Submitted to: Avian Diseases
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/14/2002
Publication Date: 10/1/2002
Citation: Witter, R.L. 2002. Induction of strong protection by vaccination with partially attenuated serotype 1 marek's disease viruses. Avian Diseases. 46:925-937. Interpretive Summary: Marek's disease (MD), a virus-induced cancer-like disease of chickens, is considered as a major disease problem in commercial poultry. Vaccination has dramatically reduced the incidence of the disease, losses still occur and very little is known about the best ways to derive better vaccines. The objective of this research was to determine if vaccines with mild virulence would provide better immunity than vaccines with no virulence. We have determined that mild vaccines provide better protection and, thus, have identified a new strategy for deriving highly effective vaccines. This important information about vaccine development will help scientists in academia and industry understand the best ways to derive better vaccines and eventually lead to better control of the disease.
Technical Abstract: Four of 5 partially attenuated preparations derived from the 2 Marek's disease (MD) virus strains (584A and 648A) or the previously attenuated Md11 strain induced 28-62% higher levels of protection in ab+ chickens against virulent MD challenge than the fully attenuated counterpart viruses. Confirmatory data from 2 subsequent trials in ab+ chickens showed that protection induced by the partly attenuated (passage 80) was 79 and 118% higher, respectively, than that induced by the fully attenuated (passage 100) preparations of strain 648A. However, in one trial with ab- chickens, no difference in protection between partially and fully attenuated virus was observed. Strong protection (up to 85%) against highly virulent challenge also was provided by preparations of 648A at passages 40-60, which were moderately oncogenic when used alone. Partially attenuated strains tended to replicate to higher titers in both ab+ and ab- chickens compared to fully attenuated vaccines. These studies establish for MD the principle that judiciously selected, partially attenuated MD viruses may replicate better and induce stronger immunity against virulent challenge than fully attenuated preparations of the same strain. Moreover, depending on passage level, some partially attenuated vaccine strains may be relatively safe for ab+ chickens, causing few or no lesions.