High-virulence Marek's disease virus infection in chickens previously infected with low-virulence virus.

Abstract

The pathogenicity of high-virulence (JM-10 isolate) Marek's disease virus (MDV) was compared in White Leghorn chickens with and without prior infection with a naturally low-virulence MDV isolate (CU-2) or the avirulent FC126 strain of turkey herpesvirus (HVT-4). Infection with CU-2 protected against JM-10 challenge. Route of infection of CU-2 was not a factor in protectivity. CU-2 infection was not protective against simultaneous challenge with JM-10, but only 2 days' prechallenge time with CU-2 infection conferred maximal protection in susceptible birds. One-day-old chickens were no more susceptible than those 4 weeks old to CU-2 infection, but the younger CU-2-infected birds were not protected against JM-10 challenge. Bursectomy at hatching time had 2 effects. It increased the incidence of MD with either CU-2 or JM-10 infection (statistically significant only with the latter). Also, it altered the lesion spectrum in CU-2-infected chickens that developed MD; visceral tumors occurred in bursectomized birds, but only neural lesions were seen in intact birds. There was less viral antigen in lymphoid organs, and degenerative lesions were prevented in JM-10- challenged birds with prior CU-2 or HVT-4 infection. Cell-associated infectivity (both HVT-4 and JM-10) was present in spleen cells of those inoculated with both viruses, and the level of JM-10 infectivity was similar to that of chickens receiving only JM-10.