The German language has a saying “as healthy as a fish in water”. It sounds good, but it doesn’t stand up to scrutiny because fishes get sick just as often as we do. In the wild, however, unhealthy fishes are only rarely noticed because they are hardly visible in the water anyway and once sick soon fall prey to predators. The situation is quite different in aquaculture where the fishes swimming in tanks and ponds are constantly monitored. Normally it is not very difficult to recognize disorders or health problems in fishes. Lack of appetite, a change in colour, changed swimming patterns or other behavioural abnormalities such as headstands, surface breaking behaviour, frequent jumps, or rubbing against solid structures are clear signs that a fish may be affected by disease. But this can be quite a problem, for a lot of symptoms of fish diseases are nonspecific. A person needs a certain level of experience to recognize with certainty whether a fish is at the surface because of a lack of oxygen in the water or because it is really sick. Reliable diagnoses are often only possible for trained fish farmers and specialized veterinarians… particularly since the fishes do not only have to be examined externally (behaviour, condition of fins, ulcers or swellings, dull skin, excessive mucus production, fungal infection) but in much more detail, for a lot of diseases are accompanied by changes in the internal organs. Swelling, inflammation, bleeding, cysts or pus in the liver, spleen, intestine, stomach or swim bladder are always clear indications of serious diseases.
There are several possibilities and reasons why the health status of a fish can get out of control. The interdependencies that exist between the fish’s immune system (which protects it against numerous diseases), latent pathogens in the environment, and the animal’s current living conditions are highly complex. There are many factors that can trigger disease within aquaculture systems. The spectrum ranges from stress due to poor farming conditions, parasitic infestation and external injuries, to genetic impacts such as malformations or hereditary diseases, and various fish diseases caused by pathogenic germs, bacteria and viruses. When people talk about fish diseases they almost always mean these kinds of infections. Diseases resulting from pathogens are particularly dangerous because they often lead to huge losses and, in the case of inadequate isolation of the infected communities, can easily spread to other fish farms, for example when several fish farming facilities are positioned successively in a river’s course. That is why some particularly dangerous fish diseases are subject to mandatory reporting, both at national level and within the EU. Notifiable diseases include, for example, IHN (infectious hematopoietic necrosis), VHS (viral haemorrhagic septicaemia) and IPN (infectious pancreatic necrosis) in salmonids, as well as koi herpes viral infections in carp, mostly KHV for short although the correct abbreviation would actually be CNGNV, standing for carp nephritis and gill necrosis virus.
Stress and parasites weaken the immune system
Although stress is in itself not an illness it interferes with the fishes’ rhythm of life, weakens their resilience and thus creates the necessary conditions for subsequent diseases. Among the most important measures for disease prophylaxis are thus the avoidance of any kinds of stress, from daily handling procedures to water quality or the suitable design of the fish tanks. Environment-related physical and chemical damages, i.e. health damages that are not caused by pathogen germs but by unfavourable water parameters, constitute an extensive complex of risk sources. Too much or too little oxygen, temperature shocks, poisoning from substances that are dissolved in the water, or high pH values which can lead to gill necrosis (GN), for example, in carp. Too little oxygen and carbon dioxide in the water can cause gill swelling in trout and is just as serious as oversaturation of the water with dissolved gases which is considered to be the cause of gas bubble disease. Sunburn can also occur in fishes if the aquaculture facility does not offer sufficient shade. Damages caused by constructional or functional defects in a farm’s technology and can lead to considerable losses are also termed “technopathies”.
Parasitologists will often tell you that if no parasites have been found in a fish it has not been examined carefully enough. And indeed, there are hardly any fishes without parasites. Whilst ectoparasites (from the Greek ecto – outside) which colonise on the skin, fins, gills and external body cavities, endoparasites (from the Greek endo – inside) are to be found in the abdominal cavity as well as in and on internal organs, in the intestine, the liver, the kidney and the brain. Like stress, most parasites are not considered an illness but they weaken the infected fish and its immune defence mechanisms which facilitates secondary infections. Parasitic tapeworms, flukes, hookworms und annelids damage the host organism to different degrees depending on the infection intensity. The fishes lose weight, growth is poor, and their general well-being is affected. But there are also parasites which can be the direct cause of illness if they appear in large enough numbers. An example of such parasites is Ichthyophthiriose (white spot disease), which originates from parasitic paramecia. Whirling disease in trout, a disorder which leads to the fish swimming abnormally in an awkward corkscrew-like pattern, a darkening of the rear part of the body, and spinal distortion is caused by a single cell parasite (Myxosoma cerebralis).
In the case of other parasites such as swim bladder worm in eels it is not even clear what concrete effects the parasites might have on the fish. Originally the parasite Anguillicola crassus came from Japanese eel but in the mid-1980s it was discovered in the swim bladders of European eel as well. In the meantime it seems to have infected the majority of eels. Eels with swim bladder worms become stunted and often display abnormal swimming patterns. In professional circles it is even discussed whether the parasite might be partially to blame for the overall decline of the eel population because mature animals perhaps no longer manage to migrate to their spawning grounds in the Sargasso Sea. A further negative accompanying effect which is above all caused by parasites that sting and bite the fish, sit on the skin or the gills, and tap the blood capillaries or parts of the gill epithelium, is the transmission of infectious diseases. The carp louse Argulus, for example, is considered to be the carrier of spring viral anaemia in carp (SVC). In Chile the salmon louse is considered partially responsible for the spreading of the ISA virus among salmon cultures.
Mycoses are indications of poor husbandry
Diseases caused by fungi, called mycoses, are not only detrimental to the fish’s appearance but they can also have serious economic consequences because if they remain untreated they almost always lead to the death of the infected animals. Mycoses are secondary infections that can occur everywhere where the protective mucus layer on the skin’s surface is damaged. For example through mechanical injuries or after illnesses that have damaged the skin structure. Such mucus-free areas are ideal targets for fungal attack. The fungi develop on the skin to form a cotton-like film. Probably the most frequent and most widely spread fish fungus or mould is Saprolegnia which occurs everywhere in almost all waters and can attack all fish species as well as their spawn.
Control of fish parasites and fungi is difficult, not only because the use of effective agents in natural waters and fish farming facilities requires very large quantities and is accordingly expensive but also because hardly any drugs are authorized and readily available. Malachite green is an effective fungicide which has been banned for treatment of food fish since 2004 and the remaining substances mostly contain copper sulphate or copper chloride as effective agents which is lethal to many invertebrates and some fish species in small doses. Some parasites such as Myxobolus cerebralis whose larvae lead to whirling disease in fishes can only be treated through prevention by disinfecting against the intermediary host Tubifex or sludge worm with quicklime or calcium cyanamide (CaCN2). Individual infected animals can be bathed in weakly concentrated formalin, potassium permanganate, iodine-free cooking salt or lysol.
Vaccines prevent bacterial and viral diseases
Probably the greatest danger to a fish stock, however, comes from bacterial and virus-based diseases that are encouraged by a lack of hygiene, overstocking, or organic contamination of the water. The chief bacterial diseases in trout and salmon farms are cold water vibriosis, furunculosis, and fish mycobacteriosis, bacterial fin rot and enteric redmouth disease ERM). BKD (bacterial kidney disease), a disease that is usually fatal, must also be mentioned here. In the case of carp, carp erythrodermatitis (CE or ED), previously known as the ulcerative form of infectious ascites, and carp furunculosis are among the frequent bacterial diseases. Fighting these diseases is difficult for it requires exact diagnosis and demands special medicinal feeds, and often antibiotics which are, however, in many countries in Europe not approved for use in the fish sector. A promising method for reducing the curative use of antibiotics and medication is prophylactic vaccination which strengthens the fish’s immune system and provides protection against bacterial attacks. Through the use of such vaccinations the Norwegian aquaculture sector has succeeded in being able to almost completely do without the use of antibiotics on salmon farms.
Viral infectious diseases in farmed fish are even more dangerous for there are up to now hardly any effective therapies or effective drugs to combat them. Neither VHS (viral haemorrhagic septicaemia) nor IHN (infectious haematopoietic necrosis) nor IPN (infectious pancreatic necrosis) is currently curable and the occurrence of these diseases usually involves partial or complete loss of the fish stock. This is also true of SVC, spring viraemia of carp, although a live vaccination against this disease already exists in some non-EU countries. The most important steps for avoiding virus-caused fish diseases are thus strict hygiene, regular controls, and preventive disinfection measures within the aquaculture facilities. Fry should only be purchased from companies that are free from pathogen viruses and whose status the vendors can verify with tests (for example IFAT, ELISA, PCR). This applies in particular to fry and food fish that are traded beyond national borders within the European community. The appendix of EU Directive 91/67/EEC lists all diseases that are notifiable and whose transmission and spreading are to be prevented through strict monitoring and control measures. Just how disastrous the effects of a fish disease that is allowed to spread unhindered are can be seen from the severity of the ISA crisis (infectious salmon anaemia) which nearly halved Chile’s salmon production in 2007 and 2008 and destroyed nearly 70% of the Salmo salar biomass in the country’s salmon farms. Because there are no effective drugs against a lot of virus diseases companies in which these diseases are rampant are mostly immediately isolated and it is not uncommon that the fish stock is destroyed. This ruthless method is often the only practicable way to prevent the spreading of the disease to other facilities and to wild fish stocks.
Global trade advances the introduction of fish diseases
Taking care not to overstock, ensuring suitable water treatment and hygiene, high-quality feed and prophylaxis (which includes above all the regular disinfection of nets, buckets, animal transport containers and fish tanks) are the easiest, cheapest and most certain way to prevent outbreaks of fish diseases in aquaculture facilities. The effort is considerable, but it pays off in the end for healthy fish stocks mean less losses and ultimately less expenditure for medication and treatment. Healthy fishes also facilitate trade relations with other companies and customers. Special care should always be taken when importing foreign fish species that from a fish disease and hygiene point of view are mostly critical. The list of diseases, parasites and other ecological and health problems that were carried with allochthonous fish and crustacean species to Europe and other regions is long.
With Directives 2006/88/EG and 2007/345/EG the EU Council is trying to prevent and combat some of the diseases that face aquatic animals today. Fish farms and processing companies have to be approved by the responsible authorities of the member states, fish imports are only permitted if they come from authorized, controlled, disease-free countries and regions. In spite of these precautionary measures the risk remains high, however, that a disease will break out in an aquaculture facility. In such cases qualified veterinary care and support become necessary as well as the use of drugs to help the infected animals and also protect the fish stock against further infection. But things look bleak in this area as the pink list of the EU (VO 37/2010 EU) shows. This list contains all the active agents and drugs that are authorized for animals that are used for human consumption. Fishes are considered “minor species” and only a few drugs are authorized for them. The pharmaceutical industry shows little interest in applying for drug approval since the procedure is a costly process. “Fish drugs” aren’t worthwhile because the market is relatively small. The consequences for aquaculture in the EU are dire: a state of emergency nearly everywhere where therapy is needed.
Necessary treatments can often only be carried out after clinical and diagnostic testing in the laboratory under veterinary supervision. Fishes and fish products are in the EU subject to national residue control plans. They are taken off the market if the drug residues exceed the authorized maximum permitted levels. This is in keeping with consumer protection requirements.