Short review by Dr. PANOS VARVARIGOS
Author: Dr. Panos Varvarigos
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Taxonomy, elements of biology and mode of transmission
Phylum: Arthropoda (the species-richest phylum on Earth)
Fish parasites among arthropods occur in the crustacean orders: Copepoda, Branchiura, Isopoda. The Branchiura are exclusively parasitic, while most of the Copepoda and Isopoda are free-living. About 450 species of isopods are parasites of marine and freshwater fish.
Family: Cymothoidae Family: Anilocridae
In the isopod families Cymothoidae and Anilocridae, which number about 200 species, are included parasitic species that remain on the fish body throughout their life. The main genus/specie of interest to the Greek marine fish farmers comprises:
Species: Ceratothoa oestroides
Note that other isopod species (Flabellifera- Cymothoidae, Anilocridae) have been reported to infest net-pen reared sea bass and/or sea bream, such as: Anilocra physodes, Nerocila orbignyi, Emetha audouini, Ceratothoa parallela. Although the name "Anilocra" is used widely by farmers for any type of isopods infesting their fish, Anilocra and Nerocila adults are externally attaching to the skin and fins of fish. Only their juveniles may be found occasionally in the buccal cavity of the fish.
Isopods are Malacostraca; their body is dorso ventrally flattened and is lacking a carapace. The isopod thorax consists of 7 free segments with 7 pairs of thoracic legs. As a result of the well-sheltered environment of the buccal cavity, species that establish there have evolved a thinner cuticular mineralisation and the pleopods of the three last pairs have transformed into respiratory organs. Paired eyes consist of numerous eyelets. On its ventral side, between the swimming legs, the female bears a brood pouch or "marsupium", shielded by special plates, called "oostegites", to carry the eggs and the larvae for some time after hatching.
A mature gravid female isopod releases about 400-550 larvae at a time. Ceratothoa sp., are constantly fertile, remaining in reproductive condition throughout the year. There is little information as regards the generation time under different water temperatures and the fluctuations in fecundity. Fecundity and hatching rate increase in warmer temperatures, July being the period of optimum isopod proliferation in the Mediterranean.
Members of these families (Cymothoidae, Anilocridae) are protandrus hermaphrodites, i.e. an individual develops and functions first as a male and then may become a female, the presence of mature females being inhibitory for a further development of males in their neighbourhood.
Eggs are laid into the marsupium, on the ventral side of the female, between its thoracopods. In the marsupium the eggs hatch into "pullus larvae", which are sexually non-differentiated and undergo a number of moults prior to reaching the infective free swimming stage, when they abandon the marsupium and actively search for a host. The entire larval development takes place in the marsupium. Sexual differentiation takes place after the pulli II larvae have left the marsupium.
The first pullus larva (pullus 1 stage, pullus primus, prehatch 1) is only found inside the marsupium. It metamorphoses by moulting into the second pullus (pullus II larva, pullus 2 stage, pullus secundus, prehatch 2), its thoracopods armed with hooks and with cuticle strongly pigmented by numerous chromatophores. The second pullus has 6 pairs of legs and is also not sexually differentiated. When released in search for a new host, the pulli II can be termed "manca larvae". Manca larvae either in the plankton or attached to a fish differ from juveniles since they have six pereonal segments and sets of legs. After a subsequent moult, the 7th segment and pair of legs appear and then the isopod becomes a "juvenile". They function first as males and subsequently as females, according to circumstances.
It is difficult to define when a juvenile becomes an "adult" isopod. Stages of males, transitional females and females (ovigerous and non-ovigerous) are referred to in the literature. Since they are protandrous hermaphrodites, then technically, only the females may be safely termed adults. The Flabellifera isopod parasites adhere in pairs to different spots on the fish skin and fins and may also live in the mouth and gill cavities.
Regarding the most sensitive age of the fish hosts when the parasites attach in their buccal cavity, empirical observations agree with research results that there is a significant correlation between the rate of infection and the length of the host, fish fry being the evident target for isopod attachment. For Ceratothoa oestroides pulli II, it had been thought that younger fish seem to consider the juvenile isopods as attractive prey, however, recent studies in aquaria have shown that the pulli II larvae swim in search of a host and that they actively attack fish. After attaching themselves on the base of the tail fin or on the flank, the young isopods progress to the anterior part of the body, go beneath the operculum and settle in the buccal cavity. The whole process from attachment onto a host until settling in the buccal cavity takes up about two hours.
There is strong competition among the pulli seeking attachment in the mouth of the host. Only two pulli may settle in the buccal cavity of the host comprising the pair of future adults. Thus, although in the first phase of infection a fish may be attacked and carry more than two pulli on its body surface and gill cavity, eventually no super-infection is possible and mostly two isopods may be hosted in the buccal cavity on any one fish.
Ceratothoa oestroides pulli II larvae remain free swimming and capable of infecting a host for about 7 days at 22°C. During this period, even in case that after successful settlement the host dies, the isopod larvae immediately abandon the dead fish and are still capable of actively seeking another (not yet moulted to the sessile pre-adult form). Later, after firm establishment in the buccal cavity of the host, the parasites are incapable of migrating to another and begin blood sucking (haematophagous). It is not precisely known what is the future of adults or gravid female isopods if forced to abandon their dead hosts.
Adult female Ceratothoa eostroides Adult ovigerous females (three on the left side) and ready to release larvae (far right).
Eggs, pulli II larvae, adult male (smaller) and adult female Ceratothoa eostroides
Parasitic isopods are
fairly common crustacean infestations of wild tropical marine fish. They are less
common in cold marine waters and not often found on freshwater fish. Parasitic
isopod fauna is rare in polar waters. The distribution of parasitic isopods, as
that of all other parasites, is closely related to the occurrence and ecology
of their hosts. Demersal fish in coastal waters are
infected most often, the parasites being rarer in bathy- and pelagic fish.
Members of the following families are most often isopod-infected: the Sparidae, Lutianidae, Serranidae, Trichiuridae, Bramidae.
Intensive fish farming in coastal waters in the Mediterranean provides a close to ideal environment for isopod parasites, hence, farmed sea bream (Sparus auratus), but mostly sea bass (Dicentrarchus labrax) infestations by isopods comprise a frequent problem in the Mediterranean. Ceratothoa oestroides is the most common among the isopod parasites and inflicts major damage. The adults are found in pairs and occur mainly in the buccal cavity of the sea bass, while the infective larval stages (pulli II /manca larvae) and the juveniles are present in the buccal and opercular cavities, on the fish head, behind the eye or behind the operculum, above the lateral line, on the caudal fin and the caudal penducle of both sea bass and sea bream. These comprise actual pathogens provoking acute tissue inflammation and necrosis. It seems that the anatomy of the buccal cavity and the dentition of sea bream inhibit the establishment of the isopods, thus adults are rarely found on sea bream.
In the wild, the usual hosts of parasitic isopods are mullets (Mugil spp., Liza spp.), bogues (Boops boops), goldlines (Boops salpa), striped breams (Lithognathus mormyrus), white breams (Diplodus sargus). These fish species abound in the vicinity of sea bream (Sparus auratus) and sea bass (Dicentrarchus labrax) net pens feeding on waste feed and comprise the vectors for the transmission of the parasites to the farmed species. None of the cymothoid species reported on farmed bass and bream are known to parasitise them in the wild. It seems that these isopods are non host-specific parasites transferred to cultured fish from the feral fish around the cages carrying adult pairs of isopods, one of which is the ovigerous female. The increase in the volume of farmed sea bass may have created a new host-parasite association whereby Ceratothoa oestroides has effected a complete host shift. The parasites proliferate mostly when the sea-water temperature increases during the summer peaking during July and August, when the prevalence of infection in the cages may exceed 50%.
Pathogenesis and diagnosis
Ceratothoa oestroides has become a major pest, primarily for sea bass, but also for sea bream reared in net pens. Strong regional pertinence is observed in the Aegean Sea (mainly the Eastern Aegean Greek islands and along the Turkish coast). Other Greek regions are vulnerable as well, among which the North and South Evian gulf are notable.
On young sea bass and bream, pulli II larvae may be found in various numbers on the skin, gills, gill and mouth cavities. The adult parasites are found paired, attached to the buccal epithelium mainly of larger sea bass.
Heavy infestations of parasitic larvae may kill smaller fish when they first infect them seeking permanent attachment. Pulli II larvae and juveniles attack relatively younger fish, about 5g-20g of weight and cause considerable damage to the skin around the head, the eyes and the gill epithelium by injuring the gill lamellae. Their voracious haematophagy and the mechanical damage of their hooks lead to severe inflammation and necrosis of head, eye and gill tissues. The infested fish are usually apathetic and anorexic and may show respiratory distress. The haemorrhagic and necrotic head tissues are evident when observing the fish in their cage. When the sick fish are removed from the water, several isopod larvae may be seen in their buccal and gill cavities and/or on the skin near the opercula.
Injured tissues are frequently invaded by secondary bacterial pathogens, such as Aeromonas spp., Tenacibaculum spp., Vibrio spp. and this may lead to severe escalation of mortality. In young stocks, the cumulative mortality due to parasitism by the pulli II larvae may run as high as 15% even without any bacterial implications.
The adult isopods are haematophagus (feed on blood) and cause anaemia. The parasitised fish have significantly lower erythrocyte counts as well as haematocrit and haemoglobin values. The leukocyte counts are increased, obviating the host's immune response to the presence of the isopods. In addition, the established adult isopods can cause considerable damage to the mouth tissues with their biting and sucking mouth parts, or their copulation activity. Their large size (up to 6 cm in length) may cause atrophy of the tongue, dysplasia of teeth and slackening of the cartilagenous tissues leading to a "bag-shaped" lower jaw. Invariably, the presence of large adult parasites in the buccal cavity interferes with feeding, causes chronic stress and results in growth retardation and a predisposition to bacterial and/or endo-parasitic invasions.
Isopod infestation is confirmed by gross observation of the parasites on the skin, mouth, or in the gill chamber of the fish. In addition, they often produce the lesions described above that characterise their presence.
Adult isopods in pairs (large female, smaller male) in the buccal cavity of sea bass. Slackening of the tissues underlying the attached parasites often results in deformations -"bag-shape"- of the lower jaw area (left).
Pullus II stage Ceratothoa oestroides larvae on the gills of a sea bass (seen on the pharyngeal teeth/gill rakers and among the gill lamellae). Gill inflammation and focal necrosis is evident. The operculum has been removed to facilitate observation. The fish must be examined fast after removal from the water, as the larvae tend to abandon quickly their dead host.
Inflamed and necrotic gills (left) and necrotic head and eye tissues (right) caused on sea bass by Pullus II isopod larvae.
Prevention and treatment
Recommended prevention would be by means of stock management measures. Excessive fish densities in the fry holding pens must be avoided. Often, in cases of heavy parasitism and mortality, reducing the fish density is enough by itself to remedy the situation. Additional preventive measures would be to: a) Avoid placing the young fish in close proximity with the adult sea-bass, which are most likely to harbour adult parasites in reproductive phase. b) Prefer deep sites with sufficient currents, which disperse the juvenile parasites in a direction away from the main body of the cage mooring.
It is worth noting that on farms where injection vaccination of sea bass is routinely performed, manual delousing of the anaesthetised fish, by means of small blunt forceps, prior to injecting results in a sharp drop of fish retaining adult isopods. Hence, there is a subsequent sharp reduction to the number of larval isopods and very little damage on the fish fry in the next season. In addition, the anaesthetic used prior to vaccination, has been seen to sedate the adult isopods, many of which lose their grip and drop, still alive, in the tarpaulin.
Treatment of isopod larvae infestations has been attempted with considerable success by means of hourly formalin baths, at concentrations of about 150ppm, subsequent to enclosing the fish in a tarpaulin and providing ample water oxygenation. Nonetheless, re-infestation occurs soon after unless the stocking densities are reduced.
treatments by means of hydrogen peroxide, dichlorvos
or pyrethroids, such as deltamethrin
or cypermethrin (ExcisTM, BetamaxTM) lack adequate, controled experimental field data in the Mediterranean.
However, empirical commercial scale information exists as regards the application methodology, the efficacy on adult isopods and larvae as well as the toxicity to the fish. Field data on environmental implications or about the potential acquisition of resistance by the isopods against these compounds is lacking. Resistance has been demonstrated to develop in the case of salmon lice Lepeophtheirus salmonis.
Laboratory experiments with deltamethrin (pyrethroid) have indicated that the minimal in vitro dose that kills Ceratothoa oestroides adults in 2 hours is 0.05mg/litre. Experimental field data by the manufacturer of AlphamaxTM in offshore net pens of sea bass indicated that deltamethrin at 3ppb for 30 minutes is efficacious and safe for treating small bass (<10g) against the juvenile isopods. Larger sea bass parasitised with adult Ceratothoa were effectively and safely treated with 7.5ppb deltamethrin for 30 minutes.
empirical/commercial data exists on the efficacy of cypermethrin:
5-10ppb for 60 min (ExcisTM)
as well as azamethiphos 50% w/w (SalmosanTM) on all age classes of sea bass and
bream at many different locations in the Mediterranean. Azamethiphos
and cypermethrin have been proven both effective
against all stages of the isopods and safe for the fish treated. Nevertheless,
the effects on other marine arthropods in the vicinity of the net pens have not
In all cases a treatment tarpaulin is required as well as a continuous supply of oxygen, especially during the warm season.
Environmental toxicity studies for these organophosphate compounds in the warm Mediterranean waters are necessary.
Against the salmon lice, the usual doses for bath treatments using tarpaulins, at approximately 10oC water temperature are as follows:
> Hydrogen peroxide:1500ppm
> Dichlorvos: 1ppm for 60 min
> Cypermethrin: 5ppb for 60 min
> Azamethiphos: 0.2ppm for 30 min
Against the isopods, the usual doses for bath treatments of bass and bream using tarpaulins, at 16-25oC water temperature are as follows:
10ppb for 60 min
> Deltamethrin: 10ppb for 30-60 min
> Azamethiphos: 0.4ppm for 45 min
Bath treatments have serious drawbacks. Even assuming a safe for the fish and reasonably potent chemical compound for use by no more than an hourly bath treatment, the following practical problems would be encountered:
A. Cost, risk, labour, time:
product to buy in the large volumes required for repetitive use.
A2. Risk of accidental fish kills (mishandling, asphyxiation).
A3. Labour intensive and time-consuming operation, prohibitive on sites with many large circular net pens.
A4. Re-infestations from wild fish are likely demanding repeat treatments.
B. End product quality: (see also the economic implications section)
periods should be adhered to hence, re-infestation may occur prior to harvest.
B2. Impossible to achieve 100% success rate, hence grading at harvest would still be necessary.
B3. Killed parasites may remain in the buccal cavity for some time (unknown) after treatment.
B4. The use of chemicals in aquaculture damages the perception of fish as healthy and wholesome food.
C. Environmental compatibility:
C1. Adverse effects to the ecosystem.
The advent of a potent in-feed treatment is required that could alleviate drawbacks A2 and A3 from the above lists. Empirical trials with emamectin benzoate (SliceTM) with sea bass in the field did not produce sustainable good results.
The costs associated with the isopod infestations of farmed sea bass may be categorised as follows:
1. Direct mortality of young stock due
to infestation by isopod larvae.
2. Accidental killing of fish during bath treatments (accidental handling loss).
3. Veterinary and medication expenses (Vet. & med.) plus labour (see previous section).
4. Rejections at harvest (degraded fish).
5. Extra labour for fish grading at the packing plant (quality).
6. Chronic stress and high propensity to other diseases (indirect mortality).
7. Growth retardation.
Although there has been no attempt to quantify these costs across fish farms, it is obvious that the losses associated with the first four items on the list above could be found in the farm records or diaries. The costs associated with chronic stress and the propensity to other diseases are difficult to quantify (item 6).
At harvest, fish that have survived parasitism are usually of inferior body condition, they may have developed the "bag-shaped" jaw and the adult parasites may be found attached in their mouth. Frequently, a number of larvae may also be found in their mouth and gill cavity having been released from the adult female. These repelling characteristics render such fish unsuitable for the market. The cost of rejects may run high in cases of heavy infestations (usually 1%, but also anything up to 25% of prevalence among harvest-size fish). Besides, there is the considerable extra labour associated with grading, or manual delousing in the packing plants performed by experienced operators (item 5 above).
A regards growth retardation (item 7 above), research to-date in the Adriatic and the Aegean Sea has shown that for fish of the same age class, parasitism by Cymothoids significantly stunts both body length and weight when comparing parasitised with non-parasitised fish. Parasitism may result in fish that are 7% shorter and 20% lighter on average.
Isopod infestations do not pose any risk to consumer health.
There are currently no regulations in place.
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Author: Dr. Panos Varvarigos
Freelance Veterinarian - Fish Pathologist, Athens, Greece.
of this website (or parts of it) is illegal and strictly forbidden.
No rights can be derived from this website.
Every effort has been made to ensure that the information is accurate until the date of last editing. It is based upon the accumulated personal experience of applied veterinary work. The author cannot take responsibility for incorrect interpretation or any resulting consequences. The contents may be used as an educational guide and are definitely not meant to become a stand-alone diagnostic tool or operations manual.
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