Influenza 101: Everything You Need to Know About Influenza

July 19 2023, by AussiePharmaDirect, 8 min reading time

The impact of influenza on the world is significant and multifaceted. Seasonal influenza outbreaks result in substantial illness, hospitalisations, and deaths globally. In severe cases, influenza can lead to pneumonia, organ failure, and even death, particularly among vulnerable populations such as the elderly, young children, pregnant women, and individuals with underlying health conditions. In addition, influenza strains with pandemic potential, such as the 1918 Spanish flu, the 1957 Asian flu, the 1968 Hong Kong flu, and the 2009 swine flu, have caused widespread illness and mortality on a global scale. These flu pandemics were mainly caused by specific strains of influenza A virus.

Influenza A, however, isn’t just the only one as there are other types of influenza as well. Influenza, commonly known as the flu, encompasses several types of viruses that can cause respiratory infections in humans and animals. The main types of influenza that generally infect humans and animals are influenza A, B, and C. Influenza D, on the other hand, is primarily known to infect animals.

To gain a more comprehensive understanding of the different types of flu and their impact on human and animal health, it is essential to delve deeper into the intricacies of these viral infections.

When was influenza discovered?

Influenza, as an infectious disease, has been documented throughout history, and it is difficult to pinpoint an exact starting point. The origins of influenza can be traced back to ancient times, with records of respiratory illnesses resembling influenza-like symptoms found in historical texts.

However, the specific pathogens causing influenza were not identified until much later. The true understanding of influenza and its causative agents began to emerge in the late 19th and early 20th centuries. In 1933, British researchers Wilson Smith, Christopher Andrews, and Patrick Laidlaw successfully isolated the influenza A virus for the first time. This breakthrough laid the foundation for further research and identification of different influenza strains and subtypes.

Influenza A

Influenza A viruses are responsible for the majority of influenza outbreaks in humans and animals. They have the potential to cause pandemics and large-scale epidemics due to their ability to undergo genetic changes and reassortment, resulting in the emergence of new strains. The viruses are primarily transmitted through respiratory droplets generated when an infected person coughs, sneezes, or talks. These droplets can be inhaled by nearby individuals, leading to infection. The virus can also spread by touching contaminated surfaces and then touching the mouth, nose, or eyes.

Influenza A viruses are further classified into subtypes based on the presence of two surface proteins: hemagglutinin (HA) and neuraminidase (NA). Currently, 18 different HA subtypes and 11 different NA subtypes have been identified. The combination of these HA and NA subtypes gives rise to various influenza A subtypes, such as H1N1, H3N2, H5N1, and so on.

The viruses are known to undergo genetic changes through two main mechanisms: antigenic drift and antigenic shift. Antigenic drift refers to gradual genetic changes in the influenza virus over time. These changes occur through mutations in the viral genome, particularly in the genes encoding the HA and NA proteins. As a result, the virus may acquire slight variations in these proteins, making it less recognisable to the immune system, potentially leading to seasonal influenza outbreaks.

Antigenic shift, on the other hand, is a more significant genetic recombination event that occurs when two different influenza A viruses infect the same host cell and exchange genetic material. This reassortment can lead to the emergence of novel influenza A subtypes with new combinations of HA and NA proteins, posing a higher risk of pandemics. One well-known example is the 2009 H1N1 pandemic, which resulted from the reassortment of human, avian, and swine influenza A viruses.

The duration of an influenza A infection can vary from person to person. In general, most people with influenza A will experience symptoms for about 7 to 10 days.

Influenza B

Influenza B is a type of influenza virus that primarily infects humans, although it can also affect certain animal species. It is one of the main causes of seasonal influenza outbreaks, often occurring alongside influenza A viruses. While they generally cause less severe illness compared to influenza A viruses, they still contribute to a considerable burden of influenza-related hospitalisations and deaths.

Influenza B viruses belong to the Orthomyxoviridae family, like influenza A and C viruses. However, unlike influenza A, which is further classified into subtypes based on the HA and NA proteins, Influenza B viruses are classified into two main lineages: the Victoria lineage and the Yamagata lineage. These lineages represent distinct genetic groups of influenza B viruses that circulate among humans:

Victoria Lineage: The Victoria lineage of influenza B viruses was named after the location where it was first identified, Victoria, Australia. This lineage has been associated with influenza B outbreaks and epidemics in various parts of the world. The Victoria lineage was predominant during the 2011-2012 flu season and has continued to circulate since then.
Yamagata Lineage: The Yamagata lineage of influenza B viruses was named after the location where it was initially discovered, Yamagata, Japan. This lineage has been responsible for influenza B outbreaks in different regions. The Yamagata lineage circulated predominantly prior to the emergence of the Victoria lineage.

The genetic diversity of influenza B viruses is characterised by minor genetic changes known as antigenic drift, rather than the significant reassortment events seen in influenza A viruses.

The influenza infectious period is similar to that influenza A. Most individuals with influenza B will experience symptoms for about 7 to 10 days.

Influenza C

Influenza C is a type of influenza virus that primarily infects humans but can also infect pigs and dogs. It is generally associated with mild respiratory illness and does not cause the widespread epidemics or pandemics observed with influenza A and B viruses.

Influenza C viruses can infect individuals of all ages, but they are most commonly found in children. While influenza C infections can occur throughout the year, they tend to be more prevalent during the winter months. Influenza C infections generally result in milder symptoms compared to influenza A and B viruses. However, in individuals with underlying health conditions or weakened immune systems, the infection can occasionally lead to more severe respiratory illness.

The genetic diversity of influenza C viruses is relatively limited compared to influenza A and B viruses. They do not undergo significant antigenic drift or antigenic shift, which are major mechanisms for the generation of new influenza strains. As a result, influenza C viruses generally exhibit a more stable genetic makeup over time.

So, how long does the influenza last? The duration of an influenza C infection can vary from person to person, but it is generally shorter compared to influenza A and B infections. In most cases, the illness caused by influenza C is self-limiting and resolves on its own within a few days. The duration of the infection is usually around 3 to 7 days, with symptoms gradually improving during this period.

Influenza D

Influenza D is a type of influenza virus that primarily infects cattle and, to a lesser extent, swine. It was first identified in 2011 and represents a distinct influenza virus lineage (D/OK, D/660, D/Yama2016, and D/Yama2019). Influenza D viruses belong to the Orthomyxoviridae family, like influenza A, B, and C viruses. However, their genetic makeup differs significantly from the other influenza types. Unlike influenza A, B, and C, which can infect humans, influenza D has not been shown to infect humans on a significant scale.

Influenza D viruses primarily circulate among domestic and wild ruminants, such as cattle, goats, and sheep. They can also infect swine, although the prevalence and impact of influenza D in swine populations appear to be lower compared to cattle. Influenza D viruses are believed to be transmitted among these animals through close contact and respiratory secretions.

Although rare cases of human infection with influenza D have been reported, these instances have been isolated and do not suggest widespread transmission or sustained human-to-human spread. The reasons for the limited infectivity of influenza D in humans are not yet fully understood. It is thought that the receptors on human respiratory cells that the virus binds to are not well-suited for influenza D viruses, which limits their ability to efficiently enter and infect human cells.

How to detect influenza A and B?

Detecting influenza, particularly influenza A and B, can be done through various methods, including both laboratory-based tests and influenza test kits designed for home use. These tests provide a convenient option for individuals to self-diagnose and take appropriate self-care measures for milder flu symptoms while minimising the spread of the virus. However, it's important to note that if symptoms of influenza worsen or if there are underlying health conditions, seeking medical attention is crucial.