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White spot - Ichthyophthirius multifiliis Fouquet, 1876

Classification

Kingdom: Protozoa

Phylum: Ciliophora

Class: Oligohymenophorea

Order: Hymenostomatida

Family: Ophryoglenina

Genus: Ichthyophthirius

Species: I. multifiliis ref.

Geographical Distribution

'Ich' has a global distribution and has been recorded in tropical, sub-tropical and temperate regions extending north to the Arctic Circle. Outbreaks were recorded in China as early as the Sung Dynasty (960-1279AD), in Europe in the Middle Ages (400-1400AD) and the first outbreak described in North America was in 1898. Evidence suggests that it originated in Asia as a parasite of carp and was probably introduced into the UK in cultured carp. ref. ref.

Life Cycle Summary

The adult parasite (the trophont) is endoparasitic within the host fish and may undergo division whilst in the host. Each trophont appears as a small white spot on the skin of the infected fish. The duration of this stage is temperature-dependent and may last about 7 days at 20˚C. In temperate regions, the parasite can overwinter within the host for an increased duration of 3-4 months recorded at 3˚C. The exit from the host into the aquatic environment is an active process possibly involving the discharge of contractile vacuoles. In the aquatic environment the trophont encysts and the resultant tomont undergoes repeated binary fission, producing a population of approximately 50-3000 tomites. The tomites then differentiate into theronts, the infective stage of the life cycle. Tomonts can survive temperatures ranging from 2 to 27˚C and division times vary with temperature from 6 days to 10 hours as temperature increases. Theronts can survive around 22.5 hours at 20˚C although viability drastically declines after 12 hours. There are no recorded findings of sexual reproduction in I. multifillis although this seems unusual for ciliates and there are possible opportunities for conjugation within the life cycle. ref.


Adapted from ref.

Life Stages


Life Stage: Trophont (adult)

Transmission In: Direct ref.

Definitive Host: Freshwater teleosts ref.

Site Within Host: Endoparasitic within the epidermis, adjacent to the basal lamina of the skins, gills and buccal cavity. Aggregations of trophonts may arise and, except in very severe infections, the parasite occurs primarily on the dorsal surface particularly the head and fins. ref. ref.

Pathology: 'Ich' can have severe effects attributed to immunopathological events, with depletion of nutrient reserves, leading to physiological dysfunction. It can also cause secondary bacterial infections. Infected fish may "flash" or rub their bodies against objects in response to the irritation, surface and gasp for oxygen in response to gill damage and become lethargic or stop feeding. ref. ref.

Reproduction Capacity: There is some evidence for trophont division ref.

Duration Of Stage: Duration is temperature dependent - around 7 days at 20˚C or 20 days at 7˚C although I. multifiliis can overwinter in the fish host for 3-4 months at 3˚C. ref. ref.

Morphology: Contains a large horse-shoe shaped (or sometimes bar-shaped) macronucleus and at least one micronucleus.  Appears on the skin of the fish as small white spots about one millimetre in diameter with each spot usually representing just one trophont. ref.

Transmission Out: Appears to be an active process possibly involving the discharge of the contractile vacuoles. ref.


Life Stage: Tomont & Tomite

Transmission In: The emerging trophont encysts producing the tomont ref.

Free Living Environment: Aquatic freshwater environment ref.

Reproduction Capacity: The tomont undergoes binary fission giving rise to a population of between 50 and 3000 tomites. The tomites further differentiate into theronts, the infective stage. ref.

Duration Of Stage: Division times vary from 6 days to 10 hours with increasing temperatures ref.


Life Stage: Theront (infective stage)

Transmission In: The theront swims continuously with spontaneous bursts of speed, spinning anticlockwise on its longitudinal axis. It displays positive phototaxis (moves in the direction of a light stimulus) and possible chemokinetic stimulation and eventually comes into contact with the host. Once in contact with a host, penetration takes between 10 and 30 seconds with the help of histolytic and exo-enzymes. ref.

Site Within Host: Epidermis ref.

Pathology: Penetration is associated with considerable disruption and lysis of epithelial cells which can cause trauma. ref.

Duration Of Stage: Thought that the theront can survive for 22.5 hours at 20˚C with a significant decline in viability after 12 hours. ref.

Morphology:
Pyriform to fusiform shape with a tapered posterior end. Approximately 30x50μm in size (although this can vary greatly depending on the initial size of the tomont). Completely covered in cilia each of which are approximately 5μm long with a 0.2μm diameter. A longer cilium, two or three times the length of the others is located at the posterior end of the cell. The apical perforatorium is a group of apical ectoplasmic ridges that converge to form a 1.5-2μm protrusion at the tip of the theront and is used to physically force the theront into the host epithelium.  ref.



Economic Value

White spot is responsible for significant economic losses in the culture of channel catfish, rainbow trout, carp, eels, tilapia and ornamental species. Epizootics in wild communities have been recorded in rivers, reservoirs and lakes and such outbreaks can have devastating effects on fish populations, for example, Wurtsbaugh & Tapia (1988) attributed the deaths of 18 million killifish, including the commercially important Orestias agassii species, to white spot in Lake Titicaca, Peru. ref.

Management

Current control and treatment methods rely heavily upon chemicals such as formalin, malachite green, chloramine-T and toltrazuril although the safety of such treatments for fish to be used for food is questionable. Recent research is providing a basis for a molecular approach to the control of the disease with a possibility of using the related facultative fish parasite Tertrahymena pyriformis as a novel vaccine. Experiments have shown that, under continual exposure, fish hosts such as carp, rainbow trout, and channel catfish, can acquire, and sustain, immunity to I. multifiliis for at least 8 months. When free from exposure to the parasite, juvenile carp have been shown to lose this protection within 3 months. Controlled exposure to the parasite may therefore help to prevent devastating outbreaks. ref.

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