Thelohanellus toyamai (syn. Myxobolus toyamai) infecting the gills of koi Cyprinus carpio in the eastern United States.

A myxozoan resembling species of Thelohanellus was isolated from the gills of koi (Cyprinus carpio) cultured in North Carolina. Plasmodia measuring ∼200 µm in diameter contained tear-shaped myxospores containing a single pyriform polar capsule. The spore body was concave on one side, measuring 16.2 (14.7-16.8) µm long and 5.6 (4.5-6) µm wide. The polar capsule was 6.4 (5.8-7.2) µm long and 4.2 (3.4-4.6) µm wide, containing a polar filament coiled perpendicular to the longitudinal axis of the spore body making 8 turns. Occasionally, an oblong, irregularly shaped mass of protoplasm was observed between the polar capsule and spore capsule. Analysis of 18S small-subunit ribosomal DNA sequence demonstrated this isolate as a 99.9% match with Myxobolus toyamai from gills of C. carpio. However, the case isolate lacked the characteristic second polar capsule of Myxobolus, morphologically placing it within the Thelohanellus. Here we supplement genetic sequence data with histopathology, an amended morphological description of the agent, and a review of the original classification. For future reference, we suggest this organism be referred to as Thelohanellus toyamai Kudo, 1933, in accordance with the original classification and the nomial M. toyamai be avoided because it is at best outdated and, at worst, incorrect.