title: An investigation into an outbreak of Rift Valley fever on a cattle farm in Bela-Bela, South Africa, in 2008.
abstract: 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.
authors: Department of Veterinary Tropical Diseases, University of Pretoria, South Africa.
authors: Lourenço P
authors: LP
authors: Mapaco
authors: Jacobus A W
authors: JA
authors: Coetzer
authors: Janusz T
authors: JT
authors: Paweska
authors: Estelle H
authors: EH
authors: Venter
mesh_terms: D000818:Animals; D000914:Antibodies, Viral / Q000097:blood; D002417:Cattle; D002418:Cattle Diseases / Q000453:epidemiology*; D004196:Disease Outbreaks / Q000662:veterinary*; D004797:Enzyme-Linked Immunosorbent Assay / Q000662:veterinary; D007074:Immunoglobulin G / Q000097:blood; D007075:Immunoglobulin M / Q000097:blood; D012295:Rift Valley Fever / Q000453:epidemiology / Q000662:veterinary*; D012756:Sheep; D012757:Sheep Diseases / Q000453:epidemiology; D013019:South Africa / Q000453:epidemiology*
chemical_list: D000914:Antibodies, Viral; D007074:Immunoglobulin G; D007075:Immunoglobulin M