| Ichthyophthirius | |
|---|---|
| Scientific classification | |
| Domain: | Eukaryota |
| (unranked): | Alveolata |
| Phylum: | Ciliophora |
| Class: | Oligohymenophorea |
| Order: | Hymenostomatida |
| Family: | Ichthyophthiriidae |
| Genus: | Ichthyophthirius |
Ichthyophthirius multifiliis (commonly known as freshwater white spot disease, freshwater ich, or freshwater ick) is a common disease of freshwater fish.[1] It is caused by the protozoa Ichtyopthirius.[2] Ich is one of the most common and persistent diseases. The protozoan is an ectoparasite. White nodules that look like white grains of salt or sugar of up to 1 mm appear on the body, fins and gills. Each white spot is an encysted parasite.[2] It is easily introduced into a fish pond tank, or home aquarium by new fish or equipment which has been moved from one fish-holding unit to another. Once 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. [3]
Whitespot is very damaging to the gills and skin. In heavily infected fish it can cause a rapid loss of condition, considerable distress and death. Infected fish have small white spots on the skin and gills (Fig. 2 and Fig. 3) and produce excess mucus, due to irritation. Whitespot causes most damage when entering and leaving the tissues of the fish. This can lead to the loss of skin and ulcers. These wounds can harm the ability of a fish to control the movement of water into its body. Damage caused to the gill tissue of an infected fish can also reduce respiratory efficiency. This means it is more difficult for the fish to obtain oxygen from the water, and becomes less tolerant to low levels of dissolved oxygen.[4]
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I. multifiliis is one of the most prevalent protozoan parasites of fish and is an obligate parasite which means that the parasite cannot survive unless live fish are present. Adult organisms have an oval to round shape and measure 0.5 to 1.0 mm in size. The adult parasite is uniformly ciliated and contains a horseshoe-shaped nucleus which can be seen in older individuals. It is an important pathogen of ornamental and farm-raised food fish species when reared under intensive conditions.[2] Wild fish populations are also susceptible and outbreaks are occasionally seen. There are few aquarists that have not met it on one or more occasions.[5]
The ich protozoa goes though the following life stages:[1]
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).
Marine ich is a similar disease caused by a different ciliate, Cryptocaryon irritans.[6]
The white spots that can be seen on the infected hosts are trophonts. This is the feeding and growing stage. Once it has reached maturity the trophont leaves the host. Now it is known as a tomont. The tomont becomes encysted, producing a sticky capsule. This enables it to attach to any substrate that it comes into contact with, from weeds and stones to fishing equipment, such as line and nets. Within its cyst the tomont divides many times, producing up to 3000 tomites. The tomites break out of the cyst wall and are now theronts. The theronts are heavily ciliated and actively seek out a host, without which they can survive for up to 48 hours. On finding a host the theront penetrates through the skin and develops into a trophont. [7]
There is no dormant stage in the lifecycle. Ich does not lie in wait for a weakened fish to infect. However, any factor that reduces immunity like changes in water temperature and quality may, in a subclinically infected fish, accelerate an outbreak of Ich. The presence of ammonia, nitrite and high levels of nitrate in water does not in itself cause clinical cases of Ich. However, poor water quality will stress fish, allow an outbreak to spread rapidly and increase mortality rates in infected fish.
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, dark in color (due to the thick cilia covering the entire cell), and has a horseshoe-shaped nucleus which is sometimes visible under 100x magnification. The adult parasite moves slowly in a tumbling manner and is easily recognized. The immature forms [tomites] are smaller, translucent, and move quickly. [8]
Typical behaviours of clinically infected fish include:
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 will not become 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 usually is not readily seen. A subclinically infected fish may initially only have a single trophozoite.
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 1 mm in diameter. A smear should show ciliates if white spot is present.
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. Once the water quality is high enough, the fish will usually recover by themselves within 1–2 weeks. Thus, it is advisable to wait for 1–2 weeks before administering antibiotics.
Gill infection will cause breathing at the surface and fast respiration. Gill examination may reveal numbers of white spots. Wet mount of a gill biopsy may reveal mutifiliis trophozoites.
Unfortunately, an efficient prevention of the disease by vaccination is not possible, although several studies identified potential vaccine candidate proteins, i.e. i-antigens, of the parasite.[9]
Any treatment method must take into account the species of fish (some will not tolerate certain medications), how high the infection rate is, and the size and type of environment.
If it is detected before it becomes too serious, a number of different treatments can be applied. Only the free-swimming stage of the parasite is susceptible to treatment; neither the trophonts under the epithelium nor the tomont cysts can be killed.[10]
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.
For treating koi & 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.
One method of treatment for ich consists of adding aquarium salt until a specific gravity 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% (100 mg/L; pure water at 4 °C/39 °F), would require large quantities of salt. Fish can be dipped in a 0.3% (3g/L; pure water at 4 °C) solution for thirty seconds to several minutes, or they can be treated in a prolonged bath at a lower concentration (0.05% = 500 mg (0,5g)/L; pure water at 4 °C).
Chemical treatments include formalin, malachite green, chelated copper, copper sulfate, potassium permanganate and quinine. 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]. [11]All treatments target the free-living theronts and tomonts, which only survive about two to three days in the absence of a host fish.
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.
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