The embryonated chicken egg has long been widely used as a sensitive host for cultivation of influenza viruses. Influenza viruses are isolated by inoculating specimen into the allantoic and amniotic cavities of chicken eggs, which are then incubated for three days. Although the viral infection may be fatal to the embryo, it is not examined for specific pathologic changes. Instead, fluids from the amniotic and allantoic cavities are collected and tested for the presence of viral hemagglutinins. If the chorioallantoic membrane is used as the inoculation site, quantitative assay of viruses that form lesions on the CAM is feasible.
Experienced virologists can distinguish lesions produced by different viruses.
Certain viruses that are very difficult to isolate or propagate in cell cultures are successfully grown and recovered in so-called organ cultures . Organ cultures do not grow appreciably while they are maintained in the laboratory and have a limited lifetime. Because initiation of each culture requires fresh human or animal tissue, organ cultures are not suited for routine use, but they are important for research and special clinical studies. For most virology laboratories, cell cultures are the least expensive and most convenient means of isolation and cultivation of viruses from clinical specimen. Most of the more commonly encountered viruses grow and produce recognizable changes in cell cultures derived from human, monkey or small laboratory animal tissues.