Human Infection caused by Avian Influenza A (H5N1) - Chile

21 April 2023

Situation at a glance

This Disease Outbreak News is an update on the report published on 6 April 2023 concerning human infection caused by avian influenza A(H5N1) in Chile. Since then, WHO received information on the results of genomic sequencing and the ongoing public health response.

On 5 April, the results of genomic sequencing were completed by the Institute of Public Health of Chile (ISP per its acronym in Spanish), which is the National Influenza Centre (NIC) in Chile, which identified avian influenza virus A(H5N1) phylogenetic clade 2.3.4.4b. The genomic sequencing had 99.9% identity with H5 hemagglutinin sequences from Chilean birds, and complete neuraminidase (NA) had 100% identity with N1 sequences from Chilean birds. 

A total of 12 contacts (close contacts and healthcare workers) were identified. All tested negative for influenza and have completed the monitoring period. No further cases in Chile have been identified so far.

Avian influenza infection in a human can cause severe disease and is notifiable under the International Health Regulations (IHR, 2005) [1]. 

Description of the case

As reported previously, on 29 March 2023, the Ministry of Health of Chile notified WHO of the detection of human infection with avian influenza A(H5) virus.

On 7 April 2023, the Chile International Health Regulations National Focal Point reported the results of genomic sequencing completed on 5 April 2023 by the NIC, which identified avian influenza A(H5N1) phylogenetic clade 2.3.4.4b.

The patient is a 53-year-old male from the Region of Antofagasta in the north of Chile. He had no history of comorbidities or recent travel.

On 13 March 2023, the patient developed symptoms including cough, sore throat, and hoarseness. On 21 March, due to worsening symptoms, he sought care at a local hospital. On 22 March 2023, the patient developed dyspnea and was admitted to a Regional Hospital at Antofagasta (SARI Sentinel Site). A nasopharyngeal swab sample was collected as part of routine severe acute respiratory infection (SARI) surveillance and tested negative for SARS-CoV-2 by reverse transcription-polymerase chain reaction (RT-PCR). On 23 March, the patient was admitted to the intensive care unit and treatment with antivirals (oseltamivir) and antibiotics was initiated. The patient remains in respiratory isolation under multidisciplinary management, with mechanical ventilation due to pneumonia. Standard infection control precautions were kept since the detection of the case.

On 27 March, a bronchoalveolar sample was collected and tested positive for an unsubtypeable influenza A virus by PCR. The sample was sent to the ISP and tested positive for avian influenza A(H5) on 29 March. On 31 March, the NIC forwarded the patient's samples to a WHO Collaborating Centre for further characterization. 

On 5 April, the results of genomic sequencing were completed by the NIC in Chile, which identified avian influenza virus A(H5N1) phylogenetic clade 2.3.4.4b. The genomic sequencing had 99.9% identity with H5 hemagglutinin sequences from Chilean birds, and complete neuraminidase (NA) had 100% identity with N1 sequences from Chilean birds. 

Three close contacts of the case were identified, all asymptomatic, tested negative for influenza and have concluded the monitoring period. Additionally, a total of nine contacts among healthcare workers were identified, all tested negative for influenza and concluded the monitoring period on 4 April. However, on 5 April one of them developed respiratory symptoms, therefore a nasopharyngeal swab was taken, which tested negative for influenza; the monitoring period for this contact was extended for seven additional days, ending on 11 April.

Avian influenza A (H5N1) was first detected in the Region of the Americas in birds in December 2014. Between December 2022 and February 2023, highly pathogenic avian influenza (HPAI) has been detected in wild aquatic birds (pelicans and penguins) and sea mammals (sea lions) in the Antofagasta Region where the case resides. According to preliminary findings of the epidemiological investigation of this human case, the most plausible route of transmission was through environmental exposure, given the large number of dead sea mammals and wild birds found in the area close to the patient´s residence.

Epidemiology of disease

Zoonotic influenza infections in humans may range from asymptomatic or mild upper respiratory infection (fever and cough) to rapid progression to severe pneumonia, acute respiratory distress syndrome, shock, and death, depending on factors related to the virus and the host. Rarely, gastrointestinal, or neurological symptoms have been reported. Human cases of avian influenza are usually the result of direct or indirect exposure to infected live or dead poultry or contaminated environments.

In the Region of the Americas during 2022 and 2023, an increasing number of outbreaks of highly pathogenic avian influenza A(H5) have been reported in backyard poultry, farm poultry, wild birds, and wild mammals. Since the first confirmation of avian influenza A(H5N1) in the region in 2014, three human infections caused by avian influenza A(H5) have been reported: the first in the United States of America, reported in April 2022; the second in Ecuador, reported in January 2023; and this case. Globally, since 2003, 873 human infections with A(H5N1) viruses, including 458 deaths (CFR 52%), have been reported to WHO.

Public health response

  • Additional public health activities have been reported since the last update. Intersectoral activities have been carried out by the Ministry of Health of Chile, the Agricultural and Livestock Service (SAG) and the National Fisheries and Aquaculture Service of Chile (SERNAPESCA), of the Ministry of Agriculture of Chile, among others, in order to follow up on the avian influenza outbreaks in Antofagasta Region and outbreaks in other Regions.
  • Active monitoring of persons with respiratory symptoms and exposure to wild birds, poultry, and sea mammals has been undertaken.
  • Seasonal influenza vaccination is being conducted in risk groups according to the guidelines of the National Immunization Program. 
  • Regarding risk communication, the population has been informed about this case and prevention measures through messaging for different target audiences.
  • The Ministry of Health of Chile is participating actively in the Disaster Risk Management Committee of the National Service for Disaster Prevention and Response (SENAPRED) to deal with the animal health emergency.

WHO risk assessment

WHO’s risk assessment remains unchanged since the last update.

According to the preliminary findings of the local epidemiological investigation, the most plausible hypothesis about transmission is that it occurred through environmental exposure to areas where either sick or dead birds or sea mammals were found close to the residence of the case. According to the information received thus far, the virus has not been detected in other individuals.

Whenever avian influenza viruses are circulating in poultry, wild birds or mammals, there is a risk for sporadic infection and small clusters of human cases due to exposure to infected animals or contaminated environments.

Public health measures have been implemented by both the human and animal health agencies, including monitoring healthcare workers and other contacts of the laboratory-confirmed case. Currently available epidemiological and virological evidence suggests that A(H5) viruses, including H5N1, have not acquired the capacity for sustained transmission among humans, thus the likelihood of human-to-human spread is low. Based on available information, WHO assesses the risk to the general population posed by this virus to be low.

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.

The preliminary risk assessment will be reviewed as needed should further epidemiological or virological information become available.

WHO advice

Due to the constantly evolving nature of influenza viruses, and the large outbreaks among animal populations, 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) health, and of timely virus sharing for risk assessment. The diversity of zoonotic influenza viruses that have caused human infections is alarming and necessitates strengthened surveillance in both animal and human populations, thorough investigation of every zoonotic infection, and pandemic preparedness planning. Vaccination of poultry workers with seasonal influenza vaccine has been advised to prevent a viral mutation that could facilitate human-to-human transmission.

WHO does not advise special traveler screening at points of entry.  In the case of a confirmed or suspected human infection with a novel influenza virus with pandemic potential, including avian influenza and variant viruses, contact tracing should be initiated and a thorough epidemiologic investigation of history of exposure to animals and of travel should be conducted. The epidemiologic investigation should include early identification of unusual clusters of respiratory disease that could signal person-to-person transmission of the novel virus, and clinical samples collected from the time and place that the case occurred should be tested and then sent to a WHO Collaborating Centre for further characterization.

There are no approved vaccines for preventing influenza A(H5) in humans. Candidate vaccine viruses to prevent influenza A(H5) infection in humans have been developed for pandemic preparedness purposes. Given the observed extent and frequency of avian influenza cases in wild birds and some wild mammals, the public should avoid contact with animals that are sick or dead from unknown causes and should report the occurrence to the authorities.

Travelers 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 or other body fluids. Travelers should also wash their hands often with soap and water and follow good food safety and food hygiene practices.

Close analysis of the epidemiological situation, further characterization of the most recent viruses found in humans and animals, and serological investigations, are critical to assess risk and to adjust risk management measures in a timely manner.

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

Further information

[1] https://www.who.int/publications/m/item/case-definitions-for-the-four-diseases-requiring-notification-to-who-in-all-circumstances-under-the-ihr-(2005)

Citable reference: World Health Organization (21 April 2023). Disease Outbreak News; Avian Influenza A (H5N1) - Chile. Available at https://www.who.int/emergencies/disease-outbreak-news/item/2023-DON461