Taxonomic ranks under review (cf. Illustrated Guide to Protozoa, 2000.
Protista (unicellular eukaryotes)
Ciliophora (with cilia, nuclear dualism, pellicular alveoli, reproductive
Olighymenophorea (conspicuous oral and body ciliature, membranelle-bearing
Cyrtophora (curved tubular cytopharyngeal apparatus = cyrtos)
Hymenostomatida (oral ciliature comprising paroral membrane and adoral
zone of membranelles)
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.
multifiliis [this species causes whitespot disease
(‘ich’) in freshwater fishes]
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
Site of infection:
Trophonts infect the epidermis, cornea and gill filaments.
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).
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.