Prevalence of Marek's Disease Virus in Different Chicken Populations in Iraq and Indicative Virulence Based on Sequence Variation in the EcoRI-Q (meq) Gene

Abstract

A cross-sectional survey was conducted in six provinces in southern Iraq to determine the point prevalence of Marek's disease virus (MDV) in different chicken populations followed by sequencing the 'meq' gene for phylogenetic analysis and virulence-associated polymorphisms. A total of 109 samples from unvaccinated flocks were analyzed comprising 52 dust and 30 spleen samples from commercial broiler farms and 27 spleens from local layer chickens purchased in the town markets. The overall prevalence of MDV was 49.5% with no significant differences between provinces (P = 0.08) or sample types ('P' = 0.89). Prevalence ranged from 36.8% in Karbala and Nasiriyah to 65% in Amarah. The percentages of positive samples were 59.1%, 46.7%, and 48.1% in broiler dust, broiler spleen, and layer spleen, respectively. The overall mean (±SEM) Log₁₀MDV viral copy number per milligram of dust or spleen as determined by quantitative PCR was 1.78 ± 0.19, with no significant differences between provinces ('P' = 0.10) or sample types (P = 0.38). In positive samples only, the overall mean was 3.43 ± 0.18. Sequencing of the 'meq' gene from samples that showed high levels of MDV target in qPCR testing was attempted. Nine samples were sequenced. These sequences were compared with 'meq' sequences of MDVs of different pathotype. All the Iraqi MDVs had a short 'meq' gene of 897 base pairs because of the deletion of 123 bp relative to the reference strain Md5. The Iraqi 'meq' sequences also contained single-nucleotide polymorphisms, resulting in differences in the amino acid sequence. All of the nine Iraqi 'meq' genes encoded two repeats of four-proline sequences. The published negative association between four-proline repeat number and MDV virulence suggests that the Iraqi MDVs are likely to be highly virulent, but this needs to be confirmed by 'in vivo' testing. Taken together, these results indicate that MDV is common in unvaccinated commercial and village chickens in southern Iraq, that there is limited 'meq' gene sequence variation, that all sequenced samples had a short 'meq' with two four-proline repeats, and that this is consistent with a high level of virulence.