The experimental fish, Mrigal (C. cirrhosus) (Hamilton) were collected from the eight fish farms, 4 Government Farm (P1) (Bangladesh Fisheries Research Institute (BFRI), (P2) Fish Seed Multiplication Farm (Maskanda), (P3) Fish Seed Multiplication Farm (Shambhugonj), (P4) Fish Seed Multiplication Farm (Gouripur)) and 4 Private farms (Brahmaputra Fish Farm (P5), Deshbondhu Fish Farm (P6), Pankouri Fish Farm (P7) and Sornolota Fish Farm (P8) were situated at Mymensingh, Bangladesh during June, 2010 to May, 2012. Samples of 960 fish were collected by seine net from the ponds at monthly intervals. Live Samples were brought to the fish diseases at Laboratory of the Aquaculture, Department of Aquaculture, Bangladesh Agricultural University, Mymensingh, Bangladesh, in a bucket containing water. The fish were than anaesthetized by blunt trauma to the head. Opercula of fish were removed to collect portion of gill, which were then fixed in 10% neutral buffered formalin. After 24 hours of fixation, the samples were dehydrated, cleared and infiltrated in an automatic tissue processor using alcoholic series of higher concentration, xylene and paraffin wax. The samples were embedded with molten wax, steel mold and plastic holder, which were labeled previously. Sections were taken by a microtome, placed in water bath (240C) and picked over a labeled glass slide. The Sections were stained with hematoxylin and eosin, mounted with Canada balsam and over slips. The stained slides were examined with microscope. Comparative histopathological studies were recorded and expressed season wise as the summer season (February to May), the rainy season (June to September) and the winter season (October to January). A fortnightly record of temperature, PH, dissolved oxygen (DO), hardness and ammonia were made and their average values were expressed season wise. The experiment was carried out over a period of 24 months. Chi-square test used to carry out statically analysis (Zar, 1984).
Gill pathology of Mrigal: In winter months (October to January) both the primary and secondary gill lamellae had marked hypertrophy (HY) and haemorrhage (H). In primary and secondary gill lamellae, pillar cell, inter lamellar region, central axis were totally lost and degenerated (arrow) due to monogenean (MT) and protozoan cysts (PC) infestations in P1. However, in January and February, haemorrhage (H), hypertrophy (HY), protozoan cyst (PC), monogenean (MT) parasite and necrosis (N) were observed in P2. The gills of P3 in January also showed almost similar pathologies. The gills of P4 farm fish in December had lamellar missing (arrow) with the presence of monogenean (MT), protozoan cysts (PC), haemorrhage (H), hypertrophy (HY) and hyperplasia (HYP) in gill lamellae. Gill lamellae were splitted (SP), hemorrhaged (H), having monogenetic trematodes (MT) and protozoan cyst (PC)in P5. In December, January and February, hypertrophy (HY), monogenetic trematodes (MT) and protozoan cyst (PC)were found in P6. Almost similar pathological change also observed in P7. Gill had hypertrophy (HY), haemorrhage (H), vacuums (V), necrosis (N), clubbing (CB) with the presence of monogenetic trematodes (MT) and protozoan cyst (PC) in primary gill lamellae in P8. In March, primary gill lamellae were also hypertrophied (HY) and hyperplasid (HYP). Several primary gill lamellae were clubbed (CB) with the accumulation of many inflammatory cells especially at the base of the primary gill lamellae. During summer gill lamellae were normal. Pathology of skin and muscle of Mrigal In August, P1 showed loss of epidermis (arrow), dermis splited (SP) and separated from muscle, monogenean (MT) infestations, vacuums (V) and haemorrhage (H) were found in muscle layer. In September, in P2 loss of epidermis and dermis (arrow), necrosis (N), vacuums (V), numerous fungal granuloma (FG), fungal hyphae (FH) and monogenetic trematode (MT) were found in muscle. Epidermis sloughed off (arrow) from dermis, dermis splited (SP) from muscle, myotomes were necrotic (N), protozoan cyst (PC), vacuums (V) and melanomacrophage (MMC) were found in many places in October in P3. Epidermis totally lost (arrow), dermis splited (SP) and separated from muscle and muscle had melanomacrophage (MMC), Gyrodactylus (G) and protozoan cyst (PC) in October of P4 fish. Skin and muscle of P5 exhibited loss of epidermis (arrow), dermis separated (SP) from muscle, melanomacrophage (MMC), necrosis (N) and vacuums (V) were seen in muscle in November. Similar skin pathology were also observed in P6 and P7 fish farm, along with Argulus (A) and monogenean (MT) infestations in the muscle during December and January. Almost similar pathological changes were also found in P8 farm fishes along with Lernaea (L) infestation in February in mrigal. Whereas, in March, muscle structure was almost normal except few parasites. In summer, again structure of skin and muscle were found normal.