Diversity of Marek's Disease Virus Strains in Infections in Backyard and Ornamental Birds

Abstract

Marek's disease is caused by Mardivirus gallidalpha2, commonly known as Marek's disease virus (MDV). This pathogen infects various bird species resulting in a range of clinical manifestations. The meq gene, which is crucial for oncogenesis, has been extensively studied, but molecular investigations of MDV in noncommercial South American birds are limited. This study detected MDV in backyard and ornamental birds from Brazil and Peru and characterized the meq gene. MDV was confirmed in all seven outbreaks examined. Three isoforms of meq (S-meq, meq, and L-meq) and two to seven proline repeat regions (PRRs) were detected among the sequenced strains. At the amino acid level, genetic profiles with low and high virulence potential were identified. Phylogenetic analysis grouped the sequences into three distinct clusters. Selection pressure analysis revealed 18 and 15 codons under positive and negative selection, respectively. The results demonstrate significant MDV diversity in the studied birds, with both high and low virulence potentials. This study highlights the importance of monitoring and characterizing circulating MDV in backyard and ornamental birds, as they can act as reservoirs for future epidemiological outbreaks.

Keywords: Indian Giant; Indian peafowl; Mardivirus gallidalpha2; Silkie chicken; backyard chicken; meq; oncogenic virus; phylogenetic tree; red junglefowl; selective pressure.

Figure 1

PCR amplification of the meq gene in the studied cases. Lanes: M (100 bp DNA Ladder, Invitrogen, Carlsbad, CA, USA); 1 (USP-386); 2 (USP-1171); 3 (USP-1540); 4 (USP-1790); 5 (USP-1873); 6 (USP-2429); 7 (USP-2583); 8 (negative control); 9 (positive control, CVI988 vaccine). In lines 1, 6, and 9, a PCR artifact of approximately 600 bp was observed.

Figure 2

Maximum-likelihood phylogenetic tree of 387 complete meq gene sequences, including six strains from this study. The tree was inferred using a GTR + R substitution model with support values based on 1000 bootstrap replicates indicated on the branches. Additional annotations include phylogenetic cluster (inner ring), isolation country (middle ring), and number of PRRs (outer bars). The MDV strains from this study are highlighted in bold. The scale bar represents the number of substitutions per site.