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D. Palic1
1Chair for Fish Diseases and Fisheries Biology, Ludwig-Maximilians-University Munich, Munich, Germany
Prof. Dušan Palić, D.V.M., MVSc, Ph.D., CertAqV, Dipl. ECAAH
Chair for Fish Diseases and Fisheries Biology
Faculty of Veterinary Medicine, Ludwig-Maximilians- University Munich
Kaulbachstrasse 37, 80539 Munich, Germany
Fishes show a range of clinical signs when diseased. What are these, and what do they mean? There are clinical signs that are pathognomonic for certain diseases, however, many are non-specific and a step- by-step approach to working up a case is necessary. Firstly, it is very important to familiarize yourself with what is normal in terms of appearance and behavior for the species you are dealing with. Typically, healthy fish should have a good appetite, have clean, clear, vibrant body coloration, hold their fins erect and have bright red gills. They should be active and not display abnormal behavior, swimming patterns or have loss of buoyancy control. However, there are always exceptions to these rules as the types of different fish species number in the thousands. There are some fishes such as the wrasses that lie on the bottom or on their side, there are upside down catfish that swim upside down and there are
fish (goldfish) with deformities which are all ‘normal’. In the following paragraphs, I will describe pathological presentations and how they may be interpreted in terms of pathobiology and etiology. Further down, there is a table of common etiologies as they relate to the general syndromes that are discussed (Table 1).
Skin conditions can vary from erosions on the mouth or fins (usually due to Flavobacterium infection) or along their lateral line system (e.g. hole-in-the-head disease). Ulcers are common and they can originate from the outside (e.g. Aeromonas spp.), or be ulcers that originate from the inside (Mycobacterial). Ulcers may present as discrete lesions anywhere on the body including the
fins, flank and operculum. They can be circumscribed and show advancing border. They may be pale to red, depending on the depth of the ulcer. The pale ulcers are more superficial and indicate edema and the red may be due to hyperemia. A dull red appearance is evidence of deeper ulceration with exposed muscle. Such deep
ulcers may be due to fungi (e.g. Aphanomyces invadans), protozoa (e.g. Tetrahymena), or simply from a predator attack. It is more common that the skin ulcers are due to secondary bacterial infection, with the primary pathogens being either skin flukes or lice. Thus it is very important
to investigate for the primary cause. Sometimes, melanization occurs in response to injury which is commonly seen in goldfish.
Fish may have proliferative skin lesions that may be raised and smooth (e.g. carp pox and neoplastic conditions) or be fine and granular (e.g. lymphocystis). They may present with fine white spots (e.g. white spot disease) or appear as a haze (e.g. velvet disease) or larger spots (e.g. digenetic trematodes). Excessive slime production may be an indication of ectoparasitism or poor water quality issues (e.g. low pH). A change in body color patterns, whether it be pale or dark is non- specific. Fish may have tuft-like white growths which can be due to fungi (Saprolegnia) or bacteria (Flavobacteria). Hyperemia of fins or body is a common sign of stress and/or bacterial infection. Often, fish with skin lesions may present with flashing (scraping against substrate
or tank walls), have clamped fins and separate from
the group. If they are infested with particularly irritant parasites (e.g. Argulus), the fish may jump in an attempt to dislodge the parasites. Generally, those with severe disease may become lethargic and very often display respiratory signs of disease too. The reason for this
is because the gills are also in intimate contact with
the water and the external environment. Thus, many pathogenic organisms that colonize the skin will also affect the gills.
Respiration: It is always a good idea to check that the gill color is a healthy bright red. Pale gills indicate anemia, whereas dark gills indicate methemoglobin formation. Gills with excessive mucus indicate ectoparasitism or dissolved toxin. Whenever gills are damaged, they have
a limited range of response and they include formation of synechiae (secondary lamellae that ‘stick’ to each other), epithelial hyperplasia, secondary lamellar fusion and if given sufficient time, mucus cell hyperplasia. All these will decrease the efficiency of gill function and fish will display respiratory signs of disease. Fish may congregate at water inlets and ‘pipe’ or ‘gasp’ at the water surface. The opercular beat rate may initially be increased as the fish try to respire through inefficient gills, but as fish become moribund, the opercular rate will decrease.
Under the heading of ‘distended abdomen’ we have proliferative conditions (ovarian neoplasia is common in koi) or cystic conditions (polycystic kidney disease common in goldfish), ‘bloat’ and ‘dropsy’. Bloat is typical for certain cichlid fish known as ‘Malawi bloat’ which is caused by Hexamita (an intestinal flagellated
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