Classification of Marek's disease viruses according to pathotype: philosophy and methodology

The concept of pathotype in Marek's disease ( MD) probably dates from the recognition of a more virulent form of the disease in the late 1950s (Benton & Cover, 1957). Distinctions between MD virus strains were further expanded with the description of the vv pathotype in the early 1980s and of the vv + pathotype in the 1990s. Pathotype designations reflect important biological properties that correlate with the break-through of vaccinal immunity in the field. However, pathotyping methods applied by various laboratories have not been uniform, preventing critical comparison of results. Better uniformity of pathotyping procedures is desirable. The Avian Disease and Oncology Laboratory (ADOL) method is based on induction of lymphoproliferative lesions in vaccinated chickens. This method has been used to pathotype more than 45 isolates and is the basis for the current pathotype classification of MD virus strains. Its limitations include requirements for a specific type of chickens ( 15 x 7 ab +), large numbers of animals, and a statistical method to compare lesion responses to those of JM/102W and Md5 control strains. Because of these limitations, it has not been and is not likely to be used in other laboratories. Comparability in pathotyping can be improved by the comparison of field isolates with standard prototype strains such as JM/102W, Md5 and 648A ( American Type Culture Collection) or their equivalents. Data may be generated by different in vivo procedures that measure tumour induction, neurological disease ( both neoplastic and non-neoplastic lesions), or solely non-neoplastic criteria ( such as lymphoid organ weights or virus replication). Methods based on neoplastic criteria, especially when generated in MD-immunized chickens, will probably correlate most closely with that of the ADOL method and be most relevant to evolution of MD virus in the field. Based on data from several trials, a modification of the ADOL method that utilizes fewer chickens and can be conducted with commercial specific pathogen free strains is proposed. The modified method is based on "best fit'' comparisons with prototype strains, and is expected to provide results generally comparable with the original method. A variety of other alternative criteria ( see earlier) are also evaluated both for primary pathotyping and as adjuncts to other pathotyping methods. Advantages and disadvantages of alternative methods are presented.