What Are Viruses And Prions

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Steps of viral replication

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absorption

penetration

uncoating

viral protein synthesis

viral nuclei acid replication

virus assembly

viral release

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DNA Virus replication

Back 2

virus infects cell, forces it to transcribe viral DNA forming viral mRNA which then translates into viral proteins

Front 3

RNA virus replication

Back 3

viral mRNA can be directly translated into viral protein by the host cell

viral mRNA can then be copied using the virus' own RNA-dependant RNA polymerase (RNA replicase)

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Retrovirus replication

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Virus’ genetic material, viral RNA, has to undergo reverse transcription through the use of reverse transcriptase

This turns it into proviral DNA which can be integrated into the host cell’s genome, then transcribed into viral mRNA and translated into viral protein

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Incubation period

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* time interval between entry of virus into the body and the commencement of disease symptoms
* patient is unlike to transmit disease to another due to low viral numbers

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Viral shedding

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* period of time where evidence of viral replication can be found, usually before symptoms start to show
* different diseases have a different schedule of how soon the virus can be detected (viral shedding period shifts)

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Localised infections

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* virus multiplies at the epithelial surface at or near the site of entry into the body, but disease symptoms may be local or systemic
* e.g. fever and aches and pains due to cytokines produced by the body in response to influenza
* e.g. viral diarrhoea and respiratory infections

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Systemic infections

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* virus initially multiplies locally at the site of entry, but then spreads by the bloodstream (viraemia) or other means (e.g. along peripheral nerves) to distant sites, once there, they will localise in target organs (tissue tropism) causing organ damage
* e.g. hepatitis A

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Vireamia

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Presence of virus in the bloodstream, may be acute (such as in the early phase of many disseminated virus infections) or chronic (such as hepatitis B or HIV)

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Mechanisms of viral diseases

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* Cytolysis- uncontrolled viral replication causes host cell functions to be switched off, inducing apoptosis
* Immunopathological- virus itself does not kill the cell, but the host immune response kills the virus infected cell (e.g. hepatitis B)
* Oncogenesis, the induction of tumours (e.g. hepatitis B and hepatocellular carcinoma, and HPV and cervical cancer)

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Viral diagnosis methods

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Detect the virus- electron microscopy, growing virus, viral antigen detection, viral nuclei acid detection, viral inclusion bodies in histology

Detect antibody response- antibody test

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electron microscopy

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* Results can be gained in hours
* However huge amounts of virus in specimen required (at least 10^6 virus particles per mL)
* Can only be used to detect adenovirus, rotavirus or Nowalk virus in faeces from a patient with diarrhoea, or Herpes simplex or Varicella zoster virus from vesicle fluid from skin or mucosa

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Growing virus

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* Takes days to weeks
* Can be done by injecting virus into live animals (outdated method) or embryonate eggs or in a cell culture which enables the cytopathic effect of the virus to be seen

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Viral Antigen Detection

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* Takes hours to gain results
* Immunofluorescence can be used, where due to specific viral proteins, specific antibodies for those proteins can be used to detect presence of viral antigens (infected cells light up)

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Viral Nucleic Acid Detection

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* Takes hours to days
* Specific target DNA is amplified in the polymerase chain reaction- if bandings occur at the same position between known virus and sample, it is a positive result

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Viral Inclusion Bodies

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* Takes days
* In some infections specific changes occur to infected cells (e.g. the growth of Negri bodies in brain cells of patients with rabies)

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Antibody test

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* Single test will show infection that occurred anytime within patient's lifetime, hence second test is needed 2 weeks later
* Antibody titre (quanitty) is tested- if the increase within the time period is over 4 times, it is a recent infection
* Another method is to detect IgM antibodies, which disappear within 1 - 3 months, instead of IgG antibodies (which stay in out bodies after infection

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Virus transport medium

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needed to prevent sample from drying out during transport

contains proteins to maintain virus stability, antibiotics to prevent bacterial overgrowth and a pH buffer/indicator

-once placed here, sample can only be used to detect for virus- different sample needed to test for bacteria