WHO tiedotus A(H5N1) lintuviruksesta: Kahdella siipikarjatyöntekijällä Englannissa seulonnassa positiivinen A H5N1, oireettomia.

Avian Influenza A(H5N1) - United Kingdom of Great Britain and Northern Ireland

30 May 2023

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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

Animal influenza viruses normally circulate in animals but can also infect humans. Infections in humans have primarily been acquired through direct contact with infected animals or contaminated environments.

Depending on the host type, influenza A viruses can be classified as avian influenza, swine influenza, or other types of animal influenza viruses.

Avian, swine, and other animal influenza virus infections in humans may cause disease ranging from mild upper respiratory tract infection to more severe disease and may be fatal. Conjunctivitis, gastrointestinal symptoms, encephalitis and encephalopathy have also been reported.

Laboratory tests are required to diagnose human infection with influenza. WHO periodically updates technical guidance protocols for the detection of zoonotic influenza using molecular methods, e.g. Reverse transcription polymerase chain reaction (RT-PCR). Evidence suggests that some antiviral drugs, notably neuraminidase inhibitors (oseltamivir, zanamivir), can reduce the duration of viral replication and improve prospects of survival in some cases.

In 2023, Europe has been experiencing a large epidemic of A(H5N1) viruses in birds, with outbreaks reported in domestic birds, wild birds, and mammals across 24 countries. Outbreaks in wild and domestic birds continue to be reported through May 2023.

Public health response

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.

WHO risk assessment

The two reported individuals with influenza A(H5N1) detection in their samples have remained asymptomatic and tested negative for influenza in their most recent samples to date. Their close contacts were asymptomatic and the follow-up period has been completed.

Both cases were detected as part of an ongoing enhanced surveillance study of asymptomatic workers exposed to poultry infected with avian influenza. In these cases, detections may have resulted from either transient respiratory tract contamination (with no virus replication) or asymptomatic infection. Further testing (e.g., serology) is needed to confirm infection.

Whenever avian influenza viruses are circulating in birds, humans who are exposed to these birds or their environments are at risk of infection.

Sporadic human cases and transient contaminations of humans are rare, but not unexpected in such contexts. Thus far, there is no evidence of person-to-person transmission in this incident.

Although both reported cases were asymptomatic in this instance, previous A(H5N1) infections have resulted in severe infections in humans.

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.

WHO advice

The reports of these events do not change the current WHO recommendations on public health measures and surveillance of influenza.

Due to the constantly evolving nature of influenza viruses, and their widespread circulation in birds, WHO continues to stress the importance of global surveillance to detect and monitor virological, epidemiological, and clinical changes associated with emerging or circulating influenza viruses that may affect human (or animal birds and mammals) health and timely virus sharing for risk assessment.

When avian influenza viruses are circulating in an area, people involved in high-risk activities such as sampling infected birds or non-human mammals, culling, and disposing of infected animals, eggs, litters, and cleaning of contaminated premises should be trained on the proper use of and provided with appropriate PPE. All persons involved in these tasks should be registered and monitored closely by local health authorities for at least seven days following the last day of contact with infected animals or their environments. Those who develop respiratory symptoms should be rapidly sampled. Testing of asymptomatic exposed individuals could also be considered on a case-by-case basis, depending on available resources and objectives.

In the case of a confirmed or suspected human infection caused by a novel influenza virus with pandemic potential, including a variant virus (infection with a swine influenza A virus), a thorough epidemiologic investigation (even while awaiting the confirmatory laboratory results) of history of exposure to animals, of travel, and contact tracing should be conducted. The epidemiologic investigation should include early identification of unusual respiratory events that could signal person-to-person transmission of the novel virus. 

Clinical specimens collected from the case should be tested and sent to a WHO Collaboration Centre for further characterization. 

WHO recommends that all people involved in work with poultry or birds should have a seasonal influenza vaccination to reduce the risk of potential recombination events.

Travellers to countries with known outbreaks of animal influenza should avoid farms, contact with animals in live animal markets, entering areas where animals may be slaughtered, or contact with any surfaces that appear to be contaminated with animal faeces. 

General precautions include regular hand washing and following good food safety and good food hygiene practices. Should infected individuals from affected areas travel internationally, their infection may be detected in another country during travel or after arrival. If this were to occur, further community-level spread is considered unlikely as this virus has not acquired the ability to transmit easily among humans. 

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.

Further information

1. UK Health Security Agency (UKHSA). Avian flu detected in 2 individuals taking part in testing programme.
2. UK Health Security Agency (UKHSA). Management of contacts of highly pathogenic avian influenza H5N1 during the 2022 to 2023 avian influenza season
3. World Health Organization. Health topics, Influenza (avian and other zoonotic)
4. World Health Organization. Fact sheets, Influenza (Avian and other zoonotic)
5. World Health Organization. Influenza at the human-animal interface summary and assessment, 24 April 2023
6. World Health Organization. Cumulative number of confirmed human cases for avian influenza A(H5N1) reported to WHO, 2003-2023, 24 April 2023.
7. EFSA (European Food Safety Authority), ECDC (European Centre for Disease Prevention and Control), EURL (European Reference Laboratory for Avian Influenza), Adlhoch C, Fusaro A, Gonzales JL, Kuiken T, Mirinaviciute G, Niqueux É, Stahl K, Staubach C, Terregino C, Broglia A, Kohnle L and Baldinelli F, 2023. Scientific report: Avian influenza overview March–April 2023. EFSA Journal 2023;21(5):8039, 45 pp. https://doi.org/10.2903/j.efsa.2023.8039
8. Institute for International Cooperation in Animal and Biologics. The Centre for Food Security And Public Health. Highly pathogenic avian influenza
9. WHO information for molecular diagnosis of influenza virus - update

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