Pathogenicity of Low-Virulence Marek's Disease Viruses in Normal versus Immunologically Compromised Chickens

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Abstract
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 wk 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 increased the incidence of MD induced by CU-1 and CU-2 viruses. The low virulence of these viruses is a reflection of an adequate immune response by the host.