Influenza is prevalent worldwide, and the teratogenic effects of influenza infection have been suspected to occur within the developing central nervous system. We herein report the sequelae of influenza B viral infection during early chick embryogenesis. Chick embryos at Hamburger-Hamilton stage 9 were infected by an in ovo injection under the blastoderm of influenza B virus (B/Taiwan/25/99). At 48 h after infection, gross malformations of the eye and brain, ranging from 25 to 58% of 168 infected embryos, were observed, in contrast to 3–6% among 71 mock-infected controls (p < 0.0001 for both eye and brain malformations). Histological analyses showed extensive tissue degeneration and aggregates of cells in the head mesenchyme, suggesting cell death and heterotopia. Influenza B viral RNA was directly localized by in situ hybridization with probes specific for the HA segment. Viral RNA was extensively detected in the head surface ectoderm and in the lung bud. In the developing brain, viral RNA was specifically located in the anterior neural retina, habenular area, mid-thalamus, and rhombencephalon. Our data show that influenza B virus can be a teratogenic agent in neural and nonneural embryonic tissues, raising concern for transplacental infection during early pregnancy.
