◾Microsporidia are currently related to Fungi, previously thought to be Protozoa (Capella-Gutiérrez et al., 2012).

◾Spores are the infective stage which invades host cells through a specialized invasion apparatus; the polar tube (Fig. 1) (Han et al., 2020).

◾The life cycle of microsporidia comprises three phases: Phase I is the extracellular phase containing mature spores. Phase II is the first phase of intracellular development where microsporidian organisms increase in number. Phase III is the sporogonic phase, which leads to spore formation (Fig. 2).

◾The spore resists severe environmental conditions due to its thick chitinous layer (Yang et al., 2018). Spores are ovoid or ellipsoidal, ranging from 1 to 20 μm (Cali et al., 2017).

◾Phylum Microsporidia Balbiani 1882 includes more than two hundred genera and about 1300 species (Cali et al., 2017). Microsporidia can be characterized based solely on spore structure, including spore size, shape, and the number and position of polar capsules.

◾The fish-infecting genus is classified into five groups microsporidia. Group 1 is represented by the family Pleistophoridae. Group 2 is represented by the family Glugeidae. Group 3 comprises three clades: Loma and a hyperparasitic microsporidian from a myxosporean; Ichthyosporidium and Pseudoloma clade and the Loma acerinae clade. Group 4 contains two families, Spragueidae with the genus and Tetramicridae. Group 5 is represented by the family Enterocytozoonidae with the genus Nucleospora and the mammal-infecting genus Enterocytozoon (Lom and Nilsen, 2003).

◾The pathogen is mainly transmitted horizontally as well as vertically.