Gill & Body Flukes
The monogeneans (flukes) are typically ectoparasites of the skin, fins and gills of many freshwater and marine fishes. Some species become endoparasitic by inhabiting the oral cavity, cornea, nasal tissue, pharyngeal tooth pads, urogenital system (cloaca, rectal gland, oviducts) and even the vascular system. In all cases they are attached to the host’s surface by a characteristic opisthaptor (an attachment organ) provided with hooks, suckers or clamps. With the exception of the members of the viviparous family Gyrodactylidae, monogeneans usually have a simple life cycle involving hermaphroditic adults, eggs and larvae, and can proliferate rapidly on fish held in captivity. Severe infestations can be associated with morbidity and mortality of fishes in aquarium systems. Confinement probably facilitates autoinfection and infection of new hosts. In nature, most monogenean larvae fail to acquire a host.
Unlike the other monogeneans, the larva of the viviparous family Gyrodactylidae is retained in the uterus until it develops into a functional preadult, inside which a second larva is already formed, with a third larva inside that and a fourth inside the third. The steps of this sequential polyembryony are poorly understood. After its birth, this preadult larva begins feeding on its host and gives birth to the second larva remaining inside it. Only then may an egg from its own ovary become fertilised, repeating in a short time the development described above. Since this peculiar embryogenesis does not involve a free-swimming infectious larva, Gyrodactylidae depend on transmission of adults or pre-adults from one host to another by direct contact.
There have been very few studies of monogenean parasites from Australian freshwater fishes. The group is morphologically diverse and identifying them correctly is often impossible. Up until 1998 twenty-six species had been described from sixteen species of native freshwater fish. A study published in 2004 (Corlis, 2004) described another nineteen new species. These were found on Melanotaenia splendida, M. eachamensis, M. maccullochi, M. trifasciata, Rhadinocentrus ornatus, Cairnsichthys rhombosomoides, Craterocephalus stercusmuscarum, C. marjoriae, Pseudomugil signifer and P. gertrudae.
A very large group of fluke parasites infecting rainbowfishes have an intermediate live stage in molluscs and crustacea. Some of these parasites cause problems in both wild caught and captive rainbowfishes.
Monogenean parasites can be introduced into the aquarium from wild-caught fish. Although monogenean parasites are commonly found on wild fish, they are rarely a direct cause of disease or death in wild populations. Some destroy their hosts, but most are specific for one species of fish or for two or more closely related species. Because of high host specificity, most Monogenea do not spread to other fish species when they are introduced with their hosts into a new environment. However, they may become more dangerous to their hosts in the new surroundings. The establishment of exotic monogenean populations on Australian native fishes via host-switching is considered less likely than for other parasitic groups due to the generally high host specificity of monogeneans.
Transmission of monogenean parasites from fish to fish is primarily by direct contact. Oviparous monogeneans release eggs into the water which hatch and mature into adult forms within 7 days, depending on temperature.
Viviparous monogeneans release live larvae which may attach to the same host as the parent or be carried by the water circulation to another. This direct life cycle can contribute to population explosions under aquarium conditions. Under the right conditions they can literally “bloom” like algae. Not all species of monogeneans will multiply rapidly on hosts maintained in captive environments; however, those that do are usually serious pathogens. Populations of viviparous species can rapidly increase to problematic levels.
Monogenean flukes can be devastating to aquarium fishes. They appear on the gill filaments as tiny dark spots about 600 µm long. They are a group of parasites best described as flatworms - the body is usually flat and oval. Most flukes are browsers, moving about the body surfaces, and feeding on skin mucus and gill tissue. Most species are host and site-specific, requiring only one host to complete an entire life cycle. In fact, some adults will remain permanently attached to a single site on the host. Large numbers of flukes on either the skin or gills may result in significant damage and mortality. Therefore, knowledge and understanding of monogenean parasites is important for aquarium hobbyists.
Mortality of aquarium fish caused by excessive parasite infestations is usually associated with inappropriate aquarium conditions, undernourished fish or have been subjected to temperature fluctuations. Heavy infestations may elicit epidermal hyperplasia due to disruption by parasite attachment and feeding; increased mucus production may also occur. Secondary infections by protozoa, bacteria and fungi often ensue and can cause deep wounds and ulcers. In severe infestations, death occurs, usually due to loss of osmotic regulation. A few flukes on a healthy mature fish are usually not significant; however, moderate numbers on a young fish can cause significant mortalities.
Freshwater fish infested with flukes become lethargic, swim near the surface or gather near the filter outlets, and subsequent loss of appetite. Heavily infected fishes may be seen rubbing the bottom or sides of the aquarium (flashing) indicative of infections on skin or gills. The skin where the flukes are attached show areas of scale loss and may ooze a pinkish serous fluid. The gills may be swollen and pale, respiration rate may be increased, and fish will be less tolerant of low oxygen conditions, showing other signs of respiratory distress. Gulping air at the water surface may be observed in severe respiratory distress.
Treatment Quarantine is the first defense with close observation of new fishes. Prevention of fluke infestations by following appropriate quarantine practices is preferable to treating the parasites after they have become established in the main aquarium. This can be done by dipping fish prior to placing them into an established aquarium, and following a quarantine protocol whenever feasible. If quarantine is not possible, a simple way to minimise introduction of flukes, as well as other external parasites, is to dip rainbowfishes in a saltwater bath (30–35 g/L) for threeto five-minutes. Dipping rainbowfishes in a salt bath will eliminate many single-celled external parasites. This practice will not completely eliminate the risk of introducing parasites to an established tank or that no parasites will be introduced with new fish, but it will help minimise the numbers brought in.
Ideally, rainbowfishes should be quarantined for at least four weeks prior to being placed into a new system. While in quarantine, prophylactic treatment with a broad-spectrum parasiticide should be carried out. A quarantine system should be very simple so that fish are readily accessible for observation and handling, water can be easily changed, and treatments readily administered.
Treatment of flukes is usually not satisfactory unless the primary cause of increased fluke populations is found and alleviated. If the disease is not in the acute phase, a low salinity bath (5 g/L for 90 minutes) or (1 level tablespoon per 20 litres of water) is often enough to solve the problem if combined with water changes and general cleaning of the aquarium environment.
Praziquantel has been identified as the most effective “in water” treatment of infected fish. Praziquantel is harmless to fish of all species, is non toxic to plants, and has no negative filter impact. Praziquantel is a bitter tasting powder which shows good absorption directly from the treated water, and then admirable clearance of various surface and internal flukes and worms in fish. Praziquantel has been known to the hobby for many years. Praziquantel was traditionally available in the form of branded Droncit® tablets, for oral administration in dogs and cats, but is now available in a range of aquarium products.
Praziquantel used at 2–3 mg/L is very effective for control of both gill and body flukes and has a wide margin of safety for fish. Praziquantel is toxic to flukes on contact, paralysing the parasites within 15 seconds under laboratory conditions. Praziquantel preparations must be dosed high enough and long enough for effective treatment. Monogeneans can be persistent in aquarium systems necessitating regular treatments. In cool water, the parasites move through their life cycles slowly, so it is important to medicate long enough to intercept the emerging larvae. When temperatures are above 25º Celsius, treat once every 3 to 4 days for a total treatment time of 20 days. When temperatures are between 20 and 25º Celsius, treat once every 4 to 5 days for a total treatment time of 25 days.
The eggs can be resilient to chemical treatment, which make the use of multiple chemical treatments appropriate to control this group of organisms. Praziquantel can also be administered in food at a dosage of 35–125 mg/kg for up to 3 days or as a short-term bath treatment at a concentration of 10 mg/L for 3 hours.
Change 50–75% of the water in between the chemical treatments. Fish, which are obviously weak and heavily parasitised may not survive. Management to lessen the chance of infestation by these parasites includes maintaining the fish in a good nutritional state and avoiding water quality problems that might weaken the fish.
The effectiveness of the long-term use of Praziquantel has been evaluated in ornamental fish. Cumulative doses up to 10 mg/L water were tolerated without side-effects by Angel Fish (Pterophyllum scalare), Discus, and a variety of catfish species (Ancistrus sp., Corydoras sp.). It was found appropriate to start with a dosage of 2.5 mg/L and to add the same dosage every other day several times. All adult parasites and larvae were killed by this treatment.
For the complete elimination of Dactylogyridae populations in a closed aquarium system, 3 therapy-cycles (duration: 5–6 days, accumulated dosage: 2.5 mg/L/day) proved to be effective. It was important to interrupt the therapy-cycles with intervals without medication (1 to 4 weeks). However, there are reports that kissing gouramis (Helostoma temminckii) have been adversely affected by Prazifish® (Praziquantel 98.5 mg/g).
Dactycid® is another effective treatment against flukes and all species of internal worms (hookworms and roundworms) including Camallanus cotti, hydras and planarians. However, it can be toxic to snails and is harmful to sharks and rays. Flubendazole (15%) is unique and specifically designed and developed for aquatic use and approved by the Veterinary Medicines Directorate under the small animal’s exemption scheme to be sold over the counter for public sale. This medication will kill flukes, camallanus worms, tapeworms, anchor worms and other helminthics commonly found in tropical and coldwater fish. It has also been used successfully with ‘skinny disease’ in clown loaches.
Organophosphates can also be used to treat monogenean parasites, as well as leeches and crustacean ectoparasites Argulus (fish lice) and Lernaea (anchorworm). Organophosphates work by interfering with the nervous system and thus affect vital physiological processes. However, organophosphates are potentially dangerous to both fish and humans and, for a variety of reasons; their use in fish disease control has been banned in many countries, although the use of organophosphates in aquaria is still practiced.
The two most commonly used organophosphates are dichlorvos and trichlorfon. The concentrations of active ingredients vary in the different preparations. Historically, trichlorfon was used at a concentration of 0.25 mg/L active ingredient. When trichlorfon is added to water it degrades to dichlorvos, which then slowly degrades to less toxic byproducts. The quicker it breaks down, the higher the initial dose needs to be.
The rate at which it degrades depends on various factors: light, high temperatures, aeration and high pH all speed up degradation. In mild temperatures and moderate pH the half -life can be several days, which means that it is active for longer. In alkaline conditions and high temperatures the half -life can be as short as a day. Also, organophosphate uptake and toxicity in fish is increased by low oxygenation of the water. These factors result in variable response of fish to exposure to organophosphates, with levels greater than 0.1 ppm being potentially toxic.
Monogenean flukes can reinfest fish between 5 and 30 days after treatment with organophosphates, depending on water temperatures and degree of pond contamination with eggs. Up to three treatments, 21 days apart may be required to eradicate or reduce monogenean numbers to an insignificant level.
Spinal deformities have been reported in rainbowfishes (Melanotaenia pygmaea) after treatment with trichlorfon, at a rate of 0.5-2 ppm. Organophosphates exert their toxic effect by inhibition of acetylcholinesterase, an enzyme involved in terminating neurotransmission at cholinergic synapses in the central nervous system, some peripheral autonomic junctions and neuromuscular junctions. In intoxicated fish that don’t die acutely from central nervous system dysfunction, the muscle spasms produced by excessive and prolonged stimulation of neuromuscular junctions of the muscles of the body are thought to be sufficiently severe to result in spinal fracture and lesions.
Other choices include potassium permanganate, formaldehyde and maybe others. These chemicals can also be dangerous when used incorrectly and should only be used by experienced aquarists. There have been reports of flukes being resistant to certain types of treatment such as formaldehyde.
When considering treatments for monogenean infections it is important to remember that the eggs of egg-laying species can be relatively resistant to water treatments. Thus treatments must sometimes be repeated to kill newly hatched larvae before they can mature and lay eggs. It is also important to realise that eggs and larvae can remain viable away from the host for some time. Thus, treating infected fish without treating their environment may only provide temporary relief. It is also important to recognise the potential for secondary infections associated with severe monogenean infections.