Highly pathogenic avian influenza A (H5N1) virus or other non-human influenza subtypes (e.g. H7, H9)
Human infections with highly pathogenic avian influenza A(H5N1) virus occur through bird-to-human, possibly environment-to-human and, very rarely, limited, non-sustained human-to-human transmission. Direct contact with infected poultry, or with surfaces and objects contaminated by their droppings, is the main route of transmission to humans. Exposure risk is considered highest when there is contact with infected avian faecal material in the environment, especially during slaughter, de-feathering, butchering and preparation of poultry for cooking. There is no evidence that properly cooked poultry or poultry products can be a source of infection.
Nature of the disease
Patients usually present initially with symptoms of fever and influenza-like illness (malaise, myalgia, cough, sore throat). Diarrhoea and other gastrointestinal symptoms may occur. The disease progresses within days and many patients develop clinically apparent pneumonia with radiographic infiltrates of varying patterns. Sputum production is variable and sometimes bloody. Multi-organ failure, sepsis-like syndromes and, uncommonly, encephalopathy occur. The fatality rate among hospitalized patients with confirmed H5N1 infection has been high (about 60%), most commonly as a result of respiratory failure caused by progressive pneumonia and acute respiratory distress syndrome. Fatal outcome had also been reported for H7N7 infection in human. However, other avian influenza subtypes (e.g. H9N2) appear to cause mild diseases.
Extensive outbreaks in poultry have occurred in parts of Africa, Asia, Europe and the Middle East since 1997, but only sporadic human infections have occurred to date. Continued exposure of humans to avian H5N1 viruses increases the likelihood that the virus will acquire the necessary characteristics for efficient and sustained human-to-human transmission through either gradual genetic mutation or reassortment with a human influenza A virus. Between November 2003 and July 2008, nearly 400 human cases of laboratory-confirmed H5N1 infection were reported to WHO from 15 countries in Africa, south-east and central Asia, Europe and the Middle East.
Risk for travellers
H5N1 avian influenza is primarily a disease of birds. The virus does not easily cross the species barrier to infect humans. To date, no traveller is known to have been infected.
Neuraminidase inhibitors (oseltamivir, zanamivir) are inhibitory for the virus and have proven efficacy in vitro and in animal studies for prophylaxis and treatment of H5N1 infection. Studies in hospitalized H5N1 patients, although limited, suggest that early treatment with oseltamivir improves survival. Late intervention with oseltamivir is also justified. Neuraminidase inhibitors are recommended for post-exposure prophylaxis in certain exposed individuals. At present WHO does not recommend pre-exposure prophylaxis for travellers but advice may change depending on new findings. Inactivated H5N1 vaccines for human use have been developed and licensed in several countries but are not yet generally available; however, this situation is expected to change. Some countries are stockpiling these vaccines as a part of pandemic preparedness. Although the vaccines are immunogenic, their effectiveness in preventing the H5N1 infection or reducing disease severity is unknown.
In affected countries, travellers should avoid contact with high-risk environments such as live animal markets and poultry farms, any free-ranging or caged poultry, or surfaces that might be contaminated by poultry droppings. Travellers in affected countries should avoid contact with dead migratory birds or wild birds showing signs of disease, and should avoid consumption of undercooked eggs, poultry or poultry products. Hand hygiene with frequent washing or use of alcohol rubs is recommended. If exposure to individuals with suspected H5N1 illness or severe, unexplained respiratory illness occurs, travellers should urgently consult health professionals. Travellers should contact their local health providers or national health authorities for supplementary information.