Piscinoodiniasis
Piscinoodiniasis or velvet disease is quite common in aquarium fishes. Virtually all freshwater fishes are susceptible to piscinoodiniasis infections. Outbreaks tend to be of an explosive nature, and large numbers of fish can succumb to the disease within the course of a few days. The parasite appears to be non-specific and indiscriminately infects various fish species invading skin, fins and gills. Occasionally, the occurrence of trophonts is reported from the oesophageal epithelium and intestine, and even in the subcutis. It has historically been responsible for a large number of fish mortalities. However, aquarium hobbyists rarely have infected fish closely examined to confirm that the infection is consistent with Piscinoodiniasis.
Piscinoodiniasis is caused by the dinoflagellate Piscinoodinium pillulare, formerly known as Oodinium pillularis. Dinoflagellates are (unicellular) protists that exhibit a great diversity of form; this group includes gill and skin parasites that can cause serious disease of freshwater and marine fish e.g., Amyloodinium (saltwater), Crepidoodinium (estuarine and marine) and Piscinoodinium (freshwater). All three genera have been traditionally classified as belonging to the family Oodiniaceae. The relatedness of species belonging to these three genera is primarily based upon a similar mode of attachment to the host, i.e., attachment disc with holdfasts. Their life cycle is also similar, with each having 3 stages: a parasitic trophont; a reproductive tomont; and a freeswimming infective dinospore.
The first dinoflagellate found on a freshwater fish was Oodinium limneticum, which was identified from aquarium fish in North America (Jacobs, 1946). Schäperclaus (1951) described a second species, Oodinium pillularis, from aquarium fish in Europe. In 1981, the genus Crepidoodinium was created for Oodinium cyprinodontum and Piscinoodinium was created for Oodinium pillularis and Oodinium limneticum. However, it is unclear as to whether Oodinium limneticum is a valid species.
Piscinoodinium pillulare is a sedentary parasite that attaches to the skin, fins, and gills of fish. Infestations of the skin are usually less serious than those affecting the gills. The rhizoids of the trophont actively penetrate into the epithelium of the host fish, which responds by means of a pronounced hyperplasia, often sufficiently intense as to entrap the parasites themselves. Areas of haemorrhage and focal necrosis appear which are frequently invaded in a secondary manner by bacteria and fungi. In the gills, the response ranges from desquamation and separation of the epithelial layer to a manifest hyperplasia affecting the entire length of the gill filament. Degenerative changes and necrosis of the gills are frequently observed in severely affected fish.
Piscinoodinium pillulare is a dangerous ectoparasite of aquarium fishes and is most pathogenic to young fish. Although young fish may die quickly from piscinoodiniasis, older fish may live for 2–3 months. The pyriform, sack-like trophont is up to 160 µm long and has an amber or yellowish-green colour, visible on heavily infected fish.
The life cycle of Piscinoodinium pillulare is comprised of a parasitic feeding stage (trophont) which attaches to integumentary epithelial cells, and an encysted dividing stage (tomont) which is detached from the host. After approximately six days of growth at 25°C, the developed trophonts drop of the host’s surface, sinks to the bottom and becomes a tomont.
Tomonts undergo successive divisions and divides successively into 64 or 128 small tomites. They divide again to produce 128 or 256 cells which differentiate into free-swimming infective dinospores. The division of tomonts into dinospores, in temperatures of 23~25°C, is completed within 4~6 days. At 15– 17°C, the process of division is lengthened to 11 days. After being released, the free-swimming dinospores can infect a new host or re-infect the same host, thus compromising its health status.
There is some evidence that suggest fish recovering from the epizootic infestation through a gradual decrease in infection develop immunity against re-infections, and specific antibodies have been demonstrated in the blood serum of infected fish.
Diagnosis The diagnosis of cases of piscinoodiniasis is based on the clinical signs observed, and detection of the typical sac-shaped trophonts in wet mounts from the gills or the skin. Trophonts, when reaching the final stage of growth, are visible to the naked eye (80–100 µm diameter) as white spots (similar to that seen in ichthyophthiriasis). On the body surface, an increase in mucous production, scale loss, suffusion, ecchymosis, petechiae, and small ulcers may be observed. The gills may also present an increase of mucous production besides epithelial hyperplasia, suffusion, petechiae, congestion, oedema, and brownish areas. Visible signs begin as a light golden dusting in oblique light and then progress to more severe infestation intensity associated with dense white dusting of the skin. This surface sheen is easiest to observe by placing the fish in the dark and shining a beam of light through the water parallel to the surface of the skin.
Infected fish generally swim near the surface of the water or gather near the filter outlets, and the fins may be folded. Fish with skin infections may exhibit ‘flashing’ behaviour. Fish with heavy gill infections typically exhibit rapid respiration (spreading opercula) as large numbers of parasites compromise gill function. Other clinical signs included dyspnea, lethargy, cachexia, localised secondary infections, and erratic swimming with loss of equilibrium. Some fish exhibit anorexia and mortalities progressively increase.
Prevention Successful fish health management begins with prevention of disease rather than treatment. Regular control and monitoring of water quality is imperative and will greatly reduce the likelihood of a disease occurrence. Without this foundation, it is impossible to prevent outbreaks of opportunistic diseases. Piscinoodinium pillulare tend to be opportunistic pathogens of aquarium fish and outbreaks can be treated and also controlled by improving hygiene, water quality and reducing stocking densities. It’s possible that the infective dinospore stages can be transmitted in aerosol droplets, particularly where tanks are situated in close proximity. Therefore, proper disease preventative management should be observed. All syphon hoses, nets, brushes, and other equipment that has been in contact with the infected fish should be disinfected.
Treatment Parasites numbers on individual fish can be rapidly reduced by osmotic shock (e.g., altering either temperature or salinity beyond that tolerated by the parasite) if the fish can tolerate such treatment. Baths in sodium chloride at 30 g/L for 5-15 minutes, for fish species capable of tolerating that salinity level, are effective in dislodging the trophonts; however care should be exercised to avoid reinfestation when the fish are returned to their aquarium. The effects of salt on fish are determined both by salt concentration and duration of exposure and salt tolerance by fish may vary depending on species and age. Raising water temperatures to 33-34°C has also been reported to control infestations.
The life cycle of this parasite can be completed in 10–14 days at 22–25°C, but lower temperatures can slow the life cycle. Also, the cyst stage is highly resistant to chemical treatment. Therefore, several applications of a treatment may be necessary to eliminate the parasite. The free-swimming dinospores are usually the stage that is most susceptible to a variety of interventions including chemotherapy.
Quinine hydrochloride (at 30 mg/L), or a combination of quinine hydrochloride and malachite green (at manufacturer’s recommendation) has also been used with some success. However, of the quinine derivatives, and despite the proven antiprotozoal activity of many of these compounds, only quinine sulphate and quinacrine hydrochloride killed ciliates within 24 hours, and only at very high doses (minimal lethal concentration = 100 mg/L in both cases). Numerous authors have reported the efficacy of quinine baths for treating ectoparasitic and sub-epidermal protozoal infections of fishes. Furthermore, the continuous administration of quinine containing feed to ornamental fishes appears to be effective for eliminating the skin-inhabiting trophozoites of the ciliate Ichthyophthirius multifiliis, responsible for white spot disease (Schmahl et al. 1996).
Under laboratory conditions (Schmahl et al. 2006), the ionophoric polyether salinomycin given in the fish diet was shown to be effective against the skin-inhabiting trophozoites of Piscinoodinium pillulare. Experimentally infested swordtails (Xiphophorus hellerii) were fed once a day ad libitum food pellets containing either 60 or 90 ppm salinomycin for a 16 or 19 days period. The efficacy of the treatment was monitored by counting the numbers of trophozoites of each fish at day 0, 3, 6, 9, 12, 16 and 19, respectively. As revealed by transmission electron microscope investigations, the damages in the trophozoites caused by the treatment consisted in malformation of the trophozoites, aggregation of droplets within the cytoplasm, and the formation of electron dense bodies along the limiting membrane. Following a prolonged treatment period, ruptures in the trophozoites limiting membranes were seen and the rhizocysts were no more detectable. Under the experimental conditions described, fishes showed no signs for adverse effects.
Copper sulphate (CuSO4) will kill both dormant (destroying the chloroplasts) and the free swimming stage as well. However, copper sulphate treatment can be unpredictable under certain aquarium conditions and is extremely dangerous to some species of fish, plants, shrimp and snails. Copper sulphate may be used as a bath for up to 10 days duration, at a concentration of 0.15 ppm of copper ion. If necessary, this bath can be repeated.
Copper sulphate should never be used without testing the alkalinity of the water, carefully measuring the volume of the aquarium or pond to be treated, and weighing the amount of chemical to be applied. The concentration of copper sulphate to apply is usually calculated by determining the total alkalinity of the water and dividing that number by 100. For example, if the total alkalinity of the aquarium is 100 mg/L, then 100 ÷ 100 = 1 mg/L copper sulphate. Do not use copper sulphate if the total alkalinity is less than 50 mg/L. If you are unsure how to measure the alkalinity of your water, or have never used copper sulphate, then do not use it. Copper can also bind to the substrate in the aquarium and leach back into the water for a long time. In addition, copper levels must be monitored frequently if good results are to be expected, and this may not be practical for the average hobbyist. Therefore Copper sulphate is best used by aquarium specialists.
When using a commercially formulated copper cure, always follow the label instructions for dosage rates. Chelated copper will stay in solution longer than copper sulphate and appears to be safer to fish. You can create your own chelated copper by using two parts citric acid to one part copper sulphate, by weight. Combine both in distilled water and dissolve them together. It is important to remember that you will be treating with the copper sulphate and not the citric acids, so when weighing the formula, use only the weight of your copper sulphate in calculating dosages.
Most fish are extremely sensitive to copper. Concentrations of copper as low as 42 µg Cu/L were found to be acutely toxic to Denariusa bandata. Melanotaenia s. inornata and Ambassis species have been found to be sensitive to copper; half of the individuals tested died at copper concentrations between 120 and 200 µg Cu/L. The freshwater shrimp genus, Caridina, is extremely sensitive to copper, dying at levels of only 2 µg Cu/L. Macrobrachium species were found sensitive to copper with half the individuals dying at 160 µg Cu/L. Snails are also known to be very sensitive to copper.