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Avian Influenza A (H5N1) Outbreak in Australia

On 22 May 2024, the World Health Organization (WHO) was notified of a laboratory-confirmed case of human infection with avian influenza A(H5N1) virus (clade by the International Health Regulations (IHR) National Focal Point (NFP) of Australia. This is the first confirmed human infection caused by avian influenza A(H5N1) virus detected and reported by Australia. Although the source of exposure to the virus in this case is currently unknown, the exposure likely occurred in India, where the case had travelled, and where this clade of A(H5N1) viruses has been detected in birds in the past. According to the IHR (2005), a human infection caused by a novel influenza A virus subtype is an event that has the potential for high public health impact and must be notified to the WHO. Based on available information, WHO assesses the current risk to the general population posed by this virus as low.

Description of the situation
On 17 May 2024, the WHO Collaborating Centre (WHO CC) for Reference and Research on Influenza in Australia notified the NFP of Australia of a suspected case of human A(H5N1) avian influenza (HPAI) in Melbourne, Victoria.

The state Victorian Department of Health confirmed this case on 18 May 2024. Under Article 44, the NFP of Australia advised the NFP of India of the confirmed case on 21 May 2024. Under Article 8, the IHR NFP of Australia notified WHO of the case on 22 May 2024.

The case is a 2.5-year-old-female child with no underlying conditions. She had a history of travel to Kolkata, India from 12 to 29 February 2024. She returned to Australia on 1 March 2024.

Upon returning to Australia, the child presented at a hospital in Victoria on 2 March 2024, where she received medical care and was admitted on the same day. On 4 March, the patient was transferred to the intensive care unit at a referral hospital in Melbourne, Victoria, due to worsening symptoms, for a period of one week. The patient was discharged from hospital after a 2.5-week admission. The case is now reported to be clinically well.

The Victorian Department of Health reported on 23 May 2024 that the family advised that the child started to feel unwell on 25 February 2024, with loss of appetite, irritability and fever, and was taken to a doctor on the evening of 28 February 2024 in India. She was febrile, coughing and vomiting and was given paracetamol. It was not reported to an Australian airport biosecurity officer that the child was unwell when she arrived in Australia on 1 March 2024.

Additional information provided by the family indicates that the case did not travel outside of Kolkata, India, and did not have any known exposure to sick persons or animals while in India. It is understood that no close family contacts of the case in Australia or India developed symptoms, as of 22 May 2024.

A nasopharyngeal swab and endotracheal aspirate taken on 6 and 7 March respectively tested positive for influenza A at the referral hospital. The samples were sent to the WHO CC for further characterisation on 3 April as part of a batch, as there was insufficient knowledge from the referring practitioners at the hospital to connect the case to the H5N1 virus. Virus genetic sequence obtained from the samples confirmed the subtype A(H5N1) and indicated that the haemagglutinin (HA) gene belonged to clade, which circulates in South-East Asia and has been detected in previous human infections and in poultry.

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 original host, influenza A viruses can be classified as avian influenza, swine influenza, or other types of animal influenza viruses.

Avian influenza virus infections in humans may cause disease ranging from mild upper respiratory tract infection to more severe disease and can be fatal. Conjunctivitis, gastrointestinal symptoms, encephalitis and encephalopathy have also been reported. There have also been several detections of A(H5N1) virus in asymptomatic persons who had exposure to infected birds.

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., 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.

From 2003 to 22 May 2024, 891 cases of human infections with avian influenza A(H5N1), including 463 deaths, have been reported to WHO from 24 countries. Almost all of these cases have been linked to close contact with infected live or dead birds, or contaminated environments.

India has reported detections of avian influenza A(H5N1) in domestic birds in 2024 to the World Organisation for Animal Health (WOAH). As the virus continues to circulate in poultry, the potential for further sporadic human cases remains. This is the first human infection with avian influenza A(H5N1) reported in Australia. In this case, the exposure likely occurred in India where this clade of A(H5N1) viruses has been detected in birds in the past, although the likely source of exposure to the virus is currently unknown.

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