Reassessment of <i>Myxobolus toyamai</i> Morphology in Koi Carp Using an Enhanced Giemsa Warming Technique: A First Report From Vietnam

Myxobolus toyamai, a well-established gill parasite of common carp, Cyprinus carpio, has been implicated in infections across multiple continents. Despite numerous attempts to classify this parasite based on the structural features of its polar capsules, existing staining techniques have proven inadequate, resulting in unclear structural definitions. This study represents the first report of M. toyamai infection in koi fish in Vietnam, along with the optimisation of a staining technique using Giemsa warming to accurately define its morphological characteristics. In addition, the study includes the analysis of the 18S rRNA gene and histopathological evaluations. The results show that the Giemsa warming staining method effectively clarifies the structure of the polar capsules, revealing distinct measurements that differ from those reported in previous studies. Furthermore, sequence analysis of the 18S rDNA gene demonstrates a high degree of similarity to M. toyamai, confirming its identification in the examined specimens.