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Thelohanellus mrigalae Tripathi, 1952 is the important myxosporean parasites causing hemorrhagic thelohanellosis disease in Cirrhinus mrigala, a fresh water teleost. Infected fishes show profound swelling, formation of pus, cyst on gill filament and infiltration of pathogen into connective tissue. About 238 live specimens of Cirrhinus mrigala were examined and myxozoan parasite T. mrigalae were identified which implicates host specificity. Any significant infection was not observed in the sampling period of March-July, whereas prevalence of T. mrigalae was much higher in the period of November to February which is post-monsoon period in West Bengal. During monsoon, moderate infection was observed as rain water is the main factor for spreading spore. The plasmodia of T. mrigalae were located in the intrafilamental epithelial site of gill. The plasmodia were cylindrical and creamy-white patch and contained ellipsoidal spores as valvular shape. The spores were 20.4 – 22.1 (±6.34) μm in length and 8.5 – 10.2 (±0.87) μm in width. An oblong, irregularly shaped mass of protoplasm was observed between the polar capsule and spore capsule. The polar capsule was 10.2 – 13.5 (±0.05) μm in length and 3.4 – 4.25 (±0.5) μm in width containing a polar filament coiled perpendicular to the longitudinal axis of the spore body. Histology of gill filaments showed a series of nodules of various sizes distorting the normal architecture of the gill cartilage. Distention of the gill filament was pronounced and lamellae adjacent to the cyst were no longer present. Necrotic lesions and hyperplasia were extensively observed in the gill filaments. Infected fish had lower RBC counts, higher WBC counts and more fragile erythrocytes than control fish.  Haemorrhaging was distinctly visible.  The increase in number of mucous goblet cells on the gill lamellae of infected fish enhance the diffusion distance between water and blood haemoglobin and rapidly impair Oxygen (O2) and Carbon dioxide (CO2) exchange.

Myxosporean, plasmodia, polar capsule, polar filament, gill lamella, goblet cells

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