Pathogenicity of low-virulence Marek's disease viruses in normal versus immunologically compromised chickens.

To learn whether "low virulence" isolants of Marek's disease (MD) virus have a low inherent oncogenic potential, 3 isolants (CU-1, CU-2, S-11) were given to genetically susceptible S-strain or P-line birds with or without immunosuppressive treatments. Based on lesion incidence and type during an 8-10-week experimental period, embryonal (17-day) bursectomy and neonatal cyclophosphamide (CY) treatment were without effect on CU-1 and CU-2 infections in P-lines. S-11 virus was virulent for P-lines, and CY treatment reduced MD incidence as reported by others for virulent virus infections. Neonatal thymectomy or infection in ovo (at 8 days of incubation) or use of the more susceptible S-strain chicks all increased the incidence of MD induced by CU-1 and CU-2 viruses. It was concluded that the "low virulence" of these viruses is a reflection of an adequate immune response by the host.