Specific staining with fluorescein-labelled immune serum was used to study the progress of mumps virus infection in a series of embryos harvested at daily intervals. The results were compared with the hemagglutinin and infectivity titers of the corresponding extraembryonic fluids. The evidence obtained indicated that the multiplication of the virus was restricted to those cells which came into surface contact with infected fluid. Following intraamniotic inoculation into 8 day old embryos, these included the cells lining the amniotic membrane and the epidermal and pharyngeal epithelium. Depending apparently on the extent of the contamination of the allantoic cavity and of the extraembryonic celom in the course of inoculation, varying amounts of virus were also present in the cells lining the chorioallantoic membrane and occasionally in the peritoneum. During the later stages of the infection, staining which was principally if not entirely extracellular was seen in the gastrointestinal tract, and, after the dissolution of the tracheal plug, in the respiratory tract.

The virus was first detected 1 to 2 days after inoculation as brightly fluorescent intracytoplasmic granules of varying size. As these granules increased in size and number they gradually filled the cytoplasm of the outer one or two layers of cells lining the adjoining cavity. The extent and brightness of the staining reached a peak 4 to 6 days after inoculation and thereafter decreased progressively.

The staining of the tissues closely paralleled the rise and fall in the infectivity of the extraembryonic fluids. The development of hemagglutinins, on the other hand, provided a less sensitive measure of virus multiplication than did the staining.

No differences were detected in the mortality of the infected and control series of embryos nor was there evidence of any pathological changes.

In contrast to the 8 day old embryos, no virus multiplication was detected following the inoculation of 16 day old embryos.

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