| Newcastle disease (ND) is a contagious avian disease that causes huge economic loses inpoultry industry. Vaccination and injection hyperimmune serum are conventinal ways tocontrol the prevalence of ND. Hemagglutination inhibition assay (HI) was widely used toevaluate the antibody level of vaccinated commercial chickens.The antibody level in vaccinated chickens is related to immune protective effenciencywhen ND outbreaks. However, what exact HI titer of ND antibody for effective protection isunclear. In general, the HI titer of4log2was supposed to protect the chickens againstNewcastle disease virus (NDV) challenge. The fact is that8log2or greater HI titer was failedto completely protect the diseases and death when NDV outbroke in chicken farms.In order to determine the relationship between HI antibody level and effective protection,we used vaccinated laying hens with the HI antibody level between6log2and18log2. Thechickens were challenged with two virulent NDV strains which were isolated fromcommercial chickens in Shaanxi province, China. The clinical signs and egg production ofchallenged chickens were observed. Our results showed that all chickens were protectedagainst NDV challenge without mortality. The chickens in lower HI titer group (6log2to9log2) showed atypical symptoms,40%morbidity and misshapen eggs rate reached87.51%.Laying hens in medium HI titer group (10log2～13log2) showed no symptoms and37.5%misshapen eggs rate. However, high HI titer (≥14log2) chicken group showed no symptomsand no misshapen eggs.Chickens in lower HI antibody level group shed the viruses from oral at early infectiontime, and subsequently shed the viruses from cloacal for at least one month. Chickens in highHI titer group showed no virus shedding. The HI antibody levels of all chickens wereincreased after challenge. Surprisingly, the viruses were detected in egg yolk when challengedchickens produced soft shell eggs, but the cases were very low and need to be furtherinvestigation. |
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