Document Type : Original Articles
Authors
1
PhD student in the Faculty of Veterinary Medicine at the Shahid Chamran University of Ahvaz, Ahvaz, Iran
2
Department of Basic Sciences, Biochemistry and Molecular Biology Section, Faculty of Veterinary Medicine, Shahid Chamran University of Ahvaz, Ahvaz, Iran.
10.22092/ari.2025.370020.3736
Abstract
Equine piroplasmosis is a tick-borne disease caused by two protozoan parasites, Babesia caballi and Theileria equi (formerly Babesia equi), transmitted by Ixodid ticks. In this study, a 435 bp fragment of the 18S 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. Taxonomic analysis of the amplified sequence designated TeKh, yielded 108 matches within the phylum Apicomplexa. Of these, 104 hits belonged to the family Theileriidae, and 86 were affiliated with the genus Theileria. Comparative analysis revealed that TeKh is identical to the only reported T. equi sequence from Fars Province, Iran (MK615933). Further multiple sequence alignment showed high similarity: 98.71% to a T. equi isolate from Russia (OM475523) and 99.74% to one from Turkmenistan (OL638195). Structural predictions using the RNAfold algorithm indicated a thermodynamic free energy of −121.98 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 (AF499604) from South Africa and Theileria sp. (KF597078) from Kenya, while T. cervi (KT959227) from China formed a separate branch with moderate bootstrap support (44%). 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 18S rRNA gene can be confidently employed as a species-specific molecular marker for diagnosing T. equi in equine hosts.
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