Avian influenza

Overview

Avian Influenza, commonly known as bird flu, is a contagious viral disease caused by influenza A viruses which primarily infect wild birds and poultry. However, in rare cases, it may also infect humans and mammals. In particular, the A(H5N1) and A(H7N9) viruses have been known to cause majority of human infections to date. Aside from A(H5N1), there have also been sporadic cases of human infection with various subtypes of avian influenza (i.e. H3, H5, H7, H9, H10).

Disease epidemiology

Human infections with the avian influenza virus are rare but have occurred sporadically since its first detection in Hong Kong in 1997. There has been no sustained person-to-person transmission, although several small clusters of infections have been reported.

The avian influenza viruses are divided into several sub-types. The most frequently identified sub-types of avian influenza that have caused human infections are the A(H5N1) and A(H7N9) viruses:

• The A(H5N1) virus is highly contagious in birds and has been associated with severe disease and deaths in humans. Since its widespread re-emergence in 2003, the virus has spread in Asia, Europe, and Africa. It has become endemic in poultry populations in some countries such as China and India. Most human infections occurred after prolonged and close contact with infected poultry. Rare, limited and non-sustained person-to-person spread of this virus has been reported.

• A multistate outbreak of A(H5N1) virus among dairy cattle herds was reported across the United States (US) in March 2024. The A(H5N1) virus was also detected in udder swabs and raw (unpasteurised) milk samples from infected cows. Farm workers in different farms were infected after their exposure to the infected cows.

• Human infections with H7N9 were first reported in China in 2013. Most human infections had occurred after exposure to infected poultry or live bird markets. Similarly, rare instances of limited, non-sustained person-to-person spread have been reported in China.

• Sporadic cases of human infections with other avian influenza viruses, including H5N6 and H9N2 viruses, have also been reported in China and other countries. Spread from birds to humans is rare and no person-to-person spread has been reported.

The World Health Organization (WHO) advises that sporadic human cases of avian influenza infection are not unexpected, given that there is a risk for sporadic infections and small clusters of human cases due to exposure to infected poultry or contaminated environments, if avian influenza viruses are circulating in poultry.

Pathogen(s)

Type A influenza virus that usually circulate in birds.

Transmission

Human infection with avian influenza virus is primarily acquired through:

• Direct or close contact with potentially sick or dead birds and other animals (e.g. mammals).

• Direct exposure to secretions or excretions from infected animals, or their contaminated environment.

• Consumption or exposure to infected animals or undercooked or unprocessed animal products.

There is currently no evidence to suggest that the virus can be spread through the consumption of properly prepared poultry or eggs, although a few cases have been linked to consumption of dishes containing raw, contaminated poultry blood.

Based on current available evidence, avian influenza viruses have not developed the capability to easily infect humans and other mammals, nor to transmit easily between humans.

Incubation period: Typically 1 to 10 days; ranging from 1 to 17 days.

Clinical features

The signs and symptoms of human infection with avian influenza viruses range from mild respiratory symptoms to severe illnesses such as pneumonia.

The most common symptoms of human infection with avian influenza virus are similar to those of regular flu:

• High fever (usually over 38 degrees Celsius)

• Cough

• Sore throat

• Runny nose

• Phlegm

• Difficulty breathing

• Headaches

• Muscle or body aches

• Fatigue

Other symptoms include:

• Diarrhoea

• Nausea

• Vomiting

• Seizures

• Conjunctivitis

The disease may progress rapidly, leading to respiratory failure with acute respiratory distress syndrome (ARDS), multiorgan failure, and may result in death.

Risk factors

Risk factors include:

• Relevant travel to or residence in areas with recently reported cases of avian influenza in the past 6 months.

• Contact with infected poultry and other animals (e.g. mammals), such as slaughtering, de-feathering, handling or preparing infected birds for consumption may increase the risks of contracting the avian influenza infection.

Diagnosis

Detection of avian influenza virus in respiratory samples is through polymerase chain reaction, sequencing or virus isolation.

Treatment and management

Treatment may vary depending on the symptoms. If the disease is caught early, Tamiflu (Oseltamivir) is often the recommended antiviral treatment alongside symptomatic treatment.

Precaution, prevention, and control

Isolation measures

All suspected and confirmed human cases of avian influenza infection should be isolated in airborne infection isolation rooms (AIIR). They should be given a surgical mask to wear if experiencing respiratory symptoms prior to the transfer to AIIR.

Standard, contact and airborne precautions are recommended when attending to suspect or confirmed cases. Full PPE (gown, gloves, eye protection, and N95 mask) is required when treating suspect or confirmed human cases with avian influenza infection.

Contact management

Tamiflu may be offered to high-risk contacts who have been exposed to the virus as prophylaxis. Close contacts of cases may be contact traced and appropriately followed up (e.g., quarantine, phone surveillance or self-monitoring).

Vaccination

The seasonal influenza vaccines do not protect against avian influenza infection.

Notification

Avian influenza is a legally notifiable disease in Singapore.

Who should notify:

• Medical practitioners

• Laboratories

When to notify:

• On clinical suspicion/ laboratory confirmation

How to notify:

• Please refer to the Infectious Disease Notification for more information.

Notification timeline:

• Immediately. No later than 24 hours.

It is also recommended that all cases who test positive for atypical influenza A viruses be reported to the Ministry of Health as soon as possible.

Resources

Please refer to the Weekly Infectious Diseases Bulletin for the number of avian influenza cases in Singapore.

For more information on avian influenza, please refer to the WHO website.

For general travel advisory, please refer to Health Advice for Travellers.