[The occurrence of Marek's disease in genetically different groups of chickens]

The occurrence of Marek's disease was studied under laboratory conditions in four genetically different groups of chickens (Brown Leghorn, F1 hybrids of the CB x IA inbred lines, pullets of the paternal branch of the grandparent stock of the Hybro meat type, and final hybrids of the White Hisex layer type) after infection with the Georgia strain of Marek's disease (MD) virus, used in two doses (1600 and 16,000 PFU per one bird). MD was diagnosed on the basis of the occurrence of macroscopic tumours; when these tumours were absent in birds which had died within 105 days from infection, the dead bodies were subjected to microscopic examination (peripheral nerves, gonads, skin, bursa of Fabricius, thymus, spleen). The size of the parenterally administered dose of the virus had no significant effect on mortality, occurrence of tumours and the MD virus in any of the groups of chickens tested. However, there was a time shift in the mortality curve in chickens infected with a lower dose. The significantly highest occurrence of MD was recorded in BrL chickens (100%). A somewhat lower occurrence of MD was recorded in the Hisex White chickens (87.8%) and in the CB x IA hybrids (73.8%). However, the dead CB x IA chickens had a higher occurrence of tumours (96.6%) than the Hisex White chickens (77.1%). The lowest MD occurrence was recorded in the pullets of the Hybro meat type (25.5%). The organ most frequently affected by tumours after infection of the birds with the Georgia strain was the liver (24.1%).