. During the
outbreak, blood samples from 77.7% (233/300) of the cattle and 36.5% (73/200) of the sheep were
collected on the affected farm and 55 blood samples were taken from cattle on a neighbouring
farm. Eight weeks later, 78% of the cattle (234/300) and 42.5% of the sheep (85/200) were bled
on the affected farm only. All sera were tested by an Immunoglobulin M (IgM)-capture Enzymelinked
immunosorbent assay (ELISA) and by an indirect Immunoglobulin G (IgG) ELISA.
Selected IgM-positive (n = 14), IgG-positive (n = 23) and samples negative for both IgM and
IgG-specific antibodies against RVF virus (n = 19) were tested using the serum neutralisation
test (SNT). Sera from IgM-positive (n = 14) and negative (n = 20) animals were also tested by a
TaqMan polymerase chain reaction (PCR). On the affected farm, 7% (16/233) of the cattle were
IgM-positive and 13.7% (32/233) IgG-positive at the first bleed and 2% were IgM-positive at
the second bleed, whilst the number of cattle positive for IgG-specific antibodies increased
by 21.3% compared with the first bleed. Only 1.4% of sheep were positive for both IgM and
IgG antibodies at the first collection; at the second bleed, IgM-positive cases decreased to
1.2%, whilst IgG-positive cases increased to 2.4%. Whilst no IgM-positive cattle were found
on the neighbouring farm, 5.5% of cattle were IgG-positive. The SNT confirmed most of the
ELISA results, whilst PCR results were all negative. Although serology results indicated
virus circulation on both farms, the negative PCR results demonstrated that the animals were
not viraemic at the time they were sampled. The movement of infected mosquito vectors by
wind over long distances into a low-lying area that favoured their breeding on the Bela-Bela
farm may have led to an outbreak of the disease there, but the reason for the low level of virus
circulation amongst susceptible animals remains unclear.
The research was done as part of a Master’s dissertation by
L.P.M. (University of Pretoria). The practical work was done
by him at the DVTD and NICD, under supervision of E.H.V.
(University of Pretoria) and co-supervised by J.T.P. (National
Institute for Communicable Diseases of the National Health
Laboratory Service) and J.A.W.C. (University of Pretoria).
E.H.V. (University of Pretoria) wrote and submitted the