Situation at a glance
On 29 March 2023, the Ministry of Health of Chile notified WHO of a laboratory-confirmed case of human infection caused by avian influenza A(H5) virus in the Region of Antofagasta. This is the first human infection with avian influenza A(H5) virus reported in Chile and the third reported in the Region of the Americas to date. This is a single human infection, and no further case has been identified so far. An outbreak investigation is ongoing including determining the exposure of the case to the virus. In recent months of 2023, unprecedented outbreaks of highly pathogenic avian influenza (HPAI) A(H5N1) in animals have been reported from Chile. Avian Influenza A(H5N1) viruses have been detected among backyard poultry, farm poultry, wild birds, and sea mammals.
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
On 29 March 2023, the Ministry of Health of Chile notified WHO of the detection of human infection with avian influenza A(H5) virus, confirmed by the Institute of Public Health of Chile (ISP per its acronym in Spanish), which is the National Influenza Centre. 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, he 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 case developed dyspnea and was admitted to a Regional Hospital at Antofagasta. A nasopharyngeal swab sample was collected as part of routine severe acute respiratory infection (SARI) surveillance and tested negative for COVID-19 by reverse transcription-polymerase chain reaction (RT-PCR). On 23 March, he was admitted to the intensive care unit. On 24 March, treatment with antivirals (oseltamivir) and antibiotics was initiated. He remains in respiratory isolation under multidisciplinary management, with mechanical ventilation due to pneumonia.
On 27 March, a bronchoalveolar sample was collected and tested positive for an unsubtypeable influenza A virus by RT-PCR. The sample was sent to the ISP and tested positive for avian influenza A(H5) on 29 March. The neuraminidase type is yet to be confirmed and the clade information for the avian influenza A(H5) virus detected in this human case is not yet known. The NIC has forwarded the patient’s samples to a WHO Collaborating Centre for further characterization.
Three close contacts of the case were asymptomatic and tested negative for influenza and have concluded the monitoring period. Additionally, a total of nine contacts among health care workers were identified, all concluded the monitoring on 4 April, however on 5 April one of them developed respiratory symptoms, therefore, further testing is ongoing, and the period of monitoring was extended for 7 more days for this contact of the case.
Avian influenza A (H5N1) was first detected in the Americas in birds in December 2014. Between December 2022 and February 2023, highly pathogenic avian influenza (HPAI) was 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 in areas close to the residence of the case where either sick or dead sea mammals or wild birds were found.
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, including 458 deaths (CFR 52%), with A(H5N1) viruses have been reported to WHO. Additionally, three human infections with influenza A (H5[2]) viruses, 84 human cases of infection with A(H5N6) viruses, and seven human cases of infection with A(H5N8) viruses have been reported to WHO.
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. While further characterization of the virus from this human case is pending, currently available epidemiological and virological evidence suggests that A(H5) viruses 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.
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 (even while awaiting the confirmatory laboratory results) 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 events 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 good food hygiene practices.
Close analysis of the epidemiological situation, further characterization of the most recent viruses found in humans and poultry, 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.
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[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)
[2] Neuraminidase subtype not known.
Citable reference: World Health Organization (6 April 2023). Disease Outbreak News; Avian Influenza A (H5) – Chile. Available at https://www.who.int/emergencies/disease-outbreak-news/item/2023-DON453