- Avian influenza (AI), commonly called bird flu, is an infectious viral disease of birds.
- Most avian influenza viruses do not infect humans; however some, such as A(H5N1) and A(H7N9), have caused serious infections in people.
- Outbreaks of AI in poultry may raise global public health concerns due to their effect on poultry populations, their potential to cause serious disease in people, and their pandemic potential.
- Reports of highly pathogenic AI epidemics in poultry, such as A(H5N1), can seriously impact local and global economies and international trade.
- The majority of human cases of A(H5N1) and A(H7N9) infection have been associated with direct or indirect contact with infected live or dead poultry. There is no evidence that the disease can be spread to people through properly cooked food.
- Controlling the disease in animals is the first step in decreasing risks to humans.
Avian influenza (AI) is an infectious viral disease of birds (especially wild water fowl such as ducks and geese), often causing no apparent signs of illness. AI viruses can sometimes spread to domestic poultry and cause large-scale outbreaks of serious disease. Some of these AI viruses have also been reported to cross the species barrier and cause disease or subclinical infections in humans and other mammals.
AI viruses are divided into 2 groups based on their ability to cause disease in poultry: high pathogenicity or low pathogenicity. Highly pathogenic viruses result in high death rates (up to 100% mortality within 48 hours) in some poultry species. Low pathogenicity viruses also cause outbreaks in poultry but are not generally associated with severe disease.
Avian influenza A(H5N1) and A(H7N9) background
The A(H5N1) virus subtype, a highly pathogenic AI virus, first infected humans in 1997 during a poultry outbreak in Hong Kong SAR, China. Since its widespread re-emergence in 2003 and 2004, this avian virus has spread from Asia to Europe and Africa and has become entrenched in poultry in some countries, resulting in millions of poultry infections, several hundred human cases, and many human deaths. Outbreaks in poultry have seriously impacted livelihoods, the economy and international trade in affected countries.
The A(H7N9) virus subtype, a low pathogenic AI virus, first infected 3 humans – 2 residents of the city of Shanghai and 1 resident of Anhui province - in March 2013. No cases of A(H7N9) outside of China have been reported. Containment measures, including the closure of live bird markets for several months, have impacted the agriculture sectors of affected countries and international trade. Continued surveillance for A(H7N9) will be necessary to detect and control the spread of the virus.
Ongoing circulation of A(H5N1) and A(H7N9) viruses in poultry, especially where endemic, continues to pose threats to public health, as these viruses have both the potential to cause serious disease in people and may have the potential to change into a form that is more transmissible among humans. Other influenza virus subtypes also circulate in poultry and other animals, and may also pose potential threats to public health.
Avian influenza A(H5N1) and A(H7N9) infections and clinical features in humans
The case fatality rate for A(H5N1) and A(H7N9) virus infections in people is much higher compared to that of seasonal influenza infections. The A(H7N9) virus particularly affects people with underlying medical conditions.
In many patients, the disease caused by the A(H5N1) virus follows an unusually aggressive clinical course, with rapid deterioration and high fatality. Like most emerging disease, A(H5N1) influenza in humans is not well understood.
The incubation period for A(H5N1) avian influenza may be longer than that for normal seasonal influenza, which is around 2 to 3 days. Current data for A(H5N1) infection indicate an incubation period ranging from 2 to 8 days and possibly as long as 17 days. Current data for A(H7N9) infection indicate an incubation period ranging from 2 to 8 days, with an average of five days.1 WHO currently recommends that an incubation period of 7 days be used for field investigations and the monitoring of patient contacts.
Initial symptoms include high fever, usually with a temperature higher than 38°C, and other influenza-like symptoms (cough or sore throat). Diarrhea, vomiting, abdominal pain, chest pain, and bleeding from the nose and gums have also been reported as early symptoms in some patients.
One feature seen in many patients is the development of lower respiratory tract early in the illness. Respiratory distress, a hoarse voice, and a crackling sound when inhaling are commonly seen. Sputum production is variable and sometimes bloody.2 Complications of A(H5N1) and A(H7N9) infection include hypoxemia, multiple organ dysfunction, and secondary bacterial and fungal infections.3
Evidence suggests that some antiviral drugs, notably oseltamivir, can reduce the duration of viral replication and improve prospects of survival.
In suspected cases, oseltamivir should be prescribed as soon as possible (ideally, within 48 hours following symptom onset) to maximize its therapeutic benefits. However, given the significant mortality currently associated with A(H5N1) and A(H7N9) infection and evidence of prolonged viral replication in this disease, administration of the drug should also be considered in patients presenting later in the course of illness. The use of corticosteroids is not recommended.
In cases of severe infection with the A(H5N1) or A(H7N9) virus, clinicians may need to consider increasing the recommended daily dose or/and the duration of treatment.
In severely ill A(H5N1) or A(H7N9) patients or in patients with severe gastrointestinal symptoms, drug absorption may be impaired. This possibility should be considered when managing these patients.4 Moreover, most A(H5N1) and A(H7N9) viruses are predicated to be resistant to adamantine antiviral drugs, which are therefore not recommended for use.
Risk factors for human infection
The primary risk factor for human infection appears to be direct or indirect exposure to infected live or dead poultry or contaminated environments, such as live bird markets. Controlling circulation of the A(H5N1) and A(H7N9) viruses in poultry is essential to reducing the risk of human infection. Given the persistence of the A(H5N1) and A(H7N9) viruses in some poultry populations, control will require long-term commitments from countries and strong coordination between animal and public health authorities.
There is no evidence to suggest that the A(H5N1)and A(H7N9) viruses can be transmitted to humans through properly prepared poultry or eggs. A few A(H5N1) human cases have been linked to consumption of dishes made of raw, contaminated poultry blood. However, slaughter, defeathering, handling carcasses of infected poultry, and preparing poultry for consumption, especially in household settings, are likely to be risk factors.
Human pandemic potential
Influenza pandemics (outbreaks that affect a large proportion of the world due to a novel virus) are unpredictable but recurring events that can have health, economic and social consequences worldwide. An influenza pandemic occurs when key factors converge: an influenza virus emerges with the ability to cause sustained human-to-human transmission, and the human population has little to no immunity against the virus. With the growth of global trade and travel, a localized epidemic can transform into a pandemic rapidly, with little time to prepare a public health response.
The A(H5N1) and A(H7N9) AI viruses remain two of the influenza viruses with pandemic potential, because they continue to circulate widely in some poultry populations, most humans likely have no immunity to them, and they can cause severe disease and death in humans.
However, whether the influenza A(H7N9) virus could actually cause a pandemic is unknown. Experience has shown that some animal influenza viruses that have been found to occasionally infect people have not gone on to cause a pandemic while others have done so. Surveillance and the investigations now underway will provide some of the information needed to make this determination.
In addition to A(H5N1) and A(H7N9), other animal influenza virus subtypes reported to have infected people include avian H9, and swine H1 and H3 viruses. H2 viruses may also pose a pandemic threat. Therefore, pandemic planning should consider risks of emergence of a variety of influenza subtypes from a variety of sources.
WHO, in its capacity for providing leadership on global health matters, is monitoring avian influenza very closely, developing and adjusting appropriate interventions in collaboration with its partners. Such partners include animal health agencies and national veterinary authorities responsible for the control and prevention of animal diseases, including influenza.
Specifically, WHO, the World Organisation for Animal Health (OIE), and the Food and Agriculture Organization (FAO) collaborate through a variety of mechanisms to track and assess the risk from animal influenza viruses of public health concern, and to address these risks at the human animal interface wherever in the world they might occur. In short, WHO is monitoring the situation as it evolves, and as more information becomes available, will revise its guidance and actions accordingly.
1 Gao, H., et al. Clinical findings in 111 cases of influenza A(H7N9) virus infection. New England Journal of Medicine, 2013, 368:2277-2285.
2 The Writing Committee of the WHO Consultation on Human Influenza A/H5. Avian influenza A (H5N1) infection in humans. New England Journal of Medicine, 2005, 353:1374-1385.