As evidenced by specific staining with fluorescent antibody, the major sites of multiplication of the PR8 and Lee B strains of influenza virus in chick embryos injected by the amniotic route were in the cells lining the amnion and in the epidermal and pharyngeal epithelium. Varying amounts of virus were also present in the epithelium of the allantois and less frequently in the peritoneum. No virus was detectable in any of the other tissues of 25 embryos injected between the 7th and 11th days of incubation and examined 48 hours later. Three out of five of the embryos inoculated with the PR8 strain of influenza virus on the 12th day of incubation, on the other hand, showed in addition extensive involvement of the cells lining the respiratory tract.

Specific staining of the tissues was first detectable when the ID50 of the amniotic fluids attained a level of greater than 4.5, which corresponded to the time of the appearance of hemagglutinins. With the inocula used this was generally achieved sometime between the 18th and 24th hour of the infection with the PR8 strain of virus and between the 24th and 48th hour of the infection with the Lee B strain of virus.

Cytologically, the multiplication of the influenza viruses was characterized by a diffuse type of immunospecific staining which was first detectable in the nuclei and later in the cytoplasm of the cells. The infection progressed rapidly and despite the restricted distribution of the viruses resulted in the death of the embryo in from 3 to 6 days.

The results obtained in the present experiments are compared with the findings previously reported in similar studies of mumps virus (6).

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