The Flu

10 things to remember about Orthomyxoviridae

1.     “Orthomyxo”means true mucous.

a.     Influenza is the Italian form of latin “influential” which means influence.

2.     Similar to Paramyxoviridae

a.     Similar envelope glycoproteins

b.   For Orthomyxoviruses, HA functions as tropogen binding and fusion. Neuraminidase has an enzymatic function.

For Paramyxoviruses, HN functions as tropogen binding and enzymatic function. F serves the fusion function.

3.     5 genera

a.     Influenza A – most common in humans

b.     Influenza B – can also infect humans. A and B are the principal pathogens that infect humans.

c.      Influenza C – not usually pathogenic in humans            

d.     Thogotovirus – found in ticks but can be transmitted to humans

e.     Isavirus – found in salmon and fish. Non-human.

4.     Causes Influenza

a.     Influenza or flu is known as a syndrome, and many other illnesses are described as “flu-like symptoms.” Symptoms of influenza are rapid onset, high fever, respiratory symptoms, and generalized muscular aches (myalgias). Complications can also arise. Secondary bacterial infection (pneumonia) is the most common and results from weakened immune system.

5.     Transmission – Respiratory route

a.     Transmission occurs via aerosolized droplets. This is airborne transmission and can thus affect everybody. This causes difficulties in developing public health interventions

6.     Incubation period

a.     The incubation period is 1 to 5 days. During this incubation period, infected individuals are still contagious and can spread the virus even though they aren’t experiencing symptoms.

7.     Wide host range

a.     Influenza viruses can infect many different animals including birds and pigs, which can serve as vectors and reservoirs, respectively.

8.     Influenza is old

a.     First isolated from humans in 1933. However, it has been infecting humans for ages. The father of medicine, Hippocrates described an illness that destroyed his armies in 412 BC, and many historians believe it was influenza.  [shaw]

9.     Interesting microbiology #1 – Defective interfering particles

a.     Influenza viruses have interesting microbiology. They have an extremely high replication rate. Thus, some incomplete virions are produced, defective interfering particles (Dis). DIs are virions without the full genome because segments are missing or the genome is shortened. These Dis require coinfection with other viruses to proliferate. Since viral resources are being used to produce Dis, less viable viruses are produced.

10.  Interesting microbiology fact #2 – Hemagglutination

a.     Orthomyxoviruses are capable of causing red blood cells to clump together (agglutinate). This is because orthomyxoviruses bind to sialic acid as cell receptors and red blood cells have a lot of sialic acid cell receptors. This can be used as a simple assay method. 

Molecular biology 

  • Negative ssRNA genome
  • Helical enveloped
  • Contains 8 gene segments that code for 11 proteins. Segments range in size from 890 to 2,341 nucleotides.
  • Genome size = 13.5 kb
  • Genome:

Special Features
  • Segmented genome (7-8)
  • Cap stealing

    • Primes transcription
  • Reassortment

    • Occurs when 2 different strains of orthomyxovirus infect a cell. They switch gene segments
  • Gene expression

    • Transcription and replication occurs in nucleus [unlike most RNA viruses)
    • Alternative start codons- NB and NA
    • Overlapping reading frames – NB and NA
    • Splicing – NEP/NS2, M2
    • Nested reading frames – PB1-F2 and PB1
  • Generation of defective interfering particles

    • Von magnus phenomenon