A Spatial Analysis of Rift Valley Fever Virus Seropositivity in Domestic Ruminants in Tanzania

Rift Valley fever (RVF) is an acute arthropod-borne viral zoonotic disease primarily occurring in Africa. Since RVF-like disease was reported in Tanzania in 1930, outbreaks of the disease have been reported mainly from the eastern ecosystem of the Great Rift Valley. This cross-sectional study was carried out to describe the variation in RVF virus (RVFV) seropositivity in domestic ruminants between selected villages in the eastern and western Rift Valley ecosystems in Tanzania, and identify potential risk factors. Three study villages were purposively selected from each of the two Rift Valley ecosystems. Serum samples from randomly selected domestic ruminants (n = 1,435) were tested for the presence of specific immunoglobulin G (IgG) and M (IgM), using RVF enzyme-linked immunosorbent assay methods. Mixed effects logistic regression modelling was used to investigate the association between potential risk factors and RVFV seropositivity. The overall RVFV seroprevalence (n = 1,435) in domestic ruminants was 25.8% and speciesspecific seroprevalence was 29.7%, 27.7% and 22.0% in sheep (n = 148), cattle (n = 756) and goats (n = 531), respectively. The odds of seropositivity were significantly higher in animals sampled from the villages in the eastern than those in the western Rift Valley ecosystem (OR = 1.88, CI: 1.41, 2.51; p<0.001), in animals sampled from villages with soils of good than those with soils of poor water holding capacity (OR = 1.97; 95% CI: 1.58, 3.02; p< 0.001), and in animals which had been introduced than in animals born within the herd (OR = 5.08, CI: 2.74, 9.44; p< 0.001). Compared with animals aged 1–2 years, those aged 3 and 4–5 years had 3.40 (CI: 2.49, 4.64; p< 0.001) and 3.31 (CI: 2.27, 4.82, p< 0.001) times the odds of seropositivity. The findings confirm exposure to RVFV in all the study villages, but with a higher prevalence in the study villages from the eastern Rift Valley ecosystem.

裂谷热（Rift Valley fever, RVF）是一种主要流行于非洲的急性虫媒病毒性人畜共患病。1930年坦桑尼亚首次报告类RVF病例以来，疫情暴发主要集中于大裂谷东部生态系统。本项横断面研究旨在描述坦桑尼亚东、西裂谷生态系统选定村庄内家养反刍动物的RVF病毒（Rift Valley fever virus, RVFV）血清阳性率差异，并识别潜在危险因素。研究采用目的抽样法，从两个裂谷生态系统中各选取3个研究村庄。研究人员采集了随机选取的1435份家养反刍动物血清样本，通过RVF酶联免疫吸附试验（enzyme-linked immunosorbent assay, ELISA）检测样本中特异性免疫球蛋白G（immunoglobulin G, IgG）与免疫球蛋白M（immunoglobulin M, IgM）的存在情况。本研究采用混合效应logistic回归模型，探究潜在危险因素与RVFV血清阳性率之间的关联。在全部1435份样本中，家养反刍动物的总体RVFV血清阳性率为25.8%；其中绵羊（n=148）、牛（n=756）及山羊（n=531）的物种特异性血清阳性率分别为29.7%、27.7%与22.0%。分析结果显示：与西裂谷生态系统村庄的采样动物相比，东裂谷生态系统村庄采样动物的血清阳性优势比显著更高（优势比OR=1.88，95%置信区间CI:1.41~2.51；p<0.001）；土壤持水能力良好的村庄采样动物，其血清阳性优势比显著高于土壤持水能力较差的村庄（OR=1.97；95%CI:1.58~3.02；p<0.001）；外来引入动物的血清阳性优势比显著高于畜群内出生的动物（OR=5.08，CI:2.74~9.44；p<0.001）。与1~2岁龄的动物相比，3岁龄及4~5岁龄动物的血清阳性优势比分别为3.40倍（CI:2.49~4.64；p<0.001）与3.31倍（CI:2.27~4.82；p<0.001）。本研究结果证实所有研究村庄均存在RVFV暴露，但东裂谷生态系统的研究村庄血清阳性率更高。