Submitted to: Avian Pathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/1/2006
Publication Date: 4/1/2007
Citation: Dudnikova, E., Norkina, S., Vlasov, A., Slobodchuk, A., Lee, L.F., Witter, R.L. 2007. Evaluation of Marek's disease field isolates by the "best fit" pathotyping assay. Avian Pathology. 36(2):135-143. Interpretive Summary: Marek's disease (MD), a virus-induced cancer-like disease of chickens, is considered to be a major disease problem in commercial poultry. Vaccination has dramatically reduced the incidence of the disease, but the virus persists in chicken flocks and is becoming more capable of causing disease. The objective of this research was to evaluate a new procedure for determining the relative ability of different viruses to cause disease. We have found that a simple procedure will provide results similar to that of more complicated procedures conducted at reference laboratories in the United States. This important information will help scientists in academia and industry around the world to evaluate MD virus isolates by a uniform procedure, which will eventually lead to better control of the disease.
Technical Abstract: Although determination of pathotype is central to the study of Marek's disease field isolates, methods are not standardized and results from different laboratories may not compare well to the original Avian Disease and Oncology Laboratory (ADOL) assay. This study was designed to investigate the validity of the "best fit" pathotyping assay, a simplified method recently described by Witter et al (2005) for testing of field isolates of Marek's disease (MD) virus. Twenty serotype 1 MD viruses were isolated from 12 breeder and commercial flocks in 8 regions of the Russian Federation and were pathotyped by the best fit assay using vaccinated and nonvaccinated chickens from Schelkovo specific pathogen free (SPF) breeders. Lesion responses induced by field isolates were compared with those induced by reference strains JM/102W, Md5, and 648A representing pathotypes v, vv and vv+, respectively. Based on comparison to reference strains, we determined the pathotype of 8 isolates as vv+, 11 isolates as vv and 1 isolate as v. Lesion responses induced by the 3 reference strains consistently differentiated the respective pathotypes in non-vaccinated chickens and in chickens vaccinated with FC126 (serotype 3) alone or with a bivalent FC126 + 301B/1 vaccine (serotypes 3 and 2, respectively). Variation between reference strain responses in replicate trials was minimal. The criteria of most importance for classification of an isolate depended on its virulence. In some cases, calculation of the proportional distance between pairs of reference strains aided in the classification of field isolates. These results indicate that the "best fit" pathotyping assay can be conducted with local chicken strains and yields data that is consistent with the ADOL method. In addition, the pathogenicity of Russian isolates appeared comparable to that of United States isolates.