What is the H5N1 virus?
Bird flu (also referred to as avian flu), as the name implies, is an infection in birds caused by an influenza virus. Influenza viruses that infect birds often do not cause illness in birds. Since the viruses are highly contagious, danger to humans arises when domesticated birds (for example chickens, ducks, and turkeys) become infected. While bird flu usually poses no threat to humans, instances of transmission of bird flu to humans have been reported since 1997, and over 100 people were infected in an outbreak that began in Southeast Asia in mid-2003. In November 2010, officials in Hong Kong announced that a woman had contracted bird flu, the first reported case of the condition in seven years. The 59-year-old woman had recently traveled to mainland China but reported no contact with live poultry or visits to farms.
Birds infected with avian influenza virus can transmit the virus to humans through their excretions: saliva, nasal secretions, and feces. During an outbreak of bird flu among poultry, there is a possible risk of spread to people who have contact with infected birds or surfaces that have been contaminated by infected birds.
A recent outbreak of bird flu that has affected humans involves the influenza A (H5N1) virus, sometimes referred to as the "H5N1 virus." Outbreaks of influenza H5N1 infection occurred in poultry in eight countries in Asia during late 2003 and 2004, resulting in more than 150 million birds either dying from the disease or being killed in order to try to control the outbreak. This widespread outbreak also led to documented human infections and deaths in several countries.
By March 2004, this outbreak was considered to be under control. Beginning in late June 2004, however, new outbreaks of influenza H5N1 among poultry were reported by several countries in Asia (Cambodia, China [Tibet], Indonesia, Kazakhstan, Malaysia, Mongolia, Russia [Siberia], Thailand, and Vietnam). Most recently, influenza H5N1 has been reported among poultry in Turkey and Romania. Outbreaks in wild and domestic birds are also under investigation in other countries.
Despite efforts to contain the outbreak, the World Health Organization (WHO) now considers the virus to be endemic in birds (meaning that the infection is steadily maintained) in many parts of Indonesia and Vietnam and in some parts of China, Cambodia, and Thailand.
In April 2013, four people in China were reported to be infected with a new strain of bird flu, known as H7N9. Two of the infected people died. In 2014 and 2015, an increase in H5N1 avian influenza cases was seen in China and Egypt.
When bird flu infects humans
Bird flu symptoms in humans can vary and range from "typical" flu symptoms (fever, sore throat, muscle pain) to eye infections and pneumonia. The disease caused by the H5N1 virus is a particularly severe form of pneumonia that leads to viral pneumonia and multiorgan failure in many people who become infected. Up to 50% of humans who contract bird flu die from the infection, but the actual numbers cary widely based upon the strain of virus and the underlying health and age of the patients. During an outbreak of bird flu, people should avoid contact with domesticated bird populations and surfaces that may have been contaminated by bird excretions.
More than 800 people have become infected since the 2003 bird flu outbreak. Confirmed cases of human infections have occurred in Africa, Asia, (especially China and Southeast Asia). Most of those infected have come from rural areas where many people keep poultry flocks that often roam freely and enter dwellings. Infected birds shed virus in their feces, leading to widespread contamination of homes and surroundings. Most of the people who have become infected were previously healthy adults and children. Infection also occurs during the slaughter of infected birds.
Research has suggested that the prescription antiviral medicines approved to treat human flu virus infections (such as Tamiflu) should work in helping preventing bird flu infection in humans. However, flu viruses can become resistant to these drugs, so these medications may not always work. Additional studies are needed to prove the effectiveness of these medicines.
Why is there so much concern about bird flu?
There are many questions about bird flu that remain unanswered and are under investigation. It is known that some forms of bird flu viruses, such as H5N1, are more highly pathogenic (cause more serious illness) than others, yet the reasons for these differences are unclear. Human and bird influenza viruses have a similar structure but differ in the composition of proteins on their external surfaces. Because influenza viruses have the capacity to mutate, or undergo changes in their surface proteins, scientists are concerned that the bird flu viruses may eventually change into forms of the virus that are able to infect humans more easily.
Another issue of concern involves transmission of bird flu viruses among humans. While no one knows if this will be an imminent threat in the U.S., the potential is there. Human-to-human spread is currently very rare; the CDC has only documented it a few times, and these rare instances have only involved spread to one other person by an affected individual. This means that those humans who have been infected in the outbreaks in other countries have been poultry workers or others who have had contact with birds or surfaces contaminated by infected birds. The fear is that mutation of the virus will cause it to take on a form that will be easily spread from person to person, since flu viruses can change relatively rapidly. Should this happen, the danger for widespread infection of human beings (pandemic) would be significant.
Research studies to develop vaccines to the H5N1 bird flu virus are also under way.
For additional information about bird flu, please read the following articles:
- Tamiflu for Bird Flu (Doctor's View)
- Oseltamivir (Tamiflu) (drug information)
- Bird Flu FAQ (Health Feature)
Medically reviewed by Robert Cox, MD; American Board of Internal Medicine with subspecialty in Infectious Disease
Switzerland. World Health Organization (WHO). "Avian Influenza." <http://www.who.int/csr/disease/avian_influenza/en/>.
United States. Centers for Disease Control and Prevention. "Questions and Answers About Avian Influenza (Bird Flu) and Avian Influenza A (H5N1) Virus." May 28, 2008. <http://www.cdc.gov/flu/avian/>.