I. Introduction

Avian influenza (AI) is an acute, highly fatal infectious disease of birds caused by type A influenza viruses of the Orthomyxoviridae family. Based on the antigenic differences of surface proteins (HA and NA), it can be divided into multiple subtypes. Due to its high pathogenicity, strong transmissibility, and potential for cross-species infection, the H5 subtype Highly Pathogenic Avian Influenza (HPAI) virus is a notifiable disease to the World Organisation for Animal Health (WOAH) and is classified as a Class A animal disease in China. Since the first H5N1 outbreak in Guangdong in 1996, the virus has continued to evolve through antigenic drift and genetic reassortment. Currently, the Clade 2.3.4.4b branch has become the globally dominant epidemic strain, frequently breaching interspecies barriers. As a major poultry farming country, China faces severe challenges due to the coexistence of large-scale and backyard farming, as well as dense migratory bird flyways. This article integrates the latest surveillance data to comprehensively review the current epidemic status and key control measures for H5 subtype AI, providing a reference for related research and practice.

II. Current Epidemic Status of H5 Subtype AI Abroad

Globally, H5 subtype AI exhibits characteristics of "intercontinental spread and endemic circulation." According to WOAH reports, since 2021, the Clade 2.3.4.4b H5N1 virus has been reported in over 50 countries, spanning Europe, Asia, Africa, and the Americas. Europe and the Americas are hard-hit areas. From 2023 to 2024, over 20 EU countries reported outbreaks on commercial farms, resulting in the culling of more than 12 million poultry, and wild bird infections covered the entire European continent. From October 2024 to September 2025, 50 countries globally reported new HPAI outbreaks in poultry, and 55 countries reported new outbreaks in wild birds and mammals. This included 1,410 poultry outbreaks and 1,912 wild bird outbreaks. A new feature of "poultry-mammal cross-border transmission" has emerged in North America, with 1,084 dairy farms in the US reporting H5N1 infections, affecting over 20,000 dairy cows, alongside 71 human cases, elevating public health risks. In developing countries like Egypt and Nigeria, poor farming conditions and low vaccine coverage lead to continuous outbreaks, making them weak links in global prevention and control.

III. Current Epidemic Status of H5 Subtype AI in China

In recent years, H5 subtype AI outbreaks in China have been generally stable without large-scale epidemics, but show a trend of "multi-genotype co-circulation." According to surveillance data from the National Avian Influenza Reference Laboratory (Guangzhou), two genotypes currently circulate in China: Clade 2.3.4.4b and Clade 2.3.4.4h. The number of Clade 2.3.4.4h strains has significantly decreased, while Clade 2.3.4.4b has become the predominant epidemic branch. Furthermore, internal differentiation within both genotypes is considerable. Clade 2.3.4.4h has differentiated into four sub-branches (h1-h4), while Clade 2.3.4.4b has differentiated into seven sub-branches (b1-b7). Research by Jia Weixin et al. from South China Agricultural University indicates that since 2024, the number of H5N1 isolates from the b5 sub-branch has been increasing, showing significant divergence. Notably, the b5.5 sub-sub-branch strain exhibits marked antigenic drift, leading to reduced protection from original vaccines. Consequently, the Ministry of Agriculture and Rural Affairs issued Announcements No. 943 and No. 949 in August and September 2025, respectively, updating the vaccine strains for H5 subtype AI to effectively address the threat of antigenic changes in currently circulating strains.

IV. Transmission and Epidemic Characteristics of H5 Subtype AI

Avian influenza viruses can circulate year-round, mutate rapidly, and are difficult to prevent and control. The virus evolves quickly through antigenic drift of the HA gene and genetic reassortment. Mutations in the HA protein receptor binding site (e.g., Q226L, N193K) enhance the virus's ability to bind to mammalian α-2,6 sialic acid receptors, laying the foundation for cross-species transmission. Some variant strains have deletion mutations in the NA gene, reducing their sensitivity to neuraminidase inhibitors. Transmission is primarily "wild bird migration-driven, with human-mediated spread as secondary." Wild bird migration serves as the main vector for cross-border spread. In intensive farming systems, the virus spreads rapidly through respiratory routes, with morbidity and mortality rates exceeding 90%. Poultry transportation, human movement, and cross-contamination in live poultry markets are important transmission pathways.

The risks of H5 subtype AI transmission stem mainly from complex farming models, frequent livestock and poultry transport, close contact between wild birds and domestic poultry, and challenges in regulating live poultry markets. The epidemic shows clear seasonality, with high incidence during winter and spring (November to March). Outbreaks are concentrated along migratory bird flyways, areas with active live poultry trade, and dense farming regions. The spectrum of infected animals has expanded to include wild birds, pigs, dogs, and other species. Waterfowl, especially wild waterbirds, are the primary reservoir and transmission vector for H5 subtype AI. The virus spreads to other birds via airborne routes, direct contact, and contaminated water or environments. During winter, lower temperatures drive wild waterfowl to congregate in warmer waters, increasing the risk of viral transmission.

V. Key Control Measures for H5 Subtype AI

1 Strengthen Biosecurity Measures

Biosecurity is fundamental to preventing AI spread. Poultry farms must strictly control the entry and exit of personnel, equipment, and vehicles to isolate external contamination sources. Enhance management of the farm environment, regularly disinfect poultry houses to reduce viral survival. Particularly in winter, low temperatures and humidity allow the virus to survive longer in soil and water. Therefore, strict management of water sources and feed is crucial to avoid contamination. Additionally, follow the WOAH "Biosecurity Procedures for Poultry Production" and China's technical prevention and control specifications to establish a comprehensive biosecurity system. Locate farms away from wild bird gathering areas, implement closed management, and adopt production models featuring "three-zone separation" and "all-in, all-out." Establish a routine disinfection system, using alternating disinfectants like peroxyacetic acid and sodium hypochlorite. Promptly and safely dispose of dead birds and manure, achieving over 75% resource utilization of farming waste.

2 Strengthen Vaccination and Immunization Management

Vaccination is a key measure for AI control. In anticipation of peak AI seasons ("Spring Prevention" and "Autumn Prevention"), vaccination efforts should be planned well in advance, especially for farms and assembly points in high-risk areas. Governments should encourage and support timely vaccination of farmers with effective H5 subtype AI vaccines. Currently, the two newest H5 subtype AI vaccines using updated strains are: 1. The Harbo Veterinary Research Institute system's recombinant avian influenza (H5+H7) quadrivalent inactivated vaccine (H5N6 H5-Re15 strain + H5-Re16 strain, H7N9 H7-Re5 strain + H7-Re6 strain); and 2. The South China Agricultural University system's reassortant avian influenza virus(H5+H7) quadrivalent vaccine, inactivated(H5N6 Strain rHN503+H5N1 Strain rHN504, H7N9 Strain rHN705+Strain rHN706). Both vaccines offer good protection against currently circulating H5 subtype AI viruses. Furthermore, immunization management must be standardized, programs rationalized, and vaccine quality and efficacy ensured to avoid immunization failures due to improper use. Large-scale farms should develop personalized immunization schedules: for long-cycle commercial broilers, primary immunization at 7-10 days of age, booster at 28-35 days; for layers and breeders, primary at 7-14 days, booster at 30-40 days, a booster before lay, then every 6 months. Backyard farmers follow a spring/autumn mass immunization plus regular catch-up schedule. Conduct regular HI antibody titer monitoring, with titers≥24 considered qualified; promptly revaccinate poultry with inadequate titers. Strengthen assessment of vaccine strain matching with epidemic strains, and accelerate the R&D and application of novel vaccines like mRNA vaccines.

3 Improve Surveillance and Early Warning Systems

Establish a monitoring network combining "active + passive, sentinel + mobile" approaches. Surveillance scope should cover key sites such as poultry farms, live poultry markets, slaughterhouses, and wild bird habitats to ensure no blind spots. Create information sharing platforms, using national animal disease surveillance systems to upload real-time data for rapid dissemination and sharing. Simultaneously, enhance information sharing and collaboration among departments like Agriculture and Rural Affairs, Health, and Forestry and Grassland to form a joint prevention and control mechanism. Conduct detailed epidemiological investigations for identified AI cases, tracing the virus source, transmission routes, and infection range. Analyze AI epidemic patterns and trends based on surveillance data and investigation results to inform prevention and control decisions.

VI. Discussion and Outlook

The H5 subtype AI epidemic situation is complex. Clade 2.3.4.4b viruses continue to circulate and evolve in wild birds, making cross-border transmission risks difficult to eliminate. Establishing scientific and effective control measures requires a multi-dimensional approach: improving surveillance systems, strengthening vaccination and immunization management, enhancing biosecurity management, intensifying environmental disinfection, and reinforcing public health education. Through integrated control strategies, the occurrence and spread of AI outbreaks can be effectively reduced, ensuring the stable development of the poultry industry while safeguarding public health security. Governments at all levels, agricultural departments, and farmers must work together to address the challenges posed by AI outbreaks, supporting the high-quality development of the poultry industry.

Note: The original Chinese article, "Current Status and Key Control Measures of H5 Subtype Avian Influenza," was published in Poultry Husbandry and Disease Control, Issue No. 6, 2026.

Source: This article is translated and reprinted from the Chinese WeChat public account article "華南生物特約 禽流感專欄——H5亞型禽流感流行現狀與防控要點"(Avian Influenza Column Sponsored by SCBM -- Current Status and Key Control Measures of H5 Subtype Avian Influenza) originally published on "通禽站" (Poultry Industry News). Please refer to the original Chinese source for all references, data, and detailed author information.