Avian infectious laryngotracheitis

Real-time PCR quantification of infectious laryngotracheitis virus in chicken tissues, faeces, isolator-dust and bedding material over 28 days following infection reveals high levels in faeces and dust

Infectious laryngotracheitis (ILT) is an important disease of chickens caused by ILT virus (ILTV). We used the Australian SA2 and A20 vaccine strains of ILTV to determine tissue distribution and excretion characteristics of ILTV in specific-pathogen-free chickens and to determine whether ILTV is readily detectable in environmental samples such as faeces, bedding material and dust using real-time quantitative PCR. Three groups of 10 freshly hatched chicks were placed in isolators and infected orally with high doses of the two strains of vaccine virus or left unchallenged as controls. Over a 28-day post-infection (p.i.) period, faecal and serum samples were collected at frequent intervals from six individually identified chickens in each group. Dust and litter samples from the isolators were collected less frequently. Tissue samples were collected from three to four sacrificed or dead/euthanized birds at 6, 14 and 28 days p.i. Infection resulted in clinical ILT, a pronounced antibody response and sustained qPCR detection of the viral genome in the trachea, Harderian gland, lung and kidney up to 28 days p.i. A high level of the viral genome was also detected in faeces between 2 and 7 days p.i., declining by about approximately four orders of magnitude to low, but detectable, levels at 21 and 28 days p.i. The finding of high-level shedding of ILTV in faeces warrants further investigation into the epidemiological role of this, and the sustained high levels of ILTV observed in dust suggest that it may be a useful sample material for monitoring ILTV status in flocks.

Persistence of the tissue culture origin vaccine for infectious laryngotracheitis virus in commercial chicken flocks in Brazil

Infectious laryngotracheitis (ILT) is a respiratory disease of great importance that causes serious economic losses in the poultry industry. Its control is based on biosecurity procedures and vaccination programs that use live attenuated vaccines such as tissue culture origin (TCO), chicken embryo origin (CEO), and vectored vaccines. However, problems have been reported, such as the reversion of virulence, virus latency, and field virus outbreaks. Several molecular techniques have been developed to differentiate between the field and vaccine strains. This study was conducted to determine the presence of infectious laryngotracheitis virus (ILTV) in Brazil from 2012 to 2014. PCR-RFLP (restriction fragment length polymorphism) was used to detect and differentiate ILTV strains; DNA sequencing and predictive RFLP analysis were also used for this purpose. Molecular analysis detected the presence of ILTV in 15 samples that were characterized as strains of TCO vaccine origin. This study showed that the ILTV TCO vaccine strain has been circulating in commercial chicken flocks in Brazil since its introduction during the 2002 outbreak.

Herpesvirus-like respiratory infection in African penguins Spheniscus demersus admitted to a rehabilitation centre

Rehabilitation is an important strategy for the conservation of the Endangered African penguin Spheniscus demersus, and disease has been raised as a concern in the management of the species, both in the wild and in rehabilitation centres. We report 8 cases of herpesvirus-like respiratory infection in African penguin chicks undergoing rehabilitation between 2010 and 2013 at a facility in Cape Town, South Africa. Infection was confirmed through the identification of viral inclusions in the tracheal epithelium and demonstration of particles consistent with herpesvirus by electron microscopy, whereas virus isolation in eggs, serology and PCR testing failed to detect the virus. Only penguin chicks were affected; they were in poor body condition, and in 2 cases infection occurred prior to admission to the rehabilitation centre. The role played by the herpesvirus-like infection in the overall respiratory disease syndrome is uncertain, due to identification of lesions in only a small proportion of the chicks as well as to the occurrence of other concurrent pathological processes. Further studies are advised to characterise the specific virus involved through the development of sensitive diagnostic methods and to clarify the epidemiology and significance of these infections in wild African penguins.

Protection Induced in Broiler Chickens following Drinking-Water Delivery of Live Infectious Laryngotracheitis Vaccines against Subsequent Challenge with Recombinant Field Virus

Infectious laryngotracheitis virus (ILTV) causes acute upper respiratory tract disease in chickens. Attenuated live ILTV vaccines are often used to help control disease, but these vaccines have well documented limitations, including retention of residual virulence, incomplete protection, transmission of vaccine virus to unvaccinated birds and reversion to high levels of virulence following bird-to-bird passage. Recently, two novel ILTV field strains (class 8 and 9 ILTV viruses) emerged in Australia due to natural recombination between two genotypically distinct commercial ILTV vaccines. These recombinant field strains became dominant field strains in important poultry producing areas. In Victoria, Australia, the recombinant class 9 virus largely displaced the previously predominant class 2 ILTV strain. The ability of ILTV vaccines to protect against challenge with the novel class 9 ILTV strain has not been studied. Here, the protection induced by direct (drinking-water) and indirect (contact) exposure to four different ILTV vaccines against challenge with class 9 ILTV in commercial broilers was studied. The vaccines significantly reduced, but did not prevent, challenge virus replication in vaccinated chickens. Only one vaccine significantly reduced the severity of tracheal pathology after direct drinking-water vaccination. The results indicate that the current vaccines can be used to help control class 9 ILTV, but also indicate that these vaccines have limitations that should be considered when designing and implementing disease control programs.

Multiple Comparison Analysis of Two New Genomic Sequences of ILTV Strains from China with Other Strains from Different Geographic Regions

To date, twenty complete genome sequences of ILTV strains have been published in GenBank, including one strain from China, and nineteen strains from Australian and the United States. To investigate the genomic information on ILTVs from different geographic regions, two additional individual complete genome sequences of WG and K317 strains from China were determined. The genomes of WG and K317 strains were 153,505 and 153,639 bp in length, respectively. Alignments performed on the amino acid sequences of the twelve glycoproteins showed that 13 out of 116 mutational sites were present only among the Chinese strain WG and the Australian strains SA2 and A20. The phylogenetic tree analysis suggested that the WG strain established close relationships with the Australian strain SA2. The recombination events were detected and confirmed in different subregions of the WG strain with the sequences of SA2 and K317 strains as parental. In this study, two new complete genome sequences of Chinese ILTV strains were used in comparative analysis with other complete genome sequences of ILTV strains from China, the United States, and Australia. The analysis of genome comparison, phylogenetic trees, and recombination events showed close relationships among the Chinese strain WG and the Australian strains SA2. The information of the two new complete genome sequences from China will help to facilitate the analysis of phylogenetic relationships and the molecular differences among ILTV strains from different geographic regions.

Methylome Analysis in Chickens Immunized with Infectious Laryngotracheitis Vaccine

In this study we investigated the methylome of chickens immunized with Infectious laryngotracheitis (ILT) vaccine derived from chicken embryos. Methyl-CpG binding domain protein-enriched genome sequencing (MBD-Seq) method was employed in the detection of the 1,155 differentially methylated regions (DMRs) across the entire genome. After validation, we ascertained the genomic DMRs distribution and annotated them regarding genes, transcription start sites (TSS) and CpG islands. We found that global DNA methylation decreased in vaccinated birds, presenting 704 hypomethylated and 451 hypermethylated DMRs, respectively. Additionally, we performed an enrichment analysis detecting gene networks, in which cancer and RNA post-transcriptional modification appeared in the first place, followed by humoral immune response, immunological disease and inflammatory disease. The top four identified canonical pathways were EIF2 signaling, regulation of EIF4 and p70S6K signaling, axonal guidance signaling and mTOR signaling, providing new insight regarding the mechanisms of ILT etiology. Lastly, the association between DNA methylation and differentially expressed genes was examined, and detected negative correlation in seventeen of the eighteen genes.

In ovo delivery of CpG DNA reduces avian infectious laryngotracheitis virus induced mortality and morbidity

Endosomal toll-like receptor-21 and -9 sense CpG DNA activating production of pro-inflammatory mediators with antimicrobial effects. Here, we investigated the induction of antiviral response of in ovo delivered CpG DNA against infectious laryngotracheitis virus (ILTV) infection. We found that in ovo delivered CpG DNA significantly reduces ILTV infection pre-hatch correlating with the expression of IL-1β and increase of macrophages in lungs. As assessed in vitro, CpG DNA stimulated avian macrophages could be a potential source of IL-1β and other pro-inflammatory mediators. Since we also found that in ovo CpG DNA delivery maintains increased macrophages in the lungs post-hatch, we infected the chickens on the day of hatch with ILTV. We found that in ovo delivered CpG DNA significantly reduces mortality and morbidity resulting from ILTV infection encountered post-hatch. Thus, CpG DNA can be a candidate innate immune stimulant worthy of further investigation for the control of ILTV infection in chickens.

Glycoprotein-based enzyme-linked immunosorbent assays for serodiagnosis of infectious laryngotracheitis

For detection of infectious laryngotracheitis virus (ILTV) antibody, glycoprotein B-, C-, and D-based enzyme-linked immunosorbent assays (B-, C-, and D-ELISAs, respectively) were developed. The B- and D-ELISAs showed enhanced detection of anti-ILTV antibodies in infected chickens compared to that of the commercial ELISA. Furthermore, the D-ELISA was efficient in detecting seroconversion with vectored vaccine, using recombinant Newcastle disease virus (rNDV) expressing glycoprotein D (gD) as the vaccine vector.

Cross-protective immune responses between genotypically distinct lineages of infectious laryngotracheitis viruses

Recent phylogenetic studies have identified different genotypic lineages of infectious laryngotracheitis virus (ILTV), and these lineages can recombine in the field. The emergence of virulent recombinant field strains of ILTV by natural recombination between commercial vaccines belonging to different genotypic lineages has been reported recently. Despite the use of attenuated ILTV vaccines, these recombinant viruses were able to spread and cause disease in commercial poultry flocks, raising the question of whether the different lineages of ILTV can induce cross-protective immune responses. This study examined the capacity of the Australian-origin A20 ILTV vaccine to protect against challenge with the class 8 ILTV recombinant virus, the genome of which is predominantly derived from a heterologous genotypic lineage. Following challenge, birds vaccinated via eyedrop were protected from clinical signs of disease and pathological changes in the tracheal mucosa, although they were not completely protected from viral infection or replication. In contrast, the challenge virus induced severe clinical signs and tracheal pathology in unvaccinated birds. This is the first study to examine the ability of a vaccine from the Australian lineage to protect against challenge with a virus from a heterologous lineage. These results suggest that the two distinct genotypic lineages of ILTV can both induce cross-protection, indicating that current commercial vaccines are still likely to assist in control of ILTV in the poultry industry, in spite of the emergence of novel recombinants derived from different genotypic lineages.

A recombinant Newcastle disease virus (NDV) expressing infectious laryngotracheitis virus (ILTV) surface glycoprotein D protects against highly virulent ILTV and NDV challenges in chickens

Infectious laryngotracheitis (ILT) is a highly contagious acute respiratory disease of chickens caused by infectious laryngotracheitis virus (ILTV). Currently, modified live ILTV vaccines are used to control ILT infections. However, the live ILTV vaccines can revert to virulence after bird-to-bird passage and are capable of establishing latent infections, suggesting the need to develop safer vaccines against ILT. We have evaluated the role of three major ILTV surface glycoproteins, namely, gB, gC, and gD in protection and immunity against ILTV infection in chickens. Using reverse genetics approach, three recombinant Newcastle disease viruses (rNDVs) designated rNDV gB, rNDV gC, and rNDV gD were generated, each expressing gB, gC, and gD, respectively, of ILTV. Chickens received two immunizations with rNDVs alone (gB, gC, and gD) or in combination (gB+gC, gB+gD, gC+gD, and gB+gC+gD). Immunization with rNDV gD induced detectable levels of neutralizing antibodies with the magnitude of response greater than the rest of the experimental groups including those vaccinated with commercially available vaccines. The birds immunized with rNDV gD showed complete protection against virulent ILTV challenge. The birds immunized with rNDV gC alone or multivalent vaccines consisting of combination of rNDVs displayed partial protection with minimal disease and reduced replication of challenge virus in trachea. Immunization with rNDV gB neither reduced the severity of the disease nor the replication of challenge virus in trachea. The superior protective efficacy of rNDV gD vaccine compared to rNDV gB or rNDV gC vaccine was attributed to the higher levels of envelope incorporation and infected cell surface expression of gD than gB or gC. Our results suggest that rNDV expressing gD is a safe and effective bivalent vaccine against NDV and ILTV.

Molecular detection and characterization of infectious laryngotracheitis virus (Gallid herpesvirus-1) from clinical samples of commercial poultry flocks in India

Although the existence of infectious laryngotracheitis virus (ILTV) in India was first reported in 1964, no reports are available regarding its molecular detection and characterization. The present study was aimed to detect and characterize ILTV from recent respiratory disease complex (RDC) outbreaks of commercial poultry flocks in different parts of the country by using envelope glycoprotein G gene (US4 gene) based PCR and sequencing. A total of thirty two flocks with a history of RDC were investigated. Overall, all the strains/breeds of birds and all ages of birds are equally susceptible and depending on the severity, the clinical signs and gross lesions were varied. Out of 32 flocks investigated 10 were found positive for ILTV infection by PCR. The phylogenetic analyses of eight representative sequences in the present study deciphered that Indian ILT viruses are closely related to chicken embryo origin vaccine strains of Italy, USA, China and Brazil.

Detection and isolation of infectious laryngotracheitis virus on a broiler farm after a disease outbreak

A broiler farm in North Alabama suffered a mild infectious laryngotracheitis (ILT) outbreak, as determined by clinical disease and PCR. The poultry integrator sought help to control further outbreaks in subsequent flocks. Samples were collected from various areas of the poultry houses on the farm over an 8-wk period. The first sampling was conducted 8 days after the infected farm was depopulated; the second was conducted 2 days prior to subsequent flock placement; and the third was conducted when the new flock was 5 wk of age. Samples were examined for ILT virus (ILTV) DNA by real-time PCR and virus isolation in embryos. The infected houses were cleaned, disinfected, heated, litter composted, and curtains replaced after the first sampling and prior to placement of the next flock. Samples from all periods were positive for ILTV DNA. However, the number of positive samples and crossing point values indicated a decrease in the amount of viral DNA, while virus isolation in embryos was successful only on the first sampling. The subsequent flock was vaccinated against ILTV by in ovo route using a commercial recombinant vaccine. Cleaning and sanitation after the disease outbreak reduced the amount of ILTV on the farm and together with in ovo vaccination of the new flock may have prevented a recurrence of another ILT outbreak.

Molecular epidemiology of infectious laryngotracheitis: a review

Infectious laryngotracheitis (ILT) is an economically important respiratory disease of poultry that affects the poultry industry worldwide. The disease is caused by gallid herpesvirus I (GaHV-1), a member of the genus Iltovirus, family Herpesviridae, subfamily Alphaherpesvirinae. The current incidence of the disease is heavily influenced by live attenuated vaccines, which have been used extensively since their introduction in the mid-twentieth century. The capability of current live attenuated vaccine viruses to revert to virulence and spread from bird to bird has shaped the molecular epidemiology of ILT. Because of the antigenic homogeneity among GaHV-1 strains, differentiation of strains has been achieved by targeting genomic differences between outbreak-related isolates and vaccine strains. Numerous genes and genomic regions have been utilized in the development of DNA-based diagnostic assays to differentiate outbreak-related isolates from vaccine strains in countries where ILT outbreaks have occurred. More recently, full genome sequences have allowed determination of the origin of some of the outbreak-related isolates circulating in some poultry production countries. Overall, molecular typing data collected worldwide have identified live attenuated vaccine-related isolates as the primary source for outbreaks of the disease.

Complete genome sequence of the first Chinese virulent infectious laryngotracheitis virus

Infectious laryngotracheitis (ILT) is an acute respiratory disease caused by infectious laryngotracheitis virus (ILTV). The complete genome sequences of five attenuated ILTV vaccine strains and six virulent ILTV strains as well as two Australian ILTV field strains have been published in Australia and the USA so far. To provide the complete genome sequence information of ILTVs from different geographic regions, the whole genome of ILTV LJS09 isolated in China was sequenced. The genome of ILTV LJS09 was 153,201 bp in length, and contained 79 ORFs. Most of the ORFs had high sequence identity with homologous ORFs of reference strains. There was a large fragment deletion within the noncoding region of unique long region (UL) of ILTV LJS09 compared with SA2 and A20 strains. Though the origin binding protein of ILTV LJS09 existed, there was no AT-rich region in strain LJS09. Alignments of the amino acid sequences revealed seven mutations at amino acids 71 (Arg → Lys), 116 (Ala → Val), 207 (Thr → Ile) and 644 (Thr → Ile) on glycoprotein B, 155 (Phe → Ser) and 376 (Arg → His) on glycoprotein D and 8 (Gln→Pro) on glycoprotein L of ILTV LJS09 compared to those of virulent strain (USDA) as ILTV LJS09 did not grow on chicken embryo fibroblasts, suggesting the role of the key seven amino acids in determination of the cell tropism of ILTV LJS09. This is the first complete genome sequence of the virulent strain of ILTV in Asia using the conventional PCR method, which will help to facilitate the future molecular biological research of ILTVs.

Detection of infectious laryngotracheitis virus by real-time PCR in naturally and experimentally infected chickens

Infectious laryngotracheitis (ILT) is an acute, highly contagious upper-respiratory infectious disease of chickens. In this study, a real-time PCR method was developed for fast and accurate detection and quantitation of ILTV DNA of chickens experimentally infected with ILTV strain LJS09 and naturally infected chickens. The detection lower limit of the assay was 10 copies of DNA. There were no cross reactions with the DNA and RNA of infectious bursal disease virus, chicken anemia virus, reticuloendotheliosis virus, avian reovirus, Newcastle disease virus, and Marek's disease virus. The real-time PCR was reproducible as the coefficients of variation of reproducibility of the intra-assay and the inter-assay were less than 2%. The real-time PCR was used to detect the levels of the ILTV DNA in the tissues of specific pathogen free (SPF) chickens infected with ILTV at different times post infection. ILTV DNA was detected by real-time PCR in the heart, liver, spleen, lung, kidney, larynx, tongue, thymus, glandular stomach, duodenum, pancreatic gland, small intestine, large intestine, cecum, cecal tonsil, bursa of Fabricius, and brain of chickens in the infection group and the contact-exposure group. The sensitivity, specificity, and reproducibility of the ILTV real-time PCR assay revealed its suitability for detection and quantitation of ILTV in the samples from clinically and experimentally ILTV infected chickens.

Outbreak of infectious laryngotracheitis in large multi-age egg layer chicken flocks in Minas Gerais, Brazil

A recent (November 2010) outbreak of infectious laryngotracheitis (ILT) in a multi-age laying hen facility in Minas Gerais state, Brazil, is described. Previous ILT outbreak in laying hens was only notified in São Paulo state, Brazil, in 2002. In the outbreak described here, the affected population was approximately eight million hens, with flock sizes ranging from 100,000 to 2,900,000 chickens. The average mortality ranged from 1 to 6%, and morbidity was around 90% (most of the twenty seven farms of the area were positive for ILT virus). Three multi-age laying farms from one company were selected for this report. Clinical signs included prostration, dyspnea, conjunctivitis, occasional swelling of the paranasal sinuses and bloody mucous nasal discharge. Severely affected chickens presented with dyspnea, gasping and became cyanotic before death. At necropsy, these chickens had fibrinous exudate blocking the larynx and the lumen of cranial part of the trachea. In addition, conjunctivitis with intense hyperemia, edema and sinuses with caseous exudate were present. On histopathology, there were marked necrosis and desquamation of respiratory ephitelium and conjunctiva with numerous syncytial cells formation and fibrinous exudate. Moderate to marked non suppurative (especially lymphocytes and plasma cells) infiltration in the lamina propria also was observed. Sixteen out of 20 examined chickens, eosinophilic intranuclear inclusion bodies were observed in the syncytial cells. The DNA extracted from larynx and trachea produced positive PCR results for ILT virus (ILTV) DNA using formalin-fixed, paraffin embedded (FFPE) samples. Amplicons from a small region of ICP4 gene were submitted to sequencing and showed 100% identity with ILTV EU104910.1 (USA strain), 99% with ILTV JN596963.1 (Australian strain) and 91% with ILTV JN580316.1 (Gallid herpesvirus 1 CEO vaccine strain) and JN580315.1 (Gallid herpesvirus 1 TCO vaccine strain).

Phylogenetic and molecular epidemiological studies reveal evidence of multiple past recombination events between infectious laryngotracheitis viruses

In contrast to the RNA viruses, the genome of large DNA viruses such as herpesviruses have been considered to be relatively stable. Intra-specific recombination has been proposed as an important, but underestimated, driving force in herpesvirus evolution. Recently, two distinct field strains of infectious laryngotracheitis virus (ILTV) have been shown to have arisen from independent recombination events between different commercial ILTV vaccines. In this study we sequenced the genomes of additional ILTV strains and also utilized other recently updated complete genome sequences of ILTV to confirm the existence of a number of ILTV recombinants in nature. Multiple recombination events were detected in the unique long and repeat regions of the genome, but not in the unique short region. Most recombinants contained a pair of crossover points between two distinct lineages of ILTV, corresponding to the European origin and the Australian origin vaccine strains of ILTV. These results suggest that there are two distinct genotypic lineages of ILTV and that these commonly recombine in the field.

Infectious laryngotracheitis virus in chickens

Infectious laryngotracheitis (ILT) is an important respiratory disease of chickens and annually causes significant economic losses in the poultry industry world-wide. ILT virus (ILTV) belongs to alphaherpesvirinae and the Gallid herpesvirus 1 species. The transmission of ILTV is via respiratory and ocular routes. Clinical and post-mortem signs of ILT can be separated into two forms according to its virulence. The characteristic of the severe form is bloody mucus in the trachea with high mortality. The mild form causes nasal discharge, conjunctivitis, and reduced weight gain and egg production. Conventional polymerase chain reaction (PCR), nested PCR, real-time PCR, and loop-mediated isothermal amplification were developed to detect ILTV samples from natural or experimentally infected birds. The PCR combined with restriction fragment length polymorphism (RFLP) can separate ILTVs into several genetic groups. These groups can separate vaccine from wild type field viruses. Vaccination is a common method to prevent ILT. However, field isolates and vaccine viruses can establish latent infected carriers. According to PCR-RFLP results, virulent field ILTVs can be derived from modified-live vaccines. Therefore, modified-live vaccine reversion provides a source for ILT outbreaks on chicken farms. Two recently licensed commercial recombinant ILT vaccines are also in use. Other recombinant and gene-deficient vaccine candidates are in the developmental stages. They offer additional hope for the control of this disease. However, in ILT endemic regions, improved biosecurity and management practices are critical for improved ILT control.

Genome-wide host responses against infectious laryngotracheitis virus vaccine infection in chicken embryo lung cells

Background

Infectious laryngotracheitis virus (ILTV; gallid herpesvirus 1) infection causes high mortality and huge economic losses in the poultry industry. To protect chickens against ILTV infection, chicken-embryo origin (CEO) and tissue-culture origin (TCO) vaccines have been used. However, the transmission of vaccine ILTV from vaccinated- to unvaccinated chickens can cause severe respiratory disease. Previously, host cell responses against virulent ILTV infections were determined by microarray analysis. In this study, a microarray analysis was performed to understand host-vaccine ILTV interactions at the host gene transcription level.

Results

The 44 K chicken oligo microarrays were used, and the results were compared to those found in virulent ILTV infection. Total RNAs extracted from vaccine ILTV infected chicken embryo lung cells at 1, 2, 3 and 4 days post infection (dpi), compared to 0 dpi, were subjected to microarray assay using the two color hybridization method. Data analysis using JMP Genomics 5.0 and the Ingenuity Pathway Analysis (IPA) program showed that 213 differentially expressed genes could be grouped into a number of functional categories including tissue development, cellular growth and proliferation, cellular movement, and inflammatory responses. Moreover, 10 possible gene networks were created by the IPA program to show intermolecular connections. Interestingly, of 213 differentially expressed genes, BMP2, C8orf79, F10, and NPY were expressed distinctly in vaccine ILTV infection when compared to virulent ILTV infection.

Conclusions

Comprehensive knowledge of gene expression and biological functionalities of host factors during vaccine ILTV infection can provide insight into host cellular defense mechanisms compared to those of virulent ILTV.

Glycoprotein J of infectious laryngotracheitis virus is required for efficient egress of infectious virions from cells

Glycoprotein J (gJ) of infectious laryngotracheitis virus (ILTV) represents a major viral antigen and is dispensable for replication in cell culture and chickens. We generated gJ deletion mutants derived from the United States Department of Agriculture standard challenge strain (USDA-ch), a GFP-expressing mutant GΔgJ, a gJ deletion mutant void of any foreign DNA insertion (BΔgJ) and a gJ rescue mutant gJR with US5 restored. GΔgJ, BΔgJ and gJR were characterized in cell culture and embryonated eggs. Entry kinetic assays showed that the gJ deletion mutants did not differ in their entry kinetics from gJR. Replication kinetics strongly indicated that gJ plays an important role during egress of the virus. Differences in the abilities of the mutants to replicate in chorioallantoic membranes of chicken embryos and to release infectious virus into the allantoic fluid supported a function of gJ during the egress of ILTV from infected cells.

Comparative analysis of the complete genome sequences of two Australian origin live attenuated vaccines of infectious laryngotracheitis virus

Infectious laryngotracheitis virus (ILTV) is an alphaherpesvirus that causes acute respiratory disease in poultry. Live attenuated ILTV vaccines have been used extensively to help control outbreaks of disease. Two Australian-origin attenuated vaccine strains, SA2 and A20 ILTV, are commercially available and are in frequent use in Australia. Both these vaccines are of chicken embryo origin (CEO). The A20 ILTV strain was developed from the SA2 ILTV strain by sequential passage of SA2 ILTV in tissue culture in order to reduce its residual virulence. Previous studies in our laboratories have demonstrated the greater attenuation of A20 ILTV under controlled experimental conditions, but the genetic basis of the in vivo phenotypes of A20 and SA2 ILTV has not been elucidated. In this study, the genetic differences between A20 and SA2 ILTV were examined by performing complete genome sequencing and comparative analysis. The genome sequences were also compared to a reference sequence from another CEO ILTV vaccine (Serva ILTV: GenBank accession number HQ_630064) of European-origin. Additional in ovo studies to assess cell to cell spread were performed in order to allow further comparisons of the pathogenicity of SA2 and A20 ILTV. The sequencing results showed that the genome sizes of SA2 and A20 ILTV were 152,975 and 152,978bp, respectively, while Serva ILTV had a genome size of 152,630bp. The genomes of SA2 and A20 ILTV shared 99.9% nucleotide sequence identity with each other, but only 99.2% identity with Serva ILTV. In complete genome alignments between SA2 and A20 ILTV, a total of 24 single nucleotide polymorphisms (SNPs) were identified, but only two of these were non-synonymous. These were located in the ORF B and UL15 genes. Four indels were detected in non-coding regions. The findings from this study demonstrate the general genetic stability of ILTV, but also show that non-synonymous changes in the ORF B and UL15 genes have arisen following tissue culture passage of SA2 ILTV to produce the A20 vaccine. It is likely that these non-synonymous changes are related to the greater attenuation of A20 ILTV compared to SA2 ILTV, and to the reduced ability of A20 ILTV to spread from cell to cell, as observed in this study. The results from this study also demonstrate the divergence between the genomes of the Australian-origin ILTV vaccine strains and the Serva vaccine strain.

First complete genome sequence of infectious laryngotracheitis virus

BACKGROUND

Infectious laryngotracheitis virus (ILTV) is an alphaherpesvirus that causes acute respiratory disease in chickens worldwide. To date, only one complete genomic sequence of ILTV has been reported. This sequence was generated by concatenating partial sequences from six different ILTV strains. Thus, the full genomic sequence of a single (individual) strain of ILTV has not been determined previously. This study aimed to use high throughput sequencing technology to determine the complete genomic sequence of a live attenuated vaccine strain of ILTV.

RESULTS

The complete genomic sequence of the Serva vaccine strain of ILTV was determined, annotated and compared to the concatenated ILTV reference sequence. The genome size of the Serva strain was 152,628 bp, with a G + C content of 48%. A total of 80 predicted open reading frames were identified. The Serva strain had 96.5% DNA sequence identity with the concatenated ILTV sequence. Notably, the concatenated ILTV sequence was found to lack four large regions of sequence, including 528 bp and 594 bp of sequence in the UL29 and UL36 genes, respectively, and two copies of a 1,563 bp sequence in the repeat regions. Considerable differences in the size of the predicted translation products of 4 other genes (UL54, UL30, UL37 and UL38) were also identified. More than 530 single-nucleotide polymorphisms (SNPs) were identified. Most SNPs were located within three genomic regions, corresponding to sequence from the SA-2 ILTV vaccine strain in the concatenated ILTV sequence.

CONCLUSIONS

This is the first complete genomic sequence of an individual ILTV strain. This sequence will facilitate future comparative genomic studies of ILTV by providing an appropriate reference sequence for the sequence analysis of other ILTV strains.

A microRNA of infectious laryngotracheitis virus can downregulate and direct cleavage of ICP4 mRNA

Viral microRNAs regulate gene expression using either translational repression or mRNA cleavage and decay. Two microRNAs from infectious laryngotracheitis virus (ILTV), iltv-miR-I5 and iltv-miR-I6, map antisense to the ICP4 gene. Post-transcriptional repression by these microRNAs was tested against a portion of the ICP4 coding sequence cloned downstream of firefly luciferase. Luciferase activity was downregulated by approximately 60% with the iltv-miR-I5 mimic. Addition of an iltv-miR-I5 antagomiR or mutagenesis of the target seed sequence alleviated this effect. The iltv-miR-I5 mimic, when co-transfected with a plasmid expressing ICP4, reduced ICP4 transcript levels by approximately 50%, and inhibition was relieved by an iltv-miR-I5 antagomiR. In infected cells, iltv-miR-I5 mediated cleavage at the canonical site, as indicated by modified RACE analysis. Thus, in this system, iltv-miR-I5 decreased ILTV ICP4 mRNA levels via transcript cleavage and degradation. Downregulation of ICP4 could impact the balance between the lytic and latent states of the virus in vivo.

MicroRNAs of Gallid and Meleagrid herpesviruses show generally conserved genomic locations and are virus-specific

Many herpesviruses, including Marek's disease viruses (MDV1 and MDV2), encode microRNAs. In this study, we report microRNAs of two related herpesviruses, infectious laryngotracheitis virus (ILTV) and herpesvirus of turkeys (HVT), as well as additional MDV2 microRNAs. The genome locations, but not microRNA sequences, are conserved among all four of these avian herpesviruses. Most are clustered in the repeats flanking the unique long region (I/TR(L)), except in ILTV which lacks these repeats. Two abundant ILTV microRNAs are antisense to the immediate early gene ICP4. A homologue of host microRNA, gga-miR-221, was found among the HVT microRNAs. Additionally, a cluster of HVT microRNAs was found in a region containing two locally duplicated segments, resulting in paralogous HVT microRNAs with 96-100% identity. The prevalence of microRNAs in the genomic repeat regions as well as in local repeats suggests the importance of genetic plasticity in herpesviruses for microRNA evolution and preservation of function.

Glycoprotein G deficient infectious laryngotracheitis virus is a candidate attenuated vaccine

Infectious laryngotracheitis virus (ILTV), an alphaherpesvirus, causes respiratory disease in chickens and is currently controlled by vaccination with conventionally attenuated virus strains. These vaccines have limitations because of residual pathogenicity and reversion to virulence, suggesting that a novel vaccine strain that lacks virulence gene(s) may enhance disease control. Glycoprotein G (gG) has recently been identified as a virulence factor in ILTV. In this study the immunogenicity and relative pathogenicity of gG deficient ILTV was investigated in SPF chickens. Birds vaccinated with gG deficient ILTV were protected against clinical signs of disease following challenge with virulent ILTV and gG deficient ILTV was also shown to be less pathogenic than currently available commercial vaccine strains. Thus gG deficient ILTV appears to have potential as a vaccine candidate.

Psittacid herpesvirus 1 and infectious laryngotracheitis virus: Comparative genome sequence analysis of two avian alphaherpesviruses

Psittacid herpesvirus 1 (PsHV-1) is the causative agent of Pacheco's disease, an acute, highly contagious, and potentially lethal respiratory herpesvirus infection in psittacine birds, while infectious laryngotracheitis virus (ILTV) is a highly contagious and economically significant avian herpesvirus which is responsible for an acute respiratory disease limited to galliform birds. The complete genome sequence of PsHV-1 has been determined and compared to the ILTV sequence, assembled from published data. The PsHV-1 and ILTV genomes exhibit similar structural characteristics and are 163,025 bp and 148,665 bp in length, respectively. The PsHV-1 genome contains 73 predicted open reading frames (ORFs), while the ILTV genome contains 77 predicted ORFs. Both genomes contain an inversion in the unique long region similar to that observed in pseudorabies virus. PsHV-1 is closely related to ILTV, and it is proposed that it be assigned to the Iltovirus genus. These two avian herpesviruses represent a phylogenetically unique clade of alphaherpesviruses that are distinct from the Marek's disease-like viruses (Mardivirus). The determination of the complete genomic nucleotide sequences of PsHV-1 and ILTV provides a tool for further comparative and functional analysis of this unique class of avian alphaherpesviruses.

Rapid detection and characterization from field cases of infectious laryngotracheitis virus by real-time polymerase chain reaction and restriction fragment length polymorphism

A real-time polymerase chain reaction (PCR) assay was developed to specifically amplify infectious laryngotracheitis virus (ILTV) DNA from field samples. The 222-base-pair PCR fragment was amplified using primers located in a conserved region of the infected cell protein 4 gene that was demonstrated in this work to encompass a single nucleotide polymorphism. Subsequent restriction fragment length polymorphism (RFLP) analysis of real-time PCR amplified fragments from a range of ILTV isolates using the restriction endonuclease MspI enabled differentiation between older ILTV isolates that were prevalent in the 1960s prior to the availability of vaccine strains and more recent isolates that predominantly are identical to vaccine strains. The assay, using real-time PCR, RFLP and sequence analysis, was used to characterize two recent field cases of infectious laryngotracheitis from Northern Ireland. One of the field cases was demonstrated to be similar to older "wild-type" isolates, while the other field case was identified to have a concurrent ILTV infection of both "wild-type" and vaccinal type origin. The assay described here using real-time PCR and RFLP provides a rapid, specific method that enables detection and characterization of ILTV directly from field cases.

Persistence of arboviruses and antiviral antibodies in vertebrate hosts: its occurrence and impacts

The recent isolation of West Nile virus from a bird in mid-winter in New York immediately raised, as one of a few explanations, the possibility of long-term persistence of arboviruses in vertebrate hosts. Although it was a highly popular topic for research many years ago, generally it has since been neglected and its meaning under appreciated. This comprehensive survey of literature worldwide uncovered, contrary to the general perception that it is a rather infrequent phenomenon, a large number of important observations involving all groups of arboviruses that have been accumulating over the years without drawing much attention. In this review, the data and observations were analysed in terms of the occurrence, role in natural transmission, mechanisms and genesis of persistence, source of problems in research and impact. The outcome of the analyses clearly demonstrates that asymptomatic, long-term infection in the absence of viraemia with or without the induction of neutralising antibody, the most frequent characteristics of arboviral persistence, presents a serious question about the validity of some of the past animal experiments that were conducted without the consideration of such a possibility. Likewise, significant impacts are felt on diverse fields ranging from epidemiology to diagnostic virology and from veterinary medicine to agricultural commerce. Published in 2001 by John Wiley & Sons, Ltd.