Skin disease

image of Skin disease
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The skin of a fish provides a barrier and first line of defence against infection, osmotic pressure and mechanical injury. Being the most visible part of the fish, it is where disease is often first seen. This chapter assesses anatomy, investigations, systemic approach, colour change, visible pathogens, spots, swellings, ulcerations, texture changes and fin lesions.

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Figure 15.1 The external anatomy of a fish. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission.
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Figure 15.2 Histological structure of fish skin. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission.
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Figure 15.3 This fish, a characin, developed a discrete purple-blue coloration on its caudal peduncle following local injection of an antibiotic to treat the ulceration between its anal and caudal fins. The dramatic colour change, due to irritation of the nerves, developed in seconds but took several weeks to resolve. (By permission of .)
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Figure 15.4 Petechiation and ecchymosis are most visible on the ventral surface and pale or white areas of fish. The generalized septicaemia in this koi was confirmed by isolation of Aeromonas from an ulcer near its mouth, as well as from the kidney. (By permission of .)
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Figure 15.5 Septicaemia in goldfish often produces patches of brown pigmentation in the skin and fins, which may be localized or widespread. Following successful antibiotic treatment, these disappear and the gold coloration returns. (By permission of .)
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Figure 15.6 Local ecchymosis and petechiation on the head of a koi, which were assumed to be due to bacteraemia since the fish also had a small body ulcer. An injectable antibiotic was administered and the cranial discoloration resolved within 7 days. (By permission of .)
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Figure 15.7 This is the same goldfish as in Figure 15.5, on presentation 6 years later when it had abdominal swelling and body ulcers with a caseous discharge. Its original gold colour had disappeared completely except for a small localized area on its caudal peduncle. Postmortem examination revealed extensive internal granulomas due to mycobacteria. (By permission of .)
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Figure 15.8 Epidermal hyperplasia, common in koi, is seen as discrete smooth milky white plaques. The hyperplastic tissue will often slough periodically after several months, as was the case with the lesion on this koi’s snout. (By permission of .)
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Figure 15.9 The crustacean parasite (arrowed) can measure up to 7 mm and can be difficult to see on fish, due to its semi-transparent body. The parasites move over the surface of the fish but can swim freely in the water, laying eggs in the environment.
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Figure 15.10 is a freshwater copepod that attaches to the host with a large cephalic process and can measure up to 15 mm in length. Only the female is parasitic and she can be recognized by a pair of trailing egg sacs, which are more easily recognized while in water. (By permission of .)
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Figure 15.11 Leeches attach to the host by a large suction pad and move around with a characteristic contractile action. A common leech, , can measure up to 4 cm in length. (By permission of .)
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Figure 15.12 Infection with a fungus. The delicate fungal structure resembles cotton-wool but collapses into a slimy mass when removed from the water. Fish with extensive lesions, such as this black moor goldfish, are unlikely to recover, due to loss of osmoregulatory control. (By permission of .)
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Figure 15.13 Nuptial tubercles are multicellular keratinous nodules, visible as spots on the operculum and pectoral fin of this male goldfish (arrowed). During the breeding season, they become larger and can measure up to 2 mm.
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Figure 15.14 ‘White spot’ is caused by small white protozoan parasites (measuring up to 1 mm) that are more visible on the dark-coloured areas of the body.
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Figure 15.15 ‘Marine white spot’ is caused by infection with a ciliate protozoan, Cryptocaryon irritans. A small number of maturing parasites (trophonts) can be seen above the head of this purple moon angelfish. (By permission of .)
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Figure 15.16 ‘Black spot’. The two dark spots behind and above the head of this small characin are caused by the intermediate stage of a digenean fluke. The cluster of very small spots near the middle of the body is an area of natural pigmentation. (By permission of .)
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Figure 15.17 Vesicles, measuring up to 4 mm, on the caudal fin of a koi with a body ulcer. Following injection with an antibiotic, the lesion resolved within 48 hours.
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Figure 15.18 affecting the fins of an immature California sheepshead, a coldwater marine fish. The lesions developed shortly after arrival at a public aquarium but resolved within 6 weeks without the need for medication. (Courtesy of the National Aquarium in Baltimore.)
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Figure 15.19 Carp pox, a benign epidermal neoplasm, produces a characteristic smooth raised lesion that resembles drops of candle wax. It can affect any area of the body, and ranges from discrete masses measuring a few millimetres in size to large extensive plaques.
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Figure 15.20 Papillomas can vary in size and appearance. This mature koi has a large lesion on its head that sloughed completely on several occasions. There is epidermal hyperplasia, seen as a darker area extending over the operculum and behind the head. (Reproduced with the permission of .)
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Figure 15.21 This erythrophoroma (red pigment cell tumour) developed as an extensive plaque over the caudal peduncle of this goldfish. The lesion had been present for a few years but did not cause any clinical problems. (By permission of .)
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Figure 15.22 These two goldfish had been isolated for several years before erythrophoromas developed spontaneously on one fish and then 12 months later on the other. Due to their extensive nature, surgical treatment was not possible. One fish survived for more than 4 years, though some lesions increased in size. (By permission of .)
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Figure 15.23 A fibroma on the dorsum of a lionhead goldfish. This discrete raised mass developed over a few weeks. Following surgical excision, the fish lived for a further 3 years before dying from an unrelated problem. (By permission of .)
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Figure 15.24 lesion on the tail fin of a koi. Small lesions may resemble carp pox, but as these grow they become quite distinctive and white hyphae become visible through the epidermis. The lesions eventually rupture when they are about 1 cm in size. (By permission of .)
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Figure 15.25 Body ulcer on a koi. Early lesions start as a localized area of inflammation, which later darkens like a bruise and eventually ulcerates, exposing the underlying tissue. They are often caused by bacterial infection but they may become further infected by . (By permission of .)
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Figure 15.26 ‘Hole-in-the-head’ disease in a discus. Small ulcers (arrowed) that develop in the sensory pits on the head gradually expand and may coalesce into large lesions. (By permission of .)
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Figure 15.27 Extensive ulceration in a passer angelfish due to ‘head and lateral line erosion’. Also known as ‘marine hole-in-the-head’, this syndrome often starts soon after captivity. (By permission of .)
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Figure 15.28 This small puncture wound on the body of an orfe was the result of an attack by a grey heron () and penetrated deep into the dorsal musculature of the fish, fracturing the spinous processes of some vertebrae. (By permission of .)
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Figure 15.29 Excess mucus (‘slime disease’) gives the skin a patchy grey-white appearance. In this goldfish, a severe ‘white spot’ and infestation was identified on microscopic examination of a skin scraping.
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Figure 15.30 ‘Dropsy’ in a 10-year-old koi with protrusion of the scales. The fish displayed both abdominal distension and cutaneous oedema for several months and required euthanasia due to its deteriorating health. Postmortem examination revealed severe ascites, a fluid-filled gonad and a large hepatic tumour. (By permission of .)
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Figure 15.31 ‘Fin rot’ due to bacterial infection with . In this young koi, the caudal fin is severely eroded and the disease has progressed on to the caudal peduncle, visible as a grey-white plaque posterior to the anal fin. (By permission of .)
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Figure 15.32 Hyperaemia in fish with long delicate fins is often an indication of stress. The prominent blood vessels are easier to see when the fish is in the water and viewed against a light background.
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Figure 15.33 Nuptial tubercles occasionally become hyperplastic and develop into large verrucose masses on both pectoral fins. These lesions rarely affect the health of the fish, though they may trap strands of filamentous blanket weed in outdoor ponds. (By permission of .)
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