Isolation of very virulent Marek's disease viruses from vaccinated chickens in Australia

Abstract

Very virulent Marek's disease viruses (vvMDV), defined as isolates against which the herpesvirus of turkey (HVT) vaccine provide poor protection, have been isolated from poultry flocks in both the United States and Europe. Twenty-one samples from vaccinated Australian flocks, experiencing problems with excessive Marek's disease (MD), were tested for the presence of transmissible MD viruses (MDV). Of the 16 samples which contained a transmissible agent, 14 were pathogenic in chickens, based on the development of MD lesions or depression of the bursa/body weight ratio. Of the pathogenic isolates which have been successfully typed 10 were serotype 1, and one was serotype 2 MDV. Pathogenicity of isolates varied. Several isolates caused tumours in 20-30% of both vaccinated and unvaccinated chickens. Two isolates, MPF6 and MPF23, caused tumours in more than 50% of chickens. When MPF6 and MPF23 were tested in vaccine trials bivalent vaccine gave no better protection against development of MD lesions than a monovalent vaccine. Isolate MPF23 was so pathogenic that lesions were produced in all chickens, regardless of the vaccine protocol used. Therefore vvMDV have been isolated in Australia, and unlike the vaccines tested overseas, bivalent Australian vaccines do not appear to provide greater protection against these vvMDV.

Pathotyping of Australian isolates of Marek's disease virus and association of pathogenicity withmeqgene polymorphism

“…Despite the absence of blanket vaccination of broilers in Australia, previous studies have shown that MDVs with many characteristics of very virulent strains, against which HVT provides limited protection, were circulating in the Australian poultry industry prior to the outbreaks of MD in the 1990s. These isolates were first identified in Australia in 1985(McKimm-Breschkin et al, 1990 and MDV was subsequently isolated from outbreaks of MD in vaccinated broiler breeders, layers and broilers (Zerbes et al, 1994;De Laney et al, 1995). These findings are consistent with vaccine failure and the evolution of Australian MDVs towards greater pathogenicity over time.…”

“…It is difficult to compare the results of the present experiment directly with those of the most comprehensive work of this kind in Australia previously, that of McKimm-Breschkin et al (1990). Different chicken strains, virus challenge protocols, and HVT vaccine isolates and doses make the results difficult to compare.…”

“…The SPAFAS Australia SPF chicken strain used in the present experiment is derived from the CSIRO HWL. This strain was found by McKimm-Breschkin et al (1990) to be one of the more MD-resistant strains of birds, more resistant than the strain NBSL that was used for the subsequent pathotyping studies in that study. Bearing this in mind, some of the more virulent viruses in the McKimm-Breschkin et al (1990) study (MPF23, MPF15) produced results comparable with the more virulent viruses in the present experiment.…”

“…This strain was found by McKimm-Breschkin et al (1990) to be one of the more MD-resistant strains of birds, more resistant than the strain NBSL that was used for the subsequent pathotyping studies in that study. Bearing this in mind, some of the more virulent viruses in the McKimm-Breschkin et al (1990) study (MPF23, MPF15) produced results comparable with the more virulent viruses in the present experiment. Vaccination with HVT (250 to 1000 PFU, probably cell-free) provided no protection at all against MD gross lesions when challenged with MPF23, the most virulent virus in that study.…”

“…The overall objectives of the study were to confirm the putative pathotype range of Australian MDVs previously ascribed on the basis of less standardized studies (McKimm-Breschkin et al, 1990;De Laney et al, 1998) and to enable comparison with pathotypes reported internationally, and to investigate whether there are differences in the meq gene of Australian isolates of MDV1 that correlate with observed differences in the virulence of the isolates.…”