Quantitative pathogenicity of Marek's disease virus and vaccine virus strains in chickens: macroscopical aspects.

Two virulent strains (JM and K) and one vaccine strain (CVI 988) of Marek's disease virus (MDV), together with two vaccine strains of the herpesvirus of turkeys (HVT) (FC 126 and PB-THV 1), all in the cell-associated state, were administered intramuscularly at 3.7 log TCID50 per dose to day-old SPF White Leghorn chickens. A control group of chicks received uninfected cells. The pathological parameters studied were onset and duration of clinical symptoms, mortality, bird weight and macroscopical lesions of peripheral nerves and visceral organs. Data were obtained from females autopsied at the age of 3, 8 and 20 weeks, and from chickens which died. Virological and serological data were procured mainly from males taken at various ages. The results indicate a clear distinction between virulent and vaccine strains. MD vaccines had no significant influence on bird weight and caused no mortality or macroscopical lesions, whereas the virulent MDV strains produced all these effects. Macroscopical lesions caused by the virulent MDV strains were seen predominantly in nerves (in about 50% of birds succumbing to MD) and gonads (in 0% to 80% of such birds depending on sex and on strain of MDV). Differences between the two virulent strains could be demonstrated. Strain JM induced earlier incidence and shorter duration of clinical disease. With strain JM death occurred earlier in females than in males. Strain K caused significantly more macroscopical lesions in gonads, heart and liver. Under the conditions of the experiment, detection of macroscopical lesions after inoculation with a virulent MDV strain was possible 3 weeks after inoculation.

Absence of Marek's disease virus antigen in the feather follicle epithelium of vaccinated chicks demonstrated by immunofluorescence.

In chickens vaccinated with MDV strain CVI 988, HVT strain FC 126 or PB-THV 1, no MDV-specific antigens could be demonstrated in the feather follicle epithelium by immunofluorescence (IF). In chickens given virulent MDV strains, the epithelium of the feather follicle was positive in IF. In an experiment where chickens were vaccinated with strain CVI 988, positive IF was observed in the lung, bursa and pancreas, but not in the feather follicle epithelium, kidney, cloaca, or caecal tonsils. Absence of IF antigen in the skin may be indicative of avirulence of the strain of MDV.