Abstract
Avian influenza (AI), particularly highly pathogenic avian influenza (HPAI) subtype H5N1, emerged as a significant global threat in the mid-2000s, causing widespread outbreaks in poultry and sporadic human infections. HPAI has persisted in Nigeria since the initial A/H5N1 incursion in 2006, leading to a chronic, cyclical, and devastating crisis with major outbreaks recurring in significant waves in 2015, 2017, and a prolonged crisis from late 2020-2025. This review synthesizes published peer-reviewed literature on Avian Influenza in Nigerian poultry, encompassing wild birds and other avian species, from 2006 to 2025. It examines the epidemiology, molecular characteristics, control strategies, and socio-economic impacts of the disease outbreaks, with a specific focus on HPAI A/H5N1 and other circulating strains such as H5N2, H5N6, H5N8, and H9N2. This research reveals that AI in Nigeria is a persistent, cyclical outbreak driven not by isolated events but by systemic vulnerabilities and complex transmission pathways involving commercial and backyard poultry, as well as wild and exotic birds that facilitate viral persistence and recurrence in Nigeria. Also, it reveals Nigeria as a dynamic epicenter for viral evolution, marked by multiple introductions of A/H5N1 clades and frequent reassortment events, which increases the risk of novel, virulent strains. Epidemiologically, the triad of host, agent and environment including the physical and contextual factors that influence AI disease ecology as seen in Live Bird Markets (LBMs) scenarios and ubiquitous backyard poultry—which accounts for 60% of the national flock—act as critical mixing and dissemination hubs, which is pivotal to the control of the disease. Control efforts, primarily relying on stamping-out policies and biosecurity, have faced significant challenges due to implementation gaps, delayed or non-reporting, and the dynamic nature of the virus. This research demonstrates the urgent need for sustained, adaptive, and integrated "One Health" approaches, including enhanced surveillance, targeted biosecurity, and a re-evaluation of the policy on no-vaccination, to mitigate the on-going threat to animal health, food security, and public and environmental health in Nigeria.

Key words: Avian influenza viruses; Live bird markets; Epidemiology; Nigeria; One Health.