Ichthyophthirius multifiliis, often termed "Ich", is a parasitic ciliate. Only one species is found in the genus, which also gave name to the family. The name literally translates as "the fish louse with many children". The parasite can infect most freshwater fish species and, in contrast to many other parasites, shows low host specificity. It penetrates gill epithelia, skin and fins of the fish host and resides as a feeding stage (the trophont) inside the epidermis. It is visible as a white spot on the surface of the fish but, due to its internal microhabitat, it is a true endoparasite and not an ectoparasite. It causes a disease commonly referred to as white spot disease due to the macroscopically visible trophonts (up to 1 mm in diameter) in the skin and fins. The trophont, continuously rotating, is surrounded by host cells (epidermal cells and leukocytes), producing a minute elevation of the skin. These light-reflecting nodules are recognized as white spots. If strict bio-security rules are violated, the parasite may be introduced into a fish rearing unit by transfer of fish or equipment from infected systems. When the organism gets into a large fish culture facility, it is difficult to control due to its fast-reproductive cycle. If not controlled, the infection may lead to 100% mortality in the tank. Strict management measures including mechanical and chemical methods are generally applied and can keep the infection at an acceptable level at farms. However, these measures are costly in terms of labour, chemicals and lost fish. Research within the Horizon2020 project ParaFishControl pointed to a range of new approaches for control. For example, the fish immune system has an ability to combat invading parasites and a vaccine may be developed in the future. In addition, novel bacterial products (surfactants from Pseudomonas) can directly kill the external stages of the parasite without harming the host. Ichthyophthirius multifiliis inflicts considerable damage to gills and skin in two ways. Firstly, the theronts penetrate the host epithelia and, when the number of parasites is high in relation to the fish size, the penetration may directly kill the fish by destroying the integrity of the fish surface. Secondly, if the invasion is successful, the invading theronts transform into the trophont stage in the fish epidermis where they develop and expand their volume manifold. When the trophonts burst out from their epidermal residence, severe ulceration follows, leading to high host mortality. The osmoregulation of the fish is challenged both by penetration and by trophont escape. Damage to the host's gills also reduces the respiratory efficiency of the fish, reducing its oxygen intake from the water.
Life cycle
The life cycle of the parasite is direct, which means that no intermediate hosts are included in transmission. It includes a trophont stage residing in the fish surface (gill epithelia, skin and fin epidermis). This stage is the feeding stage which continuously ingests cellular debris and live host cells in its epidermal location, making the parasite able to grow rapidly over a short time - depending on temperature. When the trophont has reached a certain size (100-1000 μm), it breaks out of the host epidermis and swim freely as a tomont (also covered by cilia). After minutes to hours, the tomont attaches to any surface in the fishpond or fish tank and produces a thick, gelatinous cyst wall. This is termed the tomocyst stage. Within the tomocyst, a series of mitotic cell divisions take place and, depending on temperature, up to 1000 resulting daughter cells (tomites) are produced. These escape the tomocyst by penetrating the cyst wall, whereafter they swim in the fish tank water searching for a fish host, which they penetrate fast and efficiently if it is naïve and non-immunized. This life cycle is highly dependent on water temperature, and the entire life cycle takes from approximately 7 days at 25 °C to 8 weeks at 5-6 °C.
Pathology and clinical signs
Signs and symptoms 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.
Clinical signs 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.
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. 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.
Impact 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. Sturgeon are also highly susceptible to infection by Ich.
Diagnosis
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