Wild water birds are the natural reservoir of all influenza A viruses. Humans contract avian influenza – also called influenza A (H5N1) – mainly through close contact with infected birds.
Initial symptoms are similar to normal seasonal influenza A, but pneumonia rapidly develops. The same antiviral drugs used for the treatment of seasonal influenza A may also be used for the treatment of avian influenza A (H5N1).
Although most people only think of influenza A viruses in terms of seasonal flu, wild water birds are the natural reservoir of all influenza A viruses. In birds, these viruses can cause a wide spectrum of symptoms, ranging from mild illness to rapidly fatal disease.
Avian influenza A virus strains are classified as low pathogenic (LPAI; associated with mild disease) and highly pathogenic (HPAI; associated with severe illness and high mortality).
Two subtypes of influenza A virus (H5 and H7) have been associated with HPAI. While avian flu viruses don’t normally infect people, there have been instances where infection caused from mild symptoms in humans to severe respiratory distress and even death.
Avian influenza A (H5N1) first came to attention worldwide in 1997, following a massive outbreak in poultry flocks in Hong Kong. During this outbreak only 18 human cases, including six deaths, were reported.
Drastic control measures were instituted by the local authorities, including a mass culling of all poultry and strict poultry-handling and selling restrictions. This appeared to have been successful until the re-emergence of the virus in 2003. Since then avian influenza A (H5N1) has been detected in poultry and wild birds in Asia, Africa and some parts of Europe. Human cases have been reported from several countries in Asia and Africa.
The total number of reported cases as of 16 March 2010 stands at 489 (289 deaths), with the most recent cases reported from Egypt, Indonesia and Vietnam.
To date no cases of avian influenza A (H5N1) infection in animals or humans have been reported from South Africa.
Avian influenza and SARS
Avian influenza and severe acute respiratory syndrome (SARS) are entirely different disease entities caused by completely different viruses, although some of the symptoms are the same.
Transmission of avian influenza A (H5N1)
Natural transmission of influenza viruses between birds occurs directly or indirectly through contact with contaminated objects, as poultry excrete large amounts of virus in faeces and other secretions.
Most human cases of avian influenza A (H5N1) infection are associated with direct handling of infected poultry, close contact with live poultry, slaughtering or preparing sick poultry for consumption, or consumption of uncooked poultry products. Human-to-human transmission appears to be ineffective as only a few possible cases have been reported. All involved extended, close and unprotected contact with infected patients.
Fortunately transmission of avian influenza A (H5N1) between humans still appears to be ineffective. But there is concern that the virus may combine with either human seasonal influenza A or pandemic influenza A/H1N1, and could result in a virus that is highly capable of causing disease and can be transmitted effectively between humans. That’s one of the reasons why influenza viruses are so closely monitored.
Most people develop symptoms within two to four days after their last exposure to sick poultry. They have typical influenza symptoms such as fever, coughing and shortness of breath. Many people also complain of stomach symptoms such as vomiting, diarrhoea and abdominal pain.
People with severe H5N1 infection usually develop pneumonia in both lungs. The pneumonia progresses rapidly and within days they may need the help of a ventilator to breathe. About 60% of patients with avian influenza A (H5N1) infection die, mainly because of respiratory failure. Other complications of severe infections include kidney dysfunction and multi-organ failure.
Treatment and prevention
The same antiviral drugs used for the treatment of seasonal influenza A may also be used for the treatment of avian influenza A (H5N1). Patients with confirmed or strongly suspected H5N1 infection should start treatment with oseltamivir (Tamiflu) as soon as possible. Treatment with zanamivir (Relenza) may also be considered, but there is concern that it may not work equally well in patients with disseminated infection. Patients with severe H5N1 infections should be cared for in a hospital’s intensive care unit.
Oseltamivir and zanamivir may also be used to prevent avian flu, but only if you find yourself in a situation where the risk of exposure is moderate or high.
There’s currently no commercially available vaccine against avian flu, but a number of trials with candidate vaccines have been conducted. As most people have not had prior exposure to avian flu, two doses of vaccine will most likely be needed to make sure one is sufficiently immune.
Continued enhanced surveillance for the early detection of influenza A (H5N1) in poultry, and rapid implementation of contingency plans in case of introduction, are essential elements of an integrated prevention plan.
Reviewed by Dr Jane Yeats, Department of Virology, University of Cape Town 2006
Updated and reviewed by Dr Jean Maritz and Dr Leana Maree, medical virologists, Tygerberg Hospital and University of Stellenbosch, 2010