Avian flu

Influenza viruses are members of the Orthomyxoviridae family. They are single-stranded, negative-sense RNA viruses with segmented genomes.
There are four types of influenza viruses, named with the first four letters of the alphabet: A, B, C and D. 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; and influenza D viruses mainly affect cattle and transmission to humans due to them is not known.
Type A influenza viruses are of great public health importance because of 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 animal influenza.
These subtypes include avian influenza A(H5N1) and A(H9N2) and swine influenza A(H1N1) and A(H3N2). All these animal influenza type A viruses are distinct from human influenza viruses and are not easily transmitted between humans.
However, the avian influenza virus H5N1 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 cause asymptomatic or mild infection in birds, where the range of symptoms depends on the properties of the virus.
Viruses that cause severe disease in poultry and result in high mortality rates are called highly pathogenic avian influenza (HPAI). Those that cause mild disease, on the other hand, are called low pathogenic avian influenza (LPAI).

Human infections with A/H5N1 viruses occur mostly from bird to human, less frequently from the contaminated environment to individuals, and only in a few cases does interhuman transmission occur.
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 upon contact with infected fecal material or respiratory secretion in the environment, especially during slaughtering, plucking, meat processing, and its preparation for cooking. There is no evidence of infection through properly cooked meat or in 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 in addition, avian IAV lineages can spread along these migratory routes. For example, remote sensing and phylogenetic analysis showed that the distribution of H5N1 viruses in East Asia followed the migratory routes of wild birds during 2003-2012.

Epidemics of H5N1 have occurred in poultry in areas of Asia, the Middle East, Europe, and Africa since 1997 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 human influenza A virus.
Between November 2003 and August 2011, approximately 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 A (H5N1) viruses have not acquired sustained transmission capability among humans, so the probability of interhuman transmission is low.

Incubation periods are estimated to be 3 to 5 days and may 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 and nausea, malaise. Diarrhea 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 is present in the sputum. Multiorgan failure, sepsis and rarely encephalopathy also occur.
The lethality rate among hospitalized patients with confirmed H5N1 infection was high (about 60%) as a result of respiratory failure caused by progressive pneumonia and acute respiratory distress syndrome.
Inauspicious outcomes have also been reported for H7N7 infection in humans. Other subtypes of avian influenza appear to cause mild illness.

Laboratory tests are required to diagnose human infection and are performed through the examination of a sample taken from the nose or throat in the first few days of illness, 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.

Influenza-specific antiviral drugs are available for the prophylaxis 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, several clinical trials are ongoing on the topic.
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 maximize therapeutic benefit.
However, given the significant mortality currently associated with infections with virus subtypes A(H5) and A(H7N9) and the evidence of prolonged viral replication in these diseases, administration of the drug should be considered even in patients in whom the infection has already been detected in an advanced state.
Treatment is recommended for a minimum of 5 days but may be extended until satisfactory clinical improvement is achieved.
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, immunosuppression leading to bacterial or fungal superinfection.
Co-infection with bacterial pathogens may be found in critically ill patients.

Where H5N1 is endemic in poultry ( China, Egypt, Indonesia, Vietnam ), travelers should avoid high-risk environments such as live animal markets and farms, contact with free-roaming or caged poultry, surfaces likely contaminated with poultry secretions and droppings. Travelers to endemic countries should also avoid contact with dead migratory birds and wild birds showing signs of disease, and should finally avoid consumption of undercooked eggs, poultry and chicken products. Hand hygiene through frequent washing or the use of alcohol wipes is essential. Upon contact with individuals with suspected H5N1 disease or a severe, unexplained respiratory illness, travelers should monitor their health status, and in case of fever accompanied by respiratory symptoms, they should seek urgent medical attention. Vaccines available against seasonal influenza do not protect against avian influenza. Inactivated H5N1 vaccines for human use have been developed and licensed in several countries but are not yet generally available. Because of the continuing evolution of influenza viruses, WHO continues to emphasize the importance of global surveillance to detect virological, epidemiological, and clinical changes associated with circulating influenza viruses that may undermine human (or animal) health with timely sharing of these viruses and related information for further risk assessment.