Classification of viruses

ICTV system of classification

• Takes place based on properties such as:
• nucleic acid type (DNA/RNA/ss/ds)
• Symmetry of capsid
• Presence of absence of envelope
• Dimension of virion and capsid
• The classification itself specifies hierarchical levels of: Order (virales) - Family (viridae) - Subfamily (virinae) - Genus (virus) - Species (name).

Baltimore system of classification

• The system is based on the viral genome and the process to synthesise viral mRNA. There are 7 groups.
• ds DNA virus (genome by ds DNA to ds DNA; mRNA by ds DNA to mRNA)
• ss DNA virus (genome by ss DNA to dsDNA to ss DNA; mRNA by ss DNA to ds DNA to mRNA)
• ds RNA virus (genome by ds RNA to ss RNA to ds RNA; mRNA by ds RNA to mRNA)
• + strand RNA virus (Genome by +RNA to -RNA to +RNA)
• - strand RNA virus
• Retrovirus
• Reverse transcribing DNA virus

How do viruses emerge in the population

• A virus is acknowledged as an emerging infectious disease if:
• Newly appeared in a population
• Has existed, however rapidly increasing in incidence or geographic range.
• Emergence is a 2 step process:
• Introduction into a new host population
• Establishment and further dissemination of the virus through the new host population

Factors driving viral emergence

• Ecological change - putting people in contact with virus (agriculture, dams or floods are examples)
• Human demographics and behaviour - Population dencity and immunity, urbanisation and bahaviour e.g. sex patterns)
• Globalisation ≈ travel and trade
• Viral adaptation and change - molecular change between host and viral interaction, changes in genetic structure of virus, antiviral drug resistance.