Marek's disease--survey on vaccination failures

Twelve years after the initial development of efficacious vaccines against Marek's disease--prepared mostly from HVT--and their subsequent use on a large scale, there is a general opinions that failures have been increasing in incidence and in severity. Many factors have been held responsible for these failures, and, though efforts are being made to obviate them, no acceptable solutions have been found sofar in the control of the disease in the field. If mistakes in the preparation, storage handling and administration of vaccine are excepted, because these mistakes, though not infrequent, are easier to prevent, the main causes that must be considered are: 1. The heavy and very early exposure to infection, in houses that are not well disinfected or when birds are reared for many cycles in the same place without cleaning or disinfecting, or in multi-age farms, where habits become established, particularly in broilers and pullet rearing, and also for economic reasons. 2. The considerable variability in the susceptibility to MD of different genetic strains of commercial chickens. 3. The appearance of the supposed new biological variants of virus, as recently found in the U.S. The purpose of this study has been the evaluation of the influence of these causes on the more and more frequent outbreaks of MD in our country. While the importance of the genetics and of the degree of and age at exposure has been irrefutably proven, data concerning the appearance of biological variants of MD virus are still short and controversial.