Avian infectious laryngotracheitis: Virus‐host interactions in relation to prospects for eradication

Detection and molecular characterization of infectious laryngotracheitis virus (ILTV) in chicken with respiratory signs in Brazil during 2015 and 2016

Avian infectious laryngotracheitis (ILT) is a respiratory disease that causes severe economic losses in the poultry industry, mainly due to high morbidity and mortality and reduced egg production. Molecular characterization was performed on samples collected from flocks in the Brazilian States of São Paulo, Pernambuco, and Minas Gerais during 2015 and 2016 that presented clinical signs of respiratory disease. End-point PCR was used for viral detection, and DNA sequencing was used for differentiation of vaccine and field strains. Molecular analysis based on the infected cell protein (ICP4) gene separated four of the nine samples together with previous Brazilian isolates (São Paulo and Minas Gerais), one sample was grouped on the same branch as Minas Gerais strains (along with another related sample), one sample was separately branched but still related to the tissue culture origin (TCO) vaccine strain, and two samples were grouped on the same branch as the TCO vaccine strain. Molecular analysis of the thymidine kinase (TK) gene showed the existence of strains of both high and low virulence. The characterization of two fragments of the ICP4 gene and a fragment of the TK gene in this study suggested that the virus circulating in Guatapará, as well as those in Barretos and Itanhandu, that is causing respiratory problems in birds is a highly virulent field strain. The clinical signs point to a TCO vaccine strain that most likely underwent some reversal event and is a latent reactivated infection.

Molecular and genetic detection of infectious laryngeotrachitis disease virus in broiler farms after a disease outbreak in Egypt

Infectious laryngotracheitis (ILT) is a viral respiratory illness in poultry that causes massive financial losses. This research aimed to isolate and identify the ILT virus in suspected outbreaks of broiler flocks in Egypt during 2020-2021, besides investigating its genetic link with other circulating strains. Real-time-PCR was used to test 57 samples taken from unvaccinated broiler farms. Ten samples are positive for ILTV, and the virus is being isolated in SPF chicken embryos. The Sanger sequencing was used to conduct (partial) sequencing of the infected cell protein4 gene (ICP4) for eight isolates. Phylogenetic analysis conducted Maximum Likelihood, comparative sequencing analysis of ICP4 of strains under study with vaccination ILT reference strains reveled that all isolates were clustered into two major groups. The (OM291843and OM291846) clustered together with the chicken embryo origin vaccine strains (IV and V group). The remaining six strains belong to the TCO vaccine(I, II and III group). The total sequence similarity between the strains under study and the various Egyptian strains varied from (97 to 100%) while the similarity with TCO or chicken embryo origin -vaccine strains ranged from (95to 100%). There were no deletions detected in the 272-283-bp region of the ICP4 gene. Detection of arginine to methionine substitutions at position 180 (R180M) and change of Serine to Asparagine at position 227 (S227N) in the (OM291843 and OM291846) which were previously described in chicken embryo origin -vaccine strains. This reveals that field strains may have evolved from vaccine strains, notably identification of non-synonymous substitutions which might be linked to the virulence strains' attenuation. Finally, independent of geographical distribution, both chicken embryo origin-vaccine-like and TCO-Vaccine-like virus strains were circulating in Egyptian non-vaccinated broiler flocks in 2020 and 2021. Despite their genetic differences, both viruses caused significant illnesses in the field.

Host Responses Following Infection with Canadian-Origin Wildtype and Vaccine Revertant Infectious Laryngotracheitis Virus

Infectious laryngotracheitis (ILT) is caused by Gallid herpesvirus-1 (GaHV-1) or infectious laryngotracheitis virus (ILTV) and was first described in Canadian poultry flocks. In Canada, ILTV infection is endemic in backyard flocks, and commercial poultry encounters ILT outbreaks sporadically. A common practice to control ILT is the use of live attenuated vaccines. However, outbreaks still occur in poultry flocks globally due to ILTV vaccine strains reverting to virulence and emergence of new ILTV strains due to recombination in addition to circulating wildtype strains. Recent studies reported that most of the ILT outbreaks in Canada were induced by the chicken-embryo-origin (CEO) live attenuated vaccine revertant strains with the involvement of a small percentage of wildtype ILTV. It is not known if the host responses induced by these two ILTV strains are different. The objective of the study was to compare the host responses elicited by CEO revertant and wildtype ILTV strains in chickens. We infected 3-week-old specific pathogen-free chickens with the two types of ILTV isolates and subsequently evaluated the severity of clinical and pathological manifestations, in addition to host responses. We observed that both of the isolates show high pathogenicity by inducing several clinical and pathological manifestations. A significant recruitment of immune cells at both 3 and 7 days post-infection (dpi) was observed in the tracheal mucosa and the lung tissues of the infected chickens with wildtype and CEO vaccine revertant ILTV isolates when compared to uninfected controls. Overall, this study provides a better understanding of the mechanism of host responses against ILTV infection.

Detection of Laryngotracheitis Virus in Poultry Flocks with Respiratory Disorders in Slovenia

Infectious laryngotracheitis (ILT) is an acute, highly contagious infectious disease of the upper respiratory tract in chickens and other poultry species that causes significant economic losses in countries worldwide. Between 2017 and 2019, seven outbreaks of mild to severe respiratory disorders with high suspicion of ILT occurred in commercial and backyard poultry flocks in Slovenia. In all submissions, infection with ILT virus (ILTV) was confirmed by PCR, which is the first report of ILT in Slovenia. Circulating ILT strains were characterized by the sequence and phylogenetic analysis of two fragments of the ICP4 gene. Four strains—three detected in non-vaccinated flocks and one in a flock vaccinated against ILT—were identical or very similar to the chicken embryo–origin live virus vaccines, and the other three were closely related to Russian, Chinese, Australian, and American field strains and to tissue culture origin vaccine strains. As in other diseases, coinfections with other respiratory pathogens in confirmed ILT cases may cause a more severe condition and prolong the course of the disease. In our study, coinfections with Mycoplasma synoviae (7/7 tested flocks), infectious bronchitis virus (5/5 tested flocks), Mycoplasma gallisepticum (4/7 tested flocks), Ornithobacterium rhinotracheale (3/4 tested flocks), and avian pox virus (1/2 tested flocks) were confirmed, indicating the importance of these pathogens in the occurrence of ILT infections.

A Case of Infectious Laryngotracheitis in an Organic Broiler Chicken Farm in Greece

Infectious laryngotracheitis is an economically significant viral disease of chickens, that mainly affects the upper respiratory tract, and is present worldwide. This case reports the first outbreak of infectious laryngotracheitis in a four-week-old organic broiler farm and surrounding flocks in Greece, with typical clinical symptoms and lesions, allegedly provoked by a wild strain of infectious laryngotracheitis virus. Our findings contradict the general perception indicating that the disease appears mainly in older birds and that vaccine strains are the primary cause of infectious laryngotracheitis outbreaks in most continents. A recombinant vectored vaccine was administered, supplementary to biosecurity measures, containing the viral spread. The responsible strain was potentially circulating in the area; therefore, an industry-wide holistic approach was applied, including the vaccination of neighboring broilers and breeders with the same vaccine, the rapid molecular diagnosis of the disease, and strict biosecurity protocols. The results of this holistic effort were effective because, following the application of vaccine and management protocols, manifestations of the disease in regional flocks dropped significantly, and there was no recurrence to date. These findings suggest that vaccination protocols should be modified, especially for organic broilers, to include vaccination against infectious laryngotracheitis.

Infectious laryngotracheitis: Etiology, epidemiology, pathobiology, and advances in diagnosis and control - a comprehensive review

Infectious laryngotracheitis (ILT) is a highly contagious upper respiratory tract disease of chicken caused by a Gallid herpesvirus 1 (GaHV-1) belonging to the genus Iltovirus, and subfamily Alphaherpesvirinae within Herpesviridae family. The disease is characterized by conjunctivitis, sinusitis, oculo-nasal discharge, respiratory distress, bloody mucus, swollen orbital sinuses, high morbidity, considerable mortality and decreased egg production. It is well established in highly dense poultry producing areas of the world due to characteristic latency and carrier status of the virus. Co-infections with other respiratory pathogens and environmental factors adversely affect the respiratory system and prolong the course of the disease. Latently infected chickens are the primary source of ILT virus (ILTV) outbreaks irrespective of vaccination. Apart from conventional diagnostic methods including isolation and identification of ILTV, serological detection, advanced biotechnological tools such as PCR, quantitative real-time PCR, next generation sequencing, and others are being used in accurate diagnosis and epidemiological studies of ILTV. Vaccination is followed with the use of conventional vaccines including modified live attenuated ILTV vaccines, and advanced recombinant vector vaccines expressing different ILTV glycoproteins, but still these candidates frequently fail to reduce challenge virus shedding. Some herbal components have proved to be beneficial in reducing the severity of the clinical disease. The present review discusses ILT with respect to its current status, virus characteristics, epidemiology, transmission, pathobiology, and advances in diagnosis, vaccination and control strategies to counter this important disease of poultry.

Pathogenic and Transmission Potential of Wildtype and Chicken Embryo Origin (CEO) Vaccine Revertant Infectious Laryngotracheitis Virus

Infectious laryngotracheitis (ILT) is an infectious upper respiratory tract disease that impacts the poultry industry worldwide. ILT is caused by an alphaherpesvirus commonly referred to as infectious laryngotracheitis virus (ILTV). Vaccination with live attenuated vaccines is practiced regularly for the control of ILT. However, extensive and improper use of live attenuated vaccines is related to vaccine viruses reverting to virulence. An increase in mortality and pathogenicity has been attributed to these vaccine revertant viruses. Recent studies characterized Canadian ILTV strains originating from ILT outbreaks as related to live attenuated vaccine virus revertants. However, information is scarce on the pathogenicity and transmission potential of these Canadian isolates. Hence, in this study, the pathogenicity and transmission potential of two wildtype ILTVs and a chicken embryo origin (CEO) vaccine revertant ILTV of Canadian origin were evaluated. To this end, 3-week-old specific pathogen-free chickens were experimentally infected with each of the ILTV isolates and compared to uninfected controls. Additionally, naïve chickens were exposed to the experimentally infected chickens to mimic naturally occurring infection. Pathogenicity of each of these ILTV isolates was evaluated by the severity of clinical signs, weight loss, mortality, and lesions observed at the necropsy. The transmission potential was evaluated by quantification of ILTV genome loads in oropharyngeal and cloacal swabs and tissue samples of the experimentally infected and contact-exposed chickens, as well as in the capacity to produce ILT in contact-exposed chickens. We observed that the CEO vaccine revertant ILTV isolate induced severe disease in comparison to the two wildtype ILTV isolates used in this study. According to ILTV genome load data, CEO vaccine revertant ILTV isolate was successfully transmitted to naïve contact-exposed chickens in comparison to the tested wildtype ILTV isolates. Overall, the Canadian origin CEO vaccine revertant ILTV isolate possesses higher virulence, and dissemination potential, when compared to the wildtype ILTV isolates used in this study. These findings have serious implications in ILT control in chickens.

Analysis of Whole-Genome Sequences of Infectious laryngotracheitis Virus Isolates from Poultry Flocks in Canada: Evidence of Recombination

Infectious laryngotracheitis virus (ILTV) is a herpes virus that causes an acute respiratory disease of poultry known as infectious laryngotracheitis (ILT). Chicken embryo origin (CEO) and tissue culture origin (TCO) live attenuated vaccines are routinely used for the control of ILT. However, vaccine virus is known to revert to virulence, and it has been recently shown that ILT field viral strains can undergo recombination with vaccinal ILTV and such recombinant ILT viruses possess greater transmission and pathogenicity potential. Based on complete or partial genes of the ILTV genome, few studies genotyped ILTV strains circulating in Canada, and so far, information is scarce on whole-genome sequencing or the presence of recombination in Canadian ILTV isolates. The objective of this study was to genetically characterize the 14 ILTV isolates that originated from three provinces in Canada (Alberta, British Columbia and Quebec). To this end, a phylogenetic analysis of 50 ILTV complete genome sequences, including 14 sequences of Canadian origin, was carried out. Additional phylogenetic analysis of the unique long, unique short and inverted repeat regions of the ILTV genome was also performed. We observed that 71%, 21% and 7% of the ILTV isolates were categorized as CEO revertant, wild-type and TCO vaccine-related, respectively. The sequences were also analyzed for potential recombination events, which included evidence in the British Columbia ILTV isolate. This event involved two ILTV vaccine (CEO) strains as parental strains. Recombination analysis also identified that one ILTV isolate from Alberta as a potential parental strain for a United States origin ILTV isolate. The positions of the possible recombination breakpoints were identified. These results indicate that the ILTV wild-type strains can recombine with vaccinal strains complicating vaccine-mediated control of ILT. Further studies on the pathogenicity of these ILTV strains, including the recombinant ILTV isolate are currently ongoing.

Development and application of a combined molecular and tissue culture-based approach to detect latent infectious laryngotracheitis virus (ILTV) in chickens

Infectious laryngotracheitis virus (ILTV) causes severe respiratory disease in chickens. ILTV can establish latency and reactivate later in life, but there have been few investigations of ILTV latency. This study aimed to contribute to the methodologies available to detect latent ILTV. A nested PCR was developed which was more sensitive than three other molecular methods investigated in this study. This nested PCR was then used in conjunction with in vitro reactivation culture methods that were optimized and applied to trigeminal ganglia (TG) and tracheal samples from ILTV-vaccinated commercial layer birds (n = 30). ILTV DNA was detected by nested PCR in the upper respiratory tract (URT) or eye of 22 birds. Of the remaining 8 birds, ILTV could be detected by co-culture in TG of 5 birds, with reactivated virus mostly detected 6 days post-explant (dpe). ILTV was also detected in tracheal cultures by 6 dpe. In the ILTV-positive URT samples, the virus could be characterised as vaccine strains SA2 (n = 9) or A20 (n = 5). This study provides evidence for reactivation and shedding of vaccine ILTV in commercial layer birds. Moreover, this study produced a molecular and in-vitro culture method to detect latent viral infection.

Spatial and temporal variation of Marek's disease virus and infectious laryngotracheitis virus genome in dust samples following live vaccination of layer flocks

Monitoring of Marek's disease virus (MDV) and infectious laryngotracheitis virus (ILTV) genome using poultry dust can be useful to monitor on-farm vaccination protocols but there are no set guidelines for collection of this sample type. This study assessed different dust collection methods for MDV and ILTV detection in a vaccinated layer flock (n = 1700) from day-old to 50 weeks of age. Birds were vaccinated against MDV at day-old, and ILTV by drinking water at week 6 and eye drop at week 12. Dust samples were collected weekly by settle plates (1-3 plates/15 m²) or by scraping surfaces in the poultry shed and tested for ILTV and MDV genomic copies (GC) by PCR. ILTV GC were detected 4 weeks post water vaccination, peaked at weeks 12-14 and became mostly undetectable after week 18. MDV was detected in dust on week 1, peaked at weeks 3-6, declined 3 logs by week 26 and remained detectable at this level until week 50. There was no difference in the detection rates of ILTV and MDV collected from settle plates in different locations of the shed (P > 0.10). There was no difference between settle plate and scraped samples in ILTV GC load but higher MDV GC were found in scraped samples. The settle plate method appears to reflect the current level of vaccine virus in the flock while the scrape method likely represents a cumulative record of shedding. Assessment of viral GC in dust samples is a good candidate for a practical method of estimating successful vaccine administration.

Attenuated infectious laryngotracheitis virus vaccines differ in their capacity to establish latency in the trigeminal ganglia of specific pathogen free chickens following eye drop inoculation

Infectious laryngotracheitis (ILT) is a respiratory disease that affects chickens. It is caused by the alphaherpesvirus, infectious laryngotracheitis virus (ILTV). This virus undergoes lytic replication in the epithelial cells of the trachea and upper respiratory tract (URT) and establishes latent infection in the trigeminal ganglia (TG) and trachea. Live attenuated vaccines are widely used to control ILT. At least one of these vaccines can establish latent infections in chickens, but this has not been demonstrated for all vaccines. The aim of the current study was to determine the capacity of three commercially available vaccines (SA2, A20 and Serva) and a glycoprotein G deletion mutant vaccine candidate (ΔgG ILTV) to establish latent infection in the TG of specific pathogen free (SPF) chickens. Five groups of 7-day-old SPF chickens were eye-drop vaccinated with either one of the vaccine strains or mock-vaccinated with sterile media and followed until 20 or 21 days post-vaccination (dpv). ILTV DNA was detected at 20–21 dpv in the TG of 23/40 (57.5%) vaccinated SPF chickens (SA2 = 10/10; A20 = 6/10; Serva = 3/10; ΔgG = 4/10) by PCR, but virus could not be reactivated from TG co-cultivated with primary chicken embryo kidney cells. In the birds from which ILTV DNA was detected in the TG, ILTV DNA could not be detected in the URT or trachea of 3 birds in each of the SA2, A20 and Serva vaccinated groups, and in 4 birds in the ΔgG vaccinated group, indicating that these birds were latently infected in the absence of active lytic replication and virus shedding. Results from this study demonstrate the capacity of commercial ILTV vaccines to establish latent infections and underline their importance in the epidemiology of this disease.

Determination of the minimum protective dose of a glycoprotein-G-deficient infectious laryngotracheitis virus vaccine delivered via eye-drop to week-old chickens

Infectious laryngotracheitis (ILT) is an upper respiratory tract disease of chickens that is caused by infectious laryngotracheitis virus (ILTV), an alphaherpesvirus. This disease causes significant economic loses in poultry industries worldwide. Despite widespread use of commercial live attenuated vaccines, many poultry industries continue to experience outbreaks of disease caused by ILTV. Efforts to improve the control of this disease have resulted in the generation of new vaccine candidates, including ILTV mutants deficient in virulence factors. A glycoprotein G deletion mutant vaccine strain of ILTV (ΔgG ILTV), recently licenced as Vaxsafe ILT (Bioproperties Pty Ltd), has been extensively characterised in vitro and in vivo, but the minimum effective dose required to protect inoculated animals has not been determined. This study performed a vaccination and challenge experiment to determine the minimum dose of ΔgG ILTV that, when delivered by eye-drop to seven-day-old specific pathogen-free chickens, would protect the birds from a robust challenge with a virulent field strain of virus (class 9 ILTV). A dose of 10(3.8) plaque forming units was the lowest dose capable of providing a high level of protection against challenge, as measured by clinical signs of disease, tracheal pathology and virus replication after challenge. This study has shown that the ΔgG ILTV vaccine strain is capable of inducing a high level of protection against a virulent field virus at a commercially feasible dose. These results lay the foundations upon which a commercial vaccine can be developed, thereby offering the potential to provide producers with another important tool to help control ILTV.

Enhanced infection of avian influenza virus H9N2 with infectious laryngeotracheitis vaccination in chickens

Avian influenza and infectious laryngeotracheitis viruses are common causes of respiratory diseases in chickens with economical importance worldwide. In this study, we investigated the effect of experimental co-infection of avian influenza virus-H9N2 (AIV-H9N2) with infectious laryngeotracheitis virus (ILTV) live-attenuated vaccine (LAR-VAC^®) on chickens. Four experimental groups were included in this study: negative control group, AIV-H9N2 group, AIV-H9N2+LAR-VAC^® group, and LAR-VAC^® group. AIV-H9N2 was inoculated intranasally to challenged groups at 35 days of age. On the same day, LAR-VAC^® was ocularly administered to vaccinated groups. Chickens were observed for clinical signs, changes in body weight and mortality rates. Tissue samples, sera, tracheal and cloacal swabs, and blood were also collected at 3, 6, 9 and 12 days post-infection (PI). A significant increase in clinical signs and mortality rates were observed in the AIV-H9N2 + LAR-VAC^® group. Moreover, chickens coinfected with AIV-H9N2 and LAR-VAC^® showed a significant decrease in body weight and lymphoid organs indices. The tracheal gross and histopathological lesions and the shedding titer and period of AIV-H9N2 were significantly higher in AIV-H9N2 + LAR-VAC^® group when compared to other groups. Furthermore, AIV-H9N2 infection leads to humoral and cellular immunosuppression as shown by a significant decrease in the CD4⁺/CD8⁺ ratio and antibody responses to ILTV and a significant increase in H/L ratio. In conclusion, this is the first report of co-infection of AIV-H9N2 and ILTV vaccine in chickens, which leads to increased pathogenicity, pathological lesions, and AIV-H9N2 shedding titer and period, which can lead to severe economic losses due to poor weight gain and mortality.

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.

Evaluation of the Protection Efficacy of a Serotype 1 Marek's Disease Virus-Vectored Bivalent Vaccine Against Infectious Laryngotracheitis and Marek's Disease

Laryngotracheitis (LT) is a highly contagious respiratory disease of chickens that produces significant economic losses to the poultry industry. Traditionally, LT has been controlled by administration of modified live vaccines. In recent years, the use of recombinant DNA-derived vaccines using turkey herpesvirus (HVT) and fowlpox virus has expanded, as they protect not only against the vector used but also against LT. However, HVT-based vaccines confer limited protection against challenge, with emergent very virulent plus Marek's disease virus (vv+MDV). Serotype 1 vaccines have been proven to be the most efficient against vv+MDV. In particular, deletion of oncogene MEQ from the oncogenic vvMDV strain Md5 (BACδMEQ) resulted in a very efficient vaccine against vv+MDV. In this work, we have developed two recombinant vaccines against MD and LT by using BACδMEQ as a vector that carries either the LT virus (LTV) gene glycoprotein B (gB; BACΔMEQ-gB) or LTV gene glycoprotein J (gJ; BACδMEQ-gJ). We have evaluated the protection that these recombinant vaccines confer against MD and LT challenge when administered alone or in combination. Our results demonstrated that both bivalent vaccines (BACΔMEQ-gB and BACδMEQ-gJ) replicated in chickens and were safe to use in commercial meat-type chickens bearing maternal antibodies against MDV. BACΔMEQ-gB protected as well as a commercial recombinant (r)HVT-LT vaccine against challenge with LTV. However, BACδMEQ-gJ did not protect adequately against LT challenge or increase protection conferred by BACΔMEQ-gB when administered in combination. On the other hand, both BACΔMEQ-gB and BACδMEQ-gJ, administered alone or in combination, protected better against an early challenge with vv+MDV strain 648A than commercial strains of rHVT-LT or CVI988. Our results open a new avenue in the development of recombinant vaccines by using serotype 1 MDV as vectors.

Persistence and spreading of field and vaccine strains of infectious laryngotracheitis virus (ILTV) in vaccinated and unvaccinated geographic regions, in Brazil

Infectious laryngotracheitis (ILT) is a highly infectious respiratory disease that causes morbidity and mortality in commercial chickens. Despite the use of attenuated vaccines, ILT outbreaks have been described in broiler and long-lived birds. Molecular approaches, including polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) and DNA sequencing, are used to characterize ILT viruses (ILTVs) detected in vaccinated and unvaccinated geographical regions. As part of an ILT control program implemented in a region of commercial layer production, samples of conjunctiva, trachea, and trigeminal ganglia were collected from chickens in a vaccinated and quarantined region over a period of 8 years after initiation of vaccination. To determine the origin of new ILT outbreaks in unvaccinated regions, samples collected from ill chickens were also analyzed. Chicken embryo origin (CEO) vaccine viruses and the Bastos field strain were detected circulating in healthy chickens in the vaccinated region. CEO strains and field viruses molecularly related to the Bastos strain were also detected outside of the quarantined region in chickens showing clinical signs of ILT. This study reveals the persistence and circulation of a wild field strain, despite the intensive use of tissue culture origin (TCO) and CEO vaccines in a quarantined region. Spreading of CEO viruses to unvaccinated regions and the capacity of this virus to establish latent infections and cause severe outbreaks were also observed.

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.

Safety and efficacy of a turkey herpesvirus vector laryngotracheitis vaccine for chickens

Turkey herpesvirus vector laryngotracheitis vaccine (HVT/LT) expressing the glycoprotein B gene of laryngotracheitis virus (LTV) has been developed. In vitro growth kinetics of HVT/LT were similar to those of parental turkey herpesvirus (HVT), FC-126 strain. Genetic and phenotypic stabilities of HVT/LT after in vitro (in cell culture) or in vivo (in chickens) passage were confirmed by various assays, including Southern blot analysis, western blot analysis, and an indirect immunofluorescence assay. Safety of HVT/LT was assessed by an overdose study as well as by a backpassage study in specific-pathogen-free (SPF) chickens. The overdose study indicated that HVT/LT did not cause any adverse effects in chickens. The backpassage study confirmed that HVT/LT does not revert to virulence after five passages in chickens. The vaccine did not transmit laterally from vaccinated chickens to commingled nonvaccinated chickens. Efficacy of HVT/LT was evaluated in SPF layer chickens after vaccination by the subcutaneous route at 1 day of age. The majority of the vaccinated chickens (92%-100%) were protected against challenge with virulent LTV at 7 wk of age. Efficacy of HVT/LT was further evaluated in broiler chickens from a commercial source after in ovo vaccination to embryos at 18 days of incubation. After challenge with virulent LTV at 21 and 35 days of age, 67% and 87% of HVT/LT-vaccinated chickens did not develop LT clinical signs, respectively, while 100% (21 days of age) and 73% (35 days of age) of the challenge control chickens showed clinical signs of LT. These results suggest that HVT/LT is a safe and efficacious vaccine for control of laryngotracheitis (LT).

Immune responses to infectious laryngotracheitis virus

Infectious laryngotracheitis (ILT) is an upper respiratory tract disease in chickens caused by infectious laryngotracheitis virus (ILTV), an alphaherpesvirus. Despite the extensive use of attenuated, and more recently recombinant, vaccines for the control of this disease, ILT continues to affect the intensive poultry industries worldwide. Innate and cell-mediated, rather than humoral immune responses, have been identified as responsible for protection against disease. This review examines the current understandings in innate and adaptive immune responses towards ILTV, as well as the role of ILTV glycoprotein G in modulating the host immune response towards infection. Protective immunity induced by ILT vaccines is also examined. The increasing availability of tools and reagents for the characterisation of avian innate and cell-mediated immune responses are expected to further our understanding of immunity against ILTV and drive the development of new generation vaccines towards enhanced control of this disease.

Protection of chickens from infectious laryngotracheitis with a recombinant fowlpox virus expressing glycoprotein B of infectious laryngotracheitis virus

Infectious laryngotracheitis (ILT) is an economically important disease of chickens caused by a type I gallid herpesvirus, infectious laryngotracheitis virus (ILTV). The vaccines currently available are modified live viruses, which are effective in preventing disease outbreaks. However, they have often been associated with a variety of adverse effects including spread of vaccine virus to non-vaccinates, inadequate attenuation, production of latently infected carriers, and increased virulence as a result of in vivo passage. In this study, a recombinant fowlpox virus expressing glycoprotein B (gB) of ILTV (rFPV-ILTVgB) was constructed. Protection of specific pathogen free (SPF) and commercial chickens from ILT with the rFPV-ILTVgB and commercial ILTV vaccine (Nobilis ILT) were compared after challenge with a lethal dose of virulent ILTV.Both the rFPV-ILTVgB- and the Nobilis ILT-vaccinated SPF chickens were completely protected from death, while 90% of the unvaccinated chickens died after challenge. The immunized commercial chickens were also 100% protected with rFPV-ILTVgB, compared with 85% protected with Nobilis ILT. The protective efficacy was also measured by the antibody response to ILTV gB, isolation of challenge virus and polymerase chain reaction amplification of the ILTV thymidine kinase gene after challenge. The results showed that rFPV-ILTVgB could be a potential safe vaccine to replace current modified live vaccines for preventing ILT.

Optimization of a duplex real-time PCR method for relative quantitation of infectious laryngotracheitis virus

Infectious laryngotracheitis is a highly contagious respiratory disease of chickens controlled by biosecurity and vaccination with live attenuated or recombinant vaccines. Infectious laryngotracheitis virus (ILTV) infections are characterized by a peak of viral replication in the trachea followed by a steady decrease in replication that results in the establishment of latency. Estimation of viral load is an important tool to determine the stage of ILTV infection. Here, a multiplex real-time PCR was optimized for the quantification of ILTV genomes. Quantification of viral genomes was based on the amplification of the ILTV UL44 gene, and sample variability was normalized using the chicken (Gallusgallus domesticus) alpha2-collagen gene as an endogenous control in a duplex reaction.

Evaluation of the matrix effect of thermophilic anaerobic digestion on inactivation of infectious laryngotracheitis virus using real-time PCR and viral cell culture

The matrix effect of the thermophilic anaerobic digestion (TAD) process on inactivation of infectious laryngotracheitis virus (ILTV) was evaluated. Viral cell culture and real-time PCR were used for assessing removal of the viral infectivity and degradation of viral DNA, respectively. Results showed that the TAD-derived matrix alone can inactivate the virus and destroy the nucleic acid helix core of ILTV in a time-and- dose-dependent manner. No cytopathogenic effect (CPE) was observed in the cells exposed to ILTV pre-treated with TAD matrix for 1.5h in experiment 1 and for 16h in experiment 2. There was a significant statistical difference between TAD matrix treated and non-treated cultures (p<0.001, Chi-test). Amplifiable ILT viral DNA was reduced 2.27 log by 1.5h-treatment and was not present by 16h-treatment with TAD matrix, indicating complete viral DNA fragmentation. The TAD process is an environmentally friendly way for disposing of poultry biowaste and carcasses.

Transcriptional profiling of host gene expression in chicken embryo lung cells infected with laryngotracheitis virus

Background

Infection by infectious laryngotracheitis virus (ILTV; gallid herpesvirus 1) causes acute respiratory diseases in chickens often with high mortality. To better understand host-ILTV interactions at the host transcriptional level, a microarray analysis was performed using 4 × 44 K Agilent chicken custom oligo microarrays.

Results

Microarrays were hybridized using the two color hybridization method with total RNA extracted from ILTV infected chicken embryo lung cells at 0, 1, 3, 5, and 7 days post infection (dpi). Results showed that 789 genes were differentially expressed in response to ILTV infection that include genes involved in the immune system (cytokines, chemokines, MHC, and NF-κB), cell cycle regulation (cyclin B2, CDK1, and CKI3), matrix metalloproteinases (MMPs) and cellular metabolism. Differential expression for 20 out of 789 genes were confirmed by quantitative reverse transcription-PCR (qRT-PCR). A bioinformatics tool (Ingenuity Pathway Analysis) used to analyze biological functions and pathways on the group of 789 differentially expressed genes revealed that 21 possible gene networks with intermolecular connections among 275 functionally identified genes. These 275 genes were classified into a number of functional groups that included cancer, genetic disorder, cellular growth and proliferation, and cell death.

Conclusion

The results of this study provide comprehensive knowledge on global gene expression, and biological functionalities of differentially expressed genes in chicken embryo lung cells in response to ILTV infections.

Viral respiratory diseases (ILT, aMPV infections, IB): are they ever under control?

1.

The use of vaccines is the main approach to control of the economically important poultry viral respiratory diseases infectious laryngotracheitis (ILT), avian metapneumovirus (aMPV) infections and infectious bronchitis (IB). This paper appraises the current methods of vaccine control in the light of the nature of each virus and epidemiological factors associated with each disease.

2.

Infectious laryngotracheitis virus (ILTV) exists as a single type with a wide range of disease severity. It is a serious disease in certain regions of the world. Recent work has distinguished molecular differences between vaccine and field strains and vaccine virus can be a cause of disease. Vaccines have remained unaltered for many years but new ones are being developed to counter vaccine side effects and reversion and reactivation of latent virus.

3.

Avian metapneumoviruses, the cause of turkey rhinotracheitis and respiratory disease in chickens exists as 4 subtypes, A, B, C and D. A and B are widespread and vaccines work well provided that accurate doses are given. Newer vaccine developments are designed to eliminate reversion and possibly counter the appearance of newer field strains which may break through established vaccine coverage.

4.

IB presents the biggest problem of the three. Being an unstable RNA virus, part of the viral genome that codes for the S1 spike gene can undergo mutation and recombination so that important antigenic variants can appear irregularly which may evade existing vaccine protection. While conventional vaccines work well against homologous types, new strategies are needed to counter this instability. Molecular approaches involving tailoring viruses to suit field challenges are in progress. However, the simple use of two genetically different vaccines to protect against a wide range of heterologous types is now a widespread practice that is very effective.

5.

None of the three diseases described can claim to be satisfactorily controlled and it remains to be seen whether the newer generations of vaccines will be more efficacious and cost effective. The importance of constant surveillance is emphasised and the testing of novel vaccines cannot be achieved without the use of vaccine-challenge experiments in poultry.

Epizootiology of infectious laryngotracheitis and presentation of an industry control program

Despite improved biosecurity and vaccination programs in recent years, infectious laryngotracheitis continues to emerge in the field on a regular basis in poultry producing states. Evidence is mounting that most field outbreaks are caused by viruses indistinguishable from chicken-embryo-origin vaccine strains and, for that reason, broiler outbreaks are often referred to in the field as "vaccinal laryngotracheitis" (VLT). Cooperative industry programs are described, in which the poultry industry, along with government, laboratories, and other sectors involved with poultry live production work together to control and contain VLT outbreaks. These programs take into account many epizootiologic aspects of the disease and, when diligently followed, are successful in most instances at keeping the numbers of cases low and the outbreaks under control. The programs include the rapid diagnosis of the disease, the use of geographic information system technology, biosecurity, vaccination, and communications between all stakeholders.

Comparison of the safety and protective efficacy of vaccination with glycoprotein-G-deficient infectious laryngotracheitis virus delivered via eye-drop, drinking water or aerosol

Infectious laryngotracheitis virus (ILTV), an alphaherpesvirus, causes respiratory disease in chickens and is commonly controlled by vaccination with conventionally attenuated virus strains. These vaccines have limitations due to residual pathogenicity and reversion to virulence. To avoid these problems and to better control disease, attention has recently turned towards developing a novel vaccine strain that lacks virulence gene(s). Glycoprotein G (gG) is a virulence factor in ILTV. A gG-deficient strain of ILTV has been shown to be less pathogenic than currently available vaccine strains following intratracheal inoculation of specific pathogen free chickens. Intratracheal inoculation of gG-deficient ILTV has also been shown to induce protection against disease following challenge with virulent virus. Intratracheal inoculation, however, is not suitable for large-scale vaccination of commercial poultry flocks. In this study, inoculation of gG-deficient ILTV via eye-drop, drinking water and aerosol were investigated. Aerosol inoculation resulted in undesirably low levels of safety and protective efficacy. Inoculation via eye-drop and drinking water was safe, and the levels of protective efficacy were comparable with intratracheal inoculation. Thus, gG-deficient ILTV appears to have potential for use in large-scale poultry vaccination programmes when administered via eye-drop or in drinking water.

Replication and transmission of live attenuated infectious laryngotracheitis virus (ILTV) vaccines

The aim of this study was to evaluate the replication of live attenuated infectious laryngotracheitis virus vaccines in selected tissues and their ability to transmit to contact-exposed birds. Four-week-old specific-pathogen-free chickens were eye drop-inoculated with tissue culture origin (TCO) and chicken embryo origin (CEO) vaccines. Contact-exposed chickens were housed in direct contact with eye drop-inoculated chickens from the first day postinoculation. Virus isolation and real-time polymerase chain reaction were used to detect the presence of live virus and viral DNA, respectively, in the trachea, trigeminal ganglia, eye conjunctiva, cecal tonsils, and cloaca from eye drop-inoculated and contact-exposed birds at days 2, 4, 5 to 10, 14, 18, 21, 24, and 28 postinoculation. No differences were observed in the ability of the TCO and CEO vaccines to replicate in the examined tissues. Both vaccines presented a localized replication in the eye conjunctiva and the trachea. Both vaccines were capable of transmitting to contact-exposed birds, attaining peaks of viral DNA as elevated as those observed in inoculated birds. The CEO vaccine replicated faster and reached higher viral genome copy number than the TCO vaccine in the conjunctiva and trachea of eye drop-inoculated and contact-exposed birds. The viral DNA from both vaccines migrated to the trigeminal ganglia during early stages of infection. Although the CEO and TCO vaccines were not recovered from the cecal tonsils and the cloaca, low levels of viral DNA were detected at these sites during the peak of viral replication in the upper respiratory tract.

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.

Evaluation of the efficacy of a live fowlpox-vectored infectious laryngotracheitis/avian encephalomyelitis vaccine against ILT viral challenge

Infectious laryngotracheitis (ILT) is caused by an alphaherpesvirus, and latency can be produced by previous exposure to vaccine virus. The main sites of latency for the ILT virus have been shown to be the trigeminal ganglion and the trachea. Reactivation of latent virus is one factor related to the production of clinical signs. The development of a genetically engineered ILT vaccine has been suggested for many years as a tool to eliminate viral latency. Several approaches have been suggested. Included among them is the development of a thymidine kinase-deficient mutant or the insertion of ILT viral glycoproteins into a viral vector such as a poxvirus. A commercially available, live, fowlpox-vectored infectious laryngotracheitis + avian encephalomyelitis (FP-LT+AE) vaccine was used in field trials in leghorn pullet flocks and evaluated by tracheal challenge in a laboratory setting with the use of the National Veterinary Services Laboratory (Ames, IA) ILT challenge virus. Interference of the pigeon pox vaccine, which is often administered concurrently with fowlpox vaccine, was also evaluated when given in conjunction with the FP-LT+AE vaccine. Overall, the results indicate that the FP-LT+AE vaccine provides adequate protection against ILT viral challenge. Proper administration is essential. In one flock, inadequate protection was most likely a result of either poor vaccine administration or previous exposure to pox virus. In addition, the simultaneous administration of pigeon pox vaccine did not appear to interfere with protection against ILT viral challenge.

Detection of infectious laryngotracheitis virus in trigeminal ganglia by avidin-biotin complex method in chickens: short communication

Detection of infectious laryngotracheitis virus (ILTV) by avidin-biotin complex (ABC) method was studied in trigeminal ganglia (TRG) during the acute and post-acute or latent period in chickens inoculated with an A96 strain of the antigen. TRG tissue samples were collected from uninoculated (10 chickens) and inoculated (30 chickens) animals at various intervals post inoculation (PI), and stained by the ABC method. The results indicated that no ILTV antigen was detected in the TRG between PI days 3 and 11. However, the antigen was detected in most of the chickens between PI days 13 and 41. It is concluded that the ABC method can be used successfully for the detection of ILTV antigens in TRG during post-acute or latent period.

Serological monitoring of 40 Swiss fancy breed poultry flocks

Rapid serum agglutination, haemagglutination inhibition and enzyme-linked immunosorbent assays were used to screen Swiss fancy breed chicken flocks for antibodies against 12 avian infectious agents. For this purpose, 1,002 blood samples from 40 flocks were collected and tested. Ten percent of the samples were positive for Salmonella gallinarum-pullorum and 62.5% of the flocks were affected. More than 75% of the flocks had antibodies against Mycoplasma gallisepticum/Mycoplasma synoviae, infectious bronchitis, infectious bursal disease, avian encephalomyelitis, infectious chicken anaemia and reoviral arthritis. Low prevalence of antibodies was recorded for Salmonella enteritidis, avian influenza, avian leukosis and Newcastle disease (2.0 to 4.0%).

Detection of infectious laryngotracheitis virus in formalin-fixed, paraffin-embedded tissues by nested polymerase chain reaction

Infectious laryngotracheitis virus (ILTV) is routinely diagnosed by histopathologic examination of trachea, eyelid, and lung tissues. Lesions consistent with infectious laryngotracheitis (ILT) infection include syncytial cell formation with intranuclear inclusion bodies. These changes are present during the acute phase of infection. To increase the sensitivity of detecting ILT, a nested polymerase chain reaction (PCR) was developed for detection of ILTV DNA. Nested PCR assay was specific for the amplification of ILTV DNA and did not amplify a variety of other avian pathogens. To further validate the ability of this assay to detect ILT, nested PCR was performed in formalin-fixed, paraffin-embedded tissues from 35 cases of respiratory disease. Of the 35 cases, 12 were considered ILT suspects on the basis of initial clinical observation. Eleven of the 12 ILT-suspect cases were diagnosed as ILT, and the remaining 24 were diagnosed as nonspecific tracheitis (NST) by histopathologic examination. Histopathologically positive samples were confirmed by direct fluorescent antibody test and virus isolation. Of the 11 ILT-positive cases, 10 were positive by nested PCR. In addition, ILTV DNA was detected in 7 of the 24 samples diagnosed as NST upon histopathologic examination. Therefore, by nested PCR, ILTV DNA was detected in tissues independently of the presence of syncytial cells, intranuclear inclusions, or both. ILT nested PCR is a specific and sensitive assay capable of detecting ILT at different stages of infection and can be utilized in combination with histopathological examination to accelerate the diagnosis of ILT infection.

Respiratory diseases of gallinaceous birds

Respiratory conditions constitute many of the diseases affecting all avian species, including gallinaceous birds. Clinical signs and gross lesions of different respiratory diseases are often similar, and establishing a definitive diagnosis may require ancillary laboratory testing. Determination of a specific diagnosis allows practitioners to select the most effective therapies and to prescribe a management program that prevents recurrence.

The non-essential UL50 gene of avian infectious laryngotracheitis virus encodes a functional dUTPase which is not a virulence factor

The DNA sequence of the infectious laryngotracheitis virus (ILTV) UL50, UL51 and UL52 gene homologues was determined. Although the deduced UL50 protein lacks the first of five conserved domains of the corresponding proteins of mammalian alphaherpesviruses, the ILTV gene product was also shown to possess dUTPase activity. The generation of UL50-negative ILTV mutants was facilitated by recombination plasmids encoding green fluorescent protein (GFP), and expression constructs of predicted transactivator proteins of ILTV (alphaTIF, ICP4) were successfully used to increase the infectivity of viral genomic DNA. A GFP-expressing UL50-deletion mutant of ILTV showed reduced cell-to-cell spread in vitro, and was attenuated in vivo. A similar deletion mutant without the foreign gene, however, propagated like wild-type ILTV in cell culture and was pathogenic in chickens. We conclude that the viral dUTPase is not required for efficient replication of ILTV in the respiratory tract of infected animals. The replication defect of the GFP-expressing ILTV recombinant is most likely caused by toxic effects of the reporter gene product, since spontaneously occurring inactivation mutants exhibited wild-type-like growth.