Bird flu is a viral disease that can be transmitted from birds to people. Since 2003, more than 350 people have been infected with the virus and more than half have died. To date, most cases are caused by close contact with poultry. In addition, there are limited instances of transmission from wild birds to domestic poultry and from person to person. Some experts believe the virus’ method of transmission could change, allowing the disease to rapidly infect a large percentage of the global population.
Bird flu is present in many wild birds without the bird showing signs of infection. The virus is found in the animal's saliva, nasal secretions and feces. It can be spread through direct contact with an infected animal or through contaminated surfaces (e.g., cages, dirt) or materials (e.g., water, feed).
The first indication of disease is often seen in domesticated birds (including chickens, turkeys, geese and ducks). Symptoms in poultry may range from minor (e.g., ruffled feathers, reduced egg production, mild respiratory effects) to severe (e.g., hemorrhagic disease). The more serious form may kill between 90 and 100 percent of infected birds within two days.
Flu viruses typically infect a single species. Human instances of disease caused by bird flu are historically rare and have typically resulted in minor symptoms. This has changed somewhat with H5N1, the recent viral strain that has infected people. It hospitalized 18 people in Hong Kong in 1997. Of that group, six died. Since 2003, cases of human infection have been reported in 14 countries. The most cases have occurred in Indonesia, Vietnam, Egypt, China, Thailand and Turkey. According to the World Health Organization, the H5N1 virus has been linked to 351 cases of infection and 217 deaths between 2003 and the end of 2007. Each case was attributed to probable patient exposure to infected birds or possibly limited person-to-person transmission.
Like all viruses, the one that causes bird flu is a tiny particle that depends on infecting the cells of other organisms to reproduce. It is a member of the same group of viruses that causes “typical” flu each year in people.
When viewed through a microscope, influenza viruses look like a ball with a spiked surface. Inside the ball is the genetic material (RNA or ribonucleic acid) necessary for the virus to reproduce. The ball itself is made largely of a mix of proteins that form the virus membrane. Scientists classify the virus as influenza A, B or C, based on which proteins are in the membrane. The influenza A viruses are responsible for the majority of flu outbreaks and include bird flu.
The “spikes” seen on influenza A viruses are surface proteins embedded in the membrane of the ball. There are two types of surface proteins present, one of 16 varieties of hemagglutinin (H1 to H16) and one of nine varieties of neuraminidase (N1 to N9). Many combinations of proteins are possible. Those that include H1, H2 and H3 are typically involved in human disease. Viruses that cause bird flu that have also been transmitted to people have included H5, H7 and H9.
Viruses reproduce by invading cells and forcing them to create new viruses. The body’s immune system responds to viruses and other antigens by producing antibodies. The antibodies recognize the viruses by the surface proteins and therefore know which particles to remove without damaging normal body cells. These antibodies stay in the body to prevent future infection.
Influenza viruses are able to change or mutate their structure over time to bypass the antibodies produced by the immune system. This allows influenza outbreaks to occur each year and results from two processes. Through antigenic drift, the surface proteins of the virus change structurally and existing antibodies do not recognize the particles as infecting agents. The immune system must then create new antibodies to fight infection of these changed viruses.
Antigenic shifts are more significant changes. They occur when viruses that typically infect different animal species are present in the same organism and trade genetic material. For example, a bird flu virus and a human virus can both exist in a pig and trade genetic material. There is no natural immunity to the resulting virus and it can quickly cause widespread disease. This has happened in the past in 1957, 1968 and 1977, and is believed to be responsible for the 1918 influenza pandemic that caused millions of fatalities worldwide.
Past epidemics are believed to have resulted from influenza strains mixing in pigs. Swine can host viruses from both birds and people, providing opportunities for antigenic shifts. Many scientists believe influenza strain mixing could also occur in people now that some forms of bird flu are known to be able to infect humans in some cases.
There are hundreds of types of flu in nature. They are known to infect humans, birds, horses, dogs, seals and whales. Of the many influenza A viruses that typically infect birds, five are known to also cause disease in humans. These include the following:
The most severe and widespread type of bird flu. It has been confirmed in birds in portions of Asia, Europe and Africa. Since 2003, more than 350 cases have been confirmed by the World Health Organization, though infection may be more widespread. Not all people who are infected show the typical respiratory disease and may not be tested for the virus. Most cases were directly linked to the patient working closely with poultry.
These three viruses have caused infection in the United States (Virginia and New York), the Netherlands and Canada. The viruses caused only mild symptoms and most cases were directly linked to working with poultry. There were also limited reports of disease transmission within the families of poultry workers and one case in which the source could not be determined.
This strain of influenza has only caused disease in mainland China and Hong Kong. The risk of infection is considered very low and it has only caused mild disease.
Bird flu is a viral disease that occurs in birds throughout the world. While the disease has a broad distribution, to date it is rarely transmitted to people.
The greatest risk factor for bird flu infection is close contact with domesticated birds in areas where the virus is present (primarily Southeast Asia). Birds spread the virus through their secretions and feces and may contaminate cages, feed and the ground. Most forms of contact with live birds occur through farming or within markets. In addition, humans can also be infected by exposure to bird feathers and droppings. Limited reports of human infection have been associated with eating wild birds or their parts.
Most cases of bird flu have occurred in people younger than 25 years old, with many cases reported in children. This may be more closely related to unsanitary practices of children (e.g., not washing hands, playing with dirt) than other factors.
Physicians continue to learn about how bird flu impacts the human body. More is understood with each new outbreak. The incubation period of the disease is likely one to five days, but it may take as long as eight days for symptoms to develop.
- Sore throat
- Muscle aches
- Abdominal and chest pain
Other signs and symptoms have also occurred in patients infected with bird flu.
- Conjunctivitis (eye infection)
- Difficulty breathing
- Lymphopenia (low level of a specific type of white blood cell)
Symptoms of bird flu may become severe and lead to life-threatening complications. As the disease progresses it may cause damage to the lower lungs, which can lead to respiratory failure.
Diagnosis by a physician of bird flu may involve a medical history and physical examination. The medical history will likely include questions on symptoms, including their duration, severity and progression. Because clinical symptoms may be similar to the flu and other respiratory diseases, patients should tell the physician about recent travel (especially within the past 10 days). This is particularly important if the patient was exposed to wild birds or poultry, especially if they were ill or dead. The physical examination will check for signs and symptoms that may not have been apparent to the patient, such as a low level of the white blood cells called lymphocytes (lymphopenia).
If bird flu is suspected based on the patient’s symptoms and recent travels, the physician may perform one or more tests to confirm the illness.
- Viral culture. The physician may take swabs of samples from the patient’s respiratory system to be grown in a laboratory setting.
- Blood test. Samples may be tested for the presence of genetic material from the virus through a process called polymerase chain reaction (PCR). Blood may also be tested for antibodies against bird flu produced by the body.
In the United States, specimens from all patients suspected to have bird flu are also sent to the U.S. Centers for Disease Control and Prevention (CDC) for confirmation.
Treatment options for bird flu continue to be developed. A class of antiviral medications called neuraminidase inhibitors has shown some effectiveness when administered in the early stages of disease. The optimal duration and dosage of this medication are subjects of current scientific study. Research also continues into other treatment options because some strains of bird flu are showing signs of resistance to neuraminidase inhibitors.
Efforts to prevent the occurrence and spread of bird flu are ongoing on a global scale. Examples of actions being taken include:
- Culling. Flocks of domesticated birds infected with the virus are destroyed to prevent the spread of disease to other birds and people. Farms where the virus is found may also be placed under quarantine.
- Surveillance. The World Health Organization and national health agencies (e.g., the U.S. Centers for Disease Control and Prevention [CDC]) collect information on suspected and confirmed cases of bird flu. They also sponsor educational campaigns and animal vaccination programs for poultry farms and markets.
- Import bans. Many nations aim to prevent the spread of bird flu by prohibiting the importation of birds and bird eggs from nations where the disease has occurred.
In addition to the global efforts, individuals can also act to prevent the occurrence of bird flu. These include proper sanitation (e.g., hand washing, cleaning possibly contaminated surfaces, access to clean water) and thoroughly cooking poultry products (e.g., meat, eggs) to kill the virus.
Travelers who are planning to visit areas where bird flu occurs may take additional action to reduce the risk of infection. Immunization with an influenza vaccine may be recommended. The effectiveness of prophylactic vaccines is not clear, but if administered within 48 hours of symptoms occurring, the extent and severity of the illness may be reduced. Recommendations for travelers include:
- Get a flu shot before travel
- Avoid domesticated birds on farms and in open air markets
- Wash hands with alcohol-based hand sanitizers or soap and water
- Do not consume raw eggs or poultry meat
Preparing questions in advance can help patients have more meaningful discussions with their health care provider regarding their conditions. Patients may wish to ask their doctor the following questions related to bird flu:
- Am I at risk for bird flu?
- Where does bird flu occur?
- What can I do to prevent bird flu?
- Are there any recommendations to prevent bird flu while traveling?
- Are vaccines available to prevent bird flu?
- How is bird flu spread?
- What kind of birds carry bird flu?
- What are some signs and symptoms of bird flu?
- How long after returning from my travels should I be concerned about bird flu?
- If infected with bird flu, what treatment options do I have?