Bird flu is an illness also caused by the influenza A virus.
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:
- Pandemic: A pandemic is a global outbreak of disease. This could occur if a new virus (for example, one which has mutated from an avian influenza virus) that causes serious illness were to emerge among humans (not birds) with the ability to spread easily from one person to another. A pandemic is caused by a new subtype that has never (or not recently) occurred in humans. The last pandemic involving humans was with an influenza virus, H1N1 (often termed the "swine flu"), that occurred in 2009.
- Epidemic: A fast-spreading, seasonal, or regional outbreak of flu among humans is called an epidemic. Epidemics may lead to pandemics; there have been several bird flu epidemics in poultry (for example, chicken and turkey) in several regions of the world (see below).
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.