Laryngotracheitis herpesvirus infection in the chicken: The role of humoral antibody in immunity to a graded challenge infection

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Abstract
The clinical responses of SPF White Leghorn chickens to graded levels of infection with virulent (wild‐strain) infectious laryngotracheitis (ILT) herpesvirus, administered by the tracheal route, were investigated in chickens from 1 day to 8 weeks of age. In 1‐day‐old chickens 40 plaque forming units (PFU) of ILT virus caused 55% mortality within 8 days. At least 500 PFU was needed to achieve comparable mortality at 3 weeks of age and this increased to 4,500 PFU of ILT virus by 6 to 8 weeks of age. A combination of surgical bursectomy at 1 day old and cyclophosphamide treatment ablated the ability of chickens to generate humoral antibody to ILT virus, but did not impair the level of protection induced by commercial ILT vaccine. Further, the passive transfer to ILT antibody‐positive serum to 2‐day‐old or 4‐week‐old chickens did not significantly alter their susceptibility to tracheal challenge with virulent virus. Serum antibody was therefore discounted as a major immune mechanism in resistance to ILT virus infection. An experimental inactivated vaccine to ILT virus was also investigated. One intramuscular injection induced low serological responses, but no significant protection to ILT. A second injection of inactivated vaccine only marginally increased the titre of humoral antibody, but seemed to reduce the degree of respiratory distress. However, the levels of protection afforded by the inactivated vaccine were not significant compared with a live commercial ILT vaccine,