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Author: Dr. Panos Varvarigos
Freelance Veterinarian - Fish Pathologist, Athens, Greece.

AquaHealthTM
Laboratory.

 

DISEASE

ISOPODOSIS
(Flabellifera, Cymothoidae)

Pathogen (name, taxonomy, description):

Ceratothoa oestroides.

Phylum: Arthropoda, subphylum: Mandibulata, class: Crustacea, subclass: Malacostraca, hyperorder: Peracanida, order: Isopoda, suborder: Flabellifera, family: Cymothoidae, genus: Ceratothoa.

Haematophagous ectoparasites without host specificity. They are protandrus hermaphrodites. An individual develops and functions first as a male and then may become a female. The presence of a female inhibits the development of more males into females in its vicinity. Adults are found paired in the buccal cavity of fish. Infective larvae (termed pulli II or manca larvae) roam on skin and gills damaging the epithelia. The isopods have been transferred from the feral fish to the farmed species due to the increasing populations of the latter. Especially, on sea bass, which has not been found parasitised in the wild, it seems that a new host-parasite association has been established with Ceratothoa oestroides.

Economic Implications:

Severe

Frequency of occurrence:

Frequent

Farmed fish species affected:

Mainly sea bass (Dicentrarchus labrax). Sea bream (Sparus auratus) to a lesser extent. The anatomy of the buccal cavity and the dentition of sea bream do not favour the establishment of the adult isopods. All other farmed species are also vulnerable. Fry of all fish species are equally subject to attack by isopod larvae.

Age/size of fish mostly susceptible:

Fry and young fish of all species susceptible to infection by the larval stages of the parasites. Adult, reproducing parasites are found attached in the buccal cavity of larger on-growing fish.

Seasonal occurrence:

The parasitosis is present all year round. However, fecundity and hatching rate of the isopoda increases in line with water temperature. The infective larvae proliferate; hence isopodosis is prominent during the summer and peaks between June and August.

Regional pertinence:

Sites in areas with high farming activity/pressure are more prone to suffer. The probability of occurrence is considerable at any site. Nevertheless, a clear regional pertinence is evident primarily in the Eastern Aegean Sea (Greek islands and Turkish coast), but also in the North and South Evian gulf (areas with relatively higher average annual sea temperature)..

Predisposing factors and mode of infection:

Infection is direct by the infective larval stages (pulli II) of the isopods, which are released by adult females, attached in the buccal cavity of on-growing fish or of feral fish around the cages. A single pair of adult Ceratothoa (considerably larger female, relatively small male) is found in the buccal cavity, mainly of sea bass. The female lays its eggs in its marsupium pocket found on the ventral side of the parasite. The eggs hatch and develop inside the marsupium into pullus larvae. The infective pullus II stage larvae are released from the female (400-550 at a given "birth") and actively seek a host remaining infective for about 7 days. 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. 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 only two isopods may be hosted in the buccal cavity on any one fish.

Predisposing factors comprise fish overcrowding, weak sea currents, proximity of vulnerable fry with on-growers carrying adult isopods, large numbers of feral fish around the cages.

Main lesions:
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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 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. 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, prevalence of the parasitism may exceed 50% and the cumulative mortality due to damage by the pulli II larvae may run as high as 15% even without any secondary bacterial implications.

Diagnosis (field, laboratory):
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Gross observation of the parasites on the skin, mouth or in the gill chamber with associated lesions. 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. The isopod larvae quickly abandon their dead hosts and may be found in the plastic wrapping of fish samples brought to the laboratory.

Consequences
(mortality, growth reduction, extra labour):

Costs associated with isopod infestations of farmed fish may be:

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 extra labour.
4. Rejections at harvest (degraded fish).
5. Extra labour to inspect and grade fish 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 succumb to other diseases is 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).

As 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.

Treatment:

Treatment of isopod larvae infestations on young fish has been attempted with 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.

Bath treatments with hydrogen peroxide, dichlorvos (AquaguardTM) and the pyrethroids deltamethrin (AlphamaxTM) or cypermethrin (ExcisTM, BetamaxTM) and azamethiphos (SalmosanTM)  lack adequate experimental field data in the Mediterranean. Commercial scale information is available on their efficacy and application methodology (e.g. 10ppb of cypermethrin during an hourly bath application, or 0.2ppm azamethiphos for 30 minutes is safe for sea bass and bream and kills all isopod stages), but environmental implications, or the potential acquisition of resistance by the isopods against these compounds are unknown as yet.

Bath treatments on large cage farms using tarpaulins often are not feasible. They are risky (accidental fish kills due to mishandling or asphyxiation), labour intensive and time-consuming operations. Besides, re-infestations from wild fish are likely demanding repeat treatments. Potent and safe in-feed treatments, if available, would alleviate much of the present scepticism to treat. However, emamectin benzoate (SliceTM) has been evaluated on sea bass in the field, but did not produce consistent results.

Management advice (prevention):

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 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 significant reduction in 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 out, still alive, in the tarpaulin These adults may not swim up to a new host.

Environmental issues:

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) and white breams (Diplodus sargus). These fish species abound in the vicinity of sea bream and sea bass net pens feeding on waste feed and on the rich benthos underneath the cages. They 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. However, the role of the fish farms in amplifying further and spreading the parasite in the sea is expected, but has not been studied and quantified. Large scale bath treatments with known toxic chemicals pose a serious hazard to the ecosystem (e.g. the effects of the pyrethroids on other arthropods). Further research and well justified regulation is necessary for such applications.

Regulations:

Currently no regulations are in place. Parasitism by isopods poses no risk for the consumer.

 

       


Author: Dr. Panos Varvarigos
Freelance Veterinarian - Fish Pathologist, Athens, Greece.

AquaHealthTM
Laboratory.

Reproduction of this website (or parts of it) is illegal and strictly forbidden.
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Disclaimer:

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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