Movement and contact patterns of long-distance free-grazing ducks and avian influenza persistence in Vietnam

Presence of ducks, and in particular of free-grazing ducks, has consistently been shown to be one of the most important risk factors for highly pathogenic avian influenza outbreaks which has compromised poultry production in South-East Asia since the early 2000s and continues to threaten public health, farmers’ livelihood and food security. Although free-grazing duck production has been practised for decades in South-East Asia, there are few published studies describing this production system, which is suspected to play an important role in the maintenance of avian influenza viruses. This study aimed at describing quantitatively the long-distance free-grazing duck production system in South Vietnam, characterising the movement and contact patterns of the duck flocks, and identifying potential associations between farming practices, movement and contact patterns and the circulation of avian influenza viruses. We conducted interviews among stakeholders involved in the free-grazing duck production system (duck farmers, transporters and rice paddy owners) in combination with a virological cross-sectional survey in South Vietnam. Results show that both direct and indirect contacts between free-grazing duck flocks were frequent and diverse. The flocks were transported extensively across district and province boundaries, mainly by boat but also by truck or on foot. A third of the investigated flocks had a positive influenza A virology test, indicating current circulation of avian influenza viruses, but none were positive for H5 subtypes. The age and size of the flock as well as its location at the time of sampling were associated with the risk of influenza A circulation in the flocks. These findings should be considered when developing risk assessment models of influenza virus spread aimed at informing the development of improved biosecurity practices leading to enhanced animal health, sustainable animal production and reliable income for farmers.

自21世纪初以来，鸭群（尤其是散养鸭（free-grazing ducks））的存在，一直被证实是高致病性禽流感暴发的最重要风险因素之一——此类暴发已重创东南亚地区的家禽养殖业，并持续威胁公共卫生、农户生计与粮食安全。
尽管东南亚地区的散养鸭养殖模式已沿用数十年，但公开报道的描述该养殖体系的研究仍寥寥无几，而该体系被认为在禽流感病毒的持续维持与传播中发挥着关键作用。
本研究旨在定量描述越南南部的长距离散养鸭养殖体系，刻画鸭群的移动与接触模式，并厘清养殖方式、移动与接触模式与禽流感病毒传播之间的潜在关联。研究团队联合开展了两项工作：一是对参与越南南部散养鸭养殖体系的利益相关方（养鸭户、转运人员与稻田所有者）进行访谈，二是开展病毒学横断面调查。
研究结果显示，散养鸭群之间的直接与间接接触均频繁且形式多样。鸭群会被广泛转运至跨区县甚至跨省的区域，主要运输方式为船只，也可通过卡车或步行转运。被调查的鸭群中有三分之一的甲型流感病毒学检测结果呈阳性，表明当前存在禽流感病毒的循环传播，但未检测到H5亚型阳性样本。鸭群的日龄、规模以及采样时的所在位置，均与鸭群内甲型流感病毒的传播风险存在显著关联。
在开发禽流感病毒传播风险评估模型、制定优化生物安全措施以提升动物健康水平、推动养殖业可持续发展以及保障农户稳定收入时，本研究结果具有重要参考价值。