THE DEVELOPMENT AND EVALUATION OF 2 TISSUE CULTURE-GROWN MAREKS-DISEASE CHALLENGE VIRUSES

Standardized challenge viruses are essential for the evaluation of Mar ek's disease ((MD) vaccines with many MD challenge preparations consis ting of lymphocytes or whole blood from infected birds. Virus present in these preparations is difficult to quantify by tissue culture assay s and, therefore, the infectious bird dose and long-term storage viabi lity cannot be assured. We report on the properties of two low-passage virulent Australian MD viruses, the Woodlands No. 1 strain and strain MPF 57, Both strains were isolated in chicken embryo kidney cultures and adapted to grow in chicken embryo fibroblast cultures for a maximu m of 14 passages. Both strains could be readily assayed in tissue cult ure and produced titres of 10(3) to 10(4) 50% tissue culture infectiou s doses per mi (TCID50), Birds inoculated at three different doses wer e observed over 10 weeks, and tissues examined for gross and histologi cal lesions, bursa:body weight ratios and the presence of viraemia, Ti ssue culture-grown preparations of both strains were only slightly les s virulent than the original lymphocyte challenge material and produce d similar pathological responses and around 80% death or gross lesions . From bursa:body weight ratios strain MPF 57 appeared to be more viru lent than the Woodlands No. 1 strain.