Orthomyxoviruses Flashcards
What are orthomyxoviruses?
Viruses with a segmented, (–) strand RNA genome.
They are responsible for the highly contagious, acute respiratory illness known as influenza, or simply, the “flu.”
How many types of influenza virus are there?
There are four types of influenza virus; A, B, C and D.
Which influenza type causes the seasonal epidemics of disease?
Influenza A and B viruses cause most of the seasonal epidemics of disease.
Which influenza virus can cause flu pandemics?
Influenza A is the only influenza virus known to cause flu pandemics.
What does influenza C cause in humans?
Influenza C causes a mild illness in humans.
What does Influenza D cause in humans?
Influenza D infects cattle and is not thought to infect humans.
How are influenza viruses typically transmitted?
Influenza viruses are typically transmitted from one person to the next by the respiratory route.
Describe the Influenza A virus particle.
The virus particle is highly pleomorphic (i.e., many shapes), most commonly spherical or ovoid in shape, and approximately 80–120 nm in diameter.
The virus has a lipid bilayer envelope with viral glycoproteins embedded in the membrane.
The glycoproteins are often called spike proteins because they protrude from the envelope

What are the two types of glycoproteins in the Influenza A virus’ envelope?
Hemagglutinin (H) and neuraminidase (N).

What is hemagglutinin?
A trimer of identical subunits.
Hemagglutinin is the anti-receptor that attaches to the host cell.
The virus attaches to the sialic acid found on host cell glycoprotein to begin the replication cycle.
Originally named because of the ability of the virus to agglutinate red blood cells.
What is neuraminidase?
Neuraminidase is a tetrameter of identical subunits that the virus uses to detach from the host cell by digesting the sialic acid after the replication cycle is completed.
Neuraminidase is important for release from the host cell in egress.
How many glycoproteins are in the lipid envelope of the influenza A virus, and what is the proportionality between H and N glycoproteins?
There are about 500 of these glycoproteins in the lipid envelope of the influenza A virus.
There are approximately 5 H glycoproteins to every N glycoprotein.
In addition to hemagglutinin and neuraminidinase, what are two other important envelope proteins for the influenza A virus?
The M2 and M1 proteins.
What is M2?
M2 is an ion channel that allows protons (H+) to enter into the interior of the virus particle.
The acidification process is important in the releasing of the virus’ genome from the M1 protein at the early stages of the replication cycle.
What do the M1 envelope proteins do?
The M1 proteins lines the inside of the lipid envelope and serves to allow the nucleocapsid, the envelope and the glycoproteins to assemble correctly during the late stages of the replication cycle.
What makes up the Influenza A genome?
10 genes distributed on 8 pieces of (–) sense RNA molecules.
Each segment is coatec with nucleoprotein and has its own RNA polymerase complex associated with it (PA, PB1, and PB2).
Six of the segments code for only one protein, and each of the other two segments code for two proteins.
Each segment is replicated and transcribed independently
Note that the segments 1, 2 and 3 are approximately the same size, segments 4, 5 and 6 are approximately the same size, and segments 7 and 8 are approximately the same size.

Which segments are approximately the same size in the Influenza A genome?
Segments 1, 2 and 3 are approximately the same size, segments 4, 5 and 6 are approximately the same size, and segments 7 and 8 are approximately the same size.
What is a RiboNucleoProtein?
The segments of the Influenza A genomes are called RiboNucleoProteins (RNPs) and consist of the (–) RNA, the nucleoprotein (NP) and an RNA-dependent RNA polymerase complex (PA, PB1, PB2).
One copy of NP binds to approximately 20 nucleotides in the RNA strand, thus there are multiple copies of this structural protein

How are Influenza A viruses classified?
By subtypes based on the properties of their H and N surface proteins.
There are 18 different hemagglutinin subtypes and 11 different neuraminidase subtypes (H1 through H18 and N1 through N11, respectively).
While there are potentially 198 different influenza A subtype combinations, only 131 subtypes have been detected in nature.
Which subtypes of influenza A viruses routinely circulate in people?
A(H1N1) and A(H3N2).
How many different H and N subtypes are there?
There are 18 different hemagglutinin subtypes and 11 different neuraminidase subtypes
(H1 through H18 and N1 through N11, respectively).
How is the nomenclature to describe isolates of influenza virus strains of human origin expressed?
In this order:
- virus type,
- geographic site where it was first isolated,
- strain number,
- year of isolation,
- virus subtype (for type A viruses only)
For example, seasonal influenza A (H3N2), A/Perth/16/2019

For influenza strains of non-human origin, how is the nomenclature to describe the virus expressed?
In this order:
- virus type,
- species of host,
- geographic site where it was first isolated,
- strain number,
- year of isolation,
- virus subtype (for type A viruses only)
For example, avian influenza A (H1N1), A/duck/Alberta/35/76.
Discuss possible resevoirs of Influenza A viruses.
All known influenza A subtypes exist in the wild bird reservoir and are associated with interspecies transfer and the emergence of new influenza strains with pandemic potential.
The link between humans and pigs is significant in the development and transmission of new flu virus strains.








































