Avian influenza, also called avian flu or bird flu, is an illness that usually affects only birds but occasionally jumps over to humans. Water and shorebirds can carry avian influenza viruses which can be spread over large distances through migration. They can cause outbreaks in domestic poultry, increasing the chances of transmission to humans.
Gobally we are also seeing increases in mass bird deaths, indicating that these viruses are becoming more prevalent. While it is relatively rare for humans to catch avian influenza and is mostly linked to direct or close contact with live or dead infected birds or their environments, there is a risk that these significantly lethal viruses adapt to humans and cause wide-spread epidemics with high mortality or even a pandemic event.
New infections per year
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What causes Avian influenza
Avian influenza is caused by a highly contagious virus that is common in birds. Like swine flu and human flu, avian influenza is caused by various strains of influenza A viruses. Wild aquatic birds are believed to be the natural reservoir or hosts of the virus while domestic poultry and other birds can also be affected.
Avian Influenza type A viruses are divided into sub-types based on two proteins on the surface of the virus: Hemagglutinin (H1-H16), Neuraminidase (N1-N9). Five sub-types that are known to infect humans are: H5N1, H7N3, H7N7, H7N9, and H9N2. Out of these, H5N1, H1N2, and H7N9 have caused most infections. Some countries have also reported sporadic human infections with swine influenza viruses, particularly the A(H1) and A(H3) subtypes.
They are also categorized into two classes: low pathogenic avian influenza (LPAI), and highly pathogenic avian influenza (HPAI). The mortality rate for HPAI viruses in birds can go up to 90-100%. This can affect poultry industry and result in mass culling of animals and restrictions of trade both locally and internationally. In humans, mortality is close to 60%.
Most cases of human infections are the result of handling dead infected birds, exposure to infected excrement or contact with their environment such as on farms. Respiratory transmission is also a possible route. There is a significant concern that strains of avian flu may mutate to become easily transmissible between humans and cause millions of deaths. The H1N1 and H5N1 strains are of particular concern.
Where is Avian influenza found
In the early 1900’s avian flu was documented in Italy due to a mass death of birds. The first case of highly pathogenic H5N1 virus was isolated in China in 1996 from a goose farm. In 1997, the first case of human infection with avian influenza H5N1 was reported in Hong Kong. First cases of human infection with Asian lineage avian influenza A(H7N9) virus were reported in China in 2013.
Since 2003, more than 700 human cases of Asian HPAI H5N1 have been reported, primarily from 15 countries in Asia, Africa, the Pacific, Europe, and the Middle East. Avian influenza is fast becoming a global problem, evidenced by increasing frequencies of mass birth deaths.
What are the symptoms of Avian influenza
Generalized signs and symptoms of avian influenza are varied among strains and are ranged from mild to severe. Some symptoms include conjunctivitis or pinkeye and seasonal influenza-like symptoms related mostly to respiratory illnesses. Many infections caused by Asian lineage H5N1 HPAI viruses, showed symptoms within 5 days while the incubation period may vary between 8 and 17 days in some cases.
H5N1 HPAI viruses – Asian lineage
Initial symptoms of the infection include high fever and upper respiratory symptoms similar to seasonal flu. Some patients may also experience mucosal bleeding, or gastrointestinal signs such as diarrhea, vomiting, and abdominal pain. Later stages of severe infection cases may develop kidney disease, encephalitis, and multiorgan dysfunction. Mild infections are particularly seen in children.
H9N2 LPAI viruses – Eurasian lineage
Most cases in H9N2 viruses are related to children and infants. The infections were similar to human influenza with fever, upper respiratory symptoms. Some patients also showed gastrointestinal signs such as vomiting and abdominal pain and mild dehydration.
H7N9 LPAI viruses – China
Most cases caused by H7N9 to date have been serious. The most common symptoms were fever and cough while a significant number of infections also had dyspnea and/or hemoptysis and severe pneumonia. Some of the patients also experienced diarrhea and vomiting. Conjunctivitis and inflammation in the brain were uncommon in H7N9 LPAI viruses. Simultaneous bacterial infections may have also contributed to the seriousness of the reported infections.
Other avian influenza viruses
Three serious infections reported related to Asian lineage H5N6 HPAI were seen in the elderly. Infections showed fever and severe respiratory symptoms. One case of a childhood infection was reported with mild symptoms and recovered promptly. Mild symptoms along with conjunctivitis and upper respiratory symptoms were reported in cases of infections in H7 LPAI or HPAI and H10N7 virus.
How can Avian influenza be prevented
The best way to prevent avian influenza viruses is to avoid the sources of infected poultry and avoiding handling of dead birds. Monitoring and controlling the source of the virus by eradicating HPAI viruses and closing infected poultry sites prevent the spread of the infection.
Proper handling of infected animals and their extractions, good hygiene around poultry, and using personal protective equipment (PPE) when handling. Proper disinfection of poultry facilities will help to minimize the survival of the virus in the environment. As HPAI is also found in the meat and eggs of several avian species, it is important to follow sanitary cooking and handling methods. More stringent precautions should be taken for hunters, field biologists and people who cull infected birds.
Health care workers preforming aerosol generating procedures should use airborne precautions. Standard contact and droplet precautions and appropriate personal protective equipment (PPE) should be used.
How is Avian influenza diagnosed
Avian influenza is difficult to diagnose based on symptoms as they are similar to those of seasonal influenza.
Diagnosis is done with polymerase chain reaction (PCR) tests, such as the ones made available by the WoIDM.
Rapid influenza diagnostic tests (RIDTs) have lower sensitivity compared to PCR and their reliability depends largely on the conditions under which they are used.
How is Avian influenza treated
Some antiviral drugs, notably neuraminidase inhibitor (oseltamivir, zanamivir), can reduce the duration of viral replication and improve prospects of survival, however ongoing clinical studies are needed. Emergence of oseltamivir resistance has been reported. Aditionally, A(H5) and A(H7N9) viruses are resistant to adamantane antiviral drugs.
In suspected and confirmed cases, neuraminidase inhibitors should be prescribed as soon as possible and ideally within 48 hours of the onset of symptoms.
Corticosteroids should not be used, as they have been associated with prolonged viral clearance and immuno-suppression leading to secondary bacterial or fungal infections.
Co-infection with bacterial pathogens is to be expected in critically ill patients.
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