Relationship between Marek's disease and the time course of viral genome proliferation in feather tips.

A total of 114 male chickens from three sire families of a commercial cross of White Leghorn chickens were infected with RB-1B Marek's disease (MD) virus at 21 days of age by exposing them to chickens previously inoculated with MD virus. The presence of virus in feather tips, feather pulp, and MD viral antibodies indicated all chickens became infected. The first virus-positive chickens were observed at 12 days postexposure (dpe). The frequency reached a maximum at 27 dpe and then decreased. At 80 dpe, when the experiment was terminated, no viral DNA was detected in the feather pulp of the surviving chickens (82%). Death from MD was first observed at 38 dpe and reached 18% by the end of the experiment, with spleen lesions being the major MD lesion. The viral genome titers in spleen extracts of chickens with MD lesions was negatively correlated with the time of death, and, similar to feather pulp, none of the surviving chickens was virus positive at the end of the experiment. Quantization of the viral genome titers in feather tip extracts at 27 and 38 dpe revealed a positive correlation with the presence of MD lesions, but only in the declining phase (38 dpe) and not at the peak (27 dpe) of the viral titer. Sire effects were significant, indicating the presence of genetic factors that affect viral proliferation. Again, significance was only observed at 38 dpe and not at 27 dpe. The results indicate that, in this commercial line, 1) all chickens were susceptible to infection via contact exposure, 2) all surviving chickens recovered from the viral infection, and 3) it is not sufficient to measure viral titers at a single time point when using viral titers as an endpoint for MD susceptibility.