Lyons N A, Stärk K D C, van Maanen C, Thomas S L, Chepkwony E C, Sangula A K, Dulu T D, Fine P E M
Department of Infectious Disease Epidemiology, London School of Hygiene and Tropical Medicine, Keppel Street, London WC1E 7HT, UK; Veterinary Epidemiology, Economics, and Public Health Group, The Royal Veterinary College, Hawkshead Lane, North Mymms, Hatfield, Hertfordshire AL9 7TA, UK.
Veterinary Epidemiology, Economics, and Public Health Group, The Royal Veterinary College, Hawkshead Lane, North Mymms, Hatfield, Hertfordshire AL9 7TA, UK.
Acta Trop. 2015 Mar;143:103-11. doi: 10.1016/j.actatropica.2014.09.010. Epub 2014 Oct 24.
During August-September 2012, an outbreak of Foot-and-mouth Disease (FMD) due to serotype Southern African Territories-2 (SAT2) occurred on a large, extensively grazed dairy farm in Nakuru County, Kenya. Over 29 days, 400/644 (62.1%) cattle were recorded as displaying clinical signs consistent with FMD. Out of the 18 management groups present, 17 had clinical cases (weighted mean incidence rate 3.5 per 100 cattle-days, 95% CI 2.4, 5.1; range 0.064-10.9). Transmission may have been encouraged when an infected group was moved to a designated isolation paddock. A four to five day minimum incubation period was apparent in five groups for which a point source exposure was evident. Further transmission was associated with the movement of individual animals incubating infection, use of a common dip and milking parlour, and grazing of susceptible groups in paddocks neighbouring to infectious cases. Animals over 18 months old appeared to be at highest risk of disease possibly due to milder clinical signs seen among younger animals resulting in reduced transmission or cases not being recorded. Cows with a breeding pedigree containing a greater proportion of zebu appeared to be at lower risk of disease. The outbreak occurred despite regular vaccination (three times per year) last performed approximately three months before the index case. Incidence risk by the lifetime number of doses received indicated limited or no vaccine effectiveness against clinical disease. Reasons for poor vaccine effectiveness are discussed with antigenic diversity of the SAT2 serotype and poor match between the field and vaccine strain as a likely explanation. Detailed field-derived epidemiological data based on individual animals are rarely presented in the literature for FMD, particularly in East-Africa and with the SAT2 serotype. This study provides a detailed account and therefore provides a greater understanding of FMD outbreaks in this setting. Additionally, this is the first study to provide field-derived evidence of poor vaccine effectiveness using a SAT2 vaccine. Further field-based measures of vaccine effectiveness in line with evaluation of human vaccines are needed to inform FMD control policy which has previously relied heavily upon experimental data and anecdotal experience.
2012年8月至9月期间，肯尼亚纳库鲁县一个大型、广泛放牧的奶牛场爆发了由南非2型（SAT2）血清型引起的口蹄疫。在29天内，记录到400/644（62.1%）的牛出现了与口蹄疫一致的临床症状。在现有的18个管理组中，17个组有临床病例（加权平均发病率为每100牛日3.5例，95%置信区间2.4，5.1；范围0.064 - 10.9）。当一个感染组被转移到指定的隔离围场时，传播可能得到了促进。在五个有明显点源暴露的组中，明显出现了四到五天的最短潜伏期。进一步的传播与感染潜伏期个体动物的移动、共用浸液池和挤奶厅以及易感组在感染病例附近围场的放牧有关。18个月以上的动物似乎患病风险最高，这可能是由于年轻动物临床症状较轻，导致传播减少或病例未被记录。含有较高比例瘤牛的繁殖系谱奶牛似乎患病风险较低。尽管在首例病例前约三个月进行了定期疫苗接种（每年三次），疫情仍发生了。按接种剂量终生计算的发病风险表明，疫苗对临床疾病的有效性有限或无效。疫苗有效性差的原因进行了讨论，SAT2血清型的抗原多样性以及田间毒株与疫苗毒株匹配性差可能是原因。基于个体动物的详细现场流行病学数据在口蹄疫文献中很少出现，特别是在东非和SAT2血清型方面。本研究提供了详细的描述，因此对这种情况下的口蹄疫疫情有了更深入的了解。此外，这是第一项使用SAT2疫苗提供现场证据证明疫苗有效性差的研究。需要进一步基于现场的疫苗有效性测量方法，与人类疫苗评估一致，为口蹄疫控制政策提供信息，此前口蹄疫控制政策严重依赖实验数据和轶事经验。
