Avian Influenza A(H5N1)

Situation at a glance

In mid-May, the United Kingdom of Great Britain and Northern Ireland reported to the World Health Organization (WHO) the detection of avian influenza A(H5) virus in a poultry worker at a farm in England where poultry was infected with high pathogenicity avian influenza (HPAI) A(H5N1) viruses. Another detection was reported in a second individual performing culling operations on the farm. Both detections were later confirmed by additional testing as A(H5N1). Both cases were asymptomatic and detected as part of an ongoing enhanced surveillance study of asymptomatic workers exposed to poultry infected with avian influenza.

All the workers at this farm and their contacts have been identified; none of the contacts have reported symptoms, and no other influenza cases have been identified. The United Kingdom Health Security Agency (UKHSA) has not detected evidence of human-to-human transmission.

Based on the available information, WHO considers these as sporadic detections of avian influenza viruses among humans with no evidence of person-to-person transmission to date. Thus, the likelihood of international disease spread through humans is considered to be low.

Given the widespread circulation in birds and the constantly evolving nature of influenza viruses, WHO stresses the importance of global surveillance to detect virological, epidemiological and clinical changes associated with circulating influenza viruses which may affect human (or animal) health.

Description of the situation

In late April, the UKHSA was notified by the Animal and Plant Health Protection Agency (APHA) of an outbreak of HPAI (H5N1) on a poultry farm in England, United Kingdom. The human cases were detected through an ongoing enhanced surveillance study of asymptomatic workers exposed to poultry infected with avian influenza.

The UKHSA Rapid Investigation Team were deployed to the farm in early May 2023 to recruit exposed participants for the study. Of the 24 eligible persons, one tested positive for influenza A (with no detection of human seasonal subtypes H1 or H3) on the first sample self-taken at the premises. Two further nasopharyngeal samples collected from the same person tested negative for influenza A by a UKHSA regional laboratory and by the UKHSA national influenza reference laboratory. The participant remained clinically asymptomatic throughout.

An update from the United Kingdom authorities to WHO in mid-May 2023, notified of an additional case from the same farm as influenza A(H5) positive on two separate samples. This second person was a poultry culler exposed to infected birds at the same farm. The poultry culler worked on the farm in early May using personal protective equipment (PPE). The case was clinically assessed and remains asymptomatic. The case was treated with oseltamivir and was negative on respiratory sampling taken on the last day of isolation.

Sequencing later confirmed the virus detected in both individuals as A(H5N1). All samples from these two individuals were negative for seasonal influenza viruses. All other study participants remain well and have tested negative for influenza A on their samples to date. Follow-up of contacts has been completed. The affected farm is one of the first recruited in the ongoing enhanced surveillance study of asymptomatic workers exposed to poultry infected with avian influenza.

Work to determine whether these are infections or not (i.e., could instead be due to transient mucosal contamination of the nose with virus particles) is underway, though it may be difficult to reach a conclusion.

Epidemiology of disease

The United Kingdom implemented the following public health measures:

Coordination and response: Since late April 2023, the standard measures for control of avian influenza in England were applied in the affected poultry farm. Situational assessment and response to human detection was coordinated by UKHSA and Public Health Scotland and undertaken by local health protection and National Health Service clinical services.

Surveillance: A follow-up response including passive and active surveillance of exposed workers was undertaken for the first detection. For the second detected case, close contacts were offered chemoprophylaxis, swabbing, and were requested to self-isolate for ten days since their last exposure, reporting information about their health conditions. This second case was isolated until the negative swab result.

Laboratory: Further samples have been obtained from the second case for analysis in a reference laboratory in Scotland. Virus characterization and genomic analysis are in process by the Respiratory Virus Unit, UKHSA/Scottish reference laboratory.

Infection prevention and control: In the affected farms, measures related to the control of the outbreak have been undertaken, including on-site slaughter of the birds, the destruction of contaminated materials that could carry the virus, and cleaning and disinfection of the facilities. All participants involved in the culling process wore personal protective equipment.

As per standard practice for confirmed infected poultry premises in the United Kingdom, a ten-kilometre surveillance zone, and a three-kilometre protection zone were declared by the United Kingdom Department for Environment, Food and Rural Affairs around the infected premises. The zone will remain in place for at least 21 days after preliminary cleansing and disinfection completion. It will not be lifted until surveillance activities, including clinical inspections of all commercial premises in the zone, have been undertaken.

Based on the available information, WHO assesses that the risk for the general population posed by this virus is low, and for occupationally exposed persons it is low to moderate.

All human infections caused by a novel influenza subtype are notifiable under the International Health Regulations (IHR) and State Parties to the IHR (2005) are required to immediately notify WHO of any laboratory-confirmed case of a recent human infection caused by an influenza A virus with the potential to cause a pandemic. Evidence of illness is not required for this report.

WHO does not recommend any restrictions on travel and/or trade for the United Kingdom based on available information on this event.

Citable reference: World Health Organization (30 May 2023). Disease Outbreak News; Avian Influenza A (H5N1) – United Kingdom of Great Britain and Northern Ireland. Available at https://www.who.int/emergencies/disease-outbreak-news/item/2023-DON468