Ichthyophthirius multifiliis is an endoparasite of freshwater fish which causes a disease commonly known as white spot disease, or ich. Ich is one of the most common and persistent diseases in fish. It appears on the body, fins and gills of fish as white nodules of up to 1Â mm, that look like white grains of salt. Each white spot is an encysted parasite. It is easily introduced into a fish pond or home aquarium by new fish or equipment which has been moved from one fish-holding unit or pond to another. When the organism gets into a large fish culture facility, it is difficult to control due to its fast reproductive cycle and its unique life stages. If not controlled, there is a 100% mortality rate of fish. With careful treatment, the disease can be controlled but the cost is high in terms of lost fish, labor, and cost of chemicals.
The protozoa damages the gills and skin as it enters the tissues, leading to ulceration and loss of skin. Severe infections rapidly lead to loss of condition and death. Damage to the gills reduces the respiratory efficiency of the fish, reducing its oxygen intake from the water. This causes the fish to become less tolerant to low oxygen concentrations in the water.
Life stages
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The ich protozoan goes through these life stages:
- Feeding stage : The ich trophozoite (a protozoan in active stage of life) feeds in a nodule formed in the skin or gill epithelium.
- After it feeds within the skin or gills, the trophozoite falls off and enters an encapsulated dividing stage (tomont). The tomont adheres to plants, nets, gravel or other ornamental objects in the aquarium.
- The tomont divides up to 10 times by binary fission, producing infective theronts, thus dividing rapidly and attacking the fish.
This life cycle is highly dependent on water temperature, and the entire life cycle takes from approximately 7 days at 25 °C (77 °F) to 8 weeks at 6 °C (43 °F).
Disease
Signs and symptoms
Typical behaviours of clinically infected fish include:
- Anorexia (loss of appetite, refusing all food, with consequential wasting)
- Rapid breathing
- Hiding abnormally
- Not schooling (in schooling fish)
- Resting on the bottom
- Flashing
- Rubbing and scratching against objects
- Upside-down swimming near the surface
A subclinically infected fish will not show any of these signs. For example, a healthy fish with a newly attached trophozoite will not yet have clinical disease. The trophozoite is not visible to the naked eye until it has fed on the fish and grown to one or two millimetres. A trophozoite attached to the gills is hard to see. A subclinically infected fish may initially only have a single trophozoite.
Skin
Visible ich lesions are usually seen as one or several characteristic white spots on the body or fins of the fish. The white spots are single cells called trophozoites or trophonts, which feed on the tissues of the host and may grow to 1Â mm in diameter. A smear should show ciliates if white spot is present.
Eyes
The eye becomes cloudy almost to the point of whiteness and the fish lose vision. The causes behind this disease can vary. An increase in parasites in the aquarium is the most common cause but severe stress, old age, or malnutrition can all lead to this condition. Treating this condition requires an investigation of water quality. When the water quality is suitable, the fish will usually recover by themselves within one to two weeks. Thus, it is advisable to wait for that time to elapse before administering antibiotics.
Gills
Gill infection may cause breathing at the surface and fast respiration. Gill examination may reveal numbers of white spots or wet mount of a gill from a biopsy may reveal the trophozoites. The fishes' breathing can slow, causing them to rest on the sand or gravel.
Predisposing factors
There is no dormant stage in the lifecycle. However, any factor that reduces immunity, such as changes in water temperature and quality, accelerate an outbreak of Ich in a subclinically infected fish. Ammonia, nitrite or high levels of nitrate in water do not in themselves cause clinical cases of Ich. However, poor water quality stresses fish, which allows an outbreak to spread rapidly and increases mortality rates.
Other abiotic factors can increase both fish and tadpole susceptibility to ich. These factors include decreased temperature, predatory cues (crowding, fighting) and increased levels of UV-B radiation.
Diagnosis
The diagnosis of "ich" can easily be confirmed by microscopic examination of skin and gills. Remove several "white spots" from an infected fish, then mount them on a microscope slide with a few drops of water and cover with glass. The mature parasite is large and dark (due to thick cilia covering the entire cell). It has a horseshoe-shaped nucleus, which is sometimes visible under 100x magnification. The adult parasite moves slowly by tumbling. The immature forms [tomites] are smaller, translucent, and move quickly.
Prevention
Prevention of the disease by vaccination is not possible, although several studies identified potential vaccine candidate proteins, i.e. i-antigens, of the parasite. The most effective prevention is quarantining new fish for two weeks and plants for four days in a separate tank. Fish that survive an ich infection may develop at least a partial immunity, which paralyzes trophonts that attempt to infect it.
Treatment
Any treatment method must take into account the species of fish (some will not tolerate certain medications), how many of the fish are affected, and the size and kind of environment. Temperature affects how quickly the parasites multiply, so increasing the temperature can force them through their life cycle more quickly, allowing treatments to target Ichthyophthirius in its tomont stage. Also, higher temperatures around 86 °F (30 °C) can prevent tomont replication.
If ich is detected before it becomes too serious, a number of different treatments can be applied. The first line of treatment for severe outbreak is usually formalin or malachite green, or a combination of the two. Copper, methylene blue, and baths of potassium permanganate, quinine hydrochloride, and sodium chloride (aquarium salt) have also been used but do not appear to offer an advantage over the more readily available formalin and malachite green products.
Total fish removal
Tomites, the motile and fish-infecting stage of the ich life cycle, exit from the trophonts that burst from affected fish and fall to the bottom of the tank. Without fish to re-attach to, however, tomites will die within 48 hours of exiting their trophonts. Thus, an effective way to clear a tank from ich is to remove all of the fish for a certain amount of time. At 80 degrees Fahrenheit, ich tomites will die at 2 days in the absence of fish, and just to be absolutely sure, some recommend keeping the tank empty of fish and at 80 degrees for 4 days (96 hours). This solution may be helpful when there are either very few fish in the tank or when tank capacity is too large to easily treat the volume of water.
Heat treatment
Heat treatment can be highly effective, and it can be combined with other treatments. However, it can only be used on fish that can tolerate high water temperatures, and is unsuitable for cold water fish like koi and goldfish, but even in those cases, a higher water temperature will accelerate the life-cycle of the parasite, allowing other treatments to take effect sooner.
Chlorine
For treating koi and goldfish, chlorine, in the form of tapwater, is very effective in removing not only the threadlike parasites, but eventually the persistent cysts. Thread like infestations on fish will disappear overnight, cysts will take a couple of weeks and possibly a couple of water changes to eliminate. Aquarium lighting is used to detect the presence of parasites, as the filament like threads fluoresce at these light frequencies.
Salt
One method of treatment for ich consists of adding aquarium salt until a specific density of 1.002 g/cm3 is achieved, as the parasites are less tolerant of salt than fish. This is not practical in ponds because even a light salt solution of 0.01% (100Â mg/L), would require large quantities of salt. Fish can be dipped in a 0.3% (3Â g/L) solution for thirty seconds to several minutes, or they can be treated in a prolonged bath at a lower concentration (0.05% = 500Â mg/L).
Chemical treatments
Chemical treatments include formalin, malachite green, methylene blue, chelated copper, copper sulfate, potassium permanganate, quinine and tosylchloramide sodium. There are also a large number of proprietary treatments available for the treatment of white spot, and the related Oodinium (velvet disease). Chemical treatment is only effective against free-swimming juvenile parasites [tomites]. All treatments target the free-living theronts and tomonts, which only survive about two to three days in the absence of a host fish.
Prognosis
When ich is diagnosed early, effective treatment is used, and stresses are minimised, mortality rates can be low. However, if the infection is at an advanced stage, treatment protocols are not followed, and the fish are stressed, higher death rates will occur. When a fish has had ich eradicated, it may develop partial resistance to reinfection.
Partially treated fish may initially harbour low numbers of unseen trophozoites, often in the gills. This subclinical carrier will cause another outbreak weeks later, most likely when stresses occur or uninfected fish are introduced to the aquarium.
See also
- Marine ich, a similar disease of marine fishes.
References
External links
- Shirlie Sharpe's Your Guide to Freshwater Aquariums
- Detailed life cycle of Ichthyophthirius multifiliis at MetaPathogen
- Treatment information for freshwater Ich at the Aquarium Wiki
- IchDB - Ichthyophthirius Genome Database Wiki
