Bird flu (also termed avian influenza or avian influenza A) is an illness that affects wild and domesticated birds that usually causes either little or no symptoms unless the bird population is susceptible, which may cause death in many birds within about 48 hours. Bird flu viruses have been isolated from more than 100 species of wild birds and are endemic in many aquatic wild bird species (for example, seagulls and terns). Bird flu influenza A viruses primarily affect birds and are not easily able to infect people. However, in the late 1990s, a new strain of bird flu arose that was remarkable for its ability to cause severe disease and death in domesticated birds, such as ducks, chickens, or turkeys. As a result, this strain was called "highly pathogenic" (meaning very severe) avian influenza (HPAI, a term seen in older publications). The first human case of illness from highly pathogenic avian influenza was identified in 1997.
Human infection with avian influenza is rare (the incidence has been that about 860 human infections have occurred with Asian H5N1 virus, according to the World Health Organization [WHO], in mainly 15 countries in Asia, Africa, Europe, the Pacific Islands, and in the near East) but frequently fatal. According to statistics published by the WHO and the U.S. Centers for Disease Control and Prevention (CDC), mortality (death) rates from infection with Asian H5N1 have been approximately 50$%-60%.
Government officials in China announced they detected a new strain of bird flu in March 2013. It was named H7N9 (also termed H7N9 Chinese bird flu). As of November 2016, the WHO reported a total of 800 laboratory-confirmed human cases of the H7N9 virus since March 2013. Mortality rates have varied from about 20%-34%. Fortunately, the virus subtypes that have caused bird flu in humans are not easily transmitted to humans. However, health experts are concerned about possible future changes in these viruses that may allow them to become more contagious.
H5N8 bird flu has been found in birds since the 1980s and in the Northern Hemisphere each year since 2014. Outbreaks in wild birds have happened since 2020 in Russia, European countries, China, Iran, South Korea, and Japan. Russian health officials announced in February 2021 that in a poultry farm, seven workers exposed to birds tested positive for the H5N8 virus. The officials claim all individuals were asymptomatic and showed no evidence of human-to-human transmission. However, this is the first record of H5N8 detected in humans. In a CDC report, an H5N8 strain isolated from a falcon in 2014 was assigned a low to moderate risk to develop human-to-human transmission. WHO claims to have a vaccine against an H5N8 strain, but it was developed for vaccinating poultry. This finding of human infection by the bird-to-human transmission of H5N8 is worrisome, but sparse data so far suggests this H5N8 strain likely will not cause severe disease, according to Dr. Pedro Piedra, a professor in molecular virology at Baylor College of Medicine (see reference 2).
First, here are some definitions to put the bird flu threat into perspective:
Birds have been affected by avian influenza in Asia, Europe, the Near East, and Africa, and the outbreak has killed millions of poultry. Bird flu from the highly pathogenic strain was found in the United States in December 2014 and eventually detected in 21 states (15 states with domestic poultry infections and six states with detection of the virus only in wild birds). No human infections were reported in these U.S. bird flu outbreaks. Human cases of bird flu have largely been confined to Southeast Asia and Africa. However, mutations (changes in the genetic material of the virus) often occur in the virus, and it is possible that some mutations could create a more contagious virus that could cause a worldwide pandemic of bird flu among humans. Fortunately, the mutations that have occurred to date in nature have not made the virus more contagious. Unfortunately, recent research work has been able to introduce genetic material into bird flu viruses that make this laboratory strains highly transmissible to humans. This information will be discussed in another section.
The virus spreads from bird to bird as infected birds shed flu virus in their saliva, nasal secretions, and droppings. Healthy birds get infected when they come into contact with contaminated secretions or feces from infected birds. Contact with contaminated surfaces such as cages might also allow the virus to transfer from bird to bird. Contact with humans occurs in the same way, mainly by flocks of poultry cultivated by farmers that are exposed to wild birds infected with bird flu. Other people are exposed to the bird flu when, for example, infected birds are processed for sale before they are cooked or if they come in contact with contaminated wild bird droppings or dead birds.
Bird flu is caused by a type of influenza A virus (for example, H5N1 and H7N9). There are many types of influenza viruses, and most prefer to live in a limited number of animal hosts. Thus, swine flu primarily infects swine, and bird flu primarily infects birds. The seasonal human influenza virus is best adapted to humans. A few rare cases may occur in an accidental host, such as when people who have extensive contact with sick birds get the "bird flu." Sometimes, a species-specific flu virus will change (mutate) in a way that makes it easily able to infect other species. If avian influenza mutated to be able to spread easily among people, it would likely cause a serious pandemic. Such a mutation occurred in the so-called "swine flu" virus (H1N1) in 2009 that triggered a pandemic.
Humans may get bird flu from contact with infected birds (chicken, for example) or their droppings or surfaces with infected droppings. Human-to-human spread of bird flu may occur but has been very rare so far. However, if the highly pathogenic strains of bird flu (H5N1, H7N9) mutate to allow them to be easily transmitted from human to human, investigators are concerned that a lethal pandemic could occur in humans. The following section shows how in nature these influenza subtypes may undergo genetic changes that could enhance the viral subtype's contagiousness and/or pathogenicity.
Influenza viruses have two surface proteins that can be recognized and attacked by the human body's defenses (immune system). The proteins are called hemagglutinin (H) and neuraminidase (N). There are many different types of hemagglutinin and neuraminidase proteins. The recent bird flu had type 5 hemagglutinin and type 1 neuraminidase. Thus, it is an "H5N1" influenza virus type. Another strain of bird flu has two different surface proteins and was termed H7N9. Also, H9 is another avian flu subtype.
A person's immune system learns to recognize these surface proteins either by being infected with a flu virus and recovering or by getting a vaccine (flu shot) that contains similar H and N proteins. After that, any virus that infects the identical H and N on its surface will usually be quickly recognized and stopped, causing either a mild illness or no illness. This type of defense is known as immunity (to a specific viral type). Unfortunately, immunity to one viral type often does not protect against other viral types.
Minor changes in the H or N components can allow the virus to evade a person's immune defense. These minor changes are so common that they are almost routinely detected each year. This is why a person can get influenza infections year after year. If the new virus is very similar to older viruses, the immune system may still be of some help in reducing the severity of the disease. This is sometimes referred to as "partial" immunity.
Major changes in H and N viral proteins are more serious because people have no immune defense at all against the new virus. If the new virus spreads easily from person to person, there is a risk of a worldwide pandemic with a very large percentage of people becoming infected and ill from the flu. One way the avian influenza virus could make such a dramatic change is if it picked up proteins from human viruses through a process known as recombination of genetic material that results in an "antigenic shift." Another way would be spontaneous changes (mutations) in the bird flu virus itself that would make it more infectious; this results in "antigenic drift." These scenarios are what concern scientists about the bird flu and are shown below for bird, human, and swine flu RNA genomes; this influenza A viruses all follow the same genetic methods that result in new influenza viral types. The schematic below shows an example of the antigenic shift and drift for bird flu strain H5N1 but represents the way genetic material is reassembled and altered in all influenza A viruses including the H7N9 bird flu.
If such a dangerous virus acquires the ability to spread easily among humans, it could cause a serious health pandemic. Fortunately, this has not happened to date. Although the highly pathogenic strain of bird flu has repeatedly changed over time, person-to-person transmission of bird flu remains very rare. Rare transfer to humans is seen in other non-flu diseases such as mad cow disease. It is hoped that person-to-person transmission remains a rare event for any strain of bird flu.
Serious pandemic influenza is uncommon. The deadliest pandemic in modern history was the 1918 influenza, also known as the "Spanish flu" (although it did not originate in Spain). The 1918 virus spread rapidly and killed tens of millions of people worldwide. The risk of mortality (death rate) was especially high in healthy young adults. Although the 1918 virus was a human influenza virus, it had many genes that suggest it had an avian ancestor.
Humans get bird flu through close contact with sick or dead poultry or by spending time in poultry markets in countries (mainly Asia, Europe, the Near East, and Africa) known to harbor the virus. Infected people experience typical flu-like symptoms, including
Some people also have symptoms of nausea, vomiting, diarrhea, encephalitis (brain infection), and/or eye infections. Children and even animals (dogs, cats, and other mammals) may develop similar symptoms. This infection can progress to pneumonia and respiratory failure.
In humans, bird flu has caused a very aggressive form of respiratory flu that has progressed to ARDS (acute respiratory distress syndrome), respiratory failure, multi-organ failure, neurologic changes, and even lethal viral pneumonia in some patients according to the CDC (http://www.cdc.gov/flu/avianflu/avian-in-humans.htm).
The incubation period (time from infection to development of symptoms) for H5N1 averages about two to five days (range is 17 days) while H7N9 averages five days with a range of one to 10 days. Although avian influenza viruses are very contagious among wild and domesticated birds, they are weakly contagious in humans, and person-to-person transfer of bird flu viruses is uncommon unless there is very close contact with an infected person (for example, a family member).
For any flu-like illness suspected to be due to the bird flu virus, call a doctor as soon as possible to see if it is necessary to take an antiviral medication (for example, oseltamivir [Tamiflu]). The medication may shorten the course of the illness or lessen the symptoms. Be sure to mention if you have had any contact with sick or dead poultry or recent travel to an area of the world currently affected by bird flu.
Health specialists who may be consulted to treat bird flu in humans include
Experts at the CDC and/or WHO should be notified to perform specialized testing and aid in infection control.
There is no way to tell what type of influenza infection a person has without doing tests. In most cases, the presumptive diagnosis of flu is determined by the symptoms, especially when these occur during the peak flu season (late fall and winter in the U.S.). Sometimes, the doctor may need to perform special tests to be sure the seasonal influenza virus is responsible for the disease and not due to H5N1 or other novel influenza viruses.
Some doctors' offices may use a rapid test that can be done in the office with the result available in 30 minutes. Some rapid tests detect only the influenza A virus, while others can detect both influenza A and influenza B virus types, and some may report unsubtypable influenza viruses or give a result presumptive for novel influenza viruses. These samples are recommended to be sent to the CDC for further examination. To identify whether a virus is present and to test for the type of influenza, a sample is taken from the back of the throat and/or nose. The doctor uses a cotton-tipped wooden stick and simply rubs the cotton tip at the back of the throat and/or inside the nose. Alternatively, samples may be obtained by rinsing saltwater (saline) solution through the nose and throat and transferring the fluid back into a specimen jar. The sample is sealed in a packet and sent to the lab for testing.
Also, some cases of flu may be missed by rapid tests. There are no commercially available tests that specifically detect bird flu. However, Chinese researchers are attempting to develop a test to identify H7N9 viral infections.
Again, routine diagnostic tests available in the doctor's office currently cannot determine whether a case of the flu is due to bird flu. The patient's samples would be sent to a reference laboratory (usually through the state health department) for special testing if bird flu is suspected (perhaps because of known exposure to infected birds). If a patient is in the hospital, the physician may recommend a bronchoscopy, which involves slipping a tube through the mouth or nose into the lungs to aspirate secretions. Most viruses can be identified fairly quickly by polymerase chain reaction (PCR), usually done at the CDC. The virus may also be cultivated in tissue culture and antibodies against it may also be detected in an infected person's serum, but these tests take time. The patient usually has either recovered or died by the time these viral culture tests are done.
Influenza is a respiratory infection. There are many recommended self-care techniques to help relieve viral flu symptoms; however, with bird flu, symptoms may progress rapidly and treatment at home would not be appropriate. If you have been exposed to bird flu and develop symptoms, seek medical help immediately and do not attempt to care for the infection at home.
Scientists are hopeful that antiviral medicines like oseltamivir (Tamiflu) might be effective against bird flu in humans. Another drug called zanamivir (Relenza) shows promise in the lab but has not been widely used in human cases of bird flu. Oseltamivir, peramivir, and zanamivir are types of medications called "neuraminidase inhibitors." Side effects include nausea, vomiting, and sometimes nervousness. Zanamivir is an inhaled drug and may make asthma worse. Oseltamivir is given as a pill while peramivir is given intravenously. Some strains of the bird flu have shown resistance to neuraminidase inhibitors, and clinicians may consider adding another drug like ribavirin (Copegus, Rebetol) or amantadine (Symmetrel) in these cases.
Oseltamivir, zanamivir, and peramivir are only available by prescription and are recommended by the CDC for the treatment of bird flu in humans. If a pandemic occurs, the CDC may authorize the distribution of antiviral medications directly to the public. Treatment should begin as soon as possible after symptoms start; if bird flu is suspected, the CDC recommends treatment be started immediately.
There is not enough experience to date with these drugs in the treatment of H7N9 to know what effect, if any, they will have on this viral infection.
A vaccine has been developed and approved by the FDA to protect humans against the H5N1 bird flu virus, although it is not available to the public at this time because the U.S. population has not experienced any bird flu outbreaks. It is unlikely that the H5N1 vaccine will offer protection against H7N9 bird flu. There is some concern that the inactivated viral vaccine preparation (killed H5N1 viruses) may not be as effective as predicted if the virus continues to mutate. The standard flu vaccine developed each year does not protect against these strains of bird flu.
Health researchers are currently developing new ways to create flu vaccines that can be rapidly prepared and may give people immunity to a wide range of influenza viruses; these new vaccines (some based on the conserved internal viral proteins) may be available in a few years. A 2013 publication on bird flu vaccine development showed some success in protecting research animals against viruses by immunizing with the N9 antigen, but it has not been tried in humans.
New and more rapid vaccine development is becoming available. The FDA approved (January 2013) a recombinant vaccine (Flublok) for treating seasonal influenza that does not use the tedious and time-consuming egg inoculation method for the preparation of the vaccine. Vaccine developers speculate that in the very near future, researchers may be able to produce safe and effective vaccines very quickly in large amounts that can be administered if needed to large human populations. However, there is no vaccine available against H5N8 infections approved for use in humans.
The prognosis (outcomes) for bird flu continues to be poor with the death rate reaching about 60% with the N7H9 strain of bird flu, there is reason to believe it, too, could have a high death rate in future outbreaks.
Prevention (see above) is the key to a good outcome. The U.S. Department of Agriculture and the CDC have banned the import of certain birds from many Asian countries affected by the H5N1 virus strain because of the potential that infected birds could infect humans. This ban includes both live and dead birds and their eggs. This ban is likely to be modified to include H7N9.
Although it is possible that highly pathogenic bird flu may mutate and spread widely to people, it is encouraging that this has not happened in the 16 years since the first human case was identified. The World Health Organization (WHO) continues to monitor public health reports for clusters of people with symptoms that might suggest a flu virus is moving from human to human (and not just from birds to humans).
Most articles do not have this section but it is included to give the reader some insight into the problems and dangers of biological research that may affect their lives. In 2011, at least two major research laboratories (in the U.S. and the Netherlands), while trying to predict what genetic changes needed to occur in avian flu to make the virus easily transmissible to humans, developed a highly lethal avian influenza virus strain that was easily transmissible to ferrets. Unfortunately, for humans, this lab strain could be transmissible to humans by "mistake" since the spontaneous transfer of the swine flu (H1N1) has been documented to occur between humans and ferrets in nature (pet ferrets caught H1N1 from humans).
Although this lab strain gives researchers a fine model to study viral genetics and viral transmission, many health researchers, clinicians, biowarfare experts, and many others consider such work to be highly dangerous because of the potential, however slight, for the virus to escape the lab by "mistake," or even worse, that terrorists could use the published data to create a biological weapon. Consequently, the publication of the data about this potentially lethal strain was delayed until there was some agreement in the worldwide scientific community about how to proceed. This delay was not only for publication but extended to further research work on the viral genome.
The work on person-to-person transmission genetics was and still is another major area of concern. An expert panel composed of WHO consultants decided in 2012 that the controversial information should be reported so now many of the details of the research are widely available. How the research will progress remains unclear. What can be done with H1N1 viruses is possible to do with avian influenza viruses, and such laboratory-produced modifications of H5N1 or H7N9 bird flu could have devastating human consequences in the future if they escape from the labs if no vaccine or effective antiviral drugs become readily available.
U.S. Centers for Disease Control and Prevention
U.S. Department of Agriculture
Animal and Plant Health Inspection Service
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