Humoral immune responses to inactivated oil-emulsified Marek's disease vaccine

When inactivated Md11/75C vaccine was inoculated into 1-day-old chickens, it stimulated antibodies detectable by enzyme-linked immunosorbent assay (at a titer of 6400) and indirect fluorescent antibody test (at a titer of 640), but lacking virus-neutralizing activity. Chickens passively inoculated with these antibodies were protected against bursal atrophy, weight loss, and early mortality when challenged with the virulent Md5 strain of Marek's disease virus (MDV). That led to the conclusion that virus-neutralizing activity is not a prerequisite for protection. In another experiment, antibody titers of adult chickens previously primed by exposure to live turkey herpesvirus and MDV did not increase after immunization with inactivated oil-emulsion MDV vaccines. This result provides little hope that Marek's disease can be controlled in progeny chickens by maternal immunity derived from hyperimmunized parents.