Introduction

Influenza, otherwise known as ‘flu’, is a contagious viral infection affecting the upper respiratory tract. Symptoms of the virus include coughing, fever, headaches and a runny nose.¹

Four types of influenza, known as influenza A, B, C and D, are known to exist. In the UK, seasonal flu during the winter is caused by influenza A and B.²^,³ When people who have the flu cough or sneeze, the droplets can transmit the disease to others. Alternatively, contact with surfaces containing viral particles can spread the flu once you touch your mouth or nose.⁴

Bird flu is a subtype of influenza which originates in wild birds, but has occasionally infected humans. This article will describe the key similarities and differences between influenza strains, and how this impacts disease control and prevention. 

Overview of bird flu (avian influenza)

Bird flu is a virus that used to solely affect poultry. But in Hong Kong, in 1997, the first outbreak was recorded in humans.⁵ Viruses can have various strains, or lineages, that differ from each other on a genetic level. Some strains which are known to infect humans include:

• H5N1
• H7N9
• H5N6
• H9N2

The naming conventions for these viral strains are based on the type of protein molecules that are present on the surface of the virus. For example, the “H” in H5N1 stands for hemagglutinin protein 5, and the “N” represents neuraminidase protein 1. These subtypes of protein allow us to distinguish bird flu strains from one another.⁶

Bird flu manifests differently from seasonal flu as it has a higher mortality rate, presents rarely in humans, and it is spread through direct contact with infected birds.

Types of influenza viruses

Many types of influenza viruses exist as a result of their ability to evolve rapidly.⁷ Only influenza A and B are significant in relation to human infections; other types, such as C and D, are more prevalent in animals such as cattle. 

Influenza A

• Presents in a diverse range of hosts including humans, birds, horses and pigs
• Subtypes are based on the hemagglutinin (H) and neuraminidase (N) proteins
• Avian influenza, or bird flu, falls under this category
• Causes seasonal flu during autumn and winter⁸

Influenza B

• Influenza B is limited to human hosts, but it is also found to infect other mammals like seals and dogs⁹
• Less severe than influenza A, and it is not associated with pandemics
• Influenza B is not categorised into subtypes, but there are two lineages (B Yamagata and B Victoria which are genetically distinct and infect older and younger people, respectively)¹⁰

Influenza C

• Can cause mild infections in swine (pigs), and it is also associated with respiratory illness in humans
• Genetically and structurally distinct from influenza A and B¹¹

Influenza D

• Primarily affects cattle
• Possible risk of cross-species (zoonotic) infection¹²

Bird flu and similarities in other influenza viruses

The bird flu virus has features in common with other influenza viruses, which make it able to spread and evolve. Viruses require a host to replicate and survive and so aren’t considered living. We feel unwell from the effects of viral reproduction, as our cells are harmed at the expense of their activity.

Shared virus structure

All influenza viruses are characterised by a segmented genome, made up of RNA. This RNA molecule plays an essential role in making proteins. In bird flu, and other influenza A or B viruses, the H proteins are what allows the virus to enter our cells.

The genetic material is inserted inside the cell nucleus, which sends out instructions to start making the virus instead. N proteins are thought to aid the degree of infectivity as they help to break down barriers which protect the lining of the respiratory tract.

Hemagglutinin-esterase-fusion (HEF) is the surface protein found on influenza C viruses, instead of H and N, although it performs broadly similar functions.¹³

Potential for mutation and reassortment

New flu vaccines are developed each year due to the H protein on influenza viruses changing over time. This is as a result of antigenic drift and antigenic shift. 

Antigens are any molecule on the surface of a pathogen, such as the H protein. The main difference between antigenic drift and antigenic shift is that the former is a gradual process, due to accumulated mutations, which results in changes over a long period of time.¹⁴

Antigenic shift occurs as the influenza genome is segmented, enabling gene reassortment. The protein structure changes rapidly and can lead to the evolution of a new virus through simultaneous infection. The genetic material from two strains can combine in the host cell to form a hybrid strain.¹⁵

Differences between bird flu and other influenza viruses

Host range

Birds are the main reservoir of the bird flu virus. Humans are more likely to be infected with other strains of type A influenza or type B influenza. To contract bird flu, someone must be in close contact with birds or contaminated objects like faeces and bedding. 

Severity in humans

Unlike other seasonal flu viruses, H5N1 and H7N9 are more deadly to humans with a higher mortality rate.¹⁶ In contrast to the usual symptoms of flu, such as coughing and a high fever, the symptoms of bird flu can be more severe and therefore lead to complications such as: 

• Acute respiratory distress syndrome 
• Pneumonia
• Multi-organ failure¹⁷

Human-to-human transmission

Unlike other seasonal influenza, bird flu is not spread easily between humans, even through droplets. Furthermore, other strains of influenza A and B are more contagious and prevalent. Bird flu is only a concern for pandemics if it mutates to spread more readily between people.

Historical context and pandemics

Influenza has been the cause of multiple pandemics throughout history. Some examples include:

• Spanish flu (H1N1), which killed 50 million people globally in 1918¹⁸
• Swine flu outbreak (also H1N1) in the UK in 2009 – an example of reassortment creating a new virus¹⁹

Bird flu could become a potential pandemic threat if it were to mutate and spread more easily among humans. To lower the risk of this occurring, ongoing surveillance and efforts to monitor birds and poultry products are in place to control the spread of infection. Bird flu and human flu viruses must be carefully tracked to prevent outbreaks and prepare for infective hybrid strains. 

Preventive measures for bird flu and other influenza viruses

We can take action to avoid infections from bird flu and other influenza viruses by:

• Getting vaccinated for seasonal flu – although unfortunately there is no bird flu vaccine
• Surveillance of flu e.g. through laboratory monitoring²⁰
• Good hand hygiene and food handling practices, whether or not you are in contact with poultry

Summary

Bird flu is a subtype of influenza A, which mainly affects poultry, but has been known to cause outbreaks in humans. The impact of bird flu on humans is more severe, however, it is rare and most cases of flu are generally caused by other strains of influenza A and B. It is difficult to contract bird flu unless you are in direct contact with infected birds and human-to-human transmission is rarely seen. 

Understanding the similarities and differences between bird flu and other strains of influenza can help guide preventative measures. Monitoring of influenza and bird flu, as well as augmented safety protocols for poultry workers, can help to reduce the risk of future pandemics.