251 Introduction Fishes are considered as one of the most important sources of animal protein all over the world. Because of numerous lakes, seas and a long river, Egypt has a very diversified fauna of fresh and marine water fishes. Vision either in wild or farmed fishes is quite different from mammalians as it has to be adapted to acquatic environment (Lee 2002). The awareness of parasites that affect fish health, growth, and survival is increasing together with a developing interest for the fish farming and production. Knowing fish parasites becomes important for a rapid and correct diagnosis. Early diagnosis is a prerequisite for implementing preventive measures, which are the best way to reduce infection spread (Abdel Ghaffar et al. 2012, Morsy et al. 2012). Myxosporidea have a great importance in ichtyopathology. They are frequently described in freshwater, brackish and marine fishes. Myxosporean parasites are the most important fish pathogens and more than 2,300 species have been reported from marine and freshwater fishes in several global areas (Manrique et  al. 2015, Manrique et  al. 2016). Myxosporea infecting fishes are a group of parasites responsible for myxosporidiosis, a serious disease of fishes (Adriano et  al. 2009, Morsy 2010). Myxobolus Butschli, 1882, is one of the largest genus of myxosporean groups with approximately 856 species. Species of Myxobolus infect fishes from all over the world (Eiras et  al. 2014). They can infect a diverse set of specific tissues including specifically the tegument, eyes, gills, glands, gonads, kidneys, muscle, digestive tract, and nervous system (Lorna and Dykova 1992). The Myxobolus dermatobius (Ishii, 1915) (Syn. M.  dermatobia) in Nile tilapia (Oreochromis niloticus) causes petechial to focal haemorrhages in orbit, exophthalmia and unilateral eye opacity especially in advanced cases (Abdel‑Aal 2002) while M. dermatobia, isolated from Tilapia zillii at Giza province, causes unilateral eye opacity (Mohamed et al. 2004). The ultrastructural morphology of myxosporean species has been widely studied (Lorn and Dykova 1992). However, in Egypt, few species only have been ultrastructurally described. In this study the ultrastructural morphology of M.  dermatobius detected in Nile tilapia is described. Department of Parasitology, Faculty of Veterinary Medicine, Zagazig University, Egypt. *Corresponding author at: Department of Parasitology, Faculty of Veterinary Medicine, Zagazig University, Egypt. E‑mail: nosseur@gmail.com. Keywords Myxosporidae, Ultrastructure, Myxozoa, Nile tilapia (Oreochromis niloticus), Transmission electron microscopy. Summary A total of 1,000 cultured Nile tilapia (Oreochromis niloticus) were collected from different governmental and private fish farms and examined for detection of myxosporean parasites infection. The infected fishes showed slight unilateral exophthalmia with whitish cyst in the eye. Numerous white cysts like plasmodia of Myxobolus dermatobius were recovered from the eye of the examined fishes with low prevalence rate (1%). Small intact cyst was isolated, fixed in 3% glutaraldehyde in 0.1 M sodium cacodylate (pH 7.4) and prepared for transmission electron microscopy examination. Ultrathin sections myxospores of M. dermatobius revealed pair of capsulogenic cells at the apical pole of the developing myxospore. Single sporoplasm containing a single nucleus and sporoplasmosomes fills nearly all the space beneath the polar capsules. The later were pyriform in shape, each one had homogenous dense core and 4 turns of polar filaments. Ultrastructural characteristics of the present myxospore were described and discussed in detail. Nosseur M. El‑Sayed* Ultrastructural morphology of the Myxobolus dermatobius Ishii 1915 (Mixosporea: Myxobolideae) microspores infecting eyes of Nile tilapia (Oreochromis niloticus) in Egypt Veterinaria Italiana 2020, 56 (4), 251‑255. doi: 10.12834/VetIt.1151.6322.3 Accepted: 20.11.2016 | Available on line: 31.12.2020 252 Veterinaria Italiana 2020, 56 (4), 251‑255. doi: 10.12834/VetIt.1151.6322.3 Ultrastructural morphology of Myxobolus dermatobius microspores El‑Sayed Results The prevalence of infection with M. dermatobius was 1% (10 out of 1,000). It was found in the form of whitish cyst in the eye of Nile tilapia. Slight exophthalmia was observed in the infected eye. The myxospores were recovered from their original plasmodia found in the eye of Nile tilapia (Figure 1). Each myxospore contains a pair of capsulogenic cells, two peripherally arranged valvogenic cells and one sporoplasm cell. Capsulogenic cells are found at the apical pole of the developing spore and, together with the sporoplasm, forms a central core that is ensheathed by valvogenic cells. These cells give rise to the two shell valves surrounding each spore and the sutural ridge joining the valves. The differentiation of the capsulogenic cells starts with appearance of a club‑shaped structure ‘capsular primordium’ (Figures 2 and 3). Sporoplasm fills nearly all the space beneath the polar capsules. It contains a nucleus, small vesicles Materials and methods A thousand of live Nile tilapia were collected from different governmental and private fish farms in Sharkia Governorate, Egypt. The collected fishes were transported to the laboratory and dissected. The different organs were examined macroscopically and microscopically for detection of any visible myxosporean cysts. Several small intact cysts with minimum surrounding tissue isolated from ten positive fish samples were fixed in 3% glutaraldehyde in 0.1 M sodium cacodylate (pH 7.4) for at least 24 hours, washed in the same buffer and post‑fixed with 2% OsO 4 in the same buffer. The specimens were dehydrated in series of graded ethanol, transferred to propylene oxide and, finally, embedded in araldite. Ultrathin sections were stained with uranyle acetate and lead citrate and observed in a Philips (208) TE Moperated at 80‑100 kV (Vital et al. 2003). Figure 1. Specimen of Nile tilapia (Oreochromis niloticus), showing white plasmodia and exophthalmia (arrows) in the eye due to Myxobolus dermatobius infection. Figure 2. Transmission electron microscopy of M. dermatobius premature myxospore parasite from the eye of Oreochromis niloticus showing two capsulogenic cells (CC) containing capsular primordium (CP), sporoplasm (SP), valvogenic cell (VC), and suture valve (arrow). Scale bar = 300 nm. Figure 3. Transmission electron microscopy of M. dermatobius nearly mature myxospore parasite from the eye of Oreochromis niloticus showing two capsulogenic cells (CC), primordia of polar filaments (arrow head), sporoplasm (SP) with sporoplasmosomes (arrow) and glycogen body (G). Scale bar = 300 nm. Figure 4. Transmission electron microscopy of the transverse section of M. dermatobius mature, parasite from the eye of Oreochromis niloticus, showing two polar capsules (PC), sporoplasm (SP) containing sporoplasmosomes (arrows), one nucleus (N), polar filaments and small vesicles (arrow head). Scale bar = 300 nm. 253Veterinaria Italiana 2020, 56 (4), 251‑255. doi: 10.12834/VetIt.1151.6322.3 El‑Sayed Ultrastructural morphology of Myxobolus dermatobius microspores El‑wafa (Abu El‑wafa 1988) identified M. spheroidalis and M. ocularis from eye. Mazen (Mazen 1994) described M. heterosporus from eye and gills. Abdel‑Ghaffar and colleagues (Abdel‑Ghaffar et  al. 1995) described Myxobolus sp. from the inner wall of cornea, the base of the gill arch, and roof of the mouth. Hegazy (Hegazy 1999) isolated M.  cornealis from the eye while, Abd El‑Aal (Abd El‑Aal 2002) and El‑Mansy (El‑Mansy 2005) described M. dermatobia and M. heterosporus from eye and cornea, respectively. M. dermatobia was isolated from eye of Tilapia zilli at Giza province (Mohamed et al. 2004). Most of the Egyptian Myxobolus species morphological descriptions have been mainly based on light microscopy and diagrammatic drawings and just few ultrastructural descriptions are available. In accordance with Abd El‑Aal (Abd El‑Aal 2002) and Abdel‑Ghaffar (Abdel‑Ghaffar et al. 2005), the ultrastructural characteristics of M.  dermatobius revealed that the spore developed from five cells. The capsulogenic cell of the present species showed capsular primordial to that described in M. stomum and Myxobolus sp. by Ali and colleagues (Ali et  al. 2003) and Abdel‑Ghaffar and colleagues (Abdel‑Ghaffar et  al. 2005), respectively. The valvogenic cells gave rise to shell valve surrounding each spore and sutural ridge joining the valves were similar to M. dermatobia described by Abd El‑Aal (Abd El‑Aal 2002). The sporoplasm of the investigated species was composed of single mono‑nucleated cell as in M.  dermatobia described by Abd El‑Aal (Abd El‑Aal 2002) but different from bi‑nucleated sporoplasm observed in other Myxobolus species (Ali et  al. 2003, Casal et  al. 1996, Abdel‑Ghaffar et  al. 1994, Abdel‑Ghaffar et  al. 2005). The sporoplasmosomes of the present species complied with a similar dense body found in M.  cotti reported by EI‑Matbouli and colleagues (EI‑Matbouli et  al. and, sometimes, exhibits dense matrices known as sporoplasmosomes. A small area of sporoplasm is occupied by a glycogen body (Figure 4). Two polar capsules are pyriform in shape, equal size, located side by side at the same level and occupy approximately half of the total spore length. Each polar capsule has a homogenous core of medium electron‑density containing polar filaments, surrounded by an electron‑lucent layer and an outer layer of medium density (Figure 5). Four turns of polar filament coils are probably in each capsule. The apical portion of each mature polar capsule is plugged by a cap‑like cover (Figure 6). Discussion Myxobolus Bütschli, 1882, contains over 450 of the 1,700 species described within phylum Myxospora (Myxozoa). These parasites primarily infect fishes, but a small number of species have been found parasitizing amphibians and reptiles (Lom and Dyková 1992). The infected fishes showed slight unilateral exophthalmia with whitish cyst in the eye. Similar lesion was observed in the eye of tilapia nilotica by Abd El‑Aal (Abd El‑Aal 2002) and Mohamed and colleagues (Mohamed et  al. 2004). Numerous species of Myxobolus have been reported among different African tilapia species; Myxobolus agolus, M. brachysporus, M. clarii, M. cichlidarum, M.  heterosporus, M. tilapiae and M. camerounensis and M. kainjiae appeared to be common in gills, fins, eyes and teguments of Oreochromis niloticus (Ousman et al. 2007). In Egypt, several species of Myxobolus were isolated from O. niloticus. Faisal and Shalaby (Faisal and Shalaby 1987) identified M. nilei (syn. Myxosoma tilapiae) from eyes, skin, gills, kidney, spleen and pancreas while, Abed (Abed 1987) described M.  heterosporus from eye, muscle and kidney. Abu Figure 5. Longitudinal section through Myxobolus dermatobius well developed polar capsule showing an electron-dense outer layer (arrow); a central translucent layer (LU) and inner dense core (C) with polar filament coils (PF). Scale bar =500 nm. Figure 6. Transmission electron microscopy of longitudinal section through M. dermatobius parasite from the eye of Oreochromis niloticus, showing well developed polar capsule, four turns of polar filament (PFt) and apical cap (arrow). Scale bar = 500 nm. 254 Veterinaria Italiana 2020, 56 (4), 251‑255. doi: 10.12834/VetIt.1151.6322.3 Ultrastructural morphology of Myxobolus dermatobius microspores El‑Sayed et  al. 2005), M. stomum (Ali et  al. 2003), M. cotti (El‑Matbouli et  al. 1990) and M. sciades (Azevedo et  al. 2010). The homogenous core of each polar capsule that contains four turns of polar filaments was identical to that of M. dermatobia described by Abd El‑Aal (Abd El‑Aal 2002). The same number of polar filament coils was reported in M. hetersporus by El Mansy (El Mansy 2005) while different numbers of polar filament turns were mentioned in M. maculatus (14‑15), in Myxobolus sp. (5), in M.  stomum (5‑6), in M. sciades (9‑10), in M. sclerii (4‑5), in M. brachysporus (6‑7) and in M. cuneus (7‑8) (Casal et al. 2002, Abdel‑Ghaffar et al. 2005, Ali et al. 2003, Azevedo et  al. 2010, Kaur and Singh 2010, Abdel‑Baki et al. 2015, Manrique et al. 2016). 1990); M.  dermatobia (Abdel‑Aal 2002), M.  stomum (Ali et  al. 2003) and Myxobolus  sp. (Abdel‑Ghaffar et  al. 2005). The glycogen body noticed in the sporoplasm is essential in the myxosporean spore as it provides the energy necessary for further developmental stages in the life cycle. It was similar to that reported in M. dermatobia (Abd El‑Aal 2002), Myxobolus sp. (Abdel‑Ghaffar et al. 1994) and M. cotti (El‑Matbouli et al. 1990). The fully developed polar capsule was surrounded by an electron‑lucent layer and an outer layer of medium density and covered by a cap‑like structure at its apical end. A similar finding was reported in many species of Myxobolus as M. dermatobia (Abd El‑Aal 2002), Myxobolus sp. 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