Bird Flu in Chickens (Avian Influenza)

Bird flu (avian influenza) is a highly transmissible virus that has been around for a long time.

Orthomyxoviruses, the same type of virus that causes flu in humans, pigs, horses, and other animals, are responsible for causing bird flu.

Numerous virus strains have developed over time and are divided into low-path (low pathogenicity) and high-path categories (high pathogenicity).

Low-Path Bird Flu

The majority of bird flu viruses in the world, including those in the US, are low-path variants.

Low-path variants typically have minimal or no effects on chickens and present few health risks to people.

However, low-path strains can mutate as they travel across a huge population of chickens packed in a small space, leading to the development of high-path viruses.

High-Path Bird Flu

High-path variants have the potential to infect people, are more likely to kill chickens, and spread more quickly.

A high-path virus is one that kills 6 out of 10 injected chicks in a lab.

High-path strains were found in industrial flocks in the United States in 1924, 1983, and 2004. Sadly, these chickens were put in quarantine and killed in order to halt the spread of the lethal illness.

Luckily, there were no reported cases of infected people associated with these outbreaks.

Type A

The influenza virus comes in three varieties: Type A, Type B, and Type C. Orthomyxovirus, the virus that causes avian influenza, is a Type A virus.

There are 144 Type A variants that are capable of infecting many species, including humans and chickens.

With a few rare exceptions, every variant is typically host-specific. As a result, the variants that cause illnesses in chickens may spread easily from one chicken to another but not easily from chickens to people or other animals.


Out of the 144 subtypes of Type A variants, the high-path H5N1 variant is the one that receives the most media attention. Ten out of 10 infected chicks die from H5N1.

H5N1 can be low-path or high-path. The low-path virus is also known as North American H5N1. While the Asian H5N1 is another name for the high-path virus.

Humans are typically not affected by H5N1. Although there have been isolated occurrences of serious respiratory infections in humans since the first bird flu outbreak in 1997.

People who contracted the disease had continuous, close contact with diseased birds. And many individuals who came into contact with sick chickens, but did not get sick themselves, developed antibodies to the virus.

H5N1 is sporadic and can lead to severe pneumonia in people. Sadly, nearly 60% of those who contract this flu pass away.

High-path H5N1 has not yet been reported in the United States.

Bird Flu Symptoms

The symptoms of bird flu can take various forms and range greatly. (Symptoms usually last between 14 and 21 days.)

Symptoms of low-path bird flu include:

  • No symptoms at all, or…
  • Lethargy
  • Coughing
  • Sneezing
  • Rattling, congested sounds coming from the lungs
  • Watery eyes
  • Nasal discharge
  • Unkept feathers
  • Loss of appetite
  • Diarrhea (occasionally)
  • Hens lay less and tend to become broody more easily.
  • Occasionally, death (mortality rate is usually less than 5%)

Sometimes the abrupt deaths of many seemingly healthy chickens are the first indication of a high-path virus outbreak.

Symptoms of high-path bird flu include:

  • Lethargy
  • Loss of appetite
  • Swollen and darker-looking wattles and comb
  • Reddish, swollen legs
  • Soft-shelled or shellless eggs
  • Rapid reduction of egg-laying
  • Sometimes coughing, sneezing
  • Rattling, congested sounds coming from the lungs
  • Bloody discharge from the mouth and nose (Occasional)
  • Diarrhea
  • Trembling of the neck and head
  • Neck arched to one side or bent backward
  • Incapable of standing
  • Death within about 48 hours

Fortunately, the typical backyard flock is unlikely to become infected with high-path bird flu.

Chicken-to-Chicken Bird Flu Transmission

Direct contact with infected chicken poop and poop-contaminated surfaces, such as on equipment or shoes, is the most common way that bird flu spreads from chicken to chicken.

Consequently, it is more likely to happen in industrial operations where numerous chickens are carelessly crammed into unhygienic conditions.

Additionally, the virus is transmitted by nasal, oral, and ocular (eye) secretions. So, it can spread from chicken to chicken through direct contact, as well as by coughing and sneezing.

Recovering chickens continue to be carriers of bird flu.

Chicken-to-Human Bird Flu Transmission

Bird flu normally does not transfer from one person to another. It infects people through close contact with infected or dead chickens, or through contact with the chickens’ poop.

The majority of those affected by bird flu live in  Eastern Europe, Asia, and the Middle East where a lot of chickens are packed together in unhealthy shelters. Even then, infection in people is rare, and those who do become ill have had a lot of close contact with sick chickens.

The likelihood that you will contract bird flu from your backyard chickens is extremely small.

Bird Flu Vaccine

For use in the case of an outbreak, a vaccination has been developed to defend chickens against known high-path variants. While infected chickens are being culled, this vaccination stops the flu from spreading.

But bird flu viruses frequently evolve (similar to human flu viruses). So, current bird flu vaccines may not shield chickens against future bird flu variants.


Low-path bird flu can spread undetected through a small flock before dying out and leaving the chickens immune to getting infected with that particular variant of the virus in the future.

A treatment is sadly unavailable for high-path bird flu. Following the quarantining and killing of infected chickens, the only thing you can do is thoroughly clean and disinfect the property.