Long-Term Studies of Marek's Disease Infection in Individual Chickens

Parameters of infection with Marek's disease herpesvirus (MDV) were studied in a flock of naturally infected White Leghorn chickens during a 76-week period. Infection appeared to persist indefinitely in the host chicken although the virus titers were variable and frequently low. Transmission of infection by contact was demonstrated for some 76-week-old survivor birds and seemed related to the titer of viremia and presence of virus in the integument. Titers of Marek's disease (MD) precipitins peaked prior to the 28th week, and in a few cases eventually disappeared; however, antibody detected by the indirect fluorescent antibody test remained undiminished. The persistence of precipitins appeared related to persistence of viral antigen. Birds with high virus titers died more frequently of MD than other birds, but virus pathogenicity could not be implicated as a determinant of survival. Duration of survival was related directly, although not causally, to antibody titer. Response differences among sire families were noted, but progeny tests showed that absence of detectable viremia was unreliable as a marker for genetic resistance.