4 │ Discussion
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(Payne et
al., 1992; Zhang et al., 2020). 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(Payne & Nair, 2012; Sun et al., 2016; Wang et al., 2016a,
2016b). The outbreak of hemangioma associated with ALV-J was reported
between 2006 and 2010 in China in commercial layer chickens(Cheng, Liu,
Cui, & Zhang, 2010; Lai et al., 2011). 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(Sun et al., 2020). 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(Y. Li et al.,
2017), 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 (Dong et al., 2017; Meng et al.,
2016), which allows the development of new variants with changes in
antigenicity, tissue tropism, host range and pathopoiesis (Lupiani,
Hunt, Silva, & Fadly, 2000). Gao found that ALV-J is subject to greater
selective pressure in the hen’s follicles, which can promote the
evolution of the virus(Gao et al., 2020). 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.