Microsporidian infections represent a constant threat to aquaculture. These parasites are widely distributed in seawater, freshwater and estuaries causing major losses. They are a widespread and diverse group that infects many species, from protists to humans. Horizontal transmission can happen between hosts that are related or unrelated to the same or different life stages and from the same or different species. On the other hand, vertical transmission occurs within a host family, where parasites are passed down through generations of hosts. (Dunn and Smith, 2001). Microsporidia commonly transmit through the ingestion of spores, which infect the gut and spread to other tissues. Transmission and virulence are strongly linked, with high parasite burden and pathogenicity in horizontal transmission, and low virulence in vertical transmission.

In Egypt, various microsporidia species have been identified from marine and freshwater fishes e.g., Red porgy, Pagrus pagrus, Gilthead seabream, Sparus aurata, Flathead grey mullet Mugil cephalus, and African sharptooth catfish Clarias gariepinus (Eissa 1995 &2002; Shehab El-Din,  2008;  Morsy et al., 2013; Abd Rabo, 2017, Abd El-Lateif and Torra 2020)