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Context: Myxosporean endoparasites (phylum cnidarian) are critical pathogens that affect both wild and cultured freshwater and marine water fishes globally causing huge economic losses and high mortalities., Study Objective: The present study investigated myxosporean infections in Nile tilapia and African catfish collected from the natural resources., Methods: A total of four hundred Nile tilapia with an average weight (60 ± 5 g) and two hundred African catfish with an average weight (185 ± 30 g) were collected seasonally from Qalyubia Governorate, Egypt for parasitological and molecular diagnosis of isolated myxozoan species., Results: Microscopic examination revealed Myxobolus heterosporous, Myxobolus brachysporus, Myxobolus tilapiae, and Myxobolus amieti in Nile tilapia and Henneguya suprabranchiae, and Myxobolus brachysporus in African catfish. Sequencing of 18S rDNA gene for isolated Myxozoan spp. from Nile tilapia revealed Myxobolus tilapiae deposited in GenBank under accession numbers (OR766325 and OR766326). In African catfish, the isolated Myxobolus brachysporus sequence was deposited under accession numbers (OR766327 and OR766328). Henneguya suprabranchiae was also identified in African catfish (accession. No. OR763724 and OR763433)., Conclusion: Overall, these results indicate a high prevalence of myxozoan infection in naturally inhabiting Nile tilapia and African catfish. Curiously, Henneguya suprabranchiae was detected in the digestive tract and kidneys of African catfish, which is considered a rare form., Implication: This study highlighted the importance of parasitic surveys in natural resources that impact fish production., Competing Interests: Declarations. Ethical approval: The study was conducted following the protocol involving the use of animals that was approved by the Benha University, Faculty of Veterinary Medicine Animal Care and Use Committee (BUFVTM 13-10-22). All fish-handling procedures and regulations followed the ARRIVE guidelines for Animal Care and Use. All applicable institutional and governmental rules relating to the ethical use of animals were followed. No written consent was obtained from the fishermen for used fish in the study. Consent for publication: Not applicable. Competing interests: The authors declare no competing interests., (© 2024. The Author(s).)

In the present study, the morphology and morphometric characterization of Thelohanellus niloticus sp. nov., a new myxozoan belonging to genus Thelohanellus Kudo, 1933 (Myxosporea, Bivalvulida) infecting the gills of Labeo niloticus (Osteichthyes, Cyprinidae), were described for the first time from the River Nile at El-Minia Governorate, Egypt. Forty-one out of 78 (52.6 %) of the examined fish were infected. The infection was observed as irregular, milky whitish, cyst-like plasmodia (up to 0.8 mm in diameter) attached to the gill filaments of the host fish. These plasmodia contained tear-shaped myxospores with slightly tapering anterior and rounded posterior ends. Each spore has a single pyriform polar capsule. Spores measured about 23.3 ± 0.3 (20.4-27.1) μm long and 13.4 ± 0.4 (11.5-14.2) μm wide. The polar capsule was 11.7 ± 0.3 (9.2-12.5) μm long and 4.7 ± 0.3 (3.5-6.2) μm wide, containing a polar filament coiled perpendicular to the longitudinal axis of the spore body making eight turns. Occasionally, an oblong, irregular-shaped mass of protoplasm with a slightly oval nucleus (1.4 μm in diameter) and a small iodinophilous vacuole measured 0.85 ± 0.2 μm (0.73-1.2 μm) were observed in the spore. Due to the lack of the second polar capsule characterizing Myxobolus sp., the present parasite is placed within the genus Thelohanellus. Based on morphological differences (compared with other members of Thelohanellus Kudo, 1933) and the host specificity, this species is described as a new one of the genus Thelohanellus recorded for the first time in Egypt.

Forty-three out of 120 (35.8%) Clarias garipenus fish were found to be naturally infected with Henneguya suprabanchiae. The infection appeared as clusters of ovoid to ellipsoidal plasmodia being embedded within the hyaline cartilage of the suprabranchial organ of the fish. Histological studies indicate tissue distortion at the sites of infection. The continuous growth of the plasmodium led to deformation of the filament structure as well as disorganization and displacement of the gill lamellae. Also, a severe atrophy occurred in the hyaline cartilage of the gills and also in the gill lamellae. Based on the structure and measurements of fresh spores by light microscopy, this parasite was identified as H. suprabranchiae. Spores are oval in shape, and they measure 13 (11-14) × 3 (2-5) μm length by width. It has two polar capsules inside, each measures 3 (2.5-5) × 1 (1.5-4) μm length by width. Each polar capsule has a spirally coiled (7-9 turns) polar filament. The ultrastructural analysis showed that the plasmodia were surrounded by single-unit membrane. The generative cells and the early developmental stages were arranged at the periphery of the plasmodia, while immature and mature spores were centrally arranged. The developmental stages characterizing sporogenesis, capsulogenisis, and valvogenesis of the present parasite were ultrastructurally studied. The small subunit 18 S rDNA (SSU rDNA) gene sequences from different isolates was sequenced and compared with the sequence of the same gene from the Henneguya sp. isolated from GenBank. The phylogenetic position of the present Henneguya sp. within the genus was determined using sequence analysis of all related taxa available in GenBank, and the phylogenetic tree derived from this study is elucidated and compared with the current taxonomy of the available myxosporeans. Comparison of the nucleotide sequences and divergence showed that the SSU rDNA gene of this Henneguya species revealed 92.2% sequence identity with Henneguya exilis (acc. no. AF021881) differing in 30 nucleotides with lower divergence value.

The validity, impacts and seasonal prevalence of Henneguya suprabranchiae infecting Clarias gariepinus from the River Nile were studied. The overall prevalence was 35% (84/240) with maximum rate of infection in spring and minimum rate of infection in summer. The development of the cyst-like plasmodia was "the intrafilamental type'. Henneguya suprabranchiae is a pathogenic species as the parasite showed high intensity of infection which led to deformation of the filament structure and complete disappearance of the gill lamellae. The validity of H. suprabranchiae compared to the other described species of the same host was discussed.

A new myxosporean, Chloromyxum aegypticus n. sp., is described from the testes of the Egyptian toad Bufo regularis, captured from El Mansoura locality. C. aegypticus is identified on the basis of cytology, electron microscopy and histopathology. It is distinguished from all previously reported Chloromyxum spp. by its shape, dimensions of the mature spore (9.1 ± 0.1 (9.0-9.2) μm in length  × 7.9 ± 0.1 (7.8-8.0) μm in width), polar capsules, external ridge, sporoplasm nuclei, undulating suture, locality and host. Accumulation of several hundreds of plasmodia (0.8 ± 0.3 (0.5-1.1) mm in length × 0.5 ± 0.3 (0.2-0.7) mm in width) in the testes causes their enlargement. Parasites cause destruction of the seminiferous tubule cells and complete loss of spermatozoa. Since spermatogenesis stops, this seems to be a form of "parasitic castration". This is the first known record of the genus Chloromyxum in amphibian testes.

Light microscopic description was presented for a new myxosporean species, Auerbachia bajadi. The parasite was collected from the gallbladder of orangespotted trevally Carangoides bajad (Forsskål, 1775) in Al-Quseir city, Red Sea coast, Egypt. The prevalence was 30/80 (37%) and the infection was reported as an enormous amount of free mature spores floating in the bile. Spores were club-shaped with a broad anterior part and a narrow caudal part. Shell valves were asymmetrical and the larger valve was drawn into a wide caudal projection. The mean spore measurements were 22.7 mum in total length and 9.5 mum in width. Polar capsule was single and elliptical-shaped with five polar filament turns. The polar capsule measured 9.5 mum in length and 4.5 mum in width. The spores were distinctly different from the four recorded species of genus Auerbachia. One of the cited species of this genus was excluded as it was related to genus Coccomyxa than genus Auerbachia. Also, the accurate citation date of this genus was discussed.

The study of Myxosporidia infecting Clarias pachynema Boulenger, 1903 (Clariidae), Ctenopoma petherici Günther, 1864 (Anabantidae) and Parachanna obscura (Günther, 1861) (Channidae) from the basin of Nkam in Cameroon, revealed the presence of two new species of Myxosporidia of the genus Myxidium Bütschli, 1882, all found in the gall bladder of their hosts. These new species were: Myxidium nkamensis n. sp., found in Clarias pachynema and M. sangei n. sp. found in the gall bladder wall of Parachanna obscura. Two other species, Myxidium petrocephali Fomena and Bouix, 1986 and M. distichodi Kostoïngué, Faye and Toguebaye, 1998 were found in new hosts. The list of Myxidium species parasitizing African fresh water fishes is given.