Immune responses upon in ovo HVT-IBD vaccination vary between different chicken lines

The genotype of chickens is assumed to be associated with variable immune responses. In this study a modern, moderate performing dual-purpose chicken line (DT) was compared with a high-performing layer-type (LT) as well as a broiler-type (BT) chicken line. One group of each genotype was vaccinated in ovo with a recombinant herpesvirus of turkeys expressing the virus protein VP2 of the infectious bursal disease virus (HVT-IBD) while one group of each genotype was left HVT-IBD unvaccinated (control group). Genotype associated differences in innate and adapted immune responses between the groups were determined over five weeks post hatch. HVT-IBD vaccination significantly enhanced humoral immune responses against subsequently applied live vaccines compared to non-HVT-IBD vaccinated groups at some of the investigated time points (P < 0.05). In addition HVT-IBD vaccination had depending on the genotype a significant impact on splenic macrophage as well as bursal CD4⁺ T-cell numbers (P < 0.05). On the other hand, the detectable genotype influence on Interferon (IFN) γ and nitric oxide (NO) release of ex vivo stimulated spleen cells was independent of HVT-IBD vaccination. The results of our study suggest considering a genotype specific vaccination regime in the field.

Serological study reveal different antigenic IBDV strains prevalent in southern China during the years 2000-2017 and also the antigenic differences between the field strains and the commonly used vaccine strains

The aim of this study was to determine the antigenic relatedness of Infectious Bursal Disease Viruses (IBDVs) in the field in southern China during the period 2000-2017, as well as the antigenic relationship between the field strains and the most commonly used vaccine strains by using a virus neutralization (VN) test in vitro. The antigenic relatedness (R) value and the difference in VN titers were analyzed, and the antigenic index based on the sequences of the hypervariable region of VP2 (vVP2) of the strains was further evaluated. As a result, the R value of representative field strains showed that there were three subtypes present in the field strains examined, with 7 strains belonging to subtype 1, while strains BH11 and JS7 belonged to subtype 2 and subtype 3, respectively. The commonly used vaccine strains B87 and FW2512 belonged to subtype 1. The analysis of the VN titer differences revealed that all the 136 field strains were classified into subtype 1, except BH11 and JS7. All the field strains in subtype 1 have been divided into at least 5 subgroups, suggesting the antigenic diversity among these strains. The antigenic index based on IBDV-VP2 sequences further confirmed the antigenic differences between the three subtype strains and also the antigenic diversity among the subtype 1. The results demonstrated the antigenic diversity of field IBDVs in southern China during the years 2000-2017 and the antigenic differences between the field strains and the commonly used vaccine strains. This would indicate that the commonly used vaccines are only partially effective. These results enhance our understanding of IBDV genetic evolution and should help to develop more effective vaccines for the control of this disease in the future.

Differences in smolt status affect the resistance of Atlantic salmon (Salmo salar L.) against infectious pancreatic necrosis, while vaccine-mediated protection is unaffected

In today's aquaculture of Atlantic salmon (Salmo salar L.), a majority of viral disease outbreaks occur after seawater transfer. A relevant question is how the parr-smolt transformation influences the efficacy of viral vaccines and the innate resistance against viral diseases. In this study, vaccinated and unvaccinated A. salmon parr were exposed to different photoperiodic regimens (1-, 3- or 6-week continuous light-WCL). Fish groups at different stages in the smoltification process were induced, as demonstrated by differences in morphological and physiological smolt parameters. At the time of seawater transfer, the 6-WCL group had reached a more pronounced stage in the smoltification process than the 1-WCL group. In unvaccinated fish, the subsequent cohabitation challenge with infectious pancreatic necrosis virus (IPNV) gave a significantly higher accumulated mortality in the 6-WCL group (87%) compared to the 1-WCL group (39%). In the vaccinated groups, this effect was not apparent and there were no differences in accumulated mortality between the 1 WCL, 3 WCL and 6-WCL groups. These data suggest that the resistance to IPN in A. salmon was negatively influenced by smoltification, while vaccine-mediated protection to IPN was maintained equally well irrespective of smolt status.

Salmon cells SHK-1 internalize infectious pancreatic necrosis virus by macropinocytosis

We have previously shown that infectious pancreatic necrosis virus (IPNV) enters the embryo cell line CHSE-214 by macropinocytosis. In this study, we have extended our investigation into SHK-1 cells, a macrophage-like cell line derived from the head kidney of Atlantic salmon, the most economically important host of IPNV. We show that IPNV infection stimulated fluid uptake in SHK-1 cells above the constitutive macropinocytosis level. In addition, upon infection of SHK-1 cells, IPNV produced several changes in actin dynamics, such as protrusions and ruffles, which are important features of macropinocytosis. We also observed that the Na+/H+ pump inhibitor EIPA blocked IPNV infection. On the other hand, IPNV entry was independent of clathrin, a possibility that could not be ruled out in CHSE 214 cells. In order to determine the possible role of accessory factors on the macropinocytic process, we tested several inhibitors that affect components of transduction pathways. While pharmacological intervention of PKI3, PAK-1 and Rac1 did not affect IPNV infection, inhibition of Ras and Rho GTPases as well as Cdc42 resulted in a partial decrease in IPNV infection. Further studies will be required to determine the signalling pathway involved in the macropinocytosis-mediated entry of IPNV into its target cells.

Progress on chicken T cell immunity to viruses

Avian virus infection remains one of the most important threats to the poultry industry. Pathogens such as avian influenza virus (AIV), avian infectious bronchitis virus (IBV), and infectious bursal disease virus (IBDV) are normally controlled by antibodies specific for surface proteins and cellular immune responses. However, standard vaccines aimed at inducing neutralizing antibodies must be administered annually and can be rendered ineffective because immune-selective pressure results in the continuous mutation of viral surface proteins of different strains circulating from year to year. Chicken T cells have been shown to play a crucial role in fighting virus infection, offering lasting and cross-strain protection, and offer the potential for developing universal vaccines. This review provides an overview of our current knowledge of chicken T cell immunity to viruses. More importantly, we point out the limitations and barriers of current research and a potential direction for future studies.

Detection of infectious bursal disease virus (IBDV) antibodies using chimeric plant virus-like particles

The aim of the present study is to use Physalis mottle virus (PhMV) coat protein (CP) as a scaffold to display the neutralizing epitopes of Infectious bursal disease virus (IBDV) VP2. For this, three different chimeric constructs were synthesized by replacing the N-terminus of PhMV CP with tandem repeats of neutralizing epitopes of IBDV VP2 and expressed in Escherichia coli. Expression analysis revealed that all the three recombinant chimeric coat protein subunits are soluble in nature and self-assembled into virus-like particles (VLPs) as evidenced through sucrose density gradient ultracentrifugation. The chimeric VLPs were characterized by various biochemical and biophysical techniques and found that they are stable and structurally sound. When the chimeric VLPs were used as coating antigen, they were able to detect IBDV antibodies. These results indicated that the chimeric VLPs can be used as potential vaccine candidates for the control of IBDV, which needs to be further evaluated in animal models.

First examination of the Lough Neagh European eel (Anguilla anguilla) population for eel virus European, eel virus European X and Anguillid Herpesvirus-1 infection by employing novel molecular techniques

Lough Neagh is home to the largest wild-caught European eel (Anguilla anguilla) commercial fishery in the EU, producing 14% of the EU catch and worth £3.2 million to the local economy. Viral infections have been suggested to play a contributory role in the decline of the worldwide eel stock, but previous studies of the Lough Neagh European eel population had not observed either acute or chronic viral signs. Eel virus European (EVE), Eel virus European X (EVEX) and Anguillid herpesvirus-1 (HVA) have been detected throughout Europe and as the Lough Neagh eel fishery is supplemented by re-stocking of eels from France, Spain and the United Kingdom and these viral infections may be asymptomatic, it is vital that the viral pathogen prevalence in the Lough is accurately determined. This study aimed to ascertain the presence of these viruses in the Lough Neagh European eel population by employing novel molecular techniques testing specifically for the presence of EVE, EVEX and HVA. No evidence was found of HVA infection, whereas EVE and EVEX were found, albeit at a very low prevalence.

The effectiveness of fishery net dips: Advice for the improvement of biosecurity measures

Disease poses a major threat to aquaculture and commercial and recreational fisheries globally. Biosecurity measures have been implemented; however, empirical evidence of their efficacy in situ is lacking. Here, we present the results from a study conducted to examine the effectiveness of disinfectant net dips. Samples were collected from disinfectant net dips at 25 recreational fisheries in south-west England and assessed to determine (a) the level of bacterial contamination and (b) the reduction in titre of a target virus (infectious pancreatic necrosis virus, IPNV) following a contact time of 2 and 5 min. In addition, the study examined the reduction in target virus titre following exposure to laboratory prepared Virkon^® , representing "clean," "dirty" and "diluted and dirty" conditions, for 2 and 5 min. Bacterial contamination was high in 64% of disinfectant samples, and, 76% of disinfectant samples did not effectively reduce the target virus titre in 2 or 5 min. Virus titre was successfully reduced following exposure to laboratory prepared Virkon^® for 2 or 5 min, although dilution and contamination reduced the effectiveness. These results suggest that disinfectant net dips may not be working effectively on a high proportion of fishery sites. We provide recommendations for improving biosecurity.

Molecular characterization and phylogenetic analysis of very virulent infectious bursal disease virus circulating in Morocco during 2016-2017

Very virulent infectious bursal disease virus (vvIBDV), the cause of significant economic losses in many poultry-producing areas, has been present in Morocco since 1991. In spite of the introduction of vaccination, disease outbreaks are frequently observed. To ascertain if vaccines failure may be due to the emergence of new strains, the aim of this study was to perform for the first time the molecular characterization of vvIBDV strains circulating in Morocco by focusing on the hypervariable region (HVR) of the VP2 protein, which is frequently used for molecular epidemiology and phylogenetic studies. Field samples of haemorrhagic bursae of Fabricius were collected for molecular characterization in different parts of the country during 2016-2017 from 48 chicken flocks showing symptoms of disease. In a phylogenetic tree, nucleotide sequences containing the VP2 HVR of 13 samples that were positive for vvIBDV formed a common branch with those of vvIBDV references strains published in GenBank, but they clearly grouped into a distinct subcluster. An alignment of the deduced amino acid sequences, in addition to confirming the presence of the "signature" typical of the vvIBDV HVR, also revealed the presence of substitutions in hydrophilic loops that are known to be involved in the elicitation of neutralizing antibodies. One of these substitutions is unique to the Moroccan isolates. These results represent the first molecular characterization of vvIBDV isolates in Morocco and may indicate that one of the causes of vaccine ineffectiveness is antigenic drift.

Molecular epidemiological study on Infectious Pancreatic Necrosis Virus isolates from aquafarms in Scotland over three decades

In order to obtain an insight into genomic changes and associated evolution and adaptation of Infectious Pancreatic Necrosis Virus (IPNV), the complete coding genomes of 57 IPNV isolates collected from Scottish aquafarms from 1982 to 2014 were sequenced and analysed. Phylogenetic analysis of the sequenced IPNV strains showed separate clustering of genogroups I, II, III and V. IPNV isolates with genetic reassortment of segment A/B of genogroup III/II were determined. About 59 % of the IPNV isolates belonged to the persistent type and 32 % to the low-virulent type, and only one highly pathogenic strain (1.79 %) was identified. Codon adaptation index calculations indicated that the IPNV major capsid protein VP2 has adapted to its salmonid host. Under-representation of CpG dinucleotides in the IPNV genome to minimize detection by the innate immunity receptors, and observed positive selection in the virulence determination sites of VP2 embedded in the variable region of the main antigenic region, suggest an immune escape mechanism driving virulence evolution. The prevalence of mostly persistent genotypes, together with the assumption of adaptation and immune escape, indicates that IPNV is evolving with the host.

Differential activation of intraepithelial lymphocyte-natural killer cells in chickens infected with very virulent and vaccine strains of infectious bursal disease virus

To gain insights into the role of CD3^-/28.4⁺ intraepithelial lymphocytes-natural killer (CD3^-/28.4⁺IEL-NK) cells during infectious bursal disease virus (IBDV) infection, characterisation of the cells was performed following infection with different strains of the virus. In vitro treatment with IL-18 or ionomycin/PMA successfully stimulated and activated the cells via a significant increase in the expression of CD69, B-Lec, CHIR-AB1 and NK-lysin. Similarly, chickens infected with the vaccine strain of IBDV also up-regulated the expression of CD69, B-Lec, CHIR-AB1 and NK-lysin in CD3^-/28.4⁺ IEL-NK cells up to 3 days post infection (dpi) and down-regulated the expression of the inhibitory receptor B-NK at 3 dpi. On the contrary, infection with the very virulent IBDV (vvIBDV) strain lead to a reduced activation of the cells by down-regulating the expression of the CD69, CHIR-AB1 and NK-lysin especially at 1 dpi. These findings altogether demonstrate the differential activation of CD3^-/28.4⁺IEL-NK cells in chicken following infection with the vaccine or very virulent strains of IBDV. The study therefore provides an important clue into the differential pathogenesis of IBDV infection in chicken. Further studies are however required to determine the functional importance of these findings during IBDV vaccination and infection.

Sequence analysis of VP2 hypervariable region of the field isolates of infectious bursal disease viruses from southern region of India

Infectious bursal disease virus isolates obtained from southern parts of India were subjected to comparative sequencing and phylogenetic analysis of 743bp hypervariable region of VP2. The sequence analysis showed that among eight isolates, only HY12 showed the characteristic conserved amino acid residues at 256I, 294I, and 299S of vvIBDV. Six isolates BGE14, PY12, NKL14, VCN14, RPM14 and EDE14 had conserved amino acid residues at 256I and 299S, whereas at residue 294, isoleucine was substituted by valine. The remaining isolate MB11 had leucine at residue 294 and asparagine at residue 299 similar to classical strain 52/70. The serine-rich heptapeptide sequence SWSASGS adjacent to the second hydrophilic region was conserved in all seven Indian IBDV isolates except isolate MB11. Conservation of this sequence was earlier reported to be an indication of a virus isolate being pathogenic in nature. The reported heptapeptide sequence of the classical strain is 'SWSARGS'. In the present study, 'SWSARGS' heptapeptide sequence was observed in MB11 isolate. The pathogenicity trials conducted with these isolates further confirmed the genome analysis in classification. This study further reveals that the circulating IBDV strains in India could be diverse in nature.

Infectious bursal disease virus infection leads to changes in the gut associated-lymphoid tissue and the microbiota composition

Infectious bursal disease (IBD) is an acute, highly contagious and immunosuppressive poultry disease. IBD virus (IBDV) is the causative agent, which may lead to high morbidity and mortality rates in susceptible birds. IBDV-pathogenesis studies have focused mainly on primary lymphoid organs. It is not known if IBDV infection may modify the development of the gut associated lymphoid tissues (GALT) as well as the microbiota composition. The aim of the present study was to investigate the effects of IBDV-infection on the bursa of Fabricius (BF), caecal tonsils (CT) and caecum, and to determine the effects on the gut microbiota composition in the caecum. Commercial broiler chickens were inoculated with a very virulent (vv) strain of IBDV at 14 (Experiment 2) or 15 (Experiment 1) days post hatch (dph). Virus replication, lesion development, immune parameters including numbers of T and B lymphocytes, macrophages, as well as the gut microbiota composition were compared between groups. Rapid IBDV-replication was detected in the BF, CT and caecum. It was accompanied by histological lesions including an infiltration of heterophils. In addition a significant reduction in the total mucosal thickness of the caecum was observed in vvIBDV-infected birds compared to virus-free controls (P < 0.05). vvIBDV infection also led to an increase in T lymphocyte numbers and macrophages, as well as a decrease in the number of B lymphocytes in the lamina propria of the caecum, and in the caecal tonsils. Illumina sequencing analysis indicated that vvIBDV infection also induced changes in the abundance of Clostridium XIVa and Faecalibacterium over time. Overall, our results suggested that vvIBDV infection had a significant impact on the GALT and led to a modulation of gut microbiota composition, which may lead to a higher susceptibility of affected birds for pathogens invading through the gut.

Surface IgM λ light chain is involved in the binding and infection of infectious bursal disease virus (IBDV) to DT40 cells

Infectious bursal disease virus (IBDV) is an important immunosuppressive virus in chickens. Surface immunoglobulin M (sIgM)-bearing B lymphocytes act as the major targets of IBDV in the bursa of Fabricius, and sIgM may function as one of the membrane binding sites responsible for IBDV infection. Recently, using the virus overlay protein binding assay, the chicken λ light chain of sIgM was identified to specifically interact with IBDV in a virulence-independent manner in vitro. To further investigate sIgM λ light chain-mediated IBDV binding and infection in pre-B cells, the cell line DT40, which is susceptible to both pathogenic and attenuated IBDV, was used. Based on the RNA interference strategy, the DT40 cell line whose λ light chain of sIgM was stably knocked down, herein termed DT40LKD, was generated by the genomic integration of a specific small hairpin RNA and a green fluorescence protein co-expression construct. Flow cytometry analysis indicated that the binding of IBDV to DT40LKD cells was significantly reduced due to the loss of sIgM λ light chain. In particular, reduced viral replication was observed in IBDV-incubated DT40LKD cells, and no viral release into cell culture medium was detected by the IBDV rapid diagnostic strips. In addition, the rescue of sIgM λ light chain expression restored viral binding and replication in DT40LKD cells. These results show that sIgM λ light chain appears to be beneficial for IBDV attachment and infection, suggesting that sIgM acts as a binding site involved in IBDV infection.

Infectious pancreatic necrosis virus (IPNV) serotype Sp is prevalent in Turkish rainbow trout farms

Infectious pancreatic necrosis virus (IPNV) is a common pathogen of rainbow trout (Oncorhynchus mykiss) in Turkey. We found that 455 of 1,676 sample pools tested were IPNV positive. Positive samples were found in all geographical regions where sampling was conducted. Sequence and phylogenetic analyses of VP2 from 30 isolates representing all regions showed that the viruses were highly similar in sequence and grouped within Genogroup 5 (serotype Sp-A2). No correlations between sequences, sampling sites or geographical origins were identified. Although clinical disease was evident in several farms, analyses of the amino acid sequence of VP2 showed that all virus strains harboured the P₂₁₇ T₂₂₁ motif, assumed to be associated with low virulence. We conclude that IPNV is prevalent in Turkish rainbow trout farms and that the viruses are very homogenous and likely to be of European origin. Frequent exchange of eggs and live fish within the farming industry may explain the homogeneity of the IPNV.

Bursal transcriptome profiling of different inbred chicken lines reveals key differentially expressed genes at 3 days post-infection with very virulent infectious bursal disease virus

Infectious bursal disease is a highly contagious disease in the poultry industry and causes immunosuppression in chickens. Genome-wide regulations of immune response genes of inbred chickens with different genetic backgrounds, following very virulent infectious bursal disease virus (vvIBDV) infection are poorly characterized. Therefore, this study aims to analyse the bursal tissue transcriptome of six inbred chicken lines 6, 7, 15, N, O and P following infection with vvIBDV strain UK661 using strand-specific next-generation sequencing, by highlighting important genes and pathways involved in the infected chicken during peak infection at 3 days post-infection. All infected chickens succumbed to the infection without major variations among the different lines. However, based on the viral loads and bursal lesion scoring, lines P and 6 can be considered as the most susceptible lines, while lines 15 and N were regarded as the least affected lines. Transcriptome profiling of the bursa identified 4588 genes to be differentially expressed, with 2985 upregulated and 1642 downregulated genes, in which these genes were commonly or uniquely detected in all or several infected lines. Genes that were upregulated are primarily pro-inflammatory cytokines, chemokines and IFN-related. Various genes that are associated with B-cell functions and genes related to apoptosis were downregulated, together with the genes involved in p53 signalling. In conclusion, bursal transcriptome profiles of different inbred lines showed differential expressions of pro-inflammatory cytokines and chemokines, Th1 cytokines, JAK-STAT signalling genes, MAPK signalling genes, and their related pathways following vvIBDV infection.

Differential gene expression in chicken primary B cells infected ex vivo with attenuated and very virulent strains of infectious bursal disease virus (IBDV)

Infectious bursal disease virus (IBDV) belongs to the family Birnaviridae and is economically important to the poultry industry worldwide. IBDV infects B cells in the bursa of Fabricius (BF), causing immunosuppression and morbidity in young chickens. In addition to strains that cause classical Gumboro disease, the so-called 'very virulent' (vv) strain, also in circulation, causes more severe disease and increased mortality. IBDV has traditionally been controlled through the use of live attenuated vaccines, with attenuation resulting from serial passage in non-lymphoid cells. However, the factors that contribute to the vv or attenuated phenotypes are poorly understood. In order to address this, we aimed to investigate host cell-IBDV interactions using a recently described chicken primary B-cell model, where chicken B cells are harvested from the BF and cultured ex vivo in the presence of chicken CD40L. We demonstrated that these cells could support the replication of IBDV when infected ex vivo in the laboratory. Furthermore, we evaluated the gene expression profiles of B cells infected with an attenuated strain (D78) and a very virulent strain (UK661) by microarray. We found that key genes involved in B-cell activation and signalling (TNFSF13B, CD72 and GRAP) were down-regulated following infection relative to mock, which we speculate could contribute to IBDV-mediated immunosuppression. Moreover, cells responded to infection by expressing antiviral type I IFNs and IFN-stimulated genes, but the induction was far less pronounced upon infection with UK661, which we speculate could contribute to its virulence.

Virus mutations and their impact on vaccination against infectious bursal disease (Gumboro disease)

Infectious bursal disease (also known as Gumboro disease) is an immunosuppressive viral disease specific to chickens. In spite of all the information amassed on the antigenic and immunological characteristics of the virus, the disease has not yet been brought fully under control. It is still prevalent in properly vaccinated flocks carrying specific antibodies at levels normally high enough to prevent the disease. Common causes apart, failure of vaccination against infectious bursal disease is associated mainly with early vaccination in flocks of unknown immune status and with the evolution of viruses circulating in the field, leading to antigenic drift and a sharp rise in pathogenicity. Various highly sensitive molecular techniques have clarified the viral determinants of antigenicity and pathogenicity of the infectious bursal disease virus. However, these markers are not universally recognised and tend to be considered as evolutionary markers. Antigenic variants of the infectious bursal disease virus possess modified neutralising epitopes that allow them to evade the action of maternally-derived or vaccine-induced antibodies. Autogenous or multivalent vaccines are required to control antigenic variants in areas where classical and variant virus strains coexist. Pathotypic variants (very virulent viruses) remain antigenically related to classical viruses. The difficulty in controlling pathotypic variants is linked to the difficulty of eliciting an early immune response, because of the risk of the vaccine virus being neutralised by maternal antibodies. Mathematical calculation of the optimal vaccination time and the use of vaccines resistant to maternally-derived antibodies have improved the control of very virulent viruses.

Genetic and pathologic characteristics of infectious bursal disease viruses isolated from broiler chickens in Iran during 2014-2015

Infectious bursal disease (IBD) virus causes a highly contagious immunosuppressive disease in chickens. A total number of 12 pooled bursal samples were collected during 2014-2015 from broiler farms in different regions of Iran. Typical macroscopical and histopathological lesions of the bursa of Fabricius were found similar to reports by other researchers. A 474-bp part of hypervariable region of VP2 (hvVP2) was sequenced and analyzed. Ten isolates had the characteristic amino acid residues of very virulent IBD (vvIBD) viruses and the other two were identified as attenuated (vaccine) strains. The vvIBD isolates had a unique G to S mutation at position 254, compared to other Iranian vvIBD isolates. Two attenuated isolates had the mutation 253Q, not found in D78 strain, creating virulent variant of vaccine strains. Degree of similarity among the studied vvIBD isolates was relatively high (97.6-100%), proposing a common ancestor for them. However, they were partly different from previous Iranian and neighbor countries' isolates (96.2-97.3% similarity to Shiraz isolate and 95.7-96.7% to Iraq and Turkey isolates). In phylogenetic analysis, the studied vvIBD isolates classified as a separate subgroup in the group of isolates from Iran and neighbor countries. More studies on genetic and antigenic characteristics of these isolates as well as probable modifications in their pathogenicity are needed to evaluate the significance of the mentioned differences.

In vivo study of viral haemorrhagic septicaemia virus and infectious pancreatic necrosis virus coexistence in Senegalese sole (Solea senegalensis)

The effect of IPNV-VHSV coinfection and superinfection on the mortality caused by both viruses in Senegalese sole has been analysed. No effect was observed after coinfection. However, a clear viral interference was recorded between a primary IPNV and a subsequent VHSV infection, which led to a survival increase in the infected sole of 50% points when compared with fish infected only with VHSV. The significantly higher Mx transcriptional values in the fish pre-exposed to IPNV (at least at first days after superinfection) and the increased daily mortality when low Mx transcriptional levels were recorded suggest that Mx may be involved in the protective effect against VHSV infection. However, in fish subjected to VHSV primary/IPNV secondary infection, no interference was observed.

Design and validation of a RT-qPCR procedure for diagnosis and quantification of most types of infectious pancreatic necrosis virus using a single pair of degenerated primers

Infectious pancreatic necrosis virus (IPNV) is an important virus which affects the salmonid aquaculture industry worldwide; therefore, it is important to develop rapid and reliable methods of diagnosis to detect the disease at early stages. Nowadays, RT-qPCR is replacing other methods because it provides additional information on the viral load, which is important to have a better understanding of the virus replication level and of the stage of the infection and its risk level. The main problem stems from the high diversity of this virus, which can compromise the reliability of the diagnosis. In this study, we have designed an RT-qPCR procedure for diagnosis and quantification of IPNV based on a single pair of primers targeted to segment B. The procedure has been validated, in vitro and in vivo, testing two different types of standards against seven reference strains and 23 field isolates from different types. The procedure is reliable for the detection of any type, with a detection limit of 31 TCID₅₀  mL^-1 , 50 pfu mL^-1 or 66 RNA copies mL^-1 , depending on the standard. All the standard curves showed high reliability (R²  > 0.95). The results support the high reliability of this new procedure for the diagnosis and quantification of IPNV.

LEI0258 microsatellite variability and its association with humoral and cell mediated immune responses in broiler chickens

Major histocompatibility complex (MHC) has a profound influence on disease resistance or susceptibility, productivity and important economic traits in chicken. Association of the MHC with a wide range of immune responses makes it a valuable predictive factor for the disease pathogenesis and outcome. The tandem repeat LEI0258 is a genetic marker which is located within the B locus of chicken MHC and strongly associated with serologically defined haplotypes. LEI0258 microsatellite marker was applied to investigate the MHC polymorphism in Ross 308 broiler chicken (N=104). Association of LEI0258 alleles with humoral and cell mediated immune responses to Newcastle disease (ND), Infectious bursal disease (IBD) and Avian influenza (AI) vaccines were also examined. LEI0258 polymorphism was determined by PCR-based fragment analysis, and association of LEI0258 alleles with immune responses were evaluated using multivariate regression analysis and GLM procedures. A total of seven alleles ranging from 195 to 448bp were found, including two novel alleles (263 and 362bp) that were unique in Ross 308 broiler population. Association study revealed a significant influence of MHC alleles on humoral and cellular immune responses in Ross population (P<0.05). Alleles 385 and 448bp were associated with increased peripheral blood lymphocyte proliferation response. Alleles 300, 362 and 448bp had a positive effect on immune responses to Infectious bursal disease vaccine, and allele 263bp was significantly correlated with elevated antibody titer against Newcastle disease vaccine. Results obtained from this study confirmed the important role of MHC as a candidate gene marker for immune responses that could be used in genetic improvement of disease-resistant traits and resource conservation in broiler population.

Infectious bursal disease: outbreak investigation, molecular characterization, and vaccine immunogenicity trial in Ethiopia

The study was conducted with the objective of isolation and molecular characterization of infectious bursal disease virus (IBDV) circulating in Ethiopia and to assess the immunogenicity of different commercially available live attenuated IBD vaccines and finally to select the appropriate vaccine strain for the existing IBDV. Outbreak samples collected from different poultry farms with IBD infection between 2013 and 2015 were used for the virus isolation and molecular characterization. IBD vaccine immunogenicity test was conducted using four different commercially available live attenuated IBD vaccine strains: namely D78, B2K, LC75, and EXTREM. Day-old Bowman brown chickens purchased from commercial farm in Debre Zeit were used for the experiment. Serum samples were collected at days 14 and 21 and screened for the presence of maternal IBDv antibodies. The screening test result revealed that most of the chickens from vaccinated progeny were positive at the age of day 14 with mean antibody titer of .42, but declined at day 21 to 0.049 below cut-off point (S/P < 0.3). Chickens were divided into five different groups (four vaccinal and one control) and vaccinated at the age of day 21 and boosted after 14 days. Serum samples were collected and all of them were challenged at their 42 days of age with locally isolated very virulent infectious bursal disease virus (vvIBDV). From four of the vaccine strains used for immunogenicity study, the intermediate plus strains (LC75 and EXTREM) found to be superior and efficiently cross protect against the challenge with locally isolated vvIBDV. The development of clinical signs was studied and post-mortem examinations were conducted both on dead and sacrificed birds. From a total of 25 tissue samples processed for virus isolation on chicken fibroblast cell culture, 95% (18/20) of bursa and 80% (4/5) of the spleen samples showed visible cytopathic effect (CPE). The positive samples were tested by PCR and 19 of them had the expected band (643 bp). Further 11 representative samples were sequenced and confirmed that the circulating virus among poultry population in the country is vvIBDV. The study has recommended to produce vaccine using intermediate plus strains to prevent and control currently circulating vvIBDV.

Non-Lytic Egression of Infectious Bursal Disease Virus (IBDV) Particles from Infected Cells

Infectious bursal disease virus (IBDV), a member of the Birnaviridae family, is responsible for a devastating immunosuppressive disease affecting juvenile domestic chickens. IBDV particles are naked icosahedrons enclosing a bipartite double-stranded RNA genome harboring three open reading frames (ORF). One of these ORFs codes for VP5, a non-structural polypeptide dispensable for virus replication in tissue culture but essential for IBDV pathogenesis. Using two previously described recombinant viruses, whose genomes differ in a single nucleotide, expressing or not the VP5 polypeptide, we have analyzed the role of this polypeptide during the IBDV replication process. Here, we show that VP5 is not involved in house-keeping steps of the virus replication cycle; i.e. genome transcription/replication, protein translation and virus assembly. Although infection with the VP5 expressing and non-expressing viruses rendered similar intracellular infective progeny yields, striking differences were detected on the ability of their progenies to exiting infected cells. Experimental data shows that the bulk of the VP5-expressing virus progeny efficiently egresses infected cells during the early phase of the infection, when viral metabolism is peaking and virus-induced cell death rates are as yet minimal, as determined by qPCR, radioactive protein labeling and quantitative real-time cell death analyses. In contrast, the release of the VP5-deficient virus progeny is significantly abridged and associated to cell death. Taken together, data presented in this report show that IBDV uses a previously undescribed VP5-dependent non-lytic egress mechanism significantly enhancing the virus dissemination speed. Ultrastructural analyses revealed that newly assembled IBDV virions associate to a vesicular network apparently facilitating their trafficking from virus assembly factories to the extracellular milieu, and that this association requires the expression of the VP5 polypeptide.

Modified live infectious bursal disease virus (IBDV) vaccine delays infection of neonatal broiler chickens with variant IBDV compared to turkey herpesvirus (HVT)-IBDV vectored vaccine

Chickens are commonly processed around 35-45days of age in broiler chicken industry hence; diseases that occur at a young age are of paramount economic importance. Early age infection with infectious bursal disease virus (IBDV) results in long-lasting immunosuppression and profound economic losses. To our knowledge, this is the first study comparing the protection efficacy of modified live (MdLV) IBDV and herpesvirus turkey (HVT)-IBDV vaccines against early age variant IBDV (varIBDV) infection in chicks. Experiments were carried out in IBDV maternal antibody (MtAb) positive chicks (n=330), divided into 6 groups (n=50-60/group), namely Group 1 (saline), Group 2 (saline+varIBDV), Group 3 (HVT-IBDV), Group 4 (HVT-IBDV+varIBDV), Group 5 (MdLV) and Group 6 (MdLV+varIBDV). HVT-IBDV vaccination was given via the in ovo route to 18-day-old embryonated eggs. MdLV was administered via the subcutaneous route in day-old broilers. Group 2, Group 4 and Group 6 were orally challenged with varIBDV (SK-09, 3×10³ EID₅₀) at day 6 post-hatch. IBDV seroconversion, bursal weight to body weight ratio (BBW) and bursal histopathology were assessed at 19 and 35days of age. Histopathological examination at day 19 revealed that varIBDV-SK09 challenge caused severe bursal atrophy and lower BBW in HVT-IBDV but not in MdLV vaccinated chicks. However by day 35, all challenged groups showed bursal atrophy and seroconversion. Interestingly, RT-qPCR analysis after varIBDV-SK09 challenge demonstrated an early (9days of age) and significantly high viral load (∼5744 folds) in HVT-IBDV vaccinated group vs unvaccinated challenged group (∼2.25 folds). Furthermore, flow cytometry analysis revealed inhibition of cytotoxic CD8⁺ T-cell response (CD44-downregulation) and decreased splenic lymphocytes counts in chicks after HVT-IBDV vaccination. Overall, our data suggest that MdLV delays varIBDV pathogenesis, whereas, HVT-IBDV vaccine is potentially immunosuppressive, which may increase the risk of early age varIBDV infection in broilers.