Vet Times The website for the veterinary profession https://www.vettimes.co.uk My fish appear to be itchy... they re jumping about, scraping and flicking Author : FIONA MACDONALD Categories : Vets Date : August 18, 2008 FIONA MACDONALD discusses the diagnosis and treatment of skin and gill fluke infestation in the companion fish JUST as summer brings fleas, ticks and mites to the small animal clinic, companion fish, such as koi carp, also suffer their own special types of parasitic irritation (Figure 1). The main parasites that cause koi carp, in particular, to be restless and show a flicking behaviour, consist of a variety of species. These include Monogenean trematodes, otherwise known as flukes, which are divided into Gyrodactylus and Dactylogyrus species, as well as parasitic copepods such as Lernaea cyprinacea and Branchiura - Argulus. Gyrodactylus The Gyrodactylus species is commonly known as the skin fluke, and although they consist of a distinct separate species from the gill fluke, both can vary where they attach and feed on the fish. Thus, gill flukes can also be found on the skin and skin flukes may also colonise the gills. Gyrodactylus flukes reproduce by giving birth to live offspring, which can sometimes be seen within the body of the adult fluke. This means that they have the possibility to be treated with parasiticides throughout their life cycle. Their life cycle is direct, and so they can infest large numbers of susceptible fish. 1 / 14
Flukes infest fish by attaching to the skin using hooks, which causes serious irritation. The fish then responds by scraping or flicking and this is the symptom most commonly recognised by keepers. The fish also cause mechanical damage to themselves by trying to rub on objects in the pond (). Added to this, flukes, by their method of attachment, will cause skin damage, which will, in turn, allow the entry of pathogenic bacteria and possibly even viruses. Affected fish can lose their colour and become depressed. Diagnosis Diagnosis is carried out by firstly sedating the fish and then gently scraping the affected area - with either a blunt scalpel blade or even the edge of a glass microscopic slide - to remove any mucus. Transfer the scraping on to a microscopic slide and add a drop of the pond or tank water, before covering it with a cover slip and examining it with a microscope. A magnification of 40 under the microscope is usually adequate to see most of the common fish skin parasites. The Gyrodactylus parasite has two characteristic hooks at its anterior end () and is less than 1 mm in length. Occasionally, a juvenile may be seen within the uterus of the adult. Alternatively, it is possible to use a dry slide to simply take an impression smear. There is less risk of damage to the fish, but a slightly reduced chance of finding the parasites. Some keepers have sufficient expertise to carry out koi skin scraping themselves, and so it is quite possible to be presented with the diagnosis and the actual flukes (). One of the interesting aspects of veterinary fish practice is that since fish are not considered as animals under the current Veterinary Surgeons Act (1966), it is quite legal for lay people to carry out disease diagnosis. Indeed, some fish biologists can certainly bring expertise to the whole area of fish veterinary work. However, the right to prescribe POMV treatments remains strictly under the control of veterinary surgeons. Practitioners should take care to use products that have been tested on fish, and if using the cascade system, it is advisable to check with the manufacturer to avoid any nasty surprises. Treatment When considering treatment of any condition in ornamental fish, the primary consideration should be to check the water quality. Poor-quality parameters are commonly associated with disease, and simply treating the disease entity without improving the environment of the fish will lead to disappointment. Commercial test kits are readily available and testing is relatively straightforward. The most important parameters are ammonia, nitrates and nitrites. These show the relationship between food digestion, stocking density and efficiency of the bacteria in the biofilters. When water quality is 2 / 14
poor, an immediate remedy is to change at least 50 per cent of the water. This is the fish equivalent of fresh air and a good clean out. Once water conditions have been addressed, treatment should be selected. Generally, the range of licensed medicinal products specifically developed for fish are limited. This has inevitably led to the widespread use of a variety of remedies of varying efficacy - and, indeed, safety - to the fish. Historically, products such as formalin have been widely used for a range of conditions, from bacterial infections to a variety of parasites. The chemical should be used with care since it can have toxic effects - not only on the fish being treated, but also on the biofilters. Salt baths have also been used by some koi keepers to remove skin flukes. Affected fish have to be removed from their ponds to be immersed in a salt bath, because once salt is in a pond it cannot be removed and there is the risk of adverse effects of high salt levels on pond plants. As a caution, specialist aquarium salt must be used. Household salt iodinised and may cause some problems to fish as well as the anti-caking agents it contains. Parasiticides, such as mebendazole and praziquantel, are effective against flukes - the only issue here is in their presentation. Oral administration of any medicinal product to ornamental fish is always a challenge, but the immersion approach requires the product to be soluble in the tank or pond. Because of the method parasites use to reproduce, treatment should be carried out four times to completely eliminate the infestation - once every seven days for a period of 28 days. Dactylogyrus Dactylogyrus species affect the gills of the fish and reproduce by laying eggs. The rate at which they lay eggs, and become sources of infestation to other fish, varies according to the water temperature: the higher the temperature, the faster the eggs are layed. As many as 20 eggs per hour can be laid in warm water temperatures of between 22ºC and 24ºC. On release, these eggs float out from the gills into the environment and hatch into the next larval stage. Again, the time taken to hatch depends on water temperature, but can vary between 20 days in cool water to as short as four days at higher temperatures. The newly hatched larvae can attach to the next host after about four days. Approximately 10 days later, the larvae will have matured into the egg-laying stage and the cycle starts all over again. It has a direct life cycle, and once infested, the fish has the capability of further infesting large numbers within a very short time in warm conditions. The eggs of these organisms, however, are very resistant, so to fully control infestations in a standard pond set-up, treatment should be repeated at intervals of around seven days. The flukes attach to the gill surfaces and feed on blood and epithelial tissue (). Affected fish will 3 / 14
have some difficulty in breathing normally, owing to the interference with gill function, showing pale gills with extended gill covers. Not only do they cause gill damage, but the disturbance they cause to the integrity of the gill surface allows the entry of pathogenic bacteria, such as Aeromonas and Flavobacterium species, as well as viruses. Diagnosis Fish infested with gill flukes may show distended operculae with laboured breathing. Gills can look inflamed with excessive mucus production. Affected fish are often depressed and can be found lying on the bottom of the pond. A definitive diagnosis, however, may require microscopic examination of the gills. In live fish it may be difficult to obtain a successful biopsy, but often a simple skin scrape (as described previously for skin flukes) may well be sufficient, since flukes are not totally sitespecific, and infestations often involve more than one type of fluke or even ectoparasite. Dactylogyrus parasites can be up to 2 mm in length, and have distinctive eyespots. Treatment Gill fluke treatment is exactly as described for skin fluke treatment. Formalin or even salt baths are commonly used (despite the drawbacks described before), and the veterinary therapeutics of choice are mebendazole or praziquantel. Copepods Parasitic copepods affecting fish such as koi carp include Ergasilus and Lernaea. These parasites can be found on the skin, gills or fins. However, the main copepod of concern to koi keepers is Lernaea cyprinacea, otherwise known as the anchor worm. Adult females can grow to 2 cm in length and are usually the ones seen attached to the fish. Their life cycle is direct, and even low numbers of anchor worms can cause serious damage to the fish they infest. The mechanical damage caused by their attachment not only causes localised haemorrhage and inflammation, but can also act as a port of entry to secondary infections of bacteria or viruses. Diagnosis Diagnosis is fairly simple. The affected fish will show the characteristic symptoms of irritation, agitation and rubbing (flashing) and, as the parasites can be up to 2 cm in length, they are clearly visible to the naked eye. Treatment 4 / 14
As with so many fish diseases, there are only limited therapeutics available for the control of this infestation. The first priority is to address any water quality issues that may have precipitated the problem (). The pharmaceutical options are limited. One of the products widely used to control sea lice in the farmed Atlantic salmon industry is emamectin. This also controls Lernaea. It is administered in the feed (which, itself, can present some challenges to the veterinary practitioner medicating companion fish). Lufenuron has been used successfully. Care must be taken, however, to choose the suspension form, since the active ingredient is very insoluble. It is also very persistent in the environment. In the case of a serious and persistent infestation, one control option may even be to drain and disinfect the whole pond system. All stages of the parasite life cycle are susceptible to drying. Branchiura The last group of ectoparasites to be discussed in this article are the Branchiurans, of which the main concerns are the fish lice or Argulus species. Although there are a significant number of different species, the most important for companion fish in freshwater are Argulus foliaceus and Argulus japonicas. These parasites have a direct life cycle and, after mating, the females will lay their eggs in places such as aquatic plants, before returning to the fish host. As with parasitic copepods, the attachment of these parasites to the fish and their feeding behaviour will predispose the affected fish to secondary infection. The parasites themselves have also been implicated in the spread of viral disease and even some nematodes. Diagnosis The diagnosis is by observation of the parasites. They should be clearly visible to the naked eye, tending to congregate along the backs of the fish around the area of the dorsal fin. Treatment Products available for treating Argulus, as with the copepods, are those that have been developed for salmonids infested with sea lice. The main therapeutic of choice would be emamectin, which is administered orally. Summary Fish veterinary practice - although, in some quarters, almost perceived as a poor relation to other companion species - should not be discounted. It is an opportunity to practise the basic principles 5 / 14
of: observation; history taking; diagnosis and treatment; and management areas, such as biosecurity, which includes specific issues such as quarantine. Fish suffer from all the usual range of pathogens familiar in other species - bacteria, viruses and parasites. The names and classifications may be different, but the principles are the same. Access to information via the internet means that clients can be better informed, but the basic role of the veterinary surgeon to offer professional advice and have access to prescription medicines has not changed. The challenges offered by this novel area of companion animal practice may lead to some very interesting opportunities (). 6 / 14
Figure 1. This salmon has been the victim of parasitic infestation. The fish s side clearly shows where the scales have been irritated. 7 / 14
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Figure 2. If fish do become infested, they can cause further damage to themselves by trying to rub on objects in the pond. 9 / 14
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Figure 3. At less than 1 mm in length, the Gyrodactylus parasite has two characteristic hooks at its anterior end. Photo: COURTESY OF BERNICE BREWSTER. Figure 4. Some koi keepers have enough expertise to carry out skin scrapings, and so aid the veterinarian. 11 / 14
Figure 5. Once hatched, the Dactylogyrus flukes attach to the gill surfaces and feed on blood and epithelial tissue. Photo : COURTESY OF BERNICE BREWSTER. 12 / 14
Figure 6. A koi pond in an attractive garden setting. Water quality must always be addressed before treatment can begin. 13 / 14
Figure 7. Fish are a popular companion animal and an interesting challenge for the practitioner. 14 / 14 Powered by TCPDF (www.tcpdf.org)