Evolution and emergence of new viruses Flashcards
Virus evolution
viral polymerases make a lot of errors, so each copy of genome may contain errors and be different to others
Quasispecies
virus doesn’t exist in host as constant, homogenous species, and drug/environmental effects will exert selective pressures to influence alleles present
When may viruses face a bottleneck?
At transmission or during replication under limiting conditions
Those with advantageous mutations survive
Antiviral drug resistance
High mutation rate, large progeny numbers and short replication time make viral evolution in response to selective pressure very fast.
How does Anti Retroviral Therapy avoid resistance?
ART targets many HIV genes at many different points in replication cycle.
>1 drug targeting each point with different approach, so mutation doesn’t effect efficacy of all drugs
Why should a patient not be treated with a single drug?
Resistance is more likely to develop
Why is combination therapy essential?
E.g. HAART for HIV
As long as there’s drugs that target different genes in virus, if 1 virus gets a resistant mutation in 1 place, it is still susceptible to drugs targeting other genes.
Antibody selective pressure applied by neutralising antibodies
Can prevent viruses entering cells
But also select for those in the quasispecies that have antigen mutations unsusceptible to antibodies
Antigenic drift
gradual evolution of virus driven by antibody selective pressure
What does antigenic drift necessitate?
Yearly update to vaccine
What does antigenic drift allow?
Re-emergence of virus
Zoonosis
Virus emerges from animals, crosses into humans
Example of zoonotic virus
Ebola
How do new viruses emerge?
Zoonosis Genetic variation (Antigenic drift) Increased exposure- travel or world population Increased exposure- spread of vector New discoveries
Host range
Range of cells that can act as a host to a virus or bacteriophage
Examples of ‘New’ viruses that have only recently been discovered or detected
‘Non A non B’ hepatitis caused by hepatitis C virus
HPV 16 and 18 as cause of cervical cancer
HHV8 as cause of Kaposi’s Sarcoma noticed during AIDS pandemic
Merkel cell polyoma virus identified in tumours as non-human sequence
Global influences on emerging infections
Environmental modification World population Climate change Travel Farming practises; monocultures Immunosuppressed humans Medical progress
Arboviruses
Transmitted by insects
5 Examples of arboviruses
Yellow fever Dengue West Nile Zika Chikingunya
What type of viruses are arboviruses?
Flaviviruses
Alphaviruses
What kind of genome do arboviruses have?
Positive sense RNA genome
Why is there an increasing prevalence of arboviruses?
Global warming Decrease in mosquito control Imports Stagnant water in large cities Dams
What is the vector for west nile virus?
Mosquito (Culex tarsalis)
What are dead end hosts for west nile virus?
Humans
Horses
