Classification: Taxonomic ranks under review (cf. Illustrated Guide to Protozoa, 2000. Allen Press)

Protista (unicellular eukaryotes)
Ciliophora (with cilia, nuclear dualism, pellicular alveoli, reproductive conjugation)
Olighymenophorea (conspicuous oral and body ciliature, membranelle-bearing ‘holotrichs’)
Cyrtophora (curved tubular cytopharyngeal apparatus = cyrtos)
Hymenostomatida (oral ciliature comprising paroral membrane and adoral zone of membranelles)

Family: Ichthyophthiriidae
These ciliates are monoxenous (one-host) ectoparasites of fishes. Trophonts infect epithelial tissues, often causing visible lesions evident as white spots. When replete, they leave the host and form encysted stages (tomonts) in the external environment. These cysts produce hundreds of infective stages (tomites) which are released as swarmers (theronts) which actively seek new hosts.

Ichthyophthirius multifiliis [this species causes whitespot disease (‘ich’) in freshwater fishes]

Parasite morphology: The parasite forms three developmental stages: trophonts, tomonts and theronts. Trophonts variable in size (up to 1mm), horseshoe-shaped macronucleus encircling single micronucleus; subapical vestibulum with weakly developed buccal ciliature tomonts encysted on substrate, repeatedly divides to form numerous small tomites which break through cyst wall to become theronts (25-70 x 15-22µm) covered with 36-48 meridional (longitudinal) kineties (ciliary rows) converging around the pre- and post-oral sutures. ellipsoidal macronucleus and subspherical micronucleus..

Host range: Infections have been detected in numerous species of aquarium and wild freshwater fish throughout the world. There is some conjecture about the existence of different parasite races, which may have different temperature tolerances, being adapted to hosts with specific temperature preferences, or they may be geographic races varying in virulence in introduced and/or endemic fish species.

Site of infection: Trophonts infect the epidermis, cornea and gill filaments.

Pathogenesis: Theronts use an elevated pointed ridge (perforatorium) to penetrate host tissues and they discharge their pellicular mucocysts to form a stick envelope glued to the host’s epithelium. Within minutes, the parasites penetrate deeper into epithelial or epidermal tissues where they feed and grow (increasing their volume up to 3,000 times). The trophonts form greyish pustules in the skin of their hosts where they feed by ingesting host cell debris. Infected fish produce excess mucus to combat the irritation but many epidermal cells are destroyed and are sloughed. Heavy infections of the gill filaments interfere with gas exchange and may prove fatal. Lesions containing engorging trophonts appear as visible white spots covering infected fish. Fish surviving infection exhibit some protective immunity against subsequent infections.

Mode of transmission: Engorged trophonts are liberated from ruptured pustules into the water column where they settle on convenient substrates or on the bottom. They form a gelatinous cyst and undergo a series of divisions producing from 250 to 2,000 tomites which are subsequently released and actively search for new hosts. The number, size and duration of the life-cycle stages depends prevailing environmental condition, particularly temperature (no development occurs below 2°C or above 30°C). The whole life-cycle may be completed in as little as 3-8 days at 23-24°C, but it progressively takes longer at lower temperatures (up to 3 months at 4-5°C).

Differential diagnosis: Infections are diagnosed by the detection of characteristic pustules containing feeding trophonts.

Treatment and control:
Aquarium fish have been successfully treated with dilute concentrations of formaldehyde, malachite green or methylene blue. Closed culture systems are particularly at risk of sustained contamination and outbreaks. Periodic flushing of tanks and ponds with clean fresh water helps to reduce contamination levels. Avoiding overstocking also reduces stress.


< Back to top