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Chapter: Microbiology and Immunology: Virology, Virus: Laboratory Diagnosis of Viral Diseases

Direct Detection of Viruses - Methods of Laboratory Diagnosis of Viral Diseases

Viruses can be detected directly in clinical specimens by electron microscopy (EM), fluorescence microscopy, and light microscopy.

Methods of Laboratory Diagnosis

Laboratory diagnosis of viral infections can be carried out by many methods. These methods include 

(a) demonstration of virus-induced cytopathic effects (CPEs) in the cells, 

(b) direct detection of viruses, 

(c) virus isolation and viral assays, 

(d) detection of viral proteins and other enzymes, 

(e) detection of viral genome, and 

(f) viral serology (Fig. 53-1).



Direct Detection of Viruses

Viruses can be detected directly in clinical specimens by elec-tron microscopy (EM), fluorescence microscopy, and light microscopy.

 Electron microscopy

Electron microscopy is the most commonly used method for direct detection of virus in clinical specimens for diagnosis of many viral diseases. EM can be used to detect distinctive viruses or viral structures directly in appropriate clinical specimens or in biopsy for diagnosis of viral infections. EM is not a routinely used test for detection of viral infection and is performed only in the laboratories equipped with electron microscope.

 Fluorescence microscopy

Direct fluorescence microscopy (DFA), using specific monoclo-nal or polyclonal antibody, is employed to detect viral antigens on the cell surface or within the cells infected by viruses. The viral antigens can be detected (a) in the acetone-fixed cell smears, (b) in the frozen tissue sections of the cells from virus-infected cells, and (c) also in vesicular fluid. The DFA test has been employed for:

·           antemortem diagnosis of rabies by detection of rabies-virus antigen in smears from the face or nape of the neck;

·           demonstration of viral antigens in brain biopsy specimens for diagnosis of herpes simplex encephalitis and subacute sclerosing encephalitis; and

·           rapid diagnosis of infections caused by adenoviruses, paramyxoviruses, and orthomyxoviruses.

 Light microscopy

Viral antigens in infected cell cultures are demonstrated by immunoperoxidase staining.


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Microbiology and Immunology: Virology, Virus: Laboratory Diagnosis of Viral Diseases : Direct Detection of Viruses - Methods of Laboratory Diagnosis of Viral Diseases |


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