Advances on genetic and genomic studies of ALV resistance

Avian leukosis (AL) is a general term for a variety of neoplastic diseases in avian caused by avian leukosis virus (ALV). No vaccine or drug is currently available for the disease. Therefore, the disease can result in severe economic losses in poultry flocks. Increasing the resistance of poultry to ALV may be one effective strategy. In this review, we provide an overview of the roles of genes associated with ALV infection in the poultry genome, including endogenous retroviruses, virus receptors, interferon-stimulated genes, and other immune-related genes. Furthermore, some methods and techniques that can improve ALV resistance in poultry are discussed. The objectives are willing to provide some valuable references for disease resistance breeding in poultry.

Construction of chicken infectious anemia virus infectious clone and study on its pathogenicity

Chicken infectious anemia virus (CIAV) can be transmitted through contaminated live poultry vaccine. However, the pathogenicity of contaminated CIAV strains is rarely reported. Previously, the chickens showed the typical symptoms of anemia after using the attenuated live fowl pox virus (FPV) vaccine. Therefore, exogenous CIAV contamination was suspected. We detected anti-CIAV antibodies in SPF chicks vaccinated with the FPV vaccine. CIAV contamination was confirmed in the FPV vaccine, and the CIAV strain was named JS2020-FPV. This study aims to rescue JS2020-FPV by reverse genetic assays and investigate its pathogenicity. Firstly, double-copies infectious clone of JS2020-FPV was constructed. For the pathogenicity study, infectious clone of JS2020-FPV was used to inoculate 1-day-old SPF chicks. The typical symptoms of anemia were observed in the JS2020-PFV group 14  days post inoculation. The hematocrit and body weight of chicks in the JS2020-PFV group were significantly lower than those in the mock group. Notably, the thymus development index and antibody levels of NDV were lower in chicks in the JS2020-PFV group than those in the mock group. Different degrees of apoptosis of MSB1 and DF-1 were observed after inoculated with the JS2020-FPV VP3 recombinant fusion protein expressed by E. coli system, indicating that VP3 induced apoptosis in the transformed cells. Overall, the pathogenicity of the CIAV detected in the contaminated vaccine was confirmed by inoculating SPF chicks with the double-copies infectious DNA clone in this study. Our findings indicate that the dangers of vaccine contamination cannot be ignored.

Genomic Characterization of CIAV Detected in Contaminated Attenuated NDV Vaccine: Epidemiological Evidence of Source and Vertical Transmission From SPF Chicken Embryos in China

Live attenuated vaccines have been extensively used to prevent infectious disease in poultry flocks. Freedom from exogenous virus is a high priority for any veterinary vaccines. Recently, attenuated Newcastle disease virus (NDV) vaccines were detected to be contaminated with chicken infectious anemia virus (CIAV) in a routine screening for exogenous viruses. To investigate the possible source of the contamination, we conducted virological tests on a specific-pathogen-free (SPF) layer breeder flock that provide the raw materials for vaccines in this manufacturer. Firstly, CIAV antibodies in serum and egg yolks samples of the SPF laying hens were detected by ELISA assays. The results showed that CIAV antibodies in serum and egg yolks were 62% positive and 57% positive, respectively. Then, DNA was extracted from the NDV vaccines and SPF chicken embryonated eggs, and detected by molecular virology assays. The results showed that three assays for pathogens in embryonated eggs had similar positive rates (35.8%). And the sequences of CIAV from SPF embryos and NDV vaccines consisted of 2,298 nucleotides (nt) with 100% homology. The new full-length genome of CIAV was designated SDSPF2020 (Genbank accession number: MW660821). Data showed SDSPF2020 had the sequence similarities of 95.8–99.6% with reference strains, and shared the highest homology with the Chinese strain HLJ15125. These results strongly suggested that exogenous CIAV contamination is most likely caused by wild virus infection in SPF flocks and vertical transmission to chicken embryos. Collectively, this study illustrated that vertical transmission of CIAV from a SPF layer breeder flock to embryos was a non-neglible way for exogenous virus contamination in vaccine production.

Role of env gene and LTR sequence in the pathogenesis of subgroup K avian leukosis virus

Avian leukosis virus (ALV) is a retrovirus that induces tumours in infected birds; ALV is divided into different subgroups according to the env gene and cellular tropism. In general, ALV subgroup J (ALV-J) is considered to be the most pathogenic and prevalent subgroup while subgroup K (ALV-K), a newly identified subgroup, only causes mild symptoms. To illuminate the roles of the env viral gene and LTR sequence in pathogenic differences between ALV-J and ALV-K, rescued ALV-J strain rSDAU1005, rescued ALV-K strain rJS11C1, and recombinant strains rENV(J)-LTR(K) and rENV(K)-LTR(J) were characterized and investigated in this study. Among rescued viruses, rSDAU1005 had the highest replication efficiency while rJS11C1 replicated the slowest (replication efficiency rankings were rSDAU1005 >rENV(K)-LTR(J)>rENV(J)-LTR(K)>rJS11 C1). The luciferase reporter gene assay results showed that the promoter activity of ALV-K LTR was lower than that of the ALV-J LTR promoter, which may have accounted for the slower replication efficiency of ALV-K. Pathogenicity of the four rescued viruses was determined via inoculating the yolk sacs of specific-pathogen-free chickens. The results demonstrated that all four viruses were pathogenic; rSDAU1005 caused the most severe growth retardation and immunosuppression. rENV(J)-LTR(K) was more pathogenic when compared to rENV(K)-LTR(J), indicating that env and the LTR sequence play important roles in pathogenicity between ALV-K and ALV-J. Additionally, env seemed to especially play a role in ALV-K pathogenesis. This study provided scientific data and insight to improve detection methods and judgement criteria in ALV clearance and surveillance.

ALV-J-contaminated commercial live vaccines induced pathogenicity in Three-Yellow chickens: one of the transmission routes of ALV-J to commercial chickens

One avian leukosis virus of subgroup J (ALV-J) strain GX14YYA1 was isolated from a commercial bivalent Newcastle disease (ND)–infectious bronchitis (IB) vaccine in our previous study. To evaluate the pathogenicity of the ALV-J-contaminated vaccine on commercial chickens, day-old Three-Yellow chicks in group I were vaccinated with ALV-J-contaminated bivalent ND-IB live vaccine by intranasal and eye drop at 1-day-old for the primary vaccination and at 7-day-old for the secondary vaccination. Groups II and III were kept as the normal vaccination group with the noncontaminated ND-IB vaccine and blank control groups, respectively. The birds of different groups were maintained separately in isolators for 175 d. The first viremia was detected at 4 wk of age and 20% (2/10) of the birds maintained viremia during 11 to 25 wk of age. At the same time, the birds in group I experienced a significant suppression of body weight gain when compared with those of groups II and III (P < 0.05). In addition, the birds in group I showed obvious ALV-J hemangioma-type anatomical lesions in the liver and tumors were observed in the abdominal cavity. The results demonstrated that the ALV-J contaminated commercial live vaccines can induce pathogenicity in commercial Three-Yellow chickens and indicate that ALV-J-contaminated commercial live vaccines could be one of the transmission routes of ALV-J to commercial chickens.

Roles of MOV10 in Animal RNA Virus Infection

Animal epidemic diseases caused by RNA viruses are the primary threat to the livestock industry, and understanding the mechanisms of RNA virus clearance from target cells is critical to establish an effective method to reduce economic losses. As an SF-1, ATP-dependent RNA helicase in the UPF1p family, MOV10 participates in the RNA degradation of multiple viruses mediated via miRNA pathways and therefore contributes to a decrease in the replication of RNA viruses. This review primarily focuses on the bioactivity of MOV10, the mechanism of RNA virus removal, and the potential roles of MOV10 in RNA virus clearance. In addition, clues are provided to reduce animal diseases caused by RNA viruses.

Avian leukosis virus contamination in live vaccines: A retrospective investigation in China

Avian leukosis (AL) is one of the most pandemic immunosuppressive diseases and has been widely spread between 2006 and 2009 in China. The contamination of avian leukosis virus (ALV) in attenuated vaccine is considered as one of the possible transmission routes of this disease. Based on a retrospective survey of 918 batches of attenuated vaccine produced before 2010, three of them were identified as ALV-positive and corresponding ALV strains were successfully isolated from a live Fowlpox virus vaccine, a live Newcastle disease virus vaccine and a live Infectious Bursal Disease virus vaccine, respectively, and whole-genome sequencing showed that these three isolates shared the highest homology with ALV-A wild strains isolated in China (97.7%) over the same period, and the phylogenetic analysis based on their gp85 genes further confirmed that they belong to subgroup A. Meanwhile, although these three ALV-A strains isolated from contaminated vaccines shared a close genetic relationship, their U3 region of genome have a relatively low identity, suggesting that these three strains may have different sources. This study reminds us once again that the possibility of ALV infecting chickens through contaminated live vaccines, requiring us to carry out stricter exogenous virus monitoring in vaccines.

A New Strategy for the Detection of Chicken Infectious Anemia Virus Contamination in Attenuated Live Vaccine by Droplet Digital PCR

Chicken infectious anemia virus (CIAV) causes the atrophy of bone marrow hematopoietic and lymphoid tissues in chicks, leading to huge economic losses all over the world. The using of attenuated vaccine contaminated with CIAV increased the mortality and the pathogenicity of other diseases in many farms. However, it is difficult to detect the CIAV contamination by general detection technology due to the extremely low dose of CIAV in vaccines. In this study, we established a new method called droplet digital Polymerase Chain Reaction (ddPCR) to detect CIAV contamination of vaccines more sensitively and accurately. The lowest detection limitation of this method is 2.4 copies of CIAV plasmid or CIAV contamination at 0.1 EID₅₀/1000 feathers in vaccines without any positive signals of other viruses. Besides, the sensitivity of ddPCR is 100 times greater than that of conventional PCR and 10 times greater than that of real-time PCR. The ddPCR technique is more sensitive and more intuitive. Therefore, it could be valuable for the detection of CIAV contamination in vaccines.

Synergetic pathogenicity of Newcastle disease vaccines LaSota strain and contaminated chicken infectious anemia virus

Newcastle disease virus (NDV)-attenuated vaccine has been widely used to prevent ND in poultry flocks, while many reports also mentioned the exogenous virus contamination in attenuated vaccines, which might be the reason for the widespread of some contagious diseases. Recently, the chicken infectious anemia virus (CIAV) contamination in the NDV-attenuated vaccine was also found in China, though no systemic study has studied the pathogenicity or infection mechanism of this special transmission route. Accordingly, simulation experiments were launched using CIAV isolated from a contaminated NDV-attenuated vaccine. Results showed that using NDV-attenuated vaccine contaminated with CIAV could cause CIA in chickens with obvious symptoms, including anemia, hemorrhage, lymphoatrophy, and growth retardation, while the synergistic reaction of CIAV and LaSota prompted their multiplication in vivo and disturbed the production of antibodies against each other. And CIAV could significantly reduce the NDV antibody titers and decrease the protective effectiveness. This study showed the synergetic pathogenicity of CIAV and LaSota strain after using contaminated NDV-attenuated vaccine, helping us to understand how the CIAV causes infection and induces severe diseases with a relatively low dose through the mouth, as well as reminding us that the damage of an attenuated vaccine contaminated with CIAV even in extremely low dose is not insignificant.

Avian leukosis virus subgroup - J as a contaminant in live commercially available poultry vaccines distributed in Nigeria

Globally, vaccines are used to prevent and control the menace of infectious diseases in livestock with some reported to be inadvertently contaminated with extraneous agents (EAs). With the aim of screening and characterizing for some selected EAs, 44 live viral poultry vaccines were randomly selected based on availability. The vaccines comprised 14 manufacturers in 10 different countries including Nigeria were screened by Polymerase Chain Reaction. In 9% (4/44) of the vaccines, contamination with only avian leukosis virus (ALV) subgroup J (ALV-J) was recorded. Other exogenous ALV subgroups, chicken infectious anemia and infectious laryngotracheitis viruses were absent. The EAs was found in infectious bursal disease (n = 1), Fowlpox (n = 2) and Mareks disease (n = 1) vaccines. Phylogenetic analysis of the ALV-J env gene showed clustering with contemporary group I and II. The result underscores the importance of screening vaccines to avoid the introduction and spread of EAs that could pose a threat to poultry production.

Development and evaluation of a droplet digital PCR assay for the detection of fowl adenovirus serotypes 4 and 10 in attenuated vaccines

In recent years, there has been an increase in reported cases of fowl adenovirus serotype 4 (FAdV-4) in chickens in China. The use of live attenuated vaccines contaminated with FAdV-4 has been proved to be one of the important causes of massive outbreaks of hydropericardium syndrome. To detect the contamination with FAdV-4 in attenuated vaccines more promptly and accurately, a droplet digital PCR (ddPCR) assay was developed for the rapid detection of FAdV-4 and FAdV-10. The ability of this assay to detect FAdV-4 contamination in attenuated Newcastle disease virus vaccines was assessed in comparison to a quantitative real-time PCR (qPCR) and a conventional PCR assay. The findings indicated that the ddPCR assay could detect FAdV-4 contamination at 0.1 EID₅₀/1,000 feathers, while the qPCR could detect FAdV-4 contamination at 1 EID₅₀/1,000 feathers with identical genomic targets, which was 1,000-fold more sensitive than conventional PCR detection with a sensitivity of 10² EID₅₀/1,000 feathers. The ddPCR assay also showed high specificity for FAdV-4/10 and no positive signals were detected for other FAdVs. Consequently, the intuitive and rapid results were especially suitable for the detection of FAdV-4 contamination in vaccines. In this study, a ddPCR assay was developed to effectively detect and quantify low-dose FAdV-4 contamination, providing a new method for rapid detection of FAdV-4 contamination in various samples, especially vaccines.

Newcastle disease virus-attenuated vaccine LaSota played a key role in the pathogenicity of contaminated exogenous virus

Newcastle disease virus (NDV)-attenuated vaccine has been widely used since the 1950s and made great progress in preventing and controlling Newcastle disease. However, many reports mention exogenous virus contamination in attenuated vaccines, while co-contamination with fowl adenovirus (FAdV) and chicken infectious anaemia virus (CIAV) in the NDV-attenuated vaccine also emerged in China recently, which proved to be an important reason for the outbreaks of inclusion body hepatitis–hydropericardium syndrome in some flocks. It is amazing that exogenous virus contamination at extremely low doses still infected chickens and induced severe disease; thus, we speculated that there must be some interaction between the NDV-attenuated vaccine and the contaminated exogenous viruses within. Accordingly, simulation experiments were launched using FAdV and CIAV isolated from the abovementioned vaccine. The results showed that the pathogenicity of FAdV and CIAV co-infection through the contaminated vaccine was significantly higher than that of direct oral infection, while the synergistic reaction of these viruses and LaSota prompted their multiplication in vivo and disturbed the production of antibodies against each other. This study showed the interactions of FAdV, CIAV and LaSota after using contaminated NDV-attenuated vaccine, helping us to understand how the contaminated exogenous viruses cause infection and induce severe disease at a relatively low dose through the oral route.

Phylogenetic Analysis and Pathogenicity Assessment of the Emerging Recombinant Subgroup K of Avian Leukosis Virus in South China

In recent years, cases of avian leukosis virus (ALV) infection have become more frequent in China. We isolated 6 ALV strains from yellow feather broiler breeders in south China from 2014 to 2016. Their full genomes were sequenced, compared, and analyzed with other reference strains of ALV. The complete genomic nucleotide sequences of GD150509, GD160403, GD160607, GDFX0601, and GDFX0602 were 7482 bp in length, whereas GDFX0603 was 7480 bp. They shared 99.7% to 99.8% identity with each other. Homology analysis showed that the gag, pol, long terminal repeats (LTRs), and the transmembrane region (gp37) of the env genes of the 6 viruses were well conserved to endogenous counterpart sequences (>97.8%). However, the gp85 genes displayed high variability with any known chicken ALV strains. Growth kinetics of DF-1 cells infected with the isolated ALV showed viral titers that were lower than those infected with the GD13 (ALV-A), CD08 (ALV-B), and CHN06 (ALV-J) on day 7 post-infection. The infected Specific-pathogen-free (SPF) chickens could produce continuous viremia, atrophy of immune organs, growth retardation and no tumors were observed. These subgroup ALVs are unique and may be common in south China. The results suggested that updating the control and eradication program of exogenous ALV for yellow feather broiler breeders in south China needs to be considered because of the emergence of the new subgroup viruses.

Newcastle disease virus-attenuated vaccine co-contaminated with fowl adenovirus and chicken infectious anemia virus results in inclusion body hepatitis-hydropericardium syndrome in poultry

Inclusion body hepatitis-hydropericardium syndrome (IBH-HPS) induced by fowl adenovirus type 4 (FAdV-4) has caused huge economic losses to the poultry industry of China, but the source of infection for different flocks, especially flocks with high biological safety conditions, has remained unclear. This study tested the pathogenicity of Newcastle disease virus (NDV)-attenuated vaccine from a large-scale poultry farm in China where IBH-HPS had appeared with high mortality. Analysis revealed that the NDV-attenuated vaccine in use from the abovementioned poultry farm was simultaneously contaminated with FAdV-4 and chicken infectious anemia virus (CIAV). The FAdV and CIAV isolated from the vaccine were purified for the artificial preparation of an NDV-attenuated vaccine singly contaminated with FAdV or CIAV, or simultaneously contaminated with both of them. Seven-day-old specific pathogen-free chicks were inoculated with the artificially prepared contaminated vaccines and tested for corresponding indices. The experiments showed that no hydropericardium syndrome (HPS) and corresponding death occurred after administering the NDV-attenuated vaccine singly contaminated with FAdV or CIAV, but a mortality of 75% with IBH-HPS was commonly found in birds after administering the NDV-attenuated vaccine co-contaminated with FAdV and CIAV. In conclusion, this study found the co-contamination of FAdV-4 and CIAV in the same attenuated vaccine and confirmed that such a contaminated attenuated vaccine was a significant source of infection for outbreaks of IBH-HPS in some flocks.

Full-length genome sequence analysis of an avian leukosis virus subgroup J (ALV-J) as contaminant in live poultry vaccine: The commercial live vaccines might be a potential route for ALV-J transmission

One avian leukosis virus subgroup J (ALV-J) strain was isolated from 67 commercial live poultry vaccines produced by various manufacturers during 2013-2016 in China. The complete genomes of the isolate were sequenced and it was found that the genes gag and pol of the strain were relatively conservative, while the gp85 gene of the strain GX14YYA1 had the highest similarities with a field strain GX14ZS14, which was isolated from the chickens of a farm that had once used the same vaccine as the one found to be contaminated with the GX14YYA1. This is the first report of ALV-J contaminant in live poultry vaccine in China. Our finding demonstrates that vaccination of the commercial live vaccines might be a potential new route for ALV-J transmission in chickens and highlights the need for more extensive monitoring of the commercial live vaccines in China.

Genome analysis and pathogenicity of reticuloendotheliosis virus isolated from a contaminated vaccine seed against infectious bursal disease virus: first report in China

Specific-pathogen-free (SPF) chickens were inoculated with the virus seed of an infectious bursal disease virus (IBDV)-attenuated vaccine, and positive reticuloendotheliosis virus (REV) antibody levels were subsequently detected in the chicken sera, indicating potential REV contamination of the vaccine. After neutralization with IBDV-positive blood serum, the vaccine was inoculated into DF-1 cells for REV isolation and identification. An REV strain, designated IBD-C1605, was identified using an immunofluorescence assay test. Three pairs of primers were employed for the amplification, cloning and sequencing of three overlapping fragments of the IBD-C1605 genome, and the whole-genome sequence of this isolate was obtained after gene assembly. The genome was 8362 base pairs (nt) in length and its homology with the nucleotide sequences of different reference strains varied between 94.2 and 99.2 %. Isolate IBD-C1605 was inoculated into 1-day-old SPF chickens to observe its pathogenicity. Infection with this organism slowed down the weight gain of SPF chickens and caused atrophy of their immune organs, such as the bursa of Fabricius and thymus gland. Furthermore, the chicken antibody levels decreased significantly after Newcastle disease virus and avian influenza virus subtype H9 vaccine immunization. This is the first report on the isolation and identification of REV from attenuated vaccine virus seeds in China, and is also the first study on the pathogenicity of REV from a contaminated vaccine in China. Our findings contribute towards a better understanding of the detrimental effects of vaccine contamination with exogenous viruses such as REV.

Genomic Analysis of the Chicken Infectious Anemia Virus in a Specific Pathogen-Free Chicken Population in China

The antibody to chicken infectious anemia virus (CIAV) was positive in a specific pathogen-free (SPF) chicken population by ELISA test in our previous inspection, indicating a possible infection with CIAV. In this study, blood samples collected from the SPF chickens were used to isolate CIAV by inoculating into MSB1 cells and PCR amplification. A CIAV strain (SD1403) was isolated and successfully identified. Three overlapping genomic fragments were obtained by PCR amplification and sequencing. The full genome sequence of the SD1403 strain was obtained by aligning the sequences. The genome of the SD1403 strain was 2293 bp with a nucleotide identity of 94.8% to 98.5% when compared with 30 referred CIAV strains. The viral proteins VP2 and VP3 were highly conserved, but VP1 was not relatively conserved. Both amino acids 139 and 144 of VP1 were glutamine, which was in accord with the low pathogenic characteristics. In this study, we first reported that CIAV exists in Chinese SPF chicken populations and may be an important reason why attenuated vaccine can be contaminated with CIAV.