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.