Abstract:

【Objective】 Fowl adenovirus (FAdV) circulates in chicken flocks with multiple serotypes, while there is less information about the pathogenicity of all serotype strains. To understand the capability of causing disease as a primary pathogen to chicken of the FAdV-8a strain, we carry out the experiment for evaluating the pathogenic characteristics of this serotype strain, which will help making the control strategy for breeding. 【Method】 In 2017, liver tissue was collected form the diseased flocks. The liver tissue homogenates were inoculated into the embryo egg for isolating the virus. The isolate was determined as a FAdV strain after PCR detection. To classify the isolate, genomic sequencing and the genetic evolution based on the complete genome sequence and the hexon gene sequence were performed. To clarify the pathogenicity, thirty 10-day-old SPF chicks were randomly divided into 2 groups and exposed to the isolate JL/170408 via nasal inhalation and eye droppings. The clinical syndrome (including morbidity and mortality), viremia, virus shedding, circulating antibody, postmortem examination and histopathological detection at 5 days post infection (d.p.i.), viral distribution and the tropism on tissues were performed to evaluate the pathogenic capability and characteristics of JL/170408 to SPF chicks. 【Result】 The complete genome sequencing showed that there were the highest identity between the isolate and the FAdV-8a TR59 strain. They showed high identity in the genomic structure and the encoding gene. Phylogenetic analysis based on the complete genome sequence, the isolate JL/170408 was in the cluster of FAdV-E, further was grouped into the branch of serotype 8a based on the hexon gene. Consequently, the isolate JL/170408 was determined as FAdV-8a serotype within the species of FAdV-E. The clinical peak was observed from 3 to 13 d.p.i. without death. The virus shedding and viremia was detected as early as 3 d.p.i. and last for a long period at least 51d, the antibody was not positive conversion completely and the mean ELISA titer S/P<1, which didn’t provide enough neutralizing ability to eliminate the virus in the blood and the intestinal tract. At 54 d.p.i., the ELISA titer reached a peak with a mean S/P>2, with the consequence of viremia disappearing and a sudden drop of virus shedding. Postmortem examination and histopathological detection at 5 d.p.i. showed no obvious pathologic change. While the viral load was detected in 15 tissues, suggested that the isolate propagated in multiple tissues and exhibited higher tropism to gizzard. By monitoring the circulating antibody, the infected birds showed later antibody positive conversion until 15 d post infection. Not all birds showed positive seroconversion even at 51 d post infection, and the antibody level was stable. At 54 d post infection, the antibody titer reached peak, suggesting that birds may suffer a second infection. Virus neutralization test based on the antiserum of 63 d.p.i. showed that there was no obvious correlation between the circulating antibody and the neutralization antibody. 【Conclusion】 The FAdV-8a strain JL/170408 causes disease to 10-day-old chicks as single pathogen without leading to death, the isolate is determined as low pathogenic strain. JL/170408 propagates in multiple tissues with higher tropism to gizzard. The infected chicks show a long duration of virus shedding with a repetitive characteristics.

Key words: fowl adenovirus, serotype 8a, genomic characteristic, pathogenicity