Wullie Davidson’s Blog: Avian Influenza


There are various subtypes of avian influenza (bird flu) which are divided into two categories, depending on the severity of disease they cause in birds:

1) Highly pathogenic avian influenza (HPAI), which causes a large number of deaths.

2) Low pathogenic avian influenza (LPAI), which usually causes mild or asymptomatic infection.


The subtype which has been making headlines recently, and which has been the cause of an unprecedented number of bird deaths throughout the world, is (HPAI) H5N1, which was first identified in wild birds in China in 1996. It rapidly spread to domesticated poultry, and caused some concern to epidemiologists in 2003, as it began to infect people, with an apparently very high case fatality rate. However, it is very likely that the severe cases were just the small tip of a much larger iceberg of mild to moderate and asymptomatic cases which went undetected. It does not spread easily to people, and there have been only a handful of suspected cases of person to person transmission..

Since its emergence, there have been sporadic, but limited outbreaks in wild birds, particularly in coastal regions, due to migratory seabirds and waterfowl. However, the scale of the current outbreak is unprecedented, and scientist at Edinburgh University, among others, are trying to figure out why it is that this particular strain of H5N1 is having such a devastating effect.



Scotland has about two thirds of the world populations of both the northern gannet and Arctic skua, and these species have been particularly badly affected, with some colonies losing up to 85% of their birds. Gannets are colony nesters and their colonies are more densely populated than colonies of other birds. Great skuas are scavengers, and feed on the dead birds, so it’s no surprise that both species have been so badly hit. Puffin numbers have also been greatly reduced, as have the numbers of geese that overwinter in Scotland before returning to their Arctic breeding grounds in the spring. Populations on Islay and the Solway Firth appear to have been worst affected.

The Americas

In the US, the first case in the current outbreak was detected in January 2022, and it has since been recorded in 47 states. In November, it arrived in South America, and scientists believe it may have been carried there by a migratory Franklins gull. It seems inevitable that it will eventually spread to the Galapagos Islands, threatening endangered endemic species, such as the Galapagos flightless cormorant and Galapagos penguins. When it reaches the southern tip of South America, it probably won’t be long until it arrives in Antarctica. It remains to be seen how susceptible the many species of penguins in their densely packed colonies are to the disease.

Human Pandemic Potential

The big question is, will it spread to people, possibly causing a pandemic of serious disease, similar to the Spanish flu of 1918? Researchers are agreed that the chances of this happening are low. Most novel flu pandemics, such as the Hong Kong flu of 1968, are a result of an avian flu transmitting to pigs, and acquiring mammalian characteristics as a result. If the pig is already infected by a swine flu virus, ‘reassortment’ can occur, which means that the two strains swap genes, creating a novel hybrid virus. Pig viruses are much more likely to be infectious to humans than purely avian viruses, and that’s how pandemics usually get started. Farming practices in China tend to facilitate this situation.

Some mammals are more susceptible to infection with H5N1 than others. It has been detected in foxes, which scavenge the carcases of dead birds. Cats appear to be more susceptible than most other species. If it begins to spread in garden birds, cats will pick up these birds, and may become infected, although this doesn’t mean that it will spread from cats to people. Some scientific studies suggest that it is not uncommon for people to infect cats with flu, but less is known about the risk of an infected cat spreading flu to people.

If a cat was infected with a human flu virus and then became infected with H5N1, this might result in a reassortment, with the two subtypes forming a hybrid virus. This novel virus might be more capable of infecting people. Even so, it might not cause serious illness. In 2009, a new strain of H1N1 human flu emerged, which was a reassortment with a pig virus, and named ‘swine flu’. The WHO declared a pandemic, but it turned out to be no more severe than typical seasonal flu.

Other Avian Influenza Subtypes

dead bird


H7N9 emerged in China in 2013. It causes mild or asymptomatic disease in poultry, making surveillance very difficult. Because of this, and the fact that it had already acquired mammalian flu characteristics, experts were initially worried about its potential to cause a human pandemic. The number of severe human cases rapidly overtook those caused by H5N1, apparently with the same level of mortality, but later serological testing by Chinese scientists revealed a huge number of asymptomatic cases. Little has been heard of H7N9 since 2014, but it hasn’t gone away.


H9N2 is the most common avian flu subtype in Chinese poultry, usually causing mild or asymptomatic disease, but has a mortality rate of around 10%. It is known to reassort with multiple other subtypes, including H5N1 and H7N9, so is significant as a precursor for the emergence of novel flu strains. Since 1998, a total of 86 cases of human infection with H9N2 have been reported.


H5N8 is a highly lethal avian flu virus The first known outbreak was recorded in poultry in Ireland in 1983. There was another outbreak in Europe in 2016-17, which spread to Asia and Africa. In February, 2021, there were multiple outbreaks in wild birds and poultry in the UK. The first known cases of human infection with H5N8 were detected in Russia. Seven workers at a poultry farm were serologically tested and reported to be infected after an outbreak among poultry at the farm. All were mild or asymptomatic cases.


H5N6 was first detected in poultry in China in 2013, and has since spread among wild birds and poultry around the world. Since 2014, 65 human ceses have been recorded, with 29 deaths. There have been no recorded cases of person to person transmission, although some of those infected report no contact with poultry. In 2021, the first human case outside China was identified in a five year old boy from Laos. That same year, the WHO stated that wider surveillance was urgently required in view of the recent upsurge in human cases. The first known outbreak of avian influenza in the Philippines was caused by H5N6 in 2017.

Wullie Davidson, January, 2023

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