Key facts
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.
Clinical features
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.2Complications of A (H5N1) and A (H7N9) infection include hypoxemia, multiple organ dysfunction, and secondary bacterial and fungal infections.3
Antiviral treatment
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.
WHO response
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.