Importance of Virology (The studies of viruses)
Quantifying viruses contribute to the study of virus infection mechanisms. It allows for precise determination of viral concentration for monitoring, optimization and altering to give maximum yields. Quantification serves a purpose clinically and in research. For example, clone screening multiplicity of infection and cell cultures in research. Clinically, it can provide insights into: viral infection pathogenesis, diagnosis, assessment of patient’s prognosis and as therapeutic markers for monitoring the effects of antiviral treatments. Epidemiologists use it to approximate patient’s infectivity. Additionally, tests for antiviral resistance can be done. Producing viral vaccines, viral antigens and recombinant proteins using viral vectors require quantification.
The number of particles capable of forming plaques per unit volume is called a plaque-forming unit (PFU). TCID50 is a measure of infectious virus titre. Both are very accurate. TCID50 is manually observed whilst PFU is visually observed. TCID50 can take a week to measure whereas PFU can take 3-14 days. PFU is expressed as pfu/mL and assumes each plaque is representative of one infective virus particle. Results of TCID50 and pfu/mL are not equivalent. Centred on Poisson distribution, there is a theoretical relationship between the two (approx. 0.69 PFU= 1 TCID50). This may not hold in practice however.
An introduction to viruses
A comparison can be made when a virus encounter can lead to difference outcomes for cells and for organisms.
For cells, viruses can cause:
Latent infection
Acute cytopathic infection
No infection
Abortive infection
Cell transformation
Persistent infection
For organisms, viruses can cause:
Virus-induced tumour
Acute infection
Latent infection
Slowly progressive disease
Persistent infection
Chronic infection
Subclinical infection
Defining disease
With disease, there are symptoms, but not always. Symptoms can be subjective, as they can be felt by the patient, for example a running nose, an altered mental state, or a pain felt. Clinical signs are objective, they are physical findings by the examiner, for example, abnormalities in an ECG trace. Being infected does not mean you have a disease, being infected means an organism has entered, but it may be cleared before this can lead to disease. An infection is said to be subclinical or silent in the absence of disease that can be observed. When a person is infected, this leads to an immune response.
Virulence is described as the capacity to cause disease, among viruses it varies. Some are avirulent meaning they cause no disease, whilst some are lethal, causing life threatening disease. Few viruses are actually 100% lethal.
Attenuation is the process of reducing the virulence of a virus. This occurs when producing some vaccines. A microbe is taken, the lethal factor is essentially removed, and so the microbe can be inserted into a person’s bloodstream. This way an immune response can be stimulated, without the inserted microbe being pathogenic (disease causing). Pathogenicity is described as the ability to cause disease.
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