Abstract
The mortality of chicken embryos inoculated with field strains of duck hepatitis virus (DHV) has varied (5,9) and remained irregular after 35 passages in chicken embryos (2). Nevertheless, the tendency of increasing embryo mortality through serial passages of DHV in chicken embryos has been demonstrated (1,6,8). An English strain of DHV, TN90, which was the chicken embryo 90th-passage virus, has been reported to kill most 10-day-old embryos within 48 hours (1). One other DHV strain of chicken embryo origin that usually killed 7-day-old embryos in 2 to 4 days when diluted 106-fold was also reported (7), but the history of this strain was not available. Nevertheless, it is obvious that both were highly embryo-lethal laboratory strains. Because of the low mortality rate and irregular mortality 417
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