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ICH - The Fish Lice

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What is ICH?

In this article, we are going to talk about the ‘ICH’ which is a parasitic organism. It is a ciliate that was first described by French parasitologist Fouquet in 1876. The genus is the one that is often termed ICH. There is only one species in ICH, Ichthypophthirius multifiliis which is often termed as ICH, that has been found until now under this genus and because of which the name of the genus became to be called ICH. The literal translation of the name is “the fish louse with many children”.


The parasite is known for being able to infect most of freshwater species of fish. Unlike many other parasites, the host has very low specificity. It normally penetrates the gill epithelia, skin and fins of the fish host. It lives as a feeding stage, known as the trophont, inside the epidermis of the fish. You can see Ich as a white spot on fish's body surface but it mostly lives internally as it is a true endoparasite and not an ectoparasite. Due to this reason, it is also known as white spot disease - fish living in freshwater normally have it.

General Characteristics of Ich

Due to the huge macroscopically visible trophonts (up to 1 mm in diameter) in the skin and fins, it produces a disease known as white spot illness. The trophont, which rotates constantly and is surrounded by host cells (epidermal cells and leukocytes), causes a minor skin elevation. White dots are the most common form of these light-reflecting nodules.


If tight biosecurity measures are broken in any way, the parasite can be introduced into a fish rearing unit by transferring infected fish or equipment. Because of its rapid reproductive cycle, the organism is difficult to regulate once it enters a large fish production facility. If the illness is not controlled, the tank's mortality rate could reach 100%.


Strict control procedures, such as mechanical and chemical methods, are commonly used on farms to keep the infection at a manageable level. However, these methods are frequently expensive in terms of labour, chemicals, and fish loss.


According to research conducted as part of the Horizon2020 project ParaFishControl, a variety of innovative control approaches have emerged. The immune system of fish, for example, can fight parasitic invasions, and a vaccine could be created in the future. Furthermore, new bacterial compounds (surfactants from Pseudomonas) can kill the parasite's exterior stages without hurting the host.


In two ways, Ichthyophthirius multifiliis causes significant harm to gills and skin. To begin with, the manner in which theronts penetrate the host epithelia; when the number of parasites is large in comparison to the size of the fish, the penetration may later directly kill the fish by compromising the integrity of the fish surface. And if the invasion is successful, the invading theronts enter the trophont stage in the fish epidermis, where they mature and multiply their volume exponentially. When trophonts burst out of their epidermal home, significant ulceration occurs, resulting in a high rate of host mortality. The fish's osmoregulation is further hampered by both penetration and trophont escape. Damage to the host's gills also lowers the fish's respiratory efficiency, lowering its oxygen uptake from the water.

Life Cycle of ICH

The parasite's life cycle implies no intermediate hosts are involved in transmission. It includes a trophont stage that lives on the surface of the fish such as gill epithelia, skin and fin epidermis. This is the feeding stage, in which the parasite continuously ingests cellular debris and live host cells at its epidermal location, allowing it to grow very quickly in a short period of time - depending on temperature. When the trophont reaches a particular size, usually between 100 m and 1000 m, it will break free from the host skin and swim as a tomont, which can also be covered by the cilia. The tomont clings to any surface in the fishpond or fish tank and forms a thick, viscous cyst wall between minutes to hours. The tomocyst stage is the next step.


A sequence of mitotic cell divisions occurs within the tomocyst, resulting in up to 1000 daughter cells known as tomites, depending on temperature. These fish exit the tomocyst by breaching the cyst wall, then swimming in the fish tank water in search of a fish host, which they will pierce quickly and effectively if it is naive and unimmunized. The entire life cycle takes around 7 days at 25 °C to 8 weeks at 5-6 °C, and it is largely dependent on water temperature. The following diagram depicts a typical life cycle:

Prognosis and Diagnosis

The infection puts the hosts' osmoregulation and respiration under strain. Because of the disruption of epithelial linings, secondary bacterial and fungal infections are prevalent. Non-protected (non-mucous cell lined) cells become susceptible to additional infections when trophonts burst out of the epidermis. The following are examples of typical behaviour seen in sick fish:

  • Anorexia nervosa is a condition in which (loss of appetite)

  • Increased breathing rate is a common occurrence (hyperventilation)

  • Discolouration of the skin

  • Behaviour that is abnormal (inactivity and isolation)

  • Resting at the bottom of a body of water

  • A flashing light (rubbing and scratching against rough objects)

  • Disturbance of equilibrium. Swimming near the surface when upside down.

Penetration of theront may cause irregular swimming and movements, indicating irritation of the fish's surfaces. The trophont does not become apparent to the naked eye until it has consumed the fish and grown to a diameter of 0.3-0.5 mm. The white spots on white spot disease fish can grow to be more than 1 mm in diameter and can be seen on the skin and fins, whereas trophonts connected to the gills are difficult to discern because of the gill cover known as the operculum.


The following are the most prevalent infection sites and their associated clinical signs:

Skin: ICH fish disease infections frequently appear as one or more distinct white spots on the fish's body or fins. The white spots are trophonts, which are solitary cells that feed on host cells (epidermal cells and leukocytes drawn to the location) and can grow to be 1 mm in diameter. The skin becomes uneven, fluffy, and grey after severe infections and subsequent lesions caused by trophont escape.

Gills: Gill infection might result in surface breathing and increased operculate ventilation movements.


Because of the parasite's low host specificity, ICH infection has been documented in all freshwater fish systems studied. However, susceptibility and impact vary depending on the host species. Rainbow trout, catfish, and eels are among the most vulnerable fish species, with uncontrolled infections resulting in nearly 100% mortality. Some cyprinids, like zebrafish, already have higher innate protection and are able to clear the infection faster than other species.


The presence of visible trophonts (white spot on fish) on skin or fins is frequently used to make a preliminary diagnosis of I. multifiliis infection. Microscopic inspection of skin and gill smears can confirm the diagnosis. Scrapings of skin, fins, or gill surfaces (with a coverslip or scalpel) should be mounted on a microscope slide with a few drops of water under the coverslip and inspected under a light microscope providing 20-400 x magnification). The trophont rotates slowly, has a conspicuous horseshoe-shaped macro-nucleus, and is covered with fast-beating cilia. Molecular diagnosis is qualitatively accomplished by scientific biological techniques - PCR and quantitative real-time PCR and is officially based on knowledge of genes encoding the parasite's i-antigen.

ICH Treatment Strategies

Infected fish and infected fish farm systems can usually be treated with a variety of chemotherapeutics, although caution should always be exercised during any treatment. Some pharmaceuticals are hazardous to certain fish species, therefore any ich treatment plan must take that into account (some will not tolerate certain medications). Malachite green was once the medication of choice for ich treatment, but it is now prohibited due to its carcinogenicity. Formalin (30–50 mg/L) will kill infective theronts and tomonts, however, other chemotherapeutics should be used instead due to its carcinogenicity. Copper sulphate, methylene blue, and potassium permanganate are useful, although their environmental impact is debatable. Although copper is still used in some countries, it is quite easy to overdose on it. The suggested dosage is 0.15-0.3 mg/L, with a maximum concentration of 0.4 mg/L. In soft water, copper is more harmful to fish than in hard water. Drugs like metronidazole and quinine hydrochloride are also effective, but they must be prescribed by a veterinarian.


Hydrogen peroxide and hydrogen peroxide-releasing goods such as sodium percarbonate and peracetic acid are examples of environmentally favourable products. These compounds must be introduced to the fish tank water to eradicate theronts and tomonts, but they have no effect on the fish skin's trophont stage. At greater temperatures, the toxicity of hydrogen peroxide increases. When used at a dosage of at least 7.5 g/L, sodium chloride inhibits the development of infective theronts in tomocysts. The parasite can be removed from a recirculated fish farm system when employed at a concentration of 10 g/L for 14 days. A wide range of herbal extracts including garlic juice (which has a poisonous effect on theronts) has recently been proven to be useful. Biological control has also proven to be effective. Theronts, tomonts, and tomocysts have all been found to be killed by a lipopeptide released as a surfactant by the bacterium Pseudomonas H6. It is not poisonous to fish, implying that future regulation should be based on natural items that are environmentally beneficial.


It may be necessary to remove all of the fish and transfer them to clean tanks on a regular basis. Theronts, the ICH life cycle's motile and fish-infecting stage, emerge from the tomocyst near the tank's bottom. Theronts, on the other hand, will die within 48 hours if they don't find a fish to reattach to (at higher temperatures). Transferring all the fish that carry the trophonts in their skin, fins or gills to a non-infected tank every 24 hours is an effective technique to clear ICH from a fish population and for white spot fish treatment. The fish will not become infected again, and after a few days, depending on the temperature, the infection will have cleared because trophonts will have left the fish. They don't have enough time to make theronts because 24 hours is too short for released tomonts to grow into infective stages via tomocysts. These management methods must be continued for longer periods of time in colder water. Another option is to filter water mechanically using mesh sizes of 80 microns. The tomonts will be removed from the water before they settle and convert into tomocysts (the multiplication step).


Temperature rise exceeding 30 degrees Celsius can be beneficial, and it can be supplemented with other treatments. However, it can only be used on fish that can withstand these high water temperatures, thus it is not ideal for cold-water species such as trout, salmon, koi, and goldfish.


Priority must be devoted to avoiding the parasite's introduction in the first place. Warm-water fish should be confined for at least four weeks, whereas cold-water fish should be quarantined for eight weeks. Biosecurity measures for fish farm workers, such as utilising a biocide foot bath, dressing separately for each unit, using separate equipment, and sanitising hands before and after tank maintenance, will limit the danger of the parasite spreading across units.


Some protection may be provided by the host reaction. Fish that have recovered from an infection are somewhat immune to reinfection and will be able to fight off a new infection. Due to a lack of commercially available vaccines, there is currently no way to prevent the disease through immunisation. However, multiple studies have found possible vaccine candidate proteins from the parasite, such as i-antigens and others, implying that a vaccine with promise could be developed in the future.


In conclusion, Ich or scientifically known as Ichthypophthirius multifiliis is a parasite that is very detrimental, especially to freshwater fishes. From the article, it is now easy to identify the fish infected by Ich because of the white spots present on their skin. Over time, as the parasites grow it will lead to the death of the fish. Also, since the parasite is not specific to a particular genus of fish, it is high risk and saving the fish requires an immediate change of the entire surrounding water environment. Or else the fish needs to be treated medically with compounds that have deadly effects on the bacteria like the malachite green. But since malachite green itself is now found to be lethal to humans and some other biological living organisms it has been banned and hence, other alternate methods are being found to cure fish infected with Ich.

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FAQs on ICH - The Fish Lice

1. How do you treat fish with ICH?

Removing all of the fish from the ich tank for a period of time is a wonderful method. With the absence of the fish, ich tomatoes die after two days at a given temperature above 30 °C. Other suggestions include keeping the tank devoid of fish for four days and keeping the temperature at 80 °C.

2. Can a fish survive ICH?

Fish have been known to have multiple outbreaks of ich, build immunity to it, recover, and then have another outbreak - and be fine. However, if the ich parasite infects the gills, the fish will almost certainly die. If ich is present and stays in your system, this can happen at any time.


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