Orthomyxoviridae and Influenza Viruses

Orthomyxoviridae

• Orthomyxoviridae is a family of viruses that have helical nucleocapsids and are enveloped with projections (spikes) evenly distributed over their surface.
• They have single-stranded, segmented, negative-sense RNA genomes and replicate in the nucleus.
• They infect a wide range of mammals (including humans) and birds.

Influenza Viruses

• Influenza viruses are the most important species belonging to the Orthomyxoviridae family.
• There are four genera of Influenza virus: Influenza A, Influenza B, Influenza C, and Influenza D.
• Influenza A virus infects humans, mammals (pigs, horses, whales, cats, bats), and birds, with wild ducks being the main reservoir of influenza A viruses.
• Influenza B virus infects humans and seals, while Influenza C virus infects humans, pigs, and dogs.
• Influenza D virus infects pigs, cattle, sheep, and goats, but no human infections have been recorded.

Influenza A Virus Genome

• Influenza A virus has a segmented RNA genome of negative sense, organized into 8 segments.
• The segments are: Polymerase basic 2 gene (PB2), Polymerase basic 1 gene (PB1), Polymerase acidic gene (PA), Hemagglutinin gene (HA), Nucleoprotein gene (NP), Neuraminidase gene (NA), Matrix gene (M), and Non-structural gene (NS).

Antigenic Change

• Influenza viruses can undergo antigenic change in two ways: antigenic drift and antigenic shift.
• Antigenic drift is a minor change that occurs through the accumulation of mutations over time, leading to a gradual change in the surface protein structure of the virus.
• Antigenic shift (also called reassortment) is a more efficient process that occurs when two variants of the virus infect a single cell, resulting in the formation of new viruses with mixed genes from the two parent viruses.

Human (Seasonal) Influenza

• Each annual flu season is associated with a major influenza virus subtype, which changes each year due to the development of immunological resistance and mutational changes in previously circulating virus strains.
• The most frequently circulating subtypes of influenza A viruses in the human population are H1N2, H3N2, and H1N1.
• Influenza can cause mild to severe illness, and at times can lead to death.

Avian Influenza

• Avian influenza refers to the disease caused by infection with avian (bird) influenza (flu) Type A viruses.
• These viruses occur naturally among wild aquatic birds worldwide and can infect domestic poultry and other bird and animal species.
• Avian flu viruses do not normally infect humans, but sporadic human infections have occurred.

Swine Influenza

• Swine influenza (swine flu) is a respiratory disease of pigs caused by type A influenza virus.
• Swine flu viruses do not usually infect humans, but rare human infections have occurred.

Influenza Pandemic

• An influenza pandemic is an epidemic of an influenza virus that spreads on a worldwide scale and infects a large proportion of the world population.
• The most important influenza pandemics include the Spanish flu (1918 flu pandemic) and the Swine flu pandemic (2009).
• The Spanish flu pandemic caused by an unusually severe and deadly Influenza A virus strain of subtype H1N1, killing an estimated 50 million to 100 million people worldwide.
• The Swine flu pandemic (2009) was caused by a combination of viruses from pigs, birds, and humans, and was declared a pandemic by the World Health Organization (WHO) in 2009.