Marek's disease (MD) is a contagious avian viral disease that is responsible for large economic losses to farmers. The disease is caused by Marek's disease virus (species Gallid alphaherpesvirus 2), which causes neurological lesions, immune suppression, and tumor proliferation of lymphoid cells that invade a large number of organs and tissues. Despite widespread vaccination, Marek's disease virus (MDV), has shown a continuous increase in its virulence and has acquired the ability to overcome immune responses induced by vaccines. In the present study, the oncogenic serotype MDV-1 was detected by real-time PCR in DNA samples extracted from organs developing tumor infiltrations. Identification of the pathotype based on a 132-bp tandem repeat and sequencing and phylogenetic analysis of the Meq gene and its encoded protein allowed classification of the isolated viruses as "very virulent", with two new and unique mutations in the Meq gene resulting in amino acid substitutions. Sequencing of pp38, vIl-8, UL1 and UL44 genes did not reveal any new mutations that were characteristic of the Tunisian isolates or correlated with virulence. These results raised concerns about the ability of HVT and CVI988 vaccines, which are currently used in Tunisia and other countries, to protect chickens against highly virulent virus strains.

The sheeppox virus belongs to the Capripoxvirus genus, which is one of the eleven genera of the subfamily Chordopoxviruses of the family Poxviridae. The genus Capripoxvirus consists of the sheeppox virus (SPPV), goatpox virus (GTPV), and lumpy skin disease virus (LSDV), which are sources of dermatological diseases in sheep, goats, and cattle, respectively [3].The viral genome is a double-stranded linear DNA of 150 kbp [4]. Each of the three species of the genus Capripoxvirus infects their natural hosts and is named according to the host origin; nevertheless, cross-species infections among sheep, goats, and cattle have been detected [5,6]. This makes differential diagnosis more difficult, and serological diagnosis based on detection of antibodies by immunofluorescence, ELISA, or gel immunodiffusion is inefficient [7]. In addition, serological methods cannot distinguish sheeppox infection from lumpy skin or goatpox disease [8]. Only molecular methods allow distinction between all these species through sequencing and molecular analysis of some viral genes [9,10].Several trials have been carried out to establish the relationship among these virus species by constructing phylogenetic trees after sequencing RPO30, P32, ORF117, and other genes [5,[11][12][13]. None of these genes have given the consistent results required to create a reference candidate to anchor phylogenetic classification within the genus Capripoxvirus. Therefore, it is essential to find a common gene that allows more relevant phylogenetic classification and distinction between these virus species without producing discrepancies between studies. Materials and methods Sample collectionSkin specimens with crusted scab lesions from the axilla and/or tail were collected from sheep suspected to be infected with pox virus (Table 1). Eleven samples were received from different regions of Tunisia and collected between 2010 and 2016, a period in which more than 344 outbreaks were reported [2].