H7N9

H7N9 is an influenza A virus subtype that causes influenza predominantly in birds and is enzootic in many bird populations ^[1]. It is one of the avian influenza viruses that can spill over into humans, causing an acute respiratory disease in mammals and domestic poultry ^[3][1]. Classification as low pathogenic or highly pathogenic avian influenza is based on severity in domestic chickens and does not predict severity in humans ^[1].

In humans, symptoms range from mild to severe and may include fever, diarrhea, and cough; the disease can often be fatal ^[1]. The presentation varies according to the infecting strain and the species affected ^[1]. In poultry, low-pathogenic H7N9 may cause mild symptoms or no symptoms, while highly pathogenic H7N9 can cause serious breathing difficulties, reduced egg production, and sudden death ^[1].

H7N9 viruses that emerged in 2013 were nearly eliminated in China ^[4]. H5 and H7 avian influenza viruses have collectively caused 2,634 human cases worldwide, including more than 1,000 deaths, and have driven large losses in domestic birds and outbreaks in wild birds and poultry ^[4]. H7N9 is among avian influenza viruses of pandemic concern because of global spread and sporadic human infections ^[2].

The virus can spread rapidly through poultry flocks and among wild birds, and humans can become infected after exposure to infected birds ^[1]. Infected birds shed the virus in oral and nasal secretions and feces, and other infected animals may shed influenza viruses in respiratory secretions and other body fluids ^[1]. Human cases are usually linked to close contact with infected poultry or contaminated material such as feces ^[1]. Zoonotic transmission from avian species is a primary source of human influenza outbreaks, and H7N9 is among avian influenza viruses of pandemic concern ^[2].

People who have close contact with infected poultry or contaminated material, such as feces, are the main group described as at risk of human H7N9 infection ^[1]. Human infections are rare overall but are usually traced to such exposures ^[1].

Control measures for avian influenza are stratified by pathogenicity, with highly pathogenic strains necessitating stringent interventions due to severe economic impacts and high mortality in poultry populations ^[1]. Preventive strategies also encompass vaccine development, although continuous antigenic drift complicates the creation of broadly effective formulations ^[1]. In response to potential outbreaks, candidate vaccine viruses can be swiftly dispatched to production facilities, leveraging data from earlier limited clinical trials ^[1].

1. 1Wikipedia contributors. Influenza A virus subtype H7N9 - Wikipedia [Internet]. Wikipedia. cited 3 Sept 2026.Available from: https://en.wikipedia.org/wiki/Influenza_A_virus_subtype_H7N9
2. 2Cargnin Faccin F et al. Pandemic preparedness through vaccine development for avian influenza viruses. Hum Vaccin Immunother. 2024 Dec 31. PMID: 38807261. doi: 10.1080/21645515.2024.2347019.PubMed: https://pubmed.ncbi.nlm.nih.gov/38807261/
3. 3Webster RG et al. Continuing challenges in influenza. Ann N Y Acad Sci. 2014 Sep. PMID: 24891213. doi: 10.1111/nyas.12462.PubMed: https://pubmed.ncbi.nlm.nih.gov/24891213/
4. 4Shi J et al. Alarming situation of emerging H5 and H7 avian influenza and effective control strategies. Emerg Microbes Infect. 2023 Dec. PMID: 36458831. doi: 10.1080/22221751.2022.2155072.PubMed: https://pubmed.ncbi.nlm.nih.gov/36458831/