USA - Understanding how chicken embryo ILT vaccine can cause disease

The main objective of this study was to evaluate the transmission and pathogenicity of ILTV (Infectious Laryngotracheitis Virus) chicken embryo origin (CEO) vaccine derived viral subpopulations. Dr. Maricarmen GarcÌa and Dr. John Glisson, of the M.A.M. Department of Avian Medicine Poultry Diagnostic and Research Center in Athens, Ga., recently completed the research funded by the U.S. Poultry & Egg Association. Chicken embryo origin viral subpopulations ("A" and "B") were separated as individual plaques in the choriallantoic membrane of chicken embryos, plaques were purified and the glycoprotein E gene of viral subpopulations amplified and sequenced to verify their purity. Nucleotide difference at position 629 of the glycoprotein E (gE) gene was identified for each subpopulation indicating the separation of viral subpopulations, and the ability of each subpopulation to replicate independently. The transmission and pathogenicity of CEO viral subpopulations was compared to the parent CEO vaccine. The pathogenicity of subpopulations "A" "B" and parent vaccine CEO was evaluated by scoring clinical signs on inoculated and contact-exposed birds. Overall, no deaths were recorded among inoculated or contact-exposed birds in any of the three viral treatments. The most common clinical sign recorded, among all viral treatments, was swollen sinus tissues, followed by mild to moderate conjunctivitis, mild to moderate respiratory rales, and nasal exudates. Viral subpopulation "B" produced significantly higher overall clinical sign scores than viral subpopulation "A". No significant differences in lesions were observed on histopathological examination of tissues (trachea, larynx, and eye lid) from birds exposed to viral Sub-"B, Sub-"A" or the parent CEO vaccine. All three viruses produced similar lesions. The larynx was the site where more dramatic changes were observed, including significant goblet cell necrosis and lymphocytic infiltration. Intranuclear inclusions and syncytia cell formation were not detected in tissues collected from birds in any of the three viral treatments. The transmission of the viral subpopulations was evaluated on contact-exposed birds by comparing the presence of the virus, as detected by virus isolation, the presence of viral DNA, as detected by PCR, in the trachea at different times post-exposure. Virus isolation and PCR analysis indicated that Sub-"B" DNA persisted for a longer period of time in the trachea than viral Sub-"A." However, viral Sub-"A" was characterized as a highly transmissible virus, as it cross-infected subpopulation "B" exposed birds and the negative control group of birds. In addition, viral subpopulation "A" favored migration to the trigeminal ganglia, as Sub-"A" DNA was predominately detected in trigeminal ganglia, while viral Sub-"B" did not spread as efficiently to the trigeminal ganglia. These results indicated that CEO viral subpopulations "A" and "B" differ in their ability to spread from bird to bird, and in their ability to migrate to the trigeminal ganglia, suggesting that not all ILTV strains have the same ability to establish latency. Previous work in our laboratory indicated that 69 percent of the "CEO like" outbreak-related isolates are subpopulation "A." Whether the greater ability of ILTV subpopulation "A" to rapidly spread and migrate to the trigeminal ganglia is related to the loss of attenuation of CEO vaccines when circulating in the field needs to be further studied. Overall, these results have important implications in future evaluations of the efficacy and safety of ITLV vaccines.