Mapaco Lourenço P, Coetzer Jacobus A W, Paweska Janusz T, Venter Estelle H
Department of Veterinary Tropical Diseases, University of Pretoria, South Africa.
J S Afr Vet Assoc. 2012 Oct 4;83(1):132. doi: 10.4102/jsava.v83i1.132.
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.
2008年，南非林波波省贝拉贝拉地区的一个农场报告了一起裂谷热（RVF）疑似疫情。受影响的奶牛场有7头小牛死亡，该农场未实施RVF疫苗接种计划。农场里另外300头牛（包括33头小牛）和200只羊未报告明显的临床疾病。疫情期间，从受影响农场77.7%（233/300）的牛和36.5%（73/200）的羊身上采集了血样，并从邻近农场的牛身上采集了55份血样。八周后，仅在受影响农场对78%的牛（234/300）和42.5%的羊（85/200）进行了采血。所有血清均通过免疫球蛋白M（IgM）捕获酶联免疫吸附测定（ELISA）和间接免疫球蛋白G（IgG）ELISA进行检测。使用血清中和试验（SNT）对选定的IgM阳性（n = 14）、IgG阳性（n = 23）以及针对RVF病毒的IgM和IgG特异性抗体均为阴性的样本（n = 19）进行检测。还通过TaqMan聚合酶链反应（PCR）对IgM阳性（n = 14）和阴性（n = 20）动物的血清进行了检测。在受影响农场，首次采血时7%（16/233）的牛IgM呈阳性，13.7%（32/233）的牛IgG呈阳性，第二次采血时2%的牛IgM呈阳性，而IgG特异性抗体呈阳性的牛数量与首次采血相比增加了21.3%。首次采血时只有1.4%的羊IgM和IgG抗体均呈阳性；第二次采血时，IgM阳性病例降至1.2%，而IgG阳性病例增至2.4%。虽然在邻近农场未发现IgM阳性的牛，但有5.5%的牛IgG呈阳性。SNT证实了大多数ELISA结果，而PCR结果均为阴性。尽管血清学结果表明两个农场都有病毒传播，但PCR阴性结果表明动物在采样时并未出现病毒血症。受感染的蚊媒通过风远距离移动到贝拉贝拉农场一个有利于其繁殖的低洼地区，可能导致了那里的疾病爆发，但易感动物中病毒传播水平较低的原因尚不清楚。
