Since the first strain of ALV-J, HPRS-103, was isolated from meat-type breeder chickens in the UK in 1988, it has rapidly spread around the world, causing severe economic losses to the poultry industry[4, 14]. Being the most pathogenic subgroup of ALV, ALV-J could induce malignant or benign tumorigenic diseases and immunosuppression in chicken, including hemangiomas, myelomas and fibrosarcomas [2, 15–17]. The outbreak of hemangioma associated with ALV-J was reported between 2006 and 2010 in China in commercial layer chickens[7, 18]. However, with the implementation of the ALV purification project by a number of major breeder companies, successful eradication has been achieved at pedigree and multiplier levels. Therefore, there is few reports of ALV-J infection in layers in recent years. Zhao discovered that co-infection with avian hepatitis E virus and avian leukosis virus subgroup J is the cause of an outbreak of hepatitis and liver hemorrhagic syndromes in a Hy-line brown layer chicken flock in China recently. This study also found subclinical infection of ALV-J in the roosters of Hy-line layers, which reminded us that the purification of ALV can not be overlooked, and needs to be continued through monitoring by proportional sampling.
Different methods were used to detect various samples of roosters and hens in the same flock. The ELISA method is used to detect semen directly, which showed that only 50% samples are consistent with the results of virus isolation. Virus isolation is used as the gold standard for detecting ALV, so the results achieved by ELISA has false positives and false negatives, resulting in missed and false mistaken of chickens. At the same time, different infection status in semen and plasma of the same rooster were clarified, namely, viraemia, with semen positive (V + S+, 13.04%), no viraemia, but semen positive (V-S+, 4.35%), viraemia, but semen negative (V + S-, 13.04%), double-negative (V-S-, 69.56%). The positive rate of virus isolation from plasma is higher than that of semen, but the two are not one-to-one. Therefore, this suggests that we need to choose both semen and plasma for virus isolation when performing ALV purification of roosters in order to obtain a better purification effect.
Li previously found that ALV-J in semen could be transmitted to hens by insemination through animal experiments in SPF chicken, but they only discovered the antibody of ALV-J, without viraemia in the hens and their offspring. This study is based on the infected clinical flocks. The ALV-J was isolated not only in the hens, but also in its breeding eggs. Although the positive rate of virus isolation in albumens was low, one strain of ALV-J was found and sequenced. The homology analysis showed that the strain had a homology of 94.1–99.7% with that in the roosters. Up to 100% similarity means that the strain is quite likely transmitted to the hens and their offspring through insemination of the roosters. This study comprehensively described that ALV-J from roosters can be transmitted to hens and their offspring through insemination.
Previous studies have demonstrated that ALV-J displays a high level of genetic variation and recombination [20, 21], which allows the development of new variants with changes in antigenicity, tissue tropism, host range and pathopoiesis . Gao found that ALV-J is subject to greater selective pressure in the hen's follicles, which can promote the evolution of the virus. In this study, we found that the ALV-J in the semen of the same rooster had only 96% homology with that in the plasma, which indicated that semen of roosters may suffer heavy selective pressure that promoted the evolution of ALV-J;and that is why the isolated strains located in a new branch in the phylogenetic tree compared with the referential stains.
In summary, we isolated ALV-J from Hy-line brown layers, and discovered the complete chain of the transmission of ALV-J from roosters to hens and then to the offspring through insemination and vertical transmission. Semen are detected by ELISA method is not completely accurate. There are four ALV-J infection status in plasma and semen of rooster, so the purification of ALV in rooster requires simultaneous virus isolation of semen and plasma. Therefore, we speculate that the reason why there are still some sporadic findings of ALV-J in laying hens is probably due to the incomplete purification process of roosters.