The carp is naturally protected against infection by its scales and the mucous coating which covers its body. However, the external covering of the carp is constantly in contact with the surrounding water, and is therefore an ideal place for the growth of microorganisms. Although this external covering is watertight and impermeable to the transfer of pathogens to the carp’s bloodstream, any physical damage to the epidermis (exterior surface) such as mechanical damage and scale loss, will reduce the integrity of the layer as a protection, and may allow infection to take place.
The number of scales does not affect the possibility of infection, and common carp and fully-scaled mirrors are just as resistant to infection as a leather carp, since all strains are covered in a relatively thick dermis. This contains mucous glands, which secrete the mucous layer that has protective properties. Its secretion forms a slippery film over the body surface which reduces water resistance during swimming and it protects the body surface by forming a barrier to bacterial and fungal pathogens. It is considered as having antimicrobial properties, as well as protecting the body surface from abrasions. The potential problems of scale removal have been described by several authors.
Gills also contain mucous-producing cells and which are also involved in the osmoregulatory process by excreting ammonia and excess salts. Despite their mucous covering, the position of the gills, their delicate structure and close contact with the aquatic medium, makes them vulnerable to changes in environmental conditions. They also possess a large surface area and because of the rich blood supply that they receive, they are a favoured site of infection of all kinds.
Apart from the physical barriers to infection, the carp possesses an immune system composed of organs, cells and serum components. The immune response of each fish is variable depending on several factors including the strain of fish and pathogen, the age of the fish, time of the year and environmental conditions. As previously described, stress can have an important negative effect on the immune system.
Whereas mammals possess bone marrow, it is the organs involved in the immune response in fish that produce the cellular components of the immune response and the correct physiological environment that allows the antibodies to interact with antigens. The most important lymphoid organ in fish is the thymus, which contains several types of leucocytes, including lymphocytes. Some lymphocytes include antibodies equivalent to the plasma cells of mammals. The other important organ in the immune system is the kidney, and the anterior part of the kidney (called the head kidney) has lost renal function and is related to defense against infection. Since the kidney is a source of blood cell production, it has been compared with the bone marrow, and apart from sharing the cells types found in the thymus, the kidney also contains specialized macrophages that are also found in the spleen, which is also an important organ in defense against disease.
The different organs contain several leucocyte types, which are important in the both inflammatory and pathological responses. The combating of infection by the different types can include a series of killing mechanisms such as the use of enzymes to destroy invading microorganisms.
In aquaculture, routine health checks are usually carried out in order to check the stock for signs of disease, however in fisheries, this is often impractical since it is more difficult to capture the fish, and observation of behavioural changes which may indicate infection is more complicated. One further difficulty of disease diagnosis in fisheries is that specimen fish are too costly to be sacrificed in order to confirm disease outbreaks, and as a result, reports of disease usually result from direct contact with fish through fish which are caught and any external signs of disease which caught or observed fish may present.
It is therefore important that the responsible carp angler has knowledge of the basic signs of disease so that he or she is able to inform the fishery owner, since in some cases, the ability to act quickly and inform the relevant authority of a possible disease outbreak may be crucial to preventing further spread.
It is important to remember that in many cases, diseased fish result from an initial reduction in water quality or other stressful conditions such as poor nutrition, and so the appearance of disease can often indicate that disease precursors are present and these factors may need to be removed before the stock can return to good health. There may also be more than one pathogen present, which may make disease diagnosis more difficult since the clinical signs may as a result be non-specific and therefore confusing.
The beginning of a disease outbreak is often displayed as changes in the behaviour of the fish with the most obvious being that of reduced feeding (inappetance). However, other signs are increased or reduced respiration rate, shown by faster or slower opening and closing of the opercula (gill covers), changes in body pigmentation (often a darkening of the overall colour) or increased (hyperactivity) or reduced (lethargy) overall activity. Changes in swimming behaviour can occur, which may include the fish remaining at the surface, often gulping near the air-water interface, maintaining stationary in the water, or uncontrolled and uncoordinated swimming. Another behavioural sign of disease is shyness, which may include the fish becoming overly reclusive.
Clearly it is easier for the angler to recognize abnormal behaviour once he or she has become familiarized with the normal behaviour of the carp. This includes behaviour at spawning times which may indicate disease-related behaviour, but which is part of reproductive activity (e.g. appetite loss and erratic swimming). Also, the presence of certain behaviours may be strictly environmental as opposed to disease-induced, and low oxygen conditions may provoke an increased rate of respiration, particularly at higher temperatures, often accompanied by reduced appetence.
The external visual signs of disease include changes in colour, skeletal deformities, abdominal swelling, emaciation and abnormal swimming activity. The skin may show haemorrhaging caused by captures or spawning. Haemorrhages on the surface usually appear as red spots which can result from bacterial infection or in some cases parasite attachment.
Some diseases result in scale loss from the fish. However this should not be confused with the most common reason for scale loss which is that resulting from capture (netting or sportfishing). Anglers should be particularly careful to avoid the loss of scales during capture since their removal exposes the skin to entry particularly by bacteria and fungi. Ulcers can be another external characteristic in the skin of diseased fish often as a result of bacterial infection. In carp, ulcers are usually spherical, with red necrotic centers and white edges. Once the ulcers have healed, they are repaired and are often pigmented with a dark tissue called melanin.
Disease in cyprinids can also cause a number of different alterations of the skin including hyperplasia (proliferations of epithelial cells) which is thought to be caused by a herpes-like virus, and papillomas, which are similar to warts, containing connective tissue and a blood supply.
The gills are commonly checked for signs of infection by fisheries scientists since their appearance can indicate a number of things about the health of the fish. Clearly, it is not advisable for anglers to go “poking around” in the fish’s gills, however a recently deceased fish could be examined. The most common change in the gills is cell hyperplasia which can be observed through a microscope, and commonly results from pollutants and parasitic infection, and as a result of environmental stress. As water quality declines, the number of mucous-producing cells and therefore the thickness of the mucous coating of the gills increase, resulting in reduced gas exchange. The gills of diseased fish can also show the presence of dead tissue, the loss of surface cells and abnormal accumulation of fluid.
Regarding internal signs of disease, with the exception of pathological signs of disease within the oral cavity, the angler will not be able to tell if a fish is diseased from observing the exterior, unless some of the previously mentioned disease signs are apparent. Hooking lesions are a common site of infection through the entry of viruses, bacteria and fungi, and it is therefore important to use disinfectant on any new hook-holds. Also, if previous hook-holds have become infected, the same disinfectant liquid should be applied as a means of reducing the possibility of further infection. The avoidance of infection and disfiguration through hooking increases the chance of the fish continuing to feed normally.
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