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

Abstract

We report the pathotyping of six Australian isolates of Marek's disease virus-1 (MDV1) isolated between 1992 and 2004 and association of virulence with meq gene polymorphism. Unvaccinated and herpesvirus of turkeys (HVT)-vaccinated specific pathogen free chickens were challenged at day 5 with 500 plaque forming units of Marek's disease virus. The isolates induced gross Marek's disease lesions in 53 to 94% of unvaccinated chickens, and HVT induced a protective index ranging from 38 to 100% by 56 days post challenge. This experiment provides evidence that current Australian isolates of MDV1 vary significantly in pathogenicity. However, there was no clear evidence that the most virulent recent isolates were more pathogenic than isolates from the 1980s or that any of the isolates belong to the highest pathotype category of very virulent plus. Evidence is presented that virulence can be predicted by measurements taken as early as 13 days post challenge. The meq gene sequences of five of the isolates used in the experiment were determined. When compared with the very virulent US isolate Md5, there was a 177 base-pair insertion and distinct point mutations in each of the five isolates. There were no individual mutations in the meq sequences that correlated with levels of virulence. However, amino acid alignment of the five Australian and 14 international isolates revealed that the number of repeat sequences of four prolines (PPPP repeats) in the meq gene (overall range 2 to 8) was strongly associated with virulence across all isolates, with the most pathogenic isolates having the fewest number of repeats. The results suggest that the presence of the 177 base-pair insertion alone is not an indicator of attenuation. Rather, the number of PPPP repeats, independent of the presence of the insertion, is a better indicator of pathogenicity.

Pathogenic characteristics of Marek's disease virus field strains prevalent in China and the effectiveness of existing vaccines against them

“…Our results and those of other laboratories (Renz et al, 2012;Walkden-Brown et al, 2013) raised the same issue of an association between virulence and pathotype. In their studies, MPF57 was highly virulent, as evidenced by the very high incidence of MD in unvaccinated chickens.…”

Pathological investigation of Marek’s disease in a pullet farm of Bangladesh and detection of Marek’s disease virus by pcr

“…In MD, although the bursa is sometimes involved in the lympho-proliferation, the tumor is less apparent, diffuse and inter-follicular in location. Peripheral nerve lesions are not a feature of lymphoid leukosis but the lesion is predominated in MD (Renz et al, 2012).…”

“…While MD vaccines reduce virus replication and prevent tumor formation, they do not induce sterilizing immunity; vaccinated chickens are still susceptible to infection with pathogenic strains of MDV that replicate in the host and shed into the environment in the absence of clinical disease. The inability of MDV vaccines to prevent virus replication following field challenge (''imperfect vaccination'') is thought to have contributed to the worldwide increase in the virulence of MDV over the last 40 to 50 years, as has been postulated as a probable cause of change for other pathogens (Gandon et al, 2001;Renz et al, 2012). This is for the first time in Bangladesh reported the occurrence of very virulent Md/5 (MDV1 GA strain) infectivity in pullet with higher rate of morbidity and mortality.…”

“…The Meqgene is constantly expressed through latency and in all tumors (Kung et al, 2001;Liu et al, 1999). Meq gene has got an anti-apoptotic function and the potential to stimulate cell growth and transformation by MDV-1 (Liu et al,1999;Renz et al, 2012).In this study PCR amplification was carried out targeting fragment of Meq gene with the DNA extracted from liver, spleen, heart, kidney, skeletal muscle, feather follicles, sciatic nerve, intestine and lungs of infected pullets. DNA extracted from the liver, spleen, kidney, feather follicles and lungs found to generate expected (317bp) amplicon.…”

“…The Md/5 or MDV1 (GA strain) is a very virulent strain of MD virus (OIE, 2010) the viruses also causes higher rate of morbidity and mortality. To enable systematic virulence classification of MDV isolates on the basis of the pathology (Renz et al, 2012) they induce, and their ability to overcome the effects of vaccination, a standardized MDV pathotyping scheme was developed by the USDA ADOL may be used. Virulent MDV1 challenge strains were graded according to the protection induced by HVT and HVT/ MDV2 bivalent vaccines (Witter 1997;Witter et al, 2005).…”