Ichthyophthirius multifiliis

The infection challenges hosts’ osmoregulation and respiration. Secondary bacterial and fungal infections are common due to the disturbance of epithelial linings. When trophonts burst out of the epidermis, non-protected (non-mucous cell lined) cells become accessible to other pathogens.

Typical behaviour of clinically infected fish includes:

• Anorexia (loss of appetite)
• Increased breathing rate (hyperventilation)
• Discoloration
• Abnormal behaviour (inactivity, isolation)
• Resting on the bottom
• Flashing (rubbing and scratching against objects)
• Balance disturbance. Upside-down swimming near the surface.

Trophonts of Ichthyophthirius multifiliis (diameter 300 μm) in the epidermis of a rainbow trout tail fin (light microscopy with subillumination). The horseshoe-shaped macronucleus is visible. (photo: Kurt Buchman, University of Copenhagen)

Theront penetration may elicit erratic swimming and movements reflecting irritation of fish surfaces. The trophont is not visible to the naked eye until it has fed on the fish and grown to a diameter of about 0.3-0.5 millimetres. The white spots may reach more than 1 mm in diameter and are easily recognized on skin and fins whereas trophonts attached to the gills are hard to see due to the gill cover (operculum).

Skin: Ich infections are usually visible as one or several characteristic white spots on the body or fins of the fish. The white spots are single cells called trophonts, which feed on host cells (epidermal cells and leukocytes attracted to the site) and may grow to 1 mm in diameter.[3][10] Heavy infections with subsequent lesions following trophont escape leave the skin irregular, fluffy and greyish.

Gills: Gill infection may cause breathing at the surface and increased ventilation movements of operculae.

Due to the low host specificity of the parasite, Ich infection is known from all freshwater fish systems examined. However, the susceptibility and the impact differ between host species. Rainbow trout, catfish and eels are highly susceptible fish species and uncontrolled infections lead to almost 100% mortality. Some cyprinids, such as zebrafish, have a higher innate protection and may clear the infection faster than other species.

Various chemotherapeutants can be applied for the treatment of infected fish and infected fish farm systems but caution should always be observed during any treatment. Some drugs are toxic to certain fish species and any treatment method must take into account the species of fish (some do not tolerate certain medications). Malachite green was previously the drug of choice but, due to its carcinogenicity, this organic dye is now banned in some countries. Formalin when applied repeatedly (30–50 mg/L) kills infective theronts and tomonts but, due to its carcinogenicity, other chemotherapeutants should be used. Copper-sulphate, methylene blue and potassium permanganate are effective but questionable from an environmental point of view. Copper may still be applied in some countries, but it is easy to overdose with copper. The recommended dosage is 0.15-0.3 mg/L and the concentration should never exceed 0.4 mg/L. Copper is noticeably more toxic to fish in soft water than in hard water. Drugs such as metronidazole and quinine hydrochloride are effective as well, but require prescription from a veterinary authority.

Environmentally friendly products include hydrogen peroxide and hydrogen peroxide releasing products such as sodium percarbonate and peracetic acid.[6] These compounds can be added to the fish tank water and eliminate theronts and tomonts but do not affect the trophont stage in the fish skin. The toxicity of hydrogen peroxide is increased at higher temperatures. Sodium chloride when applied in a concentration of at least 7.5 g/L inhibits production of infective theronts in tomocysts.[12] When used in a concentration of 10 g/L over 14 days, the parasite can be totally eliminated from a recirculated fish farm system.

Recently, a wide series of herbal extracts have been shown as effective, including garlic juice, which has a toxic effect on theronts and trophonts.[14] Biological control has also demonstrated its potential. A lipopeptide secreted as a surfactant from the bacterium Pseudomonas H6 has been shown to kill theronts, tomonts and tomocysts.[9] It is not toxic to fish, which suggests that future control can be based on environmentally friendly, natural products.

Total fish removal and repeated transfer to clean tanks may be applied. Theronts, the motile and fish-infecting stage of the Ich life cycle, exit from the tomocyst at the bottom of the tank. However, without fish to re-attach to, theronts die within 48 hours (at higher temperatures). An effective way to clear Ich from a fish population is to transfer all of the fish carrying trophonts in their skin, fins or gills to a non-infected tank every 24 hours. Then the fish do not get re-infected and after a number of days (dependent on temperature) the fish have cleared the infection because trophonts exit within this period. They do not have sufficient time to produce theronts as 24 h is too short time for released tomonts to develop via tomocysts releasing the infective stage. Under colder water conditions, these management procedures should be continued over a longer time. Another method is to use mechanical filtration of water using mesh sizes of 80 microns. This removes the tomonts from the water before they settle and transform to the tomocyst stage (the multiplication step).[6]

Priority should be given to avoid introducing the parasite in the first place. New warm-water fish should be quarantined for at least four weeks and cold-water fish for eight weeks. Recognition of biosecurity measures for fish farm personnel including using a biocide foot bath, separate dress for the unit, using separate equipment and disinfecting hands before and after maintenance of each tank, reduces the risk of spreading the parasite between units.[11]

The host response may provide some protection. Fish recovering from an infection are partly protected against reinfection and can resist a new infection.[4] Prevention of the disease by vaccination is, at present, not possible due to the lack of a commercially available vaccine. However, several studies have identified potential vaccine candidate proteins, e.g. i-antigens and others, of the parasite, which suggests that a vaccine can be produced in the future.[7][8]