Animal viruses Flashcards
what is a virus?
obligate intracellular parasites that require a host to replicate. absolute dependence on host cell, can infect all types of life. Most ubiquitous and diverse group of organisms on the planet
disadvantage of virus’
invasion of living organisms in order to reproduce, resulting in disease/death of the host
selective adavantage they give the host
can aid immune system’s maturation, aid development of resistance to similar viral infections, can destroy bacteria and other viruses causing infections. aid evolution by transferrin genomic sequence among different species
where do viruses come from
the progressive (escape) hypothesis- mobile genetic elements that become autonomous and can move between cells- transposons The regressive (reduction) hypothesis- derived from a more complex progenitor- a complex virus, or a type of mitochondrion The virus- first hypothesis- viruses predate cells- exist as self-replicating units. this would make the viruses the "first form of life" on earth
common viral properties
- are small particles of genetic material, surrounded by a protein coat
- are build de novo in each generation
- don’t have any cellular components
- don’t have homeostasis
- lack response to environmental stimuli
- cannot be culture
- they can only reproduce within living organisms
genome structure
a virus can have a DNA, or RNA genome and sometimes both. but only one type is present at any given time
- RNA viruses carry own polymerases and usually replicate in the host-cell cytoplasm
- DNA viruses replicate in the host cell nucleus
- Their diversity complicates their classification
- there is a classification based on the nature of the viral genomes and their modes of replication
The baltimore scheme classification
Class I&VII- DNA virus- transcription of minus strand
Class II- DNA virus- synthesis of the minus strand
Class III- RNA (ssRNA+)- transcription of minus strand
Class IV- RNA (ssRNA-)- used directly as mRNA
Class V- RNA(ssRNA+)- transcription of minus strand
Class VI- RNA(ssRNA+)- reverse transcription
viral symmetry
Helical- Rod
Isometric- spherical
life cycle
host recognition and attachment penetration gene expression and viral replication further processing and assembly of new viruses cell lysis
virus replication
eclipse: genome replicated and proteins translated
Maturation: packaging of nucleic acids in capsids
Latent period: eclipse+maturation
Release: cell lysis, number of virions released
stages of viral infection
1) infection
2) incubation period- time between infection + symptoms
3) acute period - the disease at its height
4) decline period- disease symptoms are subsiding
5) convalescent period- patient regains strength and returns to normal
types of viral infections
no permissive- virus is not allowed cell entry
permissive- virus gains entry
- acute, or lysis- virus replicates, progeny is released and cell dies
- persistent- virus replicates- progeny is released by cell survives
-latent, or chronic- virus present in cell, may replicate
- transformation- virus causes cell immortalisation
- abortive- virus enters the cell, but fails to replicate
Reservoirs
species in which infectious agents remain viable and from which individuals can become infected. BAts are an example of a reservoir of “zoonotic” diseases
Carriers
pathogen-infected indiviudals with no disease symptoms, individuals in the incubation period of the disease, or convalescing. typhoid mary is an example of a carrier
co-evolution and co-adaption
host develops resitance to pathogen, pathogen develops countermeasure evasion mechanisms. a well adapted parasite must ensure survival of its resources
Antigenic drift
mutations to original viral strain that lead to a new strain. new strain can be closely related to progenitor. if so, host usually recognises the new strain and respond to it (“cross protection”). if not the host can be re infected by the new strain
Antigenic shift
two or more viral strains, or two or more different viruses, upon infection of a new host, combine and re-assort their genome, forming a novel virus
basic reproductive number
number of cases arising from a single case. how far and how fast a viral infection with spread
Endemic
constantly present at low incidence in a specific geographic area, or population
outbreak
several cases in a short time
epidemic
occurs in a larger than expected number of people at the same time in a given location
pandemic
a globalised epidemic. pandemics, can with time become endemic
incidence
number of new cases in a population in a given period of time
prevalence
number of existing and new cases in a population, in a given time
modes of disease transmission
Direct- infected individual transmits a disease directly to a susceptible host without the assistance of an intermediary
Indirect- occurs when transmission is facilitated by a living, or non living intermediary
Emergence factors
Human demographics and behaviour Technology and industry Economic development land use travel and trade evolution-adaption breakdown in public health measures climate changes
innate immunity
non specific, first line of defence, prevents disease spread, immediate response, antigen independent
Adaptive immunity
Antigen specific, second line of defence, fights disease and prevents re-emergence, long lasting, clonal expansion of T and B lymphocytes, antigen dependent
Vaccines
conventional- live attneutated viral forms, inactivated viral extracts, viral subunits
Newly developed- recombinant viral components, viral vector-based, mRNA vaccines
Herd immunity and infection transmission
resistance of a group to infection, due to immunity present in a large proportion of the group members. the whole population with be protected. Immunised people protect non-immunised people because the pathogen cannot be passed on, and the cycle of infectivity is broken
