Epidemiological investigation and experimental immunization of Non-serotype 1 duck hepatitis virus

Nine strains of duck hepatitis virus(DHV) were obtained from duckling liver samples through duck embryo by allantoic sac inoculation.All these samples were collected from ducklings diagnosed clinically dying from duck virus hepatitis in several provinces of China.The lethal rate of 10-day-old duck embryos inoculated with these isolates was 100 percent.When inoculating 6-day-old ducklings with these isolates,the mortality rate was among 0～100%.Dead ducklings showed the sign of opisthotonus,with swollen and punctuate hemorrhage in their livers.The ELD50 of these virus strains determined with duck embryos were among 10~(-7.32)～10~(-3.5).Viral neutralization test was done using standard sera against DHV type 1 and non-serotype 1 DHV.Seven of nine isolates belonged to non-serotype 1 DHV,the other two belonged to DHV type 1.A strain of non-serotype 1 DHV previously isolated from Beijing was attenuated by passage in duck embryos and designated B_(20) strain.Diluted allantotic fluid virus was used to immunize 1-day-old ducklings and the immunized ducklings were challenged with virulent non-serotype 1 10 days post immunization.The result indicated that the protection rate of 20 folds dilution of B_(20) could reach up to 100 percent.However,100-fold dilution of B_(20) provided no protective effect.The sera of the immunized ducklings were collected and the antibody titers were measured by virus neutralization test.The antibody level reached peak in 10 days post vaccination and then decreased.The results showed that B_(20) strain could provide protection against the infection with non-serotype 1 DHV and may be used as the candidate vaccine strain.