produce hemagglutinins, which are proteins projecting from the envelopes of the viruses and present in the membranes of infected cells. Hemagglutinins have the ability to adhere to ery-throcytes in a process known as hemadsorption, which is used to screen certain cell cultures for the presence of influenza and parainfluenza viruses. This test is performed by overlaying the cell monolayer with a suspension of guinea pig erythrocytes, then examining for the presence of hemadsorption after 30 minutes. Adherence of the guinea pig erythrocytes to the cell monolayer is regarded as a positive test. Influenza and parainfluenza viruses are the most commonly isolated hemadsorbing viruses, but mumps virus also gives a positive reaction.
Despite the availability of a large number of different cell culture lines, a number of clinically important viruses cannot be grown using these conventional methods. The Epstein-Barr virus (the cause of infectious mononucleosis) and human immunodeficiency virus (the cause of AIDS) require human white blood cells for growth. Other viruses, such as some coxsackie A viruses, rabies virus, and arboviruses are best isolated in mice. Because of the highly specialized nature of these procedures, such methods are generally performed only in reference laboratories. In addition, some viruses (e.g., hepatitis viruses and rotavirus) cannot be cultivated at all. Alternative procedures such as electron microscopy, antigen detection assays, or serology are used for the diagnosis of these viral infections.
Immunologic assays, such as immunofluorescence and enzyme immunoassay, are used to detect viral antigens, and nucleic acid amplification techniques are used to detect viral nucleic acids directly in patient specimens (see Exercise 19). Antigen detection assays are available for a number of different viruses including respiratory syncytial virus, herpes simplex virus, influenza A and B viruses, rotavirus, and adenovirus. Currently, nucleic acid amplification assays are limited to the detection of human papillomavirus although assays for quantifying blood levels of viruses such as HIV are available. If viral products are detected, the laboratory diagnosis of infection is established and the need to perform viral culture is eliminated. Results are often available within 10 to 60 minutes.
The earliest nonculture laboratory method used for viral diagnosis was screening for characteristic changes in infected human cells and tissues. Examination of cell smears or tissue sections stained with special tissue stains may reveal characteristic viral inclusion bodies that represent "footprints" of viral replication and are suggestive of certain viral infections. However, the diagnostic value of such an approach is limited because sensitivity is low (50 to 70%) compared with other available methods. The major application of this method is for the diagnosis of infections caused by viruses such as molluscum contagiosum (the cause of genital warts), which are not culturable. However, a gene amplification method is now commercially available for detecting these viruses in clinical samples.
Electron microscopy is a powerful tool for the study of viral morphology and size but is of limited availability in most diagnostic laboratories. Direct electron microscopy also requires specimens containing high titers (>107 per ml) of viral particles. The major diagnostic application of electron microscopy is for the detection of certain nonculturable viruses, particularly those that cause gastroenteritis (e.g., caliciviruses, astroviruses, and rotavirus).
Serological tests to identify patient's antibodies are described in more detail in Exercise 33. A variety of serological tests, however, are available for the diagnosis of many viral infections.
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