21.15109/ARP/NEOGNNGabor, CechCechGabor0000-0003-4060-4522Veterinary Medical Research InstituteNadhirah Syafiqah Suhaimi0000-0002-1575-9103Veterinary Medical Research InstituteBoglárka Sellyei0000-0002-1926-8256Veterinary Medical Research InstituteRéka Mosonyi-BorzákRékaMosonyi-Borzák0000-0003-3596-526XVeterinary Medical Research InstituteKálmán MolnárKálmánMolnár7102480528Veterinary Medical Research InstituteCsaba SzékelyCsabaSzékely7005688631Veterinary Medical Research InstituteARP2026Agricultural SciencesThelohanellusActinospore stagesLife cycleExperimental infection18S rDNAHungaryGabor, CechVeterinary Medical Research Institute2026-04-242026-04-2410.1016/j.aqrep.2026.10338020172453780064007application/octet-streamapplication/octet-streamapplication/octet-streamtext/plain1.0CC BY-NC 4.0Eighteen Thelohanellus species have been reported from the common carp, Cyprinus carpio and its subspecies C. carpio haematopterus in the Far-Eastern countries such as Russia, China, Japan, Vietnam and Korea. In contrast, only T. nikolskii, T. hovorkai and actinospore of T. kitauei have been documented in Hungary to date. During a comprehensive parasitological survey on common carp, five Thelohanellus species and three actinospore types were identified from fish farms and Lake Balaton. Out of them, three new Thelohanellus species (T. serosae n. sp., T. borkhanuddini n. sp., and T. paranikolskii n. sp.) were described by morphological, morphometric, and molecular analyses. The life cycles of T. borkhanuddini n. sp. and T. serosae n. sp. were inferred, with T. borkhanuddini n. sp. confirmed through both molecular data and experimental infection. Notably, the acti-nospore stage of T. borkhanuddini n. sp. was identified in this study, confirming that T. borkhanuddini n. sp. is associated with four distinct neoactinomyxum morphotypes. These differed primarily in the length and width of their caudal processes and were released from two oligochaete hosts: Branchiura sowerbyi and Isochaetides michaelseni. Additionally, two known species, T. hovorkai and T. nikolskii, were recorded. Thelohanellus hovorkai was found to be associated with three distinct aurantiactinomyxon morphotypes, two of which were identified in this study. These morphotypes, which varied notably in caudal process and polar capsule dimensions, were all collected from different B. sowerbyi individuals. Additionally, a novel species, T. paranikolskii n. sp., was described, resembling T. nikolskii in both morphology and 18S rDNA sequences.Stipendium Hungaricum Program