Avian influenza (Bird flu)

Avian influenza is a bird disease caused by type A influenza virus. Widespread across the globe, it is an infection that can infect almost all bird species with varying degrees of severity, ranging from mild to highly pathogenic and contagious forms that can cause acute outbreaks.

Highly pathogenic forms present with rapid onset, followed by an equally rapid death.

The main risk is linked to the possibility of the avian virus passing to humans. Fears of a new pandemic have set in motion a series of extraordinary prevention measures around the world.

CAUSES

Influenza virus belongs to the class of Orthomyxoviridae viruses. They are single-stranded, negative-sense RNA viruses with segmented genomes.

There are four types of influenza viruses, which are named after the first four letters of the alphabet: Influenza A viruses infect humans and many different animals; influenza B viruses circulate among humans and cause seasonal epidemics; influenza C viruses can infect both humans and pigs, but infections are generally mild and are rarely reported, while influenza D viruses mainly affect cattle, with no known animal-human transmission reported.

Type A influenza viruses are of great public health significance due to their pandemic potential. They are classified into subtypes based on combinations of different surface proteins, such as hemagglutinin (HA) and neuraminidase (NA).

So far, 18 different subtypes of hemagglutinin and 11 different subtypes of neuraminidase have been identified. Depending on the host of origin, influenza A viruses can be classified as causing avian, swine, or other forms of animal flu.

These subtypes include bird flu A(H5N1) and A(H9N2), and swine flu A(H1N1) and A(H3N2). All of these type A animal influenza viruses are distinct from human influenza viruses and are not easily passed to humans.

However, the H5N1 avian influenza virus is highly pathogenic, as are other nonhuman influenza subtypes (see H1, H2, H3, H7, H9).

Waterfowl are the main natural reservoir for most influenza A virus subtypes. Most of them cause asymptomatic or mild infection in birds, where the range of symptoms depends on the characteristics of the virus.

Viruses that cause severe illness in poultry and cause high mortality rates are called Highly Pathogenic Avian Influenza (HPAI). Whereas, those that cause mild illness are called Low Pathogenic Avian Influenza (LPAI).

TRANSMISSION

Human infections with A/H5N1 viruses are mostly passed from birds to humans. However, less frequently they can be passed from the contaminated environment to humans, but interhuman transmission is rare.

Direct contact with infected poultry or surfaces and objects contaminated with bird secretions is the main route of transmission to humans.

The risk of exposure increases on contact with infected faecal material or respiratory secretions in the environment, especially during slaughtering, plucking, meat processing, and food preparation. There is no evidence of infection caused by consuming properly cooked meat or poultry products.

The main natural reservoir is waterfowl, particularly Anseriformes (ducks, geese and swans) and Charadriiformes (gulls, terns and sandpipers).

Migratory birds can carry HPAI and LPAI viruses asymptomatically over long distances, and avian IAV lineages can also spread along these migratory routes. For example, remote surveillance and phylogenetic analysis showed that the distribution of H5N1 viruses in East Asia followed the migratory routes of wild birds during 2003-2012.

GEOGRAPHICAL DISTRIBUTION

Since 1997, H5N1 epidemics have been occurring in poultry in some parts of Asia, the Middle East, Europe, and Africa, and have led to sporadic infections in humans.

Continued exposure of humans to avian H5N1 viruses increases the likelihood that the virus will acquire the characteristics necessary for efficient and sustained interhuman transmission, through gradual genetic mutation or reassortment with the human influenza A virus.

Between November 2003 and August 2011, about 565 laboratory-confirmed human cases of H5N1 infection were reported to WHO from 15 countries including Africa, Southeast Asia, Central Asia, Europe and the Middle East.

To date, it appears that the A (H5N1) viruses have not acquired sustained human-to-human transmission capabilities, so the probability of interhuman transmission is low.

SYMPTOMS

The incubation period is estimated to be 3 to 5 days and can last up to 9 days.

Patients infected with H5N1 or H7N9 viruses commonly present with flu-like illness symptoms, including fever, dry cough, muscle aches, nausea, and malaise. Diarrhoea and other gastrointestinal symptoms may sometimes occur.

The disease progresses over several days, and almost all patients clinically develop pneumonia with radiographic infiltrates of variable appearance; in some cases, blood can be present in the sputum. Multiorgan failure, sepsis and, more rarely, encephalopathy can also occur.

The lethality rate among hospitalised patients with confirmed H5N1 infection was high (approx. 60%), due to respiratory failure caused by progressive pneumonia and acute respiratory distress syndrome.

Unfortunate outcomes have also been reported for H7N7 infection in humans. Other avian influenza subtypes appear to cause mild illness.

DIAGNOSIS

Laboratory tests are needed to diagnose human infection and are performed by examining a sample taken from the nose or throat in the first few days of the disease, after the onset of symptoms.

WHO, through its Global Influence Surveillance and Response System (GISRS), periodically updates technical guidance protocols for the detection of zoonotic influenza in humans using molecules, e.g., RT-PCR and other methods.

TREATMENT

Influenza-specific antiviral drugs are available for the prevention and treatment of H5N1 infection.

Evidence suggests that some antiviral drugs, particularly the neuraminidase inhibitor (oseltamivir, zanamivir), may reduce the duration of viral replication and improve survival prospects. However, various clinical studies on the topic are still in course.

Resistance to oseltamivir has been reported. In suspected and confirmed cases, neuraminidase inhibitors should be prescribed as soon as possible (ideally within 48 hours of symptom onset) to maximise the therapeutic benefits.

However, given the significant mortality currently associated with virus infections of the A(H5) and A(H7N9) subtypes, and the evidence of prolonged viral replication in these diseases, administration of the drug should be considered, even in patients where the infection is already at an advanced state.

Treatment is recommended for a minimum of 5 days but can be extended until there is satisfactory clinical improvement.

Corticosteroids should not be used routinely, unless indicated for other reasons (e.g., asthma and other specific conditions), because their use has been associated with prolonged viral clearance, and immunosuppression that can lead to bacterial or fungal superinfection.

Bacterial co-infection can present in critically ill patients.

PREVENTION

In places where H5N1 is endemic in poultry (China, Egypt, Indonesia, Vietnam), travellers should avoid high-risk environments, such as live animal markets and farms, contact with free-roaming or caged poultry, and surfaces that are likely to be contaminated with poultry secretions and excrement.

Travellers to endemic countries should also avoid contact with dead migratory birds and wild birds that show signs of disease and should also avoid eating undercooked eggs, poultry and chicken products.

Hand hygiene, including frequent washing or the use of alcohol wipes, is essential.

When in contact with individuals with suspected H5N1 disease or a severe, unexplained respiratory illness, travellers should monitor their health status and should seek urgent medical attention in the event of fever with respiratory symptoms.

Vaccines for seasonal flu do not protect against bird flu. Inactivated H5N1 vaccines for human use have been developed and licensed in several countries but are not yet widely available.

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

 

Bibliography:

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