Geographic patterns and environmental factors associated with human yellow fever presence in the Americas

Background
In the Americas, yellow fever virus transmission is a latent threat due to the proximity between urban and wild environments. Although yellow fever has nearly vanished from North and Central America, there are still 13 countries in the Americas considered endemic by the World Health Organization. Human cases usually occur as a result of the exposure to sylvatic yellow fever in tropical forested environments; but urban outbreaks reported during the last decade demonstrate that the risk in this environment still exists. The objective of this study was to identify spatial patterns and the relationship between key geographic and environmental factors with the distribution of yellow fever human cases in the Americas.
Methodology/Principal findings
An ecological study was carried out to analyze yellow fever human cases reported to the Pan American Health Organization from 2000 to 2014, aggregated by second administrative level subdivisions (counties). Presence of yellow fever by county was used as the outcome variable and eight geo-environmental factors were used as independent variables. Spatial analysis was performed to identify and examine natural settings per county. Subsequently, a multivariable logistic regression model was built. During the study period, 1,164 cases were reported in eight out of the 13 endemic countries. Nearly 83.8% of these cases were concentrated in three countries: Peru (37.4%), Brazil (28.1%) and Colombia (18.4%); and distributed in 57 states/provinces, specifically in 286 counties (3.4% of total counties). Yellow fever presence was significantly associated with altitude, rain, diversity of non-human primate hosts and temperature. A positive spatial autocorrelation revealed a clustered geographic pattern in 138/286 yellow fever positive counties (48.3%).
Conclusions/Significance
A clustered geographic pattern of yellow fever was identified mostly along the Andes eastern foothills. This risk map could support health policies in endemic countries. Geo-environmental factors associated with presence of yellow fever could help predict and adjust the limits of other risk areas of epidemiological concern.

背景
在美洲地区，城市与野生环境的毗邻性使得黄热病病毒传播始终是一项潜在威胁。尽管黄热病已在北美与中美洲几近绝迹，但仍有13个美洲国家被世界卫生组织（World Health Organization，WHO）列为黄热病流行区。人间病例通常因在热带森林环境中接触森林黄热病而发生；但近十年报告的城市暴发疫情表明，城市环境中的黄热病风险依然存在。本研究的目的在于识别美洲地区人间黄热病病例的空间分布模式，以及关键地理与环境因子与该病例分布之间的关联。
方法与主要发现
本研究采用生态学研究方法，对2000年至2014年上报至泛美卫生组织（Pan American Health Organization，PAHO）的人间黄热病病例进行分析，病例数据按二级行政单元（县）进行聚合。以各县是否存在黄热病作为因变量，选取8项地理环境因子作为自变量。通过空间分析识别并探究各县的自然环境特征，随后构建多变量逻辑回归模型。研究期内，13个流行国家中有8个报告了共计1164例人间黄热病病例。其中近83.8%的病例集中在3个国家：秘鲁（37.4%）、巴西（28.1%）与哥伦比亚（18.4%），病例分布于57个州/省范围内的286个县（占全县总数的3.4%）。研究发现，黄热病的存在与海拔、降雨量、非人灵长类宿主多样性及气温显著相关。空间自相关分析结果显示，286个黄热病阳性县中有138个（占比48.3%）呈现出聚集性的地理分布模式。
结论与意义
本研究明确了黄热病的聚集性地理分布模式主要集中于安第斯山脉东麓。本研究绘制的风险地图可为流行国家的公共卫生政策制定提供支撑。与黄热病存在相关的地理环境因子，可助力预测并调整其他具有流行病学关注价值的风险区域范围。