64 December 2011 have completely destroyed the caudal peduncle or eroded into the cranium or abdominal cavity, sometimes exposing the swim bladder (Lilley et al. 1998). Mode of Transmission The spread of EUS is thought to be the result of flooding and movement of affected and/or carrier fish. Aphanomyces invadans is considered to be the necessary cause of EUS, and is present in all cases; however, an initial skin lesion is required for the fungus to attach and invade underlying tissues. This lesion may be induced by biotic or abiotic factors. In Australia and the Philippines, outbreaks have been associated with acidified water, along with low temperatures, presence of susceptible fish and A. Invadans propagates. In other areas, where acid water does not occur, it is possible that other biological infection, such as rhabdovirus or environnemental factors, including temperature may initiate lesions. Diagnosis Gross Observations. The gross appearance of lesions varies between species, habitat and stage of lesion development. The most distinctive EUS lesion is the open dermal ulcer. However, other diseases may also result in similar clinical lesions and it is, therefore, important to confirm the presence of A. invadans to ensure accurate diagnosis. Histopathology. Histopathological studies conducted on ulcerated fish show identical histopathological manifestations. In advanced lesions there is massive necrotising granulomatous mycosis of the underlying muscle fibres, involving the distinctive branching aseptate, invasive fungal mycelium. The presence of fungal hyphae was demonstrated in the epidermis of some early stages of infected fish from India (Viswanath et al. 1997). Basically fungal granulomata occur in the dermis and hypodermis. A. invadans induced epizootics are characterized grossly by the development of deeply penetrating ulcers and microscopically by extensive myonecrosis and granulomatous myositis (Sosa et al. 2007). Haematology. The early stages of EUS have no significant effect on the hematological parameters. However, advanced stages result in alteration of the general hematological profile (Pathiratne and Rajapakshe 1998). These authors also reported that the primary erythrocytic indices, namely, total erythrocyte count, haemoglobin and hematocrit concentration are significantly reduced in severely affected fishes, leading to anaemia. Preliminary hematological studies revealed low haemoglobin content in the blood of EUS-affected fish as compared to the corresponding healthy fish of the same species (Kar et al. 1994). Biochemistry. Only a few studies have been conducted on the biochemical profile of fish affected with EUS. In one study, high values of biochemical parameters such as Alkaline phosphatase (ALP), Serum glutamic oxaloacetic transaminase (SGOT), Serum glutamic pyruvic transaminase (SGPT) and Lactate dehydrogenase (LDH), have been observe from EUS affected fish (Kar 1999). Immunology. The macrophages of the fish surround the invading mycelium and envelope it to form granulomas (Chinabut et al. 1995, Wada et al. 1996). The mechanism by Fig 3. Necrotic ulcers, causing sloughing of the scales around the lesions at the site of injection. Fig 4. Pinhead-sized, red colored spots visible mostly on the dorsal surface of the body and the caudal peduncle of the fish. Clinical Sign and Gross Pathology In general, lesions on EUS-affected fish can be separated into three groups on the basis of gross appearance (Vishwanath et al. 1997). Clinical signs in the early stage of the disease are similar. Appetite is reduced or absent and fish become lethargic, either floating just beneath the surface or swimming with the head out of the water (Lilley et al. 1998). Pinheadsized, red spots develop on the body surface, head and fins, caudal peduncle, dorsum or operculum with no noticeable haemorrhages or ulcers. The intermediate stage lesions are represented by small (2-4 cm) dermal ulcers, with associated loss of scales, haemorrhages and oedema. The advanced stage lesions appear on other parts of the fish’s body and expand into large necrotic open ulcers; resulting eventually in death. Some affected species, such as striped snakehead, can survive with much more severe, chronic lesions that may
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