Reduced Vaccinal Protection of Turkey Herpesvirus Against Field Strains of Marek's Disease Herpesvirus ,

Abstract

Four isolates of Marek's disease herpesvirus (MDV) were obtained from 21-week-old broiler breeders with lesions and mortality from Marek's disease (MD). They had been properly vaccinated with turkey herpesvirus (HVT). The isolates were designated Ala-7, 8, 9, and 10. These 4 MDV strains were compared with the GA isolate of MDV in chickens vaccinated with graded doses of HVT. Challenge with the MDV strains was also with graded doses. All 5 isolates of MDV were similar in virulence. However, protection afforded against MD lesions by HVT was significantly lower in the Ala-7, 8, and 9 isolates than with the Ala-10 or GA isolates. The Ala-7, 8 and 9 isolates of MDV induced small-cell plaques in chick embryo fibroblasts, the Ala-10 isolate induced both large and small cell per plaque types, and the GA isolate produced predominantly small cell plaques with a few plaques of the large cell type.

Pathogenicity of a Field Marek's Disease Virus Isolate (VUB-83) for Chickens of Three Genetically Different Types

“…Isolates of similar properties were described by Witter et al (1980), Eidson et al (1981) and Powell and Lombardini (1986). In contrast to the findings of Witter et al (1980), however, "the early mortality syndrome" was not observed by us in the first post-infection weeks.…”

“…A major ecological factor is the degree of pathogenicity of the causative agent -Marek's disease virus (MDV). A number of studies have demonstrated the existence in poultry populations of MDV strains of different pathogenicity, ranging from apathogenic to highly virulent strains characterized by increased oncogenicity for both vaccinated and unvaccinated chickens regardless of their genetic susceptibility or resistance (Witter et al 1980;Eidson et al 1981;Powell and Lombardini 1986).…”

Characterisation of two highly oncogenic strains of Marek's disease virus12

“…ALA-8 was a gift from C.S. Eidson (University of Georgia, Athens, GA) and was also isolated from a flock vaccinated with HVT but experiencing a MD break (Eidson et al, 1981). JM-10, a moderately oncogenic clone of the JM strain of MDV has been described before (Calnek, 1973).…”

“…Chickens vaccinated with HVT and subsequently challenged with such highly oncogenic viruses were indeed poorly protected (Witter, 1982;Eidson et al, 1981). In this study, we compared the pathogenesis of two of these, RB-1B and ALA-8 (Eidson et al, 1981), with that of the well-studied JM strain of MDV (Sevoian et al, 1962). The aim was to learn if these more highly oncogenic viruses displayed any unique pathogenetic features which would account foT their ability to induce MD in vaccinated birds.…”

Marek's disease virus isolates with unusual tropism and virulence for ocular tissues: Clinical findings, challenge studies and pathological features

“…Very virulent MD viruses have been described which are poorly protected against by vaccination with HVT (Eidson et al, 1978;Eidson et al, 1981;Schat et al, 1982;Witter, 1982Witter, , 1988Witter & Fadly, 1980). Satisfactory protection against many of these viruses has been achieved by implementing vaccination with a bivalent vaccine comprising virus strains of serotypes 2 (SB-1, a nononcogenic strain of MD) and 3 (HVT) (Calnek et al, 1983;Witter, 1982Witter, ,1983Witter, , 1984Witter et al, , 1985.…”