| Avian leukemia is a general term for a variety of tumor diseases caused by avian leukosis virus(ALV)infection.Its clinical symptoms mainly include multiple organ tumors,immunosuppression and severe growth retardation.Since its appearance at the end of the 19 th century,it has been widely spread all over the world,and repeatedly broke out in Europe,America and Asia.The disease began to spread in China at the end of the 20 th century and reached its peak in 2008,which seriously affected the breeding of commercial broilers and laying hens in China and caused huge economic losses.Like other countries in the world,in order to control the epidemic and spread of avian leukaemia,China has gradually implemented strict eradication of avian leukaemia,striving to start with the eradication of provenance and gradually eliminate avian leukaemia in the whole country.At present,this program has achieved great success,especially ALV-J almost no longer appears in commercial broiler and laying hens in China,and the economic benefits have been significantly improved.However,since 2017,avian leukemia has been reproduced in poultry farms in many provinces in China.The clinical symptoms have changed significantly,and the incidence rate and mortality rate have increased significantly,which brings potential risks of the re-outbreak of avian leukemia.In order to understand the pathogenic characteristics of it,a systematic epidemiological analysis was conducted in 21 farms with similar symptoms in Liaoning,Jilin,Tianjin,Shandong,Hebei and Jiangsu provinces.The production data showed that the clinical symptoms of the sick chickens mainly appeared before and after the production.The average onset age of the disease was 17.5weeks,and the average daily mortality rate was 6.8/10000.The main symptoms were liver tumor,spleen tumor,kidney tumor and the chine tumor,and the mortality rate in farms with chine tumor was significantly higher than that of other farms.Subsequently,virus isolation and indirect immunofluorescence assay showed that ALV-J infection was present in all the 21 farms,the positive rate was between 20% and 100%,the overall positive rate was about 60%,and the positive rate of Shandong farms with chine tumors was 100%.Subsequently,env gene sequencing and analysis of 21 representative strains were carried out.Homology comparison showed that the nucleic acid sequence homology of the virus strains isolated in this study was 97.0%-99.9%,and the amino acid sequence homology was94.7%-99.8%.It indicated that these virus strains were highly similar and may have a common ancestor.At the same time,this study confirmed that the ALV-J strains in many provinces of China had the highest homology with GD14J2 strain(Gen Bank accession number: ku500032),which was 95.77%-97.16%,which was significantly higher than other reference strains,indicating that these new strains were more closely related to GD14J2,but whether these strains have the same origin still need a further analysis.On the other hand,genetic evolution analysis showed that the strains isolated in the same province did not have obvious regional characteristics,and the strains from different sources had the same gene insertion or deletion mutation in env gene,which indicated that ALV-J strain which may cause the outbreak of avian leukemia may have multiple infectious sources,not directly from the same strain.At the same time,this study also discussed the possible role of vaccine contamination in the outbreak of ALV-J.However,the results showed that the farms with avian leukemia did not use the vaccine contaminated with ALV-J exogenous virus,indicating that the virus may come from other ways.The above studies confirmed that the virus strain causing the outbreak of avian leukemia belongs to ALV-J,and the molecular characteristics and genomic variation of the virus strain were identified,which provided a solid theoretical basis for clinical prevention and treatment of the disease. |
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