ABSTRACT
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While certain pathogens can be identified or categorized into major groups using routine hematoxylin and eosin (H&E) staining or special staining of tissue sections, others require higher levels of magnification than provided by light microscopy. Electron microscopy (EM), particularly transmission electron microscopy (TEM), has increased magnification, superior cell preservation, and greater spatial resolution (1.5 to 2.0 nm) than light microscopy (Gondos et al., 1978; Hayat, 2000). Whereas TEM was originally developed to identify subcellular structures of normal tissues, in the 1960s it became a tool for use in basic virology and for understanding the pathogenesis of diseases caused by viruses (Wills, 1983). Electron microscopy has become an important diagnostic tool to identify cellular and subcellular abnormalities as well as certain pathogens in man and animals that could not be visualized using light microscopy (Gibbs et al., 1980). TEM has been particularly useful in identifying viruses, certain bacteria, and certain protozoan parasites in ultrathin sections of tissue. Using both gold-labeled monoclonal and polyclonal antibodies produced against specific organisms, a more accurate diagnosis can be made. Negative staining electron microscopy (NSEM) also has become a valuable diagnostic tool, particularly when looking for viruses and protozoa in fecal specimens of an animal being screened. While scanning electron microscopy (SEM) has some use in diagnosing infectious agents in animals, it has more value in demonstrating the location of certain pathogens that frequent the cell surfaces of tissues.
