Avian influenza

What is it?

-Avian influenza, also known as bird flu, is a virus disease caused by influenza strains that are well-adapted to birds. There are many different strains, and most of them can infect multiple bird species, both aquatic wild birds and domesticated species such as chickens, ducks, and turkeys.

– Avian influenza is a major veterinary problem, but it’s also a threat to human health. Some bird flu strains, such as H5N1 and H7N9, occasionally make the jump to humans; the big worry is that such strains start spreading from one person to the next, which could trigger a pandemic that could sicken millions or even billions of people.


The H5N1 virus. Credit: Cynthia Goldsmith, Jacqueline Katz/CDC

What are the symptoms?

– Veterinarians distinguish two types: low pathogenic avian influenza (LPAI) and highly pathogenic avian influenza (HPAI) viruses. In birds, LPAI viruses cause mild disease or no symptoms at all; HPAI strains are often fatal and kill off entire poultry flocks.

– In people, symptoms usually start between 2 and 5 days after infection; they vary widely from one avian influenza strain to the next, and from person to person. H5N1 and H7N9 can cause very serious disease; common symptoms include a high fever, cough, and sometimes difficulty breathing. Some patients also have diarrhea, vomiting, abdominal and chest pain, and bleeding from the nose or gums.

– So far, more than half of the 850 known H5N1 patients have died. For H7N9, the mortality rate among known patients is around 30%. The real rates are probably lower, however, because some patients with these strains didn’t seek medical attention or were not properly diagnosed.

– Other bird flu strains are much milder in people. During a big outbreak of H7N7 in poultry in the Netherlands, at least 450 people fell ill but most only suffered from an eye infection named conjunctivitis. One veterinarian died in the outbreak.

– Avian influenza also has another impact on people: killing poultry (either as a result of the virus itself or to prevent its spread) robs them of a source of food and income. This is a serious problem, especially in developing countries.


How does it spread?

– Avian influenza is very easily transmitted between birds; the virus can be found in their droppings as well in secretions from the nostrils, mouth, and eyes.

– When people fall ill, it’s almost always because they have come into contact with infected birds, for instance at farms or a slaughterhouse, in their backyard, or at a live poultry market.

– There’s no evidence that the virus can be transmitted through properly cooked food, whether it’s birds or eggs.

– There are a few cases where avian influenza may have spread from an infected person to someone else in their direct environment. If this happens at all, it’s very rare, however. Avian influenza viruses simply don’t spread well between people.

– But because the flu virus changes so often, experts worry that it might develop the capacity to transmit from one person to the next, become a fully human virus, and spread far and wide. Scientists are studying the chances that this will happen.

How is it treated?

– Avian influenza is not treated in poultry; when an infection occurs, the entire flock is usually killed.

– Human patients with avian influenza can be given an antiviral drug named oseltamivir, also known under the brand name Tamiflu, which increases the chances of survival. It needs to be given as early in the infection as possible.


Scene from an H5N1 outbreak in Côte d’Ivoire, 2006. Credit: N.Denormandie / OIE


How can it be prevented?

– Poultry vaccines exist but their use is not widespread. The main means to prevent outbreaks in poultry is through ‘biosecurity’ measures. Birds should be housed isolated from wild birds, ideally in indoor housing. Vehicles and waste should not travel from farm to farm.

– Humans can reduce the risk of infection by avoiding contact with birds that have been infected, may be infected, or have died from bird flu. Workers at farms where an infection has occurred should wear protective clothing and goggles. Only healthy animals should be slaughtered for food, and exposure to live birds at animal markets should be minimized.

– Unfortunately, these things are easier said than done, especially in developing countries. Most flocks aren’t held in biosecure facilities; millions of families keep a few chickens around the house. When they get sick or die, their owners may not realize it’s from avian influenza.

A worker at a Centers for Disease Control and Prevention laboratory working with avian influenza virus. Credit: CDC/James Gathany

What’s the outlook?

– Bird flu has proven very difficult to control and it can easily spread across long distances when carried by migrating birds. Strains circulating in wild birds will continue to spill over into domestic birds, which puts people at risk.

– We have no idea how likely it is that a bird flu strain becomes transmissible between people, which could unleash a pandemic. When H5N1 spread to birds in dozens of countries in Asia, Africa, and Europe in 2003, many experts feared that the world was on the cusp of a pandemic. That still hasn’t happened. In 2016, there are similar worries about a growing outbreak of H7N9 in China. We don’t know when or where the next outbreak may strike, which is why preparedness is important. (For more on this, see the fact sheet on human influenza.)



A World Health Organization fact sheet about avian influenza.

WHO’s latest outbreak information on avian influenza.

The avian influenza portal of the World Organisation for Animal Health.

Biosecurity for birds. Tips from the U.S. Department of Agriculture.