1 INTRODUCTION
Marek’s disease (MD) causes huge economic impact by directly causing significant levels of mortality due to visceral lymphomas. In addition, the sub-clinical immunosuppression induced by virulent strains also cause production loss indirectly (Gimeno, 2014). The Marek’s disease virus (MDV), classified as gallid alphaherpes virus 2 (GaHV-2), belongs to the genus Mardivirus of the subfamilyAlphaherpesvirinae , family Herpesviridae (International Committee on Taxonomy of Viruses, 2017). MDV has been divided into three serotypes as serotype -1, -2 and -3 (Bulow and Biggs, 1975). Of these three serotypes, MDV-1 includes all oncogenic strains, whereas serotype 2 and 3 includes non-oncogenic and avirulent strains, respectively (Witter et al., 2005). The virulence of the MDV-1 has been shifted from mild (m) to virulent (v), very virulent (vv) and very virulent plus (vv+) over a period due to mutations and positive selection (Witter, 1997; Gimeno and Schat, 2018).
Although the disease is effectively controlled by vaccination and bio-security measures, outbreaks due to emerging virulent strains are observed in recent times worldwide including India (Nair, 2018). In India, both monovalent (serotype 3/ HVT) and bivalent (serotype 2 SB1/ SB303 & serotype 3/ HVT) are currently used in commercial layers and breeder flock. Despite intensive vaccination, outbreaks in vaccinated flocks are noticed in different parts of dense poultry rearing zones (Suresh et al., 2015; Gupta et al., 2016; Prathiba et al., 2018; Puro et al., 2018). Various factors are involved in the MD outbreaks of vaccinated flocks including faulty vaccination procedure, mycotoxin in feed and concurrent immunosuppressive diseases etc. Emerging virulent strains due to high mutation rates in oncogenes being one of the major factors among them.
Although in vivo pathogenicity studies are mandatory to ascertain the pathotype of emerging MDV strains (Witter et al., 2005; Dudnikova et al., 2007), the assays are tedious and complex. On the other hand, molecular analysis of certain marker genes specifically Meqoncogene has been proved valuable by recent studies (Mescolini et al., 2019). Some of the unique oncogenes namely Marek’s EcoRI-Q encoded protein -Meq (Jones et al., 1992), phosphoprotein-pp38(Cui et al., 1999) and vIL- 8 (viral interleukin-8) (Parcell et al., 2001) were associated with oncogenicity and pathogenicity. Their potential as pathotype markers has been demonstrated successfully by recent workers in emerging field strains from several countries viz., China (Tian et al., 2011), USA (Padhi and Parcell, 2016), Colombia (Lopez-Osorio et al., 2017) Japan (Abd-Ellatieff et al., 2018) and Italy (Mescolini et al., 2019).
In the present study, we investigated the Indian MDV field strains from MD visceral tumors occurred in vaccinated poultry breeder flocks and analyzed three oncogenes viz., Meq, pp38 and vIL-8 of 17 MDVs to ascertain their virulence pathotype.