[Effectiveness of a Bulgarian vaccine against Marek's disease in broilers].

Studies were carried out to establish the effectiveness of the immunization of broilers against Marek's disease using a Bulgarian vaccine produced with strain C3-1 of a turkey herpes virus. The experiments were done under the conditions of an industrial poultry dressing combine with a total of 52,450 day-old hybrid birds vaccinated at the rate of 1000 plaque-forming units (PFU), 39,450 birds vaccinated with 500 PFU each, and 91,564 unvaccinated control birds. After the 60-63-day fattening period was over the average value of the mortality rate was 2.17 per cent and 2.48 per cent in the vaccinated birds and the controls, respectively. The vaccinated groups of birds showed also a comparatively higher weight gain--by 73 g, on an average, recorded prior to slaughter--as against the control groups. No differences were found in these indices between the groups immunized with 1000 and 500 PFU. In a laboratory test with 255 birds divided into four groups immunized with 100, 500, 250, PFU and controls, challenged in the third week with a pathogenic herpes virus it was found that up to the 6th month the first two groups developed Marek's disease as accounted by 1.65 per cent of the birds, while those vaccinated at the rate of 250 PFU were not properly protected, showing a higher morbidity rate--9.23 per cent. It is concluded that the Bulgarian vaccine--strain C3-1--confers solid immunity in a dose of 500 PFU, the vaccination of broilers being economically profitable.