Submitted to: Annual Conference on Vaccine Research
Publication Type: Abstract Only
Publication Acceptance Date: May 6, 2002
Publication Date: N/A
Technical Abstract: Vaccines are the cornerstone of programs to control infectious diseases in chickens. Marek's disease (MD) is caused by a cell-associated alpha herpesvirus and is characterized by lymphomas, neurological disease and immunosuppression. The disease has a rapid onset and can result in up to 100% mortality. Licensed vaccine strains from all 3 MD viral serotypes are in use. There are several unique features of MD vaccines. Vaccines typically consist of cryopreserved suspensions of infected cell cultures and are administered to virtually all commercially-reared chickens. Vaccines are administered by inoculation at hatch or by inoculation into the amnionic sac of 18-day embryos. Immunity is usually well established 5-7 days after vaccination, even in the presence of maternal immunity. Early immunity is necessary to counter massive early exposure in the field. Vaccination protects against pathologic and immunosuppressive responses but not against infection or shedding. The efficacy of different MD vaccines varies but in practice vaccination is uncommonly effective, providing more than 95% protection in the field. Serotype 2 and 3 vaccine strains interact synergistically. Host genetic factors influence vaccine efficacy and involve interactions between Mhc alleles and vaccine serotypes. Some chickens develop MD at older ages for reasons that may involve immunosuppressive stress. The widespread use of vaccines has resulted in the emergence of highly virulent strains, which is one of the major challenges to long term control of this disease. Efforts to counter this problem are divided between the quest for more effective vaccines and strategies to slow the pace of evolutionary change.