Molecular Phylogeny of Theileria equi in Horses of Khuzestan Province Based on ۱۸S Ribosomal RNA

Introduction: Equine piroplasmosis is a tick-borne disease caused by two protozoan parasites, Babesia caballi and Theileria equi (formerly Babesia equi), which are transmitted by Ixodid ticks.Materials & Methods: In this study, a ۴۳۵ bp fragment of the ۱۸S ribosomal RNA (rRNA) gene was amplified from horse blood samples collected in Khuzestan Province, Iran, to assess the parasite’s molecular phylogeny and predicted RNA secondary structure.Results: Taxonomic analysis of the amplified sequence, designated TeKh, yielded ۱۰۸ matches within the phylum Apicomplexa. Of these, ۱۰۴ hits belonged to the family Theileriidae, and ۸۶ were affiliated with the genus Theileria. Comparative analysis revealed that TeKh is identical to the only previously reported T. equi sequence from Fars Province, Iran (MK۶۱۵۹۳۳). Further multiple sequence alignment showed high similarity: ۹۸.۷۱% to a T. equi isolate from Russia (OM۴۷۵۵۲۳) and ۹۹.۷۴% to one from Turkmenistan (OL۶۳۸۱۹۵). Structural predictions using the RNAfold algorithm indicated a thermodynamic free energy of −۱۲۱.۹۸ kcal/mol, suggesting a stable RNA secondary structure. The phylogenetic tree revealed that sequences in cluster A, which included isolates from diverse countries, showed minimal divergence, indicating high genetic conservation of T. equi across geographic regions. In contrast, cluster B exhibited greater genetic variability, including T. bicornis (AF۴۹۹۶۰۴) from South Africa and Theileria sp. (KF۵۹۷۰۷۸) from Kenya, while T. cervi (KT۹۵۹۲۲۷) from China formed a separate branch with moderate bootstrap support (۴۴%).Conclusion: In conclusion, the T. equi sequences isolated from Khuzestan and Fars provinces in Iran showed no detectable genetic differences, highlighting strong intraspecific similarity that may reflect animal movement between these regions. Although the sample size in this phylogenetic analysis was limited, the consistently high sequence identity among T. equi isolates from various geographic regions suggests that further investigation is warranted before the ۱۸S rRNA gene can be confidently employed as a species-specific molecular marker for diagnosing T. equi in equine hosts.

کلیدواژه ها:

۱۸S ribosomal RNA ، Babesia ، Horse ، Phylogeny ، Theileria equi