Electron Microscopy (EM)

Virus particles are detected and identified on the basis of morphology. A magnification of around 50,000 is normally used. EM is now mainly used for the diagnosis of viral gastroenteritis by detecting viruses in faeces e.g. rotavirus, adenovirus, astrovirus, calicivirus and Norwalk-like viruses. Occasionally it may be used for the detection of viruses in vesicles and other skin lesions, such as herpesviruses and papillomaviruses. The sensitivity and specificity of EM may be enhanced by immune electron microscopy, whereby virus specific antibody is used to agglutinate virus particles together and thus making them easier to recognize, or to capture virus particles onto the EM grid. The main problem with EM is the expense involved in purchasing and maintaining the facility. In addition, the sensitivity of EM is often poor, with at least 10⁵ to 10⁶ virus particles per ml in the sample required for visualisation. Therefore the observer must be highly skilled. With the availability of reliable antigen detection and molecular methods for the detection of viruses associated with viral gastroenteritis, EM is becoming less and less widely used.

Electronmicrographs of viruses commonly found in stool specimens from patients suffering from gastroenteritis. From left to right: rotavirus, adenovirus, astroviruses, Norwalk-like viruses.  (Courtesy of Linda M. Stannard, University of Cape Town, http://www.uct.ac.za/depts/mmi/stannard/emimages.html)