31st July 2023

Assessment and diagnosis

Diagnosis in modern medicine relies predominantly on nucleic acid-based methods and serological assays. However, microscopy-based methods such as immunofluorescence with electron microscopy are also still used.

Samples can be taken from a range of sites specific to the virus in question. For example, if viral encephalitis is suspected, a lumbar puncture is the best method to obtain a sample.

Polymerase chain reaction (PCR)

PCR is a method by which viral DNA can be replicated in order to be detected. Molecular probes known as ‘primers’ are designed that are specific to a certain virus genome. The polymerase enzyme, and heat, are then used to create a reaction in which if any viral DNA is present in the sample tested, it will replicate and be amplified to detectable levels, making the test ‘positive’.

Reverse transcription PCR (RT-PCR) is the same principle but for the detection and amplification of RNA.

Sequencing

Genetic sequencing uses the product amplified from PCR/RT-PCR and is analysed by a sequencing machine. It enables the full sequence of a viral genome to be analysed.

Immunofluorescence

This technique makes use of an antibody specific to the virus suspected in a sample of potentially infected tissue. A chemical, or specific wavelength of light, is applied to the tissue sample. This enables visualisation of the antibody, and hence the virus, if present.

Electron microscopy

Electron microscopy was the traditional mainstay of virology. However, modern techniques require less technical skill and are quicker, so they have overtaken it. The advantage is that through electron microscopy, you can visualise a whole virus, revealing its shape and structure (see image below), and it is useful if viruses do not show up on routine diagnostic tests or if suspicious results need interrogating further.

Electron micrograph of Ebola virus

Image Source: Wikimedia Commons (2011). Ebola virus virion, Licensed under a creative commons licence. Available for free at: https://commons.wikimedia.org/wiki/File:Ebola_virus_virion.jpg (Accessed: 19 September 2022).

When Ebola was first discovered in 1976, it was electron microscopy of the virion that showed it had a different structure to the yellow fever virus, which was initially thought (wrongly) to be the cause of the outbreak in the Democratic Republic of Congo.

Serology

Serological diagnosis is performed through the detection of antibodies and antigens from patient blood samples. IgM antibodies indicate an acute infection. IgG antibodies indicate prior exposure, and hence either vaccination or previous infection.