Bird flu

Poultry farm and bird flu

Avian influenza (bird flu) viruses are among the most dangerous viruses that can affect humans, with a case fatality rate ranging from around 30 to 60 per cent.

Scientists have been warning us for years about the pandemic threat posed by factory farms and wildlife markets and many fear that the next pandemic will be caused by a bird flu virus, making the jump to humans from poultry or pigs.

We’ve been here before; Ebola, HIV, SARS and MERS are all examples of zoonotic diseases that spread to humans from animals. These ‘spillover’ events can occur when humans invade wildlife habitats, at wet markets – where many different wild and domesticated animals are sold live and slaughtered – and in factory farms, where large numbers of animals are crammed into sheds, in horrific conditions.

Bird flu is a classic example of a zoonotic disease, and most pandemics can be traced back to avian influenza viruses:

  • 1918 Spanish flu

The Spanish flu pandemic was one of the deadliest ever, killing an estimated 50 million people. Caused by an H1N1 avian influenza virus, it’s not known precisely which animal it originated from but it was of avian origin, so likely came from farmed poultry or wild birds.

The next three pandemics were caused by pick-and-mix viruses of avian origin, combining elements from more than one virus. These are known as ‘reassortant viruses’ and may have evolved in pigs:

  • 1957 Asian flu

The H2N2 virus that caused the Asian flu pandemic (estimated to have killed around 1.1 million people), was probably the product of a wild duck virus combining, possibly in pigs, with one from humans. H2N2 persists in wild and domestic birds and a re-emergence in humans could pose a significant pandemic threat.

  • 1968 Hong Kong flu

The H3N2 virus responsible for the Hong Kong flu pandemic that killed up to four million people is thought to have evolved from H2N2 by combining with another avian influenza virus (again, possibly in pigs) to produce a new strain capable of infecting humans.

  • 2009 swine flu

Another H1N1 virus was responsible for the swine flu pandemic in 2009 that began in pigs in Mexico and spread rapidly across the world, killing up to half a million people. The H1N1 virus responsible contained elements of viruses from humans, birds, North American pigs and Eurasian pigs. The mixing most likely occurred in live pigs being traded internationally.

Swine flu is now one of the seasonal flu viruses that circulate each winter and if you’ve had flu in recent years, there’s a good chance it was this one.

The problem with pig farming

Pig farming has changed dramatically in recent decades and scientists warn that pigs could play an increasingly important role as vectors of pandemic threats.

The problem is, pigs are susceptible to flu infection from birds, humans and other pigs, and viruses can combine in them to produce new ones, previously unseen. This mixing of viral genes is called reassortment.

When you move factory farmed pigs around the world, you risk spreading disease. That’s exactly what happened when the international trading of live pigs led to the emergence of a completely new virus that caused the 2009 swine flu pandemic.

Scientists say that flu viruses had been circulating in pigs for at least 10 years before the 2009 pandemic, and suggest that the fact that flu viruses in pigs are not monitored, allowed this potential pandemic strain to persist and evolve for many years undetected.

The risks from poultry farming

Intensive poultry production also provides a perfect breeding ground for mutating viruses. Chickens are raised in closed, filthy, stressful and crowded industrial facilities, with little or no natural light –an important consideration as UV light harms viruses. We are literally handing viruses and other pathogens (see antibiotic resistance) the perfect opportunity to mutate into more deadly forms – a perfect storm of our own making.

The poultry industry likes to blame the spread of bird flu on migratory birds. However, whilst wild birds may contribute to the local spread of the virus, human commercial activities, particularly those associated with poultry, are the major factors responsible for the global spread of bird flu. The 2007 H5N1 outbreak in Sussex, for example, was traced to the trading of hatching eggs, birds and poultry products between the UK and Hungary.

If we are to avoid a bird flu pandemic then we need to end factory farming, before it ends us!

Frequently asked questions

How long has bird flu been around?

Bird flu viruses have been around for thousands of years, occurring naturally in wild water-birds (especially ducks, geese, swans, gulls, terns and sandpipers) without making them ill. The viruses are passed on in water from one bird to another. In water-birds, viruses have an ideal environment in which to co-exist with their host.

Where did bird flu come from?

When wild water-birds were taken to market, however, the virus that co-existed with them could no longer spread in water so had to either mutate or die. In this new, stressful environment, mutations occurred that enabled viruses to spread via the faecal, nasal, oral and eye secretions of infected birds.

What are bird flu subtypes?

Bird flu viruses are separated into subtypes based on the proteins on their surface. You may have heard the viruses referred to as H1N1 or H5N8.

What does the H and N used in the name of bird flu viruses refer to?

Flu viruses are named after the two proteins they carry on their surface; H is for hemagglutinin and N is for neuraminidase – these are the little protein spikes on the virus’s surface that help it to invade cells. There are 18 different hemagglutinin subtypes and 11 different neuraminidase subtypes (H1 through H18 and N1 through N11, respectively).

When was bird flu first reported in birds?

Some records suggest bird flu, previously known as ‘fowl plague’, was infecting domesticated birds as far back as the 1870s. In more recent times, it’s widely accepted that bird flu has been infecting poultry since the late 1950s.

When was bird flu first reported in humans?

Flu viruses, containing elements of avian origin, have been infecting humans for at least the last 100 years, illustrated by the pandemics of the 1900s. However, bird flu became the focus of intense international attention in 1996 when a highly pathogenic strain of H5N1 emerged in farmed geese in the Guangdong province in China, killing more than 40 per cent of the birds it infected. By 1997, it had spread to poultry farms and live-poultry wet markets in Hong Kong – where it infected 18 people, leading to six deaths. To try and stop the outbreak, the government ordered the slaughter of more than 1.5 million chickens. In addition to further human cases in Hong Kong in 2003, there were poultry outbreaks in mainland China and other countries in Southeast and East Asia. There have been in the region of 15,000 poultry outbreaks, and more human deaths in Vietnam, Thailand and China. Since 2003, H5N1 has infected 882 humans killing 461 of them – mostly teenagers and young adults.

What does low and high pathogenic mean?

Bird flu viruses are described as being low or highly pathogenic depending on how lethal they are to birds.

How does bird flu spread to humans?

Bird flu is spread by close contact with an infected bird (dead or alive). This includes touching infected birds, droppings or bedding as well as killing or preparing infected poultry for cooking. Markets, where live birds are sold, can also be a source of infection. It’s been suggested that slaughterhouse workers should wear personal protective equipment (PPE) to protect themselves against these viruses.

How easily is bird flu spread between people?

Luckily, not so easily at the moment but that could change at any time. Of the 800 or so people infected with H5N1, for example, only a handful of them caught it from infected humans – mainly family members caring for sick relatives.

The worry is that if H5N1 – or another virus – mutates and becomes more transmissible, like the common cold for example, it could result in the deaths of anywhere between five and 150 million people, according to David Nabarro, a senior public health expert at the World Health Organisation.

Which strain of bird flu is the deadliest to birds and people and why?

H5N1 is spreading globally after first appearing in Asia. This highly pathogenic subtype has killed tens of millions of birds and led to the culling of hundreds of millions more, in an effort to try to stop it spreading. When media stories refer to ‘bird flu’ they often mean H5N1. However, others, such as H5N8 are also a cause for concern, especially now that H5N8 has made the jump from birds to humans.

The World Health Organisation says that globally, from 2003 to March 2024, there were 882 cases of human infection with H5N1 reported from 23 countries, 461 of which were fatal. So, this virus has killed 53 per cent of those infected; one in every two!

How does bird flu spread around the world?

The poultry industry likes to blame the spread of bird flu on migratory birds. However, whilst wild birds may contribute to the spread, human commercial activities, particularly those associated with poultry, are the major factors responsible for the global spread of bird flu.

Scientists say that even if migratory birds do act as vectors, disease infection patterns show that circulation is maintained through trade of infected domestic birds.

How do factory farms promote the spread of bird flu?

Diseased birds can spread infection via bodily secretions. Contaminated equipment, vehicles, feed, cages or clothing – especially shoes – can also spread the virus within and between farms.

Other opportunities for transmission from factory farms include via poultry manure deposited on the land that contains uneaten bird feed. It draws wild birds to the faecal reservoir of domesticated poultry thus increases the spread of bird flu viruses.

There are also issues with ventilation and airborne transmission. Flu viruses carried on dust particles can transmit infection through the air to new hosts. Large farms housing tens of thousands of animals require ventilation in order to provide air-flow to ensure the animals don’t die. So, if viruses are present in the air, they will be circulated widely around the area and expelled out into the local environment.

Pig manure lagoons (giant cesspits containing pig manure) may also increase the risk of transmission via the contamination of water or by particles being blown into the air.

Is bird flu a problem in the UK?

Yes, bird flu affects wild birds, poultry and other captive birds in the UK. In late 2021, the UK the UK experienced its largest-ever outbreak, and experts have warned that infections could rise even higher in coming years. Hundreds of cases were reported in captive birds across the country and thousands of deaths have been reported in wild birds. Nearly all reported cases involved the highly pathogenic strain H5N1.

The outbreak has led to the death of 97 million birds globally (3.8 million in the UK). In response, the UK government imposed mandatory housing for all poultry, meaning that all bird keepers, with commercial flocks or just a few birds in the garden, were required by law to take biosecurity precautions and to keep all birds indoors.


What are the symptoms of bird flu?

The main symptoms can appear very quickly and include:

  • a very high temperature, feeling hot or shivery
  • aching muscles
  • a headache
  • a cough

Other symptoms may include:

  • diarrhoea
  • sickness
  • stomach pain
  • chest pain
  • bleeding from the nose and gums
  • conjunctivitis

Bird flu is not just a respiratory disease, the virus has been detected in the brains and guts as well as the lungs of people who have died following infection. In 2007, researchers found that the H5N1 bird flu virus could also pass through a pregnant woman’s placenta to infect the foetus.

How long does it take for symptoms to appear after infection?

It usually takes three to five days for the first symptoms to appear after you’ve been infected. Within days of symptoms appearing, it’s possible to develop more severe complications such as pneumonia and acute respiratory distress syndrome.

The NHS say that getting treatment quickly and using antiviral medicine, may prevent complications and reduce the risk of developing severe illness.

How does bird flu affect children?

The risk to children is determined by how deadly an individual strain of bird flu virus is and whether the individual child has had any previous exposure to it or a similar virus.

The 2009 H1N1 swine flu pandemic affected younger people much more than the over 65s. Scientists think that many older people had some protection because they would have been exposed to similar viruses descended from the H1N1 virus that caused the 1918 Spanish flu pandemic.

Some viruses appear to spread easily amongst children, while others cause more severe illness. One study looking at how different viruses behave in children found that H7N9 was more easily spread by children than H5N1, but that H5N1 caused more severe illness. H5N1 has a high death rate amongst infants and young children; another study suggests that the case fatality rate among under 15s could be as high as a shocking 89 per cent.

If a virus that is easily spread becomes more deadly, or a deadly virus becomes more easily spread – this fatal combination of factors could lead to a devastating pandemic the likes of which have not been seen for 100 years.

What other animals can catch bird flu?

Avian influenza viruses have been found in birds, pigs, horses, cats, tigers, leopards, civets, dogs and humans. According to the UN FAO, carnivores (meat-eaters) can become infected after eating infected poultry.

Why is bird flu so dangerous in pigs?

Pigs are susceptible to infection with flu viruses from other pigs, humans and birds. If pigs are infected with more than one virus, the viruses can ‘mix and match’ together to produce a new strain, previously unseen. This process of gene swapping is called reassortment and is common among flu viruses. For this reason, some scientists refer to pigs as ‘mixing vessels’ for viruses.

Reassortment occurs frequently in nature and although it rarely results in a virus with pandemic potential, scientists say that all three pandemics of the twentieth century may have been generated by a series of multiple reassortment events in pigs or humans.

How many mutations do bird flu strains need to infect us?

Mutations occur naturally in flu viruses and if they offer the virus an advantage, for example if they become more easily spread between people, the new version will prosper.

Retrospective analysis of the pandemic H1N1 (1918), H2N2 (1957) and H3N2 (1968) viruses shows that only one or two mutations were required to confer an increased binding affinity for human respiratory cells. In other words, just one or two mutations allowed the virus to infect humans more easily. Other mutations could facilitate airborne transmission more efficiently.

One study suggests that the H7N9 strain would require just three mutations to become more easily spread between humans. It normally affects birds, but H7N9 has infected at least 1,500 people in outbreaks linked to poultry markets in China. The virus is not easily spread between people at the moment, but the high mortality rate of 40 per cent of those infected, means that if it were to mutate and become more easily spread, there could be a significant risk to public health.

Scientists say that H5N1 may be as few as three mutations away from becoming more easily transmissible. As this is such a deadly disease, the results of this would be catastrophic.

The possibility that bird flu viruses further adapt to humans and subsequently acquire human-to-human transmissibility (either by reassortment with other viruses or via mutation), underlines the pandemic potential of these zoonotic viruses.

Why do flu viruses mutate so much?

A mutation in a virus is a single change in the genetic code of that virus – like a typo. Flu viruses are composed of eight single-stranded RNA segments. As is the case in all RNA viruses, mutations occur more frequently because the virus’s replication machinery does not have a proofreading capacity. On most occasions, these small changes (or mutations) in the genes of flu viruses make no or little difference, but sometimes they can lead to changes in the surface proteins of the virus that may enable them to spread more easily or infect another species. This is called antigenic drift.

Do viruses really mutate trillions of times in factory farms?

Mutations in viruses occur naturally, so it follows that as more and more birds are infected, the number of mutated viruses will increase. A mutation that helps a virus spread more easily, for the virus, is like winning the jackpot.

Imagine five people playing on a fruit machine – the chances of one of them winning the jackpot is low, but when 100 million people play, someone is bound to win! Now consider the fact that there are 26 billion chickens in the world, all providing the potential reservoir for a mutating virus to take a gamble.

To put some real numbers behind this, around 40 per cent of H5N1 outbreaks occur in poultry farms of 10,000 birds or more. However, in many countries, less than 10 per cent of flocks are that size. Again, providing evidence that factory farms are a breeding ground for disease.

Do low pathogenic viruses mutate into high pathogenic ones in factory farms?

Yes, there are many examples of such ‘conversion events’ happening around the world – but only ever in chickens, this has never been seen in water-birds. Between 1959 and 1995, 39 different cases of low pathogenic viruses becoming high pathogenic ones were reported. All but two occurred in commercial poultry farms. Most took place in high-income countries – the majority in the UK and Ireland.

In Mexico in 1994, a low pathogenic H5N2 virus became a high pathogenic one and spread to Guatemala in 2000 and to El Salvador in 2001, presumably via trade in poultry. Low pathogenic H5N2 is now endemic in domestic poultry populations in Central America.

In northern Italy, the 1999-2000 H7N1 high pathogenic epidemic was preceded by almost 200 reported outbreaks of low pathogenic H7N1 in the same region. Both the 2003 H7N7 epidemic in the Netherlands and the 2004 H7N3 epidemic in British Columbia, Canada, were caused by highly pathogenic viruses preceded by low pathogenic infections on the same farms. In all three of these outbreaks, the authorities said that the high geographic density of the poultry farms, the frequent contact among farms by trucks and the low levels of biosecurity contributed to the considerable spread of disease.

From 2006-2015, there was a considerable increase in conversion events in Europe, especially towards the latter part of the decade. Scientists say: “The probability of such a mutation is amplified in the setting of industrial poultry production due to the rapid viral replication that occurs in an environment of thousands of confined, susceptible animals.”

Why is bird flu more of a threat than normal flu?

Normal (seasonal) flu is a highly infectious disease caused by a virus. You can catch flu all year round, but it’s especially common in winter, which is why it’s known as seasonal flu. The most likely flu viruses are identified in advance and vaccines produced that closely match them. Vaccination programmes are targeted to protect the most vulnerable.

Seasonal flu kills around 0.1 per cent of those infected. In an average year, this means 7,000 to 9,000 people in the UK die of complications caused by seasonal flu. In a typically bad year, up to 20,000 people could die.

Bird flu viruses are often completely new viruses to which very few people, if any, have immunity. They may occur at any time of the year and may affect more people than seasonal flu. Some subtypes or strains of bird flu viruses are reported as killing as many as 50-60 per cent of those infected.

Does the seasonal flu vaccine provide protection against bird flu viruses?

No, the seasonal flu vaccination does not protect against bird flu viruses.

Could there be a bird flu vaccine?

Candidate vaccines, to prevent H5N1 infection for example, are being developed but are not yet ready for widespread use.

How can I protect myself against bird flu?

The best way to protect yourself against bird flu viruses is to avoid sources of exposure. If you’re visiting a country that’s had an outbreak you should wash your hands often with warm water and soap and avoid contact with live birds and poultry. Do not go near or touch bird faeces or sick or dead birds and don’t go to live animal markets or poultry farms.

The best way to protect ourselves globally against the threat of bird flu is to end factory farming. The more people who choose to go vegan, the closer we get!

How can we stop bird flu?

The answer is simple, end factory farming and remove the viral reservoir!

What should I do if I find a dead bird?

If you find dead wild waterfowl (swans, geese or ducks) or other dead wild birds – such as gulls or birds of prey – you should report them to the Defra helpline (03459 335577).

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