Infectious laryngotracheitis virus (ILTV) vaccine intake evaluation by detection of virus amplification in feather pulps of vaccinated chickens

Interactions between avian viruses and skin in farm birds

This article reviews the avian viruses that infect the skin of domestic farm birds of primary economic importance: chicken, duck, turkey, and goose. Many avian viruses (e.g., poxviruses, herpesviruses, Influenza viruses, retroviruses) leading to pathologies infect the skin and the appendages of these birds. Some of these viruses (e.g., Marek's disease virus, avian influenza viruses) have had and/or still have a devasting impact on the poultry economy. The skin tropism of these viruses is key to the pathology and virus life cycle, in particular for virus entry, shedding, and/or transmission. In addition, for some emergent arboviruses, such as flaviviruses, the skin is often the entry gate of the virus after mosquito bites, whether or not the host develops symptoms (e.g., West Nile virus). Various avian skin models, from primary cells to three-dimensional models, are currently available to better understand virus-skin interactions (such as replication, pathogenesis, cell response, and co-infection). These models may be key to finding solutions to prevent or halt viral infection in poultry.

A preliminary study of the localisation of infectious laryngotracheitis virus glycoprotein E within specific peripheral blood lymphocytes

Infectious laryngotracheitis virus (ILTV) DNA has been detected in blood fractions, but the cell phenotype with which the virus is associated is unknown. This study investigated the presence of ILTV antigen in peripheral blood cells of six acutely ILTV-infected chickens (5 or 9 days post ocular inoculation with a virulent isolate and three sham-inoculated chickens using immunofluorescent staining. Blood fractions were separated by Ficoll-Paque density gradient centrifugation, and smears were prepared from erythrocyte and leukocyte fractions. The smears were stained for ILTV glycoprotein E and the leukocyte markers CD4, CD8, Bu-1 (B cell), KUL01 (monocyte/macrophage), TCRγδ, and TCRαβ/Vβ₂ and examined under a confocal microscope. In samples from infected birds, ILTV gE specific fluorescence was localised in B cells and all evaluated T cell types, but not in monocytes and erythrocytes. The percentage of CD4, CD8, TCRγδ, TCRαβ/Vβ₁, TCRαβ/Vβ₂ and B cells colocalised with ILTV antigen ranged from 13.3% to 22.3%. None of the samples from the sham-inoculated chickens exhibited fluorescence for ILTV gE. The results of this pilot study suggest that ILTV has a tropism for peripheral blood T and B cells. Further research is required to investigate whether these cells support ILTV productive replication.

Evaluation of Recombinant Herpesvirus of Turkey Laryngotracheitis (rHVT-LT) Vaccine against Genotype VI Canadian Wild-Type Infectious Laryngotracheitis Virus (ILTV) Infection

In Alberta, infectious laryngotracheitis virus (ILTV) infection is endemic in backyard poultry flocks; however, outbreaks are only sporadically observed in commercial flocks. In addition to ILTV vaccine revertant strains, wild-type strains are among the most common causes of infectious laryngotracheitis (ILT). Given the surge in live attenuated vaccine-related outbreaks, the goal of this study was to assess the efficacy of a recombinant herpesvirus of turkey (rHVT-LT) vaccine against a genotype VI Canadian wild-type ILTV infection. One-day-old specific pathogen-free (SPF) White Leghorn chickens were vaccinated with the rHVT-LT vaccine or mock vaccinated. At three weeks of age, half of the vaccinated and the mock-vaccinated animals were challenged. Throughout the experiment, weights were recorded, and feather tips, cloacal and oropharyngeal swabs were collected for ILTV genome quantification. Blood was collected to isolate peripheral blood mononuclear cells (PBMC) and quantify CD4+ and CD8+ T cells. At 14 dpi, the chickens were euthanized, and respiratory tissues were collected to quantify genome loads and histological examination. Results showed that the vaccine failed to decrease the clinical signs at 6 days post-infection. However, it was able to significantly reduce ILTV shedding through the oropharyngeal route. Overall, rHVT-LT produced a partial protection against genotype VI ILTV infection.

Transmission of infectious laryngotracheitis virus vaccine and field strains: the role of degree of contact and transmission by whole blood, plasma and poultry dust

Understanding the mechanisms of transmission of infectious laryngotracheitis virus (ILTV) is critical to proper control as both vaccine and wild-type strains circulate within chicken flocks with potential adverse consequences. The relative efficiency of transmission by direct contact between chickens and airborne transmission has not been investigated. Furthermore, relatively high levels of ILTV DNA have been detected in poultry dust and blood but the infectivity of these is unknown. In this study, comparison of in-contact and airborne transmission of two vaccine and one field strain of ILTV revealed that all transmitted to 100% of in-contact birds by 6 days post-exposure (dpe). Airborne transmission without contact resulted in 100% transmission by 14 and 17 dpe for the wild-type and Serva vaccine virus but only 27% transmission by 21 dpe for the A20 vaccine virus. The infectivity of dust or extracts of dust and blood or plasma from infected chickens at various stages of infection was assessed by inoculation into susceptible chickens. There was no transmission by any of these materials. In conclusion, direct contact facilitated efficient ILTV transmission but the virus was unable to be transmitted by dust from infected chickens suggestive of a limited role in the epidemiology of ILTV.

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.

Air sampling for detection of infectious laryngotracheitis (ILT) in commercial poultry flocks

OBJECTIVE

Infectious laryngotracheitis (ILT) is an acute and highly contagious viral respiratory disease of poultry, caused by gallid herpesvirus 1 (ILTV), which causes significant economic losses. Due to recent outbreaks of ILT in Australia, it has been proposed that ILT could be transmitted between poultry sheds by airborne transmission; however, there has never been direct detection of ILTV from air samples. We aimed to optimize a sampling system for the detection of airborne ILTV in poultry sheds.

RESULTS

Poultry farms with a known outbreaks of ILT were used for detection of airborne ILTV. Infected chickens were verified by detection of ILTV nucleic acid in feather shafts with all farms being positive. Using a liquid cyclonic impinging device, it was found that recovery and detection of airborne ILTV was possible in alkaline PEG buffer. Additional sampling was performed at different heights to determine the presence of ILTV in the air. In farm 3, all three air samples at both heights were positive for ILTV while at farm 2 only one sample at 45 cm was positive. We envisaged in the future air sampling will be able to detect and track potential transmission of ILTV both inside and outside of the poultry shed.

Detection of infectious laryngotracheitis virus (ILTV) in tissues and blood fractions from experimentally infected chickens using PCR and immunostaining analyses

The ability of infectious laryngotracheitis virus (ILTV) to replicate in organs outside of the upper respiratory tract and conjunctiva associated-lymphoid tissues is still not well understood. This study investigated the tissue distribution of an Australian field strain of ILTV (class 9) on birds experimentally inoculated via eye-drop at 7 days of age by using quantitative PCR (qPCR) and immunohistochemistry. Tissues including conjunctiva, caecal tonsil, kidney, liver, lung, spleen, thymus, trachea and blood were collected from sham-inoculated (control group; n = 2) and ILTV-inoculated (n = 8) birds at 7 days post-inoculation (dpi). Blood was collected from 13 infected birds at 14 dpi and fractionated using ficoll-paque. At 7 dpi, the highest detection rate and genomic copies (GC) were in conjunctiva (8/8; 8.08 ± 0.48 log₁₀ GC/mg) followed by trachea (8/8; 4.64 ± 0.48) and thymus (8/8; 4.52 ± 0.48), kidney (8/8; 3.97 ± 0.48), lung (8/8; 3.65 ± 0.48), spleen (8/8; 3.55 ± 0.48), liver (8/8; 3.51 ± 0.48), caecal tonsil (7/8; 3.76 ± 0.48) and plasma (4/8; 2.40 ± 0.48 log₁₀ GC/ml). ILTV antigen was only detected in conjunctiva (7/8), trachea (6/8) and lung (4/8) samples. At 14 dpi, ILTV detection rate and genomic copies in buffy coat cells were 12/13 and 2.86 ± 0.39 log₁₀ GC/mg, respectively while those of plasma were 11/13 and 4.29 ± 0.39 log₁₀ GC/ml and red blood cell were 3/13 and 0.36 ± 0.39 log₁₀ GC/mg. In conclusion, ILTV DNA was detected in a wide range of tissues and blood fractions but ILTV antigen was only detected in respiratory organs and conjunctiva.

Development and application of a colloidal gold test strip for the rapid detection of the infectious laryngotracheitis virus

Infectious laryngotracheitis disease is an acute, highly contagious viral disease seriously affecting poultry industry worldwide. In this study, a rapid and simple immune colloidal gold test strip for detecting infectious laryngotracheitis virus (ILTV) was developed based on membrane chromatography with monoclonal antibodies (mAbs) against gJ protein of ILTV and systematically evaluated for the detection of ILTV from clinical samples. mAb 2D4 1D7 was conjugated with colloidal gold as the detector antibody on the test strip. Another mAb, 1D8 1G3, was used as the capture complex at the test line (T-line), and goat antimouse IgG antibody was used as the capture antibody at the control line (C-line). The colloidal gold test strip showed high specificity in the detection of ILTV, with no cross-reaction with other avian pathogens, including infectious bronchitis virus, infectious bursal disease virus, avian influenza virus, Newcastle disease virus, fowl adenoviruses, and Marek's disease virus. Besides, the detection limit of this method was as low as 60 ELD₅₀/mL for the ILTV Wanggang strain. Furthermore, we evaluated its application in 260 clinical samples suspected of infection with ILTV. Results from the strip test were nearly identical with those from real-time PCR (coincidence rate 99.6%) and showed higher sensitivity than conventional PCR. All the results obtained in this study indicated that the colloidal gold test strip can be applied as a simple, rapid, sensitive, and specific diagnostic tool for the detection of ILTV, especially in resource-limited areas.

Pathogenesis and tissue tropism of natural field recombinants of infectious laryngotracheitis virus

Infectious laryngotracheitis virus (ILTV) is an economically significant respiratory pathogen of poultry. Novel recombinant strains of ILTV have emerged in Australia during the last decade and currently class 9 (CL9) and class 10 (CL10) ILTV are the most prevalent circulating strains. This study conducted a comprehensive investigation of the pathogenesis of these two viral strains. Commercial broiler and specific pathogen free (SPF) chickens were inoculated with varying doses of CL9 or CL10 ILTV and subsequently evaluated for clinical and pathological signs of infection. While no difference in the levels of acute viral replication were observed across the different challenge doses, the severity of clinical signs, tracheal pathology and mortality were dose dependent. Both strains of virus persisted in the respiratory tract for up to 14 days post inoculation (dpi) and could be detected in the lung and feathers with sporadic detection in the liver, spleen or bursa. Given the prevalence of CL9 and CL10 in Australian poultry flocks, this study provides an important foundation for the development of diagnostic and therapeutic approaches for the detection and prevention of ILTV.

A practical method for assessing infectious laryngotracheitis vaccine take in broilers following mass administration in water: Spatial and temporal variation in viral genome content of poultry dust after vaccination

Infectious laryngotracheitis is an important disease of chickens caused by infectious laryngotracheitis virus (ILTV). Outbreaks commonly occur in meat chicken flocks and mass vaccination with live attenuated vaccines, usually in water, is used to control the disease in these populations. Vaccination with live virus via water and nipple drinkers requires stringent adherence to protocols to ensure success, but vaccine administration monitoring is not currently assessed due to a lack of economically viable methods. Vaccinal ILTV has been shown to be detectable in dust in experimental studies and has potential as a method of assessing vaccination success. However, the pattern of vaccinal ILTV detection in dust following vaccination under commercial conditions has not been defined. We report the longitudinal profile of ILTV genome copies (GC) in poultry house dust collected on settle plates following vaccination of 8 flocks of commercial meat chickens on four farms. ILTV GC was enumerated using quantitative real-time polymerase chain reaction (qPCR). There was considerable variation between flocks in the levels of ILTV GC detected post vaccination and this variation was significantly associated with vaccine take measured in individual birds in a companion study. There was no effect of sampling location on ILTV GC in dust but the amount of dust collected was greater in locations closer to the exhaust fans in artificially ventilated houses. Results indicate that measurement of ILTV GC in single or pooled dust samples at 7-8 days post vaccination enables detection of poor vaccine takes and provides a practical means of monitoring ILT vaccination.

Biotic concerns in generating molecular diagnosis matrixes for 4 avian viruses with emphasis on Marek's disease virus

The great advance in the field of diagnosis of avian viruses is reflecting the highly sophisticated molecular assays of the human and general virology in providing highly sensitive and fast methods of diagnosis. The present review will discuss the biotic factors and the complexities that became evident with the evolution of the novel molecular diagnostic assays with emphasis on 4 avian viruses, chicken anemia, infectious laryngotracheitis, turkey meningoencephalitis, but mainly on Marek's disease virus. To create a biologically meaningful diagnosis, attention should be dedicated to various biotic factors and not only of the diagnostic assay. Included among the important factors are, (a) the sample examined and the sampling strategy, (b) the outcomes of the pathogen amplification ex vivo, (c) the sampling time and its reflection on the disease diagnosis, (d) the impact of simultaneous multiple virus-infections regarding the ability to demonstrate all pathogens and inter- and intra-interactions between the pathogens. A concerted consideration of the relevant factors and the use of advanced molecular diagnostic assay would yield biologically significant diagnosis in real-time that would beneficiate the poultry industry.

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.

Uptake and spread of infectious laryngotracheitis vaccine virus within meat chicken flocks following drinking water vaccination

Vaccination against infectious laryngotracheitis virus (ILTV) in commercial broiler flocks in the field, which is only undertaken in the face of a local outbreak, requires mass administration techniques, usually via drinking water. This is often fraught with difficulties such as variable vaccination "reactions" and sometimes, vaccination failure. Laboratory testing of the outbreak strains however invariably shows the vaccines in use to be protective. To investigate this paradox, the dynamics of an ILT vaccine virus was examined within broiler flocks during a natural outbreak. In an initial flock, 70 birds were individually identified and had tracheal swabs collected sequentially at intervals from 1 to 26 days after vaccination and submitted for ILTV detection using qPCR. This evaluation was extended by collection of tracheal swabs from 40 to 45 random birds at 4, 7-8, 12-13 and 25-26 days post vaccination (pv) across a further 7 flocks. The results showed a very variable early uptake of vaccine virus from the drinking water (between 3% and 52% of tested birds with detectable virus in trachea at 4 days pv) and revealed that actual vaccination of the flocks relied on bird to bird transmission of the vaccine virus. In flocks with very low (<10%) initial bird uptake, successful exposure of vaccine virus to the majority of the flock can be delayed, leaving a large proportion of birds as susceptible at the likely time of possible exposure to wild virus. This may explain the cases of apparent failure of vaccination in the field. The variable bird to bird spread can be associated with reversion to virulence, this may explain the rolling vaccine reactions often observed. The variation in initial vaccine uptake may be affected by some factors involved with the administration technique and this requires further study in a larger sample size.

Glycoprotein D peptide-based diagnostic approach for the detection of avian infectious laryngotracheitis antibodies

Infectious laryngotracheitis (ILT) is a highly contagious respiratory disease of chickens, pheasants, and peafowl. It is caused by the alpha herpesvirus, infectious laryngotracheitis virus (ILTV). Glycoprotein D (gD) of ILTV is immunogenic and helps in its binding to the susceptible host cell receptor. In the present study, a recombinant gD protein was expressed in a prokaryotic system to develop a single serum dilution ELISA. In addition, two immunogenic peptides, corresponding to regions 77-89 and 317-328, were identified in gD protein. The peptides were synthesized using solid-phase peptide synthesis, purified using reversed-phase HPLC, and characterized using mass spectrometry. The peptides displayed a good titre and were found to be promising antigens to coat the ELISA plate to detect the ILTV antibodies in the serum sample. The developed ELISA showed 96.9% sensitivity, 87.5% specificity, and 95.3% accuracy as compared to OIE referenced standard indirect ILTV ELISA (whole viral coated). The assay may not differentiate vaccinated from infected birds when the flocks are administered with live attenuated vaccines. However, the assay could be useful to detect the disease condition in birds vaccinated with recombinant vaccine expressing glycoproteins other than gD. The developed ILTV single serum dilution ELISA could be an alternative to the existing diagnostics for the detection of ILTV antibodies.

Monitoring the uptake of live avian vaccines by their detection in feathers

Protection against diseases caused by the avian viruses, Marek's disease, Infectious laryngotracheitis, chicken anemia and turkey meningoencephalitis is achieved by live vaccines. The application quality is important to assure proper uptake in commercial flocks. We describe a novel evaluation method for the vaccination process by sequential monitoring the vaccine viruses in feathers. Feather collection is easy, non-invasive and non-lethal for the birds, therefore advantageous for monitoring purposes. To demonstrate the vaccine virus presence, an innovative assay of nested real-time amplification was approached because vaccine viruses presence in vivo is less abundant comparing to virulent wild-type isolates. The Marek's disease virus vaccine virus, Rispens/CVI988, in feathers of commercial flock was detected from 4 to 7 days and for at least 3 months post-vaccination, until the survey stopped. As the drinking water route was newly adopted for Infectious laryngotracheitis vaccination, one or two vaccine doses/bird were administered. The virus uptake was detected in feathers between 2 and 20 days-post-vaccination. With a doubled vaccine dose the positivity bird rate was higher. For the first time the chicken anemia vaccine virus presence in chicken feathers was demonstrated between 14 and 35 days-post-vaccination. No previous studies were available, thus in parallel to feathers the vaccine virus was demonstrated in the livers and spleens. The turkey meningoencephalitis vaccine virus uptake in turkey feather-pulps is even more innovative because this is the first turkey virus amplified from feather-pulps. The vaccine virus presence resemble the kinetics of the other 3 viruses, 3-21 days-post-vaccination. Detecting the specific antibodies following vaccination possessed a lower sensitivity than vaccine virus demonstration in feathers. In summary, the presented assay can be adopted for the quality evaluation of the vaccination process in poultry.

Differential amplification of Marek's disease CVI988 vaccine and of wild-type isolates from organs of commercial chickens using single or duplexed probes in real-time PCR

The differentiation of Marek's disease virus (MDV)-infected and vaccinated animal (DIVA) test, based on the MDV pp38 gene was described by Baigent et al. [(2016). Real-time PCR for differential quantification of CVI988 vaccine and virulent MDV strains. Journal of Virological Methods, 233, 23-36], using similar primers and alternate probes for virulent MDV-1 and the vaccine CVI988 virus. We explored the assay's applicability for commercial vaccines and commercial chickens, as the above-mentioned study employed tissue-cultured MDV strains and tissues from experimental trials. DNA of visceral organs and feathers of vaccinated or naturally infected chickens was used. Further, the applicability of the DIVA assay was evaluated using single or duplexed probes for the two viruses in the same amplification tube. Due to the high viral content in the commercial vaccines and in the clinical cases of MDV-1 infected commercial chickens, their examination by the MDV-1 DIVA real-time PCR was performed in one step. However, for the feather DNAs of commercially vaccinated birds, a step of pre-amplification was required. The MDV-1 DIVA real-time PCR performed as single probe in separate tubes using the Vir3 probe was very sensitive for virulent MDV-1 strains, but not very specific, as it also gave a clear signal with CVI988 vaccine virus. In contrast, the CVI vaccine probe was specific for CVI988, and did not recognize the MDV-1 strains. When both probes were present in one tube, the CVI probe showed a greater sensitivity for CV1988, while the Vir3 probe showed a much better specificity for virulent MDV-1.