What are the differences between influenza A, influenza B, and avian influenza?

November 26, 2024

Influenza, commonly known as the flu, is an acute respiratory illness caused by viruses of the family Orthomyxoviridae. The influenza viruses are classified into four types: A, B, C, and D, each with distinct characteristics, transmission patterns, and impacts on public health. 

 

Influenza A: The Most Versatile Pathogen

Host Range and Prevalence

Influenza A virus belongs to the genus Alphainfluenzavirus and is the most common and threatening type of influenza. It infects a wide range of hosts, including humans, birds, pigs, horses, and marine mammals. Seasonal epidemics caused by Influenza A subtypes, such as H1N1 and H3N2, are prominent in humans, particularly during winter and spring.

 

Subtype Diversity

Influenza A is unique due to its diverse subtypes, determined by two surface proteins: hemagglutinin (HA) and neuraminidase (NA). With 18 HA and 11 NA variants identified, various combinations create different virus subtypes, including avian influenza (e.g., H5N1, H7N9) and swine influenza (e.g., H1N1v, H3N2v).

 

Severity and Mutation

Influenza A viruses mutate rapidly, increasing their potential to cause severe symptoms, pandemics, and cross-species transmission. Avian influenza, a subtype of Influenza A, is particularly notable for its high case-fatality rate among individuals aged 10–39 years, though it primarily infects birds.

 

Influenza B: The Periodic Epidemic Virus

Host Range and Prevalence

Influenza B, part of the genus Betainfluenzavirus, is more limited in scope, infecting humans and seals. Unlike Influenza A, it does not affect a wide range of animals and has never caused a global pandemic.

 

Seasonal Patterns

Influenza B causes seasonal outbreaks, often appearing every few years. While symptoms are similar to those caused by Influenza A, they are generally less severe. This virus tends to be more prevalent among children than adults.

 

Lineages

Influenza B is divided into two lineages: B/Victoria and B/Yamagata. Since March 2020, B/Yamagata viruses have not been detected in global surveillance, raising questions about their continued circulation.

 

Avian Influenza: A Zoonotic Threat

Host Range and Transmission

Avian influenza, a subtype of Influenza A, predominantly infects wild birds and poultry. Human infections are rare but have occurred, primarily among those with close contact with birds. The virus spreads among birds via respiratory droplets or contaminated surfaces.

 

Severity and Subtypes

The most dangerous avian influenza subtypes include H5 and H7, which can cause severe disease in humans. Less severe subtypes, such as H9, pose minimal risk. Unlike seasonal flu, avian influenza has a much higher case-fatality rate, especially in young adults.

 

Key Differences Between Influenza A, Influenza B, and Avian Influenza

Characteristic	Influenza A	Influenza B	Avian Influenza
Hosts	Humans, animals (e.g., birds, pigs)	Humans, seals	Birds primarily, rare in humans
Mutation Rate	High	Low	Moderate
Severity	Often severe, pandemic potential	Mild to moderate	Severe with high case-fatality rate
Prevalence	Common in adults	Common in children	Rare in humans
Spread	Seasonal epidemics in humans	Periodic outbreaks	Among birds, zoonotic in humans
Subtype Division	HA and NA subtypes	Lineages: B/Victoria, B/Yamagata	HA and NA subtypes (e.g., H5, H7)