By TIMUR. G
TURKEY
Carp pox was recognised and described as a disease of cultivated carp more than 400 years ago but suspected virus has only been demonstrated by electron microscope in 1952 but has not isolated (Roberts, 1978).
Electron micrographs show viral-like genome bound in a protein coat within nucleus and cytoplasma of the infected cells. The process of replication and general morphology (complex icosahedral) and size of the particle suggests the carp pox virus belongs in the herpes virus group (Amlcher, 1986; Roberts, 1978). A similar condition also has been reported from other cyprinids such as, common bream and tench (Amlacher, 1986; Roberts, 1978).
The lesions usually begin as small white nodules on most parts of the body. They grow, spread and coalesce to produce milky or gelatinous masses (Amlacher, 1986; Roberts, 1986; Roberts, 1978). Where the lesions are large, an extensive capillary network may give the lesions a pink tinge (Roberts, 1978).
Argulus is believed to play a role as a vector of a virus causing the epithelioma. In many cases, after eradication of Argulus, affected fish recovered from the epithelioma (Amlacher, 1970; Snieszko and Axelrod, 1970).
A low mortality of mirror carp occured in a private fish farm in Antalya in April 1988. The sick animals showed milky white or transparent gelatinous lesions on most parts of the body, fins and opercula. The weight of the affected carp was between 250–500 g.
Carp pox was also seen as a second case in another private carp farm in Denizli in 1991. The infected fish were in 50–60 gram weight and 7–8cm. in total length. Similar lesions were also observed on affected fish.
A histological study was carried out to diagnose the disease in two case which induced low mortality.
For histopathological examination, the skin lesions were dissected and fixed in 10% formal saline and embedded in wax. Sections (5 um) were stained with haematoxylin and eosin (H + E), Masson's trichrome or Periodic-acid-shiff (PAS).
Affected fish exhibited an ugly appearence with milky or gelatinous masses on most parts of the body, fins and opercula. The colour of large lesions was pink rather than white. Fish were also infested with Argulus.
Histopathological examination of the skin lesions revealed multilayered neoplastic epihelium. The folds or layers were separated by thin connective tissue. Mucus cells were few but occasional groups of 4–5 cells were seen amongst neoplastic epithelium cells. The neoplastic cells were small and darkly stained in the upper folds of the lesions and large and vacuolated in the deeper folds close on the dermis. Darkly stained (basophilic) inclusion bodies were occasionally seen within the vacuolted neoplastic cells. An inflammatory infiltration and haemorrhagie was also observed among these cells and also in the dermis. Different stages of mitosis were observed in the neoplastic epithelial cells.
Although carp pox disease was recognised and described as a diseases of cultivated carp more than 400 years ago the present report is the first to describe the disease in carp farms in Turkey.
While verrucose plagues on most of the body of the infected fish are common finding of the other workers (Amlacher, 1986; Roberts, 1978).
The presence of Argulus on the body of the infected fish support the idea that Argulus plays a role as a vector of a virus cusing the viral epithelioma (snieszko and Aqelrod 1971).
Observation of basophilic inclusion bodies within the cytoplasm of some neoplastic epithelial cells also suggests the disease has a viral aetiology (Amlacher 1970), 1986; Roberts 1978).
The reduction of mucus cell numbers was also similar to the reports o other workers (Amlacher 1986).
SUMMARY
An outbreak of carp pox in two private carp farm was associated with low mortality. Milky white or transparent gelatinous lesions were observed on the body, fins and opercula of line infected fish.
Histopathological examination of the skin lesions showed them to consist of neoplastic epidermis. An inflammatory infiltration and haemorrhagie were present among neoplastic cells. Darkly stained (basophilic) inclusion bodies were ween in some neoplastic epithelial cells.
REFERENCES
Amlacher, E (1970) Textbook of fish Diseases, Translated by D. A. Conray and Herman R.L. TFH publications.
Amlacher, E (1986) Taschenbuch der Fishkrankheiten. Gustav Fisher Verlag. Stuttgart.
Roberts, R.R. (1978) Fish Pathology. Bailliere Tindall London.
Snieszko, S.F. and Axelrod, H. R. (1971) Diseases of Fishes Book 3, TFH Publications, Inc. Ltd.
By Dr. Attia El Hili HEDIA
TUNISIA
Proper nutrition is essential to the health of fishes. There are many diseases among fishes directly related to nutritional deficiencies and excesses. By another way, malnutrition is implicated in many diseases involving pathogenic organisms. Estimating the role of malnutrition in diseases is essential for diagnosis.
Most nutitional diseases are chronic in nature. Usually, diseases signs are masked by secondarily invading infectious organisms. This is one of the principal reasons why it is important to know the compound diet in point of view quantitative (rates of proteins, carbohydrates, lipids, minerals and vitamins) and qualitative (availability of the components and probable presence of toxin or antimetabolite.
PROTEINS
Proteins are large complex organic compounds which perform an essential role in the structure and functioning of plants and animals.
Proteins are composed mostly of amino-acids. Some amino acids can be synthesised by animals and others can not be synthesised and are called essential) (essential in the diet) amino acids or EAA's.
For fish a crustaceans, the EAA's are: arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine.
To synthesise the non essential amino acids, animal must have available a source of nitrogene.
The quantitative EAA content of different species varies. The EAA content of different feed ingredient varies even more widely, for exemple:
- the most plant proteins are deficinentin the sulphur-amine acids (methionine and cystine)
- meat meal, because of it poor levels isoleucine and methionnine and cystine, is a poor quality protein compared fish meal
- fish silage is poor in tryptophan
This is one of the principal reasons why a compound diet made from several ingredients is potentially more efficient than a single ingredient which may be too high or low in one or more essential amino acids.
Even when an EAA is shown by chemical to be present in sufficient quantities, it may not be biologically available to the animal. For example, the free amino group of lysine may become bound to other molecules during processing of the fatstuff, rendering it unavailable of the target animal.
Reduced protein, or the amino acids therein, in the diet of fishes affects biosynthesis of many essential nitrogenous compounds, including all enzymes, hormones such as thyroxin or adrenalin, melanin pigments, histamine, creatine and other cofactors and many others vital substances.
In fish, often proteins of amino acids deficiencies show very few pathological symptoms, mortality rates are low and the most commun sign is reduction or cessation of growth. Feeding a diet in which an indispensable amino acid has been removed will cause growth to cease until the amino acid is restored to the diet. The fiew amino acids which does show pathological symptoms is tryptophan causes scoliosis, lordosis and renal calcinosis.
CARBOHYDRATES
The carbohydrates include starches, sugars, cellulose and gums. They contain only the elements carbon, hydrogen and oxygen.
Fish and shrimp vary in their ability to digest carbohydrate effectively but they digest and metabolize them at a lower rate than higher animals. Many fish appear to be able to utilize simple carbohydrates such as sugars, more effectively than complex starches; the reserve appears to be true for shrimp and pawns. Many fish don't have the enzyme cellulase, and fibre is usually regarded as unavailable as an energy source. Cellulase, however is produced by the gut bacteria of many fish, as is chitinase in crustacea, and herbivorous fish are able to digest fibre.
Excess digestible carbohydrate in the diet of most fishes increases blood sugar, liver glycogen storage and increased liver mass, sometimes to pathological levels. Excessive glycogen in the liver of fishes, depending on severity, tends to cause liver malfunction and many contribute to kidney mal-function. Both liver and kidney malfunction may contribute to impairment of health.
LIPIDS
The lipids or fats are complex chemicals and the simpler forms are esters of glycerol and fatty acids.
Lipids are a source of energy and important elements of cell structure. In fact, the main lipid constituent of cellular membranes is phospholipids which constitute a barrier for exterior elements but can admit certain specific elements; thanks to the globular proteins found in the membrane.
Phospholipids are formed by a molecule of glycerol by 2 fatty acids in position 1 and 2 a phosphate having 4 possible bases; choline, serine, ethanolamine and inositol.
Triglycerides are forms of energy storage, in the case of fish, they are stored in the muscle and in the liver. Tiglycerides are formed by a molecule of glycerol and by 3 fatty acides. Certain of the fatty acids are essential (can not by synthesised by the animal itself) for growth and normal appearance of fishes.
Animals have 3 series polyunsaturated fatty acids (PUFA); the n-9 series (oleic series), the n-6 series (linoleic series) and the n-3 (linolenic series); only the oleic series is synthetizable by animals.
- oleic acid has 185 atoms of carbon and one double in position 9, the 9 carbon from he methyl
- linoleic acid (18:2n-6) has 2 double bounds and the first occurs on the sixth carbon atom.
- linolenic acid (18:3n-3) has eighteen carbon atoms and three double bounds, the first of which appears on the third carbon atom
The animal and fish synthetize both palmitic (16 atoms of carbon) and stearic acid (18 atoms of carbon). The latter can be converted into oleic acid. This bioconversion (desaturation) is possible in all animals.
The essential fatty acid EFA requirement of different species vary but are not yet fully understood.
- aquatic animals have a higher requirement for the n-3 series of fatty acids the terrersterial animals, for which the n-6 series is more important.
- EFA deficiencies are more noticeable in sea water than in freshwater conditions (for trout), thus salinity affects EFA requirements.
- marine fish appear to have a greater requirement for HUFA's than fresh water or anadromous species
- cold water species appear to have a greater requirement for the n-3 series fatty acids than warm water species
The best sources of essential acids for fish diets are fish oils. Vegetable oils are low in the m3 acid but generally high in w6 acid.
The more highly unsaturated fatty acids, linoleic, linolenic and arachidanic, are regarded as essential fatty acids and have a vitamine like action in the body, for example: deficiency of linolenic series family (18 : 3n-c3) causes reduced growth, skin depigmentation, fin erosion and feinting in rainbow trout.
The necessity of high dietary levels of PUFA's in aquatic animals diets makes the possibility of fats becoming rancid. These may be toxic or growth depressive. To prevent the autoxydation products, all fats and oils which contain unsaturated falls should be stabilized the time of manufacture. Antioxidant such as ethoxiquin or santoquin can be used to prevent autoxidation.
It must be underlined that food influences the fatty acid composition of both phospholipids and triglycerides. A good diet formulation concerning the required EFA supply by food is very important, particularly in vitellogentic females and all early stages of fish development. A deficient diet produces important desorders in embryo and can result in a decreased hatching ratio and larva abnormalities.
Feeding of high fat diets may cause fatty infiltration of the liver and excessive obesity. The liver of fishes with fatty infiltration of the liver are yellowish to ochre in color. Treatment of fatty infiltration is by reduction of dietary fats of the diet and increasing choline in the diet may assist the fishes in metabolizing intracellular fat as dietary is being reduced.
VITAMINS
Vitamins are complex organic compounds required in trace amounts for normal growth, reproduction, health and general metabolism, Marry vitamin deficiency symptoms have been described for fish and a few (notably vitamin C deficiency) for shrimp.
There are two types of deficiency symptoms of each vitamin in fishes:
- non specification symptoms which are: anozrexia, apathy, ;lack of growth exophtalmia and dark pigmentation
- specific symptoms which are summurized in Table 1.
The most common vitamin deficiency in fish nutrition is that of vitamin B1 or Thiamine, In fact, the feeds which contain raw aquatic animal products, notably if not fed contain raw aquatic animal products, notably if not fed immediately after manufacturer, contain enzymes called thiaminases. Theses enzymes may partially or completely inactivate the thiamine. Thiaminase levels in freshwater fish are higher than in that of marine fish.
There are also two others importants vitamins for fishes: vitamin E and C.
- the vitamin E has anti-oxidative properties and may be required at higher levels in fish and shrimps diets which are high in PUFA's as these are suscptible to rancidity. This vitamin has also immunostimulating effect.
- The vitamin C intervenes in a large number of enzymatic reactions and in the most varied cases: synthesis of collagen, cartilages, synthesis of adrenalin from where it has an antistress effect, reproduction system, immunostimulating effect,…
With salmoneids. The most spectacular manifestations are: scoliosis, Iordosis, dwarfish, gib bosity and shortening of the operculums.
With turbot and sea bream, deficiency in vitamin C provokes a granulomatous deasease
(figure 1).
With eel, deficiency in vitamin C provokes haemorragies in the head region.
MINERALS
Mineral elements are important in many aspects of fish and shrimp metabolism. They provide strength and rigidity to bones in fish and the exoskeleton of crustacea.
There are two types of minerals:
- Macro elements which are the architectural elements of the organisms (P, Ca, Mg, Na, K, S)
- Micro elements or oligo-elements which intervene as enzyme molecule compounds and as boicatalyzers. Their role can be compared to that vitamins, whith which they interfere (Fe, Cu, Mn, Zn, Co, I, Se).
Only seven elements: Ca, P, Mg, Fe, Zn, I and Se have been shown to be required or utilized by salmonids. Their role are summarised in table 2.
The water notably the sea water contain calcium and the requirement rate in fish is feeble. However, both sea water and freshwater contain very little phosphorus. On the other hand some types of phosphorus are unavailable to fish and an assessment of the availability of phosphorus in the diet is essential. As this mineral is important, it seems advisable to supply phosphorus to the diet of fish under the form of monosodic or monocalcic phosphate.
The deficiency in Zn may be induced by an excess of calcium which blocks the Zn available.
OTHERS COMPONENTS OF FEED
Feeds contain many other types of substances:
Synthetic substances such as hormones, antibiotics, pellet binders and pigment.
Natural substances such as mycotoxins, enzyme. inhibitors, vitamin destroying enzymes, haemagglutanins, products of oxidative racidity, pesticides, herbicides,…
These substances decrease the quality of the feed. The tab:3. provide a summary of some toxic and anti-metabolite elements.
CONCLUSION
To prevent nutritional diseases of fishes, some precautions must be taken:
Fishes confined under intensive fish culture conditions must be suppled a ration containing all required nutritents each day.
Conserve the food for not more than one month in a place which have 20°C of temperature. The use of a cold storage room advisable in the mediterranean climate.
Don't mix premix of vitamins with oligo-elements premix with because theses latters accelerate the process of vitamin degradation.
Add an antioxidant like ethoxiquin to the diets to prevent the formation of the peroxide.
Add vitamin C to fishes each time a decrease in performance after a treatment or after a stressing operation.
Add a therapeutic dose of vitamin when we use some nutriments or medicaments such as raw fish (anti-vitB1), sulfaguanidi (anti-vitk).
Figure 1. PRINCIPAL FUNCTIONS OF VITAMIN C
Table 1. MAJOR SIGNS OF VITAMIN DEFICIENCIES IN FISHES
| Vitamin | Signs | |
| Fat-soluble | ||
| A | Retinal alterations | |
| D | Tetany | |
| E | Muscular dystrophy - Fatty livers | |
| K | Skin hemorrages | |
| Water-soluble | ||
| B1 | Loss of equilibrium convulsions | |
| (thiamine) | Hyperirritability | |
| B2 | Cataract, keratitis | |
Table 2. SYMPTOMS ON SOME MINERAL DEFICIENCIES OF FISHES
| Mineral | Symptoms |
| Ca | loss of appetite, feeble growth poor consumption index |
| P | poor growth, low consumption index, bone deficiencies, frontal swelling, increase muscular fat |
| Mg | poor growth, anorexia, apathy muscular relaxation |
| Fe | microcytic hypochromic anemia |
| Zn | reduced growth, cataract, dwarfism wasting away of the fine, mortality |
Table 3. SOME TOXIC AND ANTI - METABOLITE SUBSTANCES OCCURING IN FEEDS
| TYPE | FACTOR |
| Fungal toxins | Aflatoxins |
| Bacterial toxins | Botulism |
| Chemical contaminants | Organe chlorine and polychlorine |
| biphenyls | |
| Volatile N nitrosamines | |
| Natural feed | Cyclopropenoid acids |
| components | Glucosides |
| Oxalic acid | |
| Gossypol | |
| Haemagglutinins | |
| Alkaleids | |
| Toxic amine acids | |
| Vitamin antagonists | |
| Thiaminase | |
| Linatine | |
| -- | |
| -- | |
| Peroxides | Oxidised oils |
By Mustapha TALBAOUI
MOROCCO
INTRODUCTION
The important development of marine aquaculture help in knowing more about diseases and its incidences.
Even though, many technical problems have been salved, others, such as fish pathology, are not well known and are obstacles to development of aquaculture. More research on this subject and diagnosis are needed for the viability of aquaculture projects.
Fish diseases can be genetic, nutritional, environmental (physical, chemical, pollution related), infectious and parasitic.
Larval Pathology
General information: The inflation of swim bladder in sea bream (S. auratu and P. major) occurs with contact of air on water suface. The bubbles of air pass through the ((pneumatic)) canal which degenerates when the larva is 4–5 mm
Two syndromes in larval rearing in sea bass and seabream are:
Non inflation of the swim bladder.
The swim bladder is formed normally during the first ten days of the larvae: between 5th–10th days for sea-bream and 7th–10th days for sea bass.
The non inflation of the swim bladder constitues the major obstacle in larval rearing for many marine species
In such cases swim bladder does not develop and is replaced by a compact tumoural tissue or degenerative tissue
Symptoms and lesions:
- Skeletal deformations (lordosis)
- Slow growth and big susceptibility to stress
Possible etiology:
- Lipid layer on water surface prevents air contact
- Big aeration and turbulances.
The elimination of the lipidic film on the surface is achieved with a low level blower on
the surface and elimination of this film. We can also use food grade emulsifier agent
(TWEEN 80).
Hyperdilatation of the swim bladder in the sea bass.
Enlargement of the swim bladder occurs between the 20th–25th day after hatching of the larvae and involves 10 to 20% of the total population.
Stress due to manipulations, change of food, rapid temperature variation are the possible etiology. This anomaly can be solved with good environmental conditions and high quality of live food.
Mateiliosis of the flat Oyster
(Marteilia refringens)
Definition:
Marteiliosis is a parasitic disease of the digestive gland in flat oyster. It is due to the development of a haplosporid, Marteilia refringens.
M. refringens has been isolated for the first time by COMPS (1970) and HERBAACH (1971) in the flat oyster along the French Atlantic coasts.
Symptoms, lesions and diagnosis:
The sings are not specific and are similar to those of malnutrition.
- retarded growth
- transparent shell
- discoloration of the genital gland (brown to yellow)
Diagnosis is possible by histological examination which show refringent sporanges in the profond canal and in digestive cells.
Stomach epithelium and secondary canalicules are not affected. This is related to particular biochemical conditions favourable to the development and maturation of the sporanges.
Polydora sp.
They are annelids of the family Sipiorids with different species: Palydora haplura and P. ciliata in Europe P. websteri and P. ligni in North America, Australia and Japon Reproduction period: Spring and summer.
Note : Translated from the Frensh text.
By Francisco RUANO
PORTUGAL
INTRODUCTION
Considering the marine invertebrates with economic interesting aquaculture two main groups BIVALVES and CRUSTACEANS are the most important species.
Marine cephalopods and gasteropods are also important species for fisheries however the methodology for his culture at least for commercial proposals still is in improvement.
The bivalves oysters, mussels, clams and cockles are the main cultivated species on Mediterranean region. The culture of crustaceans is very recent in this region and it starts in the early eighties with the introduction of Indo-Pacific species of prawns in south of France.
THE ROLE OF PATHOLOGY
STUDY OF DEFENSE MECHANISMS OF THE ANIMAL
Pathology, using basic sciences looks for a deep and correct knowledge of each species behaviour in the presence of an external aggression: what kind of defense action (s) is involved on the control and selection of each action. The understanding of this, is crucial for our correct procedure in the presence of a diseased situation.
Let see the different defense mechanisms that both bivalves and crustaceans have and how they act.
IMMUNITY-Includes 2 different components
Humoral.
This internal defensive action is extremely elementary specially in bivalves however it is quite effective against microbial pathogenic agents.
Both bivalves and crustaceans have in the serum of its hemolymph specific proteins with a hight molecular weight such as LYSINS on bivalves and OPSONINS on crustaceans.
In crustaceans, the action is crucial, which maintains the internal milieu absolutely sterilized. It is also so powerful that in some cases, when a pathogenic bacteria e.g. reaches the hemolymphatic vessels the reaction frequently causes itself the dead of the animal provoked by a generalised congestion in hemolymphatic circulation.
In bivalves, this action is less intense and specific. Although the agglutination and lytic actions has been well proved against strange substances, it is not clear his effectiveness against certain bacterial virus or even some internal parasites.
Cellular
Closely related with humoral mechanisms, the haemocytes activity in terms of protection, is basic a phagocytosis action. The capapility for extending cytoplasm (pseudopodia), for moving in intersticial spaces and for recognise nonself substances, are it's main characteristics when phagocytosis as related to molluscan immunity, is dissected into its component parts, three major phases may be recognised (1) attraction between phagocyte and the invading material (2) surface attachment of nonself material to the phagocyte, and (3) internalization (on endocytosis).
One other defensive action very common but apparently more related with the inflammatory reaction than an humoral immune mechanism is the encapsulation of strange material. This mechanism usually involves both the haemocytes and connective tissue cells in order to confine and eventually destroy the aggressive agent.
Immune memory versus genetic selection
The existence of the immune memory in molluscs, is still a controversial question. So far the immune protection of populations challenged in laboratory with modified (attenuated, strains or several microbial agents have not been success.
In nature however exists several examples of increasing resistance in wild or cultivated populations during a second epizootic occurrence comparing with the first occurrence. As an example we can mention the increasing resistance in oysters at Chesapeake Bay against Minchinia nelsoni (MSX), during the second outbreak of the disease.
Several authors considers a genetic mechanism of the specie which creates a resistant strain against to «MSX» and not an acquired immunity.
OTHER DEFENSE MECHANISMS
CRUSTACEANS
SHELL - It's shield very effective against external aggressions. the became vulnerable
to the pathogenic agents every time the protection loses resistance, for example during the
moulting period after intensive handling or fishing and whenever some wounds are caused to
the shell.
MOBILITY - Less vulnerability against predators
MUCUS - Besides the mechanic and lubricate action carry out by the mucus, it is also a powerful
bactericidal and bacteriostatic agent.
BIVALVES
SHELL - The same action of the shell on crustaceans
MANTLE - It's a very large organ, composed by two membranes of connective tissue which separates, internally, the soft parts of the body from the internal face of the shell. Its external border is covered by tactile villosities, very sensitive organs that collects all kind of information form outside allowing the animal to against different aggressions
MUCUS - This same action than in crustaceans.
GILLS - This organs have an intensive filter activity which select and cleans all the particies carried by the water.
Besides all those defensive mechanisms it is possible to find in the internal organs of bivalves different aggressive agents in hight concentrations apparently without any trouble for the animal. Usually it is during a stress situation than morbid processes occurs in bivalves namely during the increasing of chemical pollution, rapid changes in water salinity and temperature, spawning seasons etc.
THE PRINCIPAL BIOAGRESSOR AGENTS
When we speak about parasites or microbes, pathogenic agents, we must separate two different situations:
In culture (Artificial environment)
The causes of mortality in these conditions usual are very similar both to bivalves and to crustaceans and basically they are related with cyclical outbreaks of pathogenic strains of bacteria, namely vibrio sp. and some halofic strains of Aeromonas.
Also the metamorphosis and larval stages on bivalves and larvae and juveniles, on crustaceans are the more affected phases during the production cycle.
Some fungi, namely the fusarium sp. In crustaceans and a phycomycete strain in bivalves are described as the cause of epizootics that killed most of the cultured larval populations in 2–4 days. Also in crustaceans, chitin-destroying bacteria affects all the evolutive phases of its culture occurring usually after a deficient handling.
In Nature
In nature the situations are different with bivalves and crustaceans.
BIVALVES
These species, and of course its habitat, determines a different nosological map for each one, not totally coincident with all of them. A mussel population, for example, that lives in the inter-tidal zone, is much more exposed to the variations occurred on the water collumn than a clam, permanently buried in the sediment. And the clam is also more exposed to the qualitative and quantitative variations occurred on the interface water-sediment, than other species that lives in the water collum.
One other important aspect, for the understanding of the evaluation of mortality rates on these species, in the difficulty to establish a correct correlation between dead and alive and sick animals. Due to the extreme fragility of he soft parts of its body that degrades rapidly in the water after the death of the animal the only relationship possible to be establish is between empty shells and alive animals.
This fact can change drastically the conclusions if we don't have the necessary precaution when we evaluate a massive mortality in field.
The density of each population modifies the impact of a specific disease. As an example, the Focal Necrosis in oysters caused by a Gram positive bacteria, that we will be talking about later on in this work, causes necrotic focus in the Vacuolar Connectivetissue of the mantle, and consequently the death of affected animals. This disease, is much more virulent and the mortality rate is also higher in old oyster beds not harvested frequently, with high desity and hight percentage of old empty shells, however this disease has no expression in mussel or clam populations.
Later on I will mention, in synthesis and according its importance, the different aggressor agents.
MICROBIAN
- Bacteria:
Gram negative bacteria, probably Achromobacter, have been related with
massive mortalities in Japan during 1960.
Vibrio sp - Bacillar necrosis in oysters
Achromobacter sp. - Focal necrosis in oysters
Aeromonas sp.
- Virus:
Iridovirus - Gill disease in Portuguese oyster Crassostrea angulata.
Herpesvirus- Hemocytic infection viroses of bivalves.
More than twelve different virus strains was identified in bivalves, however their virulence is very reduced and don't causes any disturbance to the host.
- Fungi:
Monilia sp. - Causal agent of «shell disease» of portuguese oyster
- Protozoa:
Haplosporidians (Minchinia nelsoni, M. costialis on Crassostrea virginica;
Martelia refringens on flat oyster Ostrea edulis; on Minchinia tapetis on
Ruditapes decussatus).
Urosporidium sp - In oysters
Bonamia ostreae - Causes hight mortality in european and North
America flat oysters populations, also recognised as «Microcell disease»
by American authors.
Steinhausii mytilovum - Causes losses on the reproductive capacity of mussels.
Ciliates, like Ancistrum sp.; Flagellates-Hexamita sp. and Gregarines-Nematopsis
sp. are very common in wild and cultivated populations of bivalves.
Mortalities caused by these agents are also related with stress factors originated
both internally (loss of resistance caused by spawning, for example) or externally
(hight densities, pollution etc.).
- Helminths
Trematodes-Bucephalus sp. Proctoeces sp. Himasthla and Gymnophallus
sp. are very common in different bivalves species, its pathogeny varies
according the incidence of he agent.
Cestodes-Larval forms of Tylocephalum.
- Parasite crustaceans
Mytilicola sp. - Parasite the digestive tract of oyster and mussels.
Pinnotheres sp. - Inhabit the shell cavity of several moluscs, where their
activities and effects suggest that they are parasites rather than commensals.
PREDATORS
- Sea stars
One single animal of this specie eats 7–9 young oysters per day.
- Anemones
It is an important predator of planktonic larvae of bivalves.
- Gasteropods
The Muricidae family is the most active and dangerous for bivalve beds.
They drill the shell and suck the meat through the hole, selecting the young
oysters. Besides that, some evolutive phases of several parasites (metacer
cariae of the genus Himasthla), are transmitted to bivalves, using these
gastropods as carriers.
- Crustaceans
Several species of crabs, depending the geographic region we are talking
about, are the main predators of bivalves fiddler crab Uca tangeri and the
Green crab Carcinus maenas causes heavy losses on the culture beds of
clams and oysters for example, in the south coast of Portugal. blue crab
Callinectes sapidus.
- CBV
(Cheasapeak Bay Virus) - Isa Picornavirus which affects specially the
ectoderm.
- Baculavirus
Affects the tubules hepathopancreas of crustaceans
- Bacteria
- the chitin-destroying gram-negative bacteria, usually cause the «Shell disease» of several species of crustaceans.
- Vibrio sp. causes frequently acute infections in larval phases, during the hatchery stage of crustaceans.
- Gaffkemia-Caused by Aerococcus viridans a gram-positive bacteria, a tetraforming encapsulated coccus, is the most virulent bacteriosis found in crustaceans.
- Fungi
- Lagenidium callinetes affects the eggs mass of blue crab females, reaching 95% of prevalence. This infection causes eggs failure during the hatch or gave rise to abnormal subsequent larval stages.
- Fusarium sp. also affects several species of crustaceans, both from fresh and salt water, causing heavy losses in ponds of Kruma shrimp in Japan. The disease promotes the blackened exoskeletal spots, namely in the gills region.
- Aphanomyces astaci cause the crayfish plague in the european populations. This fungal disease, almost vanish the indigenous species of freshwater crayfish in France in the last century. Carried by the Red crayfish Procambarus clarkii, introduced at that time in France from USA, the fungi spread out rapidly, killing the indigenous populations highly sensitive to the disease. American specis, resistant to that agent, acts as a carrier, spreading that agent through almost all european countries including Turkey.
- Protozoan:
- Microsporean :
Nosema sp. causes important lesions in gills and muscular tissues.
Thelohania sp. is the causative agent of the ((cotton disease)), affecting the
muscular tissue and also the sexual organs, reducing the reproductive capacities
of several shrimp general namely Penaeus, Pandalus and Crangon.
- Greegarines - The specie, Nematopsis sp. is common in the digestive tract of some Penaeid shrimps.
- Amoeba - The genus Pleistophora parasites specially the muscular tissue.
- Haplosporidium, Urosporidium, Sarcomastigophora and several ciliates are also causative agents of diseases in crustaceans.
- Metazoan:
- Trematodes, Metacercarean from the specie Microphallus sp.
- Nematodes-Ascarididea
OTHERS :
Predators, competitors, changes on the environmental conditions, overfishing, etc., all this factors, like I said on the Bivalves have a very negative action on the crustaceans species.
PROPHYLAXIS
In Intensive Culture Systems, several procedures are very important to maintain an healthy situation of animals.
- Preserving an healthy situation of animals.
- Preserving the good quality of water, on the facilities used for rearing the first stages of development, and for the food production it is fundamental to do the sanitary control of all production.
- The mechanic filtration of the water, (wheel water supplies, sandfilters, etc.), combined with its treatment-Sterilization-using a UV radiation system, it's a very effective proceeding to maintain a good water quality.
- The maintenance of all facilities, perfectly clean.
- The cleaning of the water supply and sewage system, as well as tanks of production must be frequent and periodic.
NOTE: The use of antibiotic drugs in a preventive dosage, it's a very common practice in several hatcheries. The ((control)) of the development of several pathogenic strains of bacteria, it is not a correct procedure from a sanitary point of view. The risks involved in that method are grater than the benefits. The great probability to produce resistant strains to future antibiotics therapies, is one of them.
In extensive production systems we must pay attention to :
- Control of the predators and competitors.
- The correct control and constant surveillance of the environmental conditions on earth ponds, ((long line)) systems, floating platform etc. in order to prevent natural or men caused damages.
- In natural or artificial sea beds of bivalves a correct management and the regulation of the fishing effort it's important for the maintenance of its productivity.
GENERAL MEASURES
- Legislation: The creation of protection areas, and specific seasons for fishing the different species, for example, are important measures to protect an endangered species.
- The implementation of artificial collectors for bivalves seed and the restocking of natural bed populations it's also important.
CONCLUSION
Massive mortalities of invertebrate species in the different regions where its cultivation takes place are explained by several adverse factors that I mentioned before: Diseases, pests, poisonings, parasites, environmental changes etc. However, it's very rare, that a single cause have been found as sufficient to explain such mortalities.
So, the different factors can be combined in order to promote the outbreaks of severe mortalities in wild populations.
On the other hand the reduction of population, under a very low level, even if the aggressive conditions has been stopped, can itself promote the extinguishment of that population, of bivalves for example.
- An explanation for this phenomena, is closely related with genetical capacity of each species that, after reaching the limit of its adaptation on adverse conditions, can not produce enough genetic varieties (strains) to survive in the new environment.
The consequence can be decreasing of a certain species from a particular ecosystem and the occupation of its ecological niche by a better adapted species. An example for this is the substitution of portuguese oyster Crassostrea angulata, by a non commercial C. Stentina, in the Tagus estuary.
By Dr. C. SOMMERVILLE
SCOTLAND
The instruments required for the practical examination of fish are typical of those used in any surgical dissection. It is important to have, in addition, a bottle of physiological saline to keep internal tissues moist, otherwise evaporation will cause the destruction of fragile parasites such as the protozoa.
Ideally, you should use a good compound microscope preferably with internal light source. Many small protozoan parasites are more easy identify under Phase Contrast and a microscope with this system should be used if available.
A stereo or dissecting microscope is also necessary as many small worms, cysts etc are not quite visible to the naked eye. If a stereo microscope is not available use a powerful hand lens.
Glass slides and coverslips should be spotlessly clean otherwise details of small parasites will be obscured and identification impossible.
Other items of equipment are as follows:
Dissecting board
Petri dishes
Pasteur pipettes
Paper towels
Paper tissues
Spare scalpel blades
ENSURE ALL ITEMS ARE PREPARED BEFORE KILLING THE FISH AS SOME PARASITES DIE VERY QUICKLY FOLLOWING THE DEATH OF THE FISH
ROUTINE SCREENING OF FISH FOR PARASITES
Ponds, tanks, cages etc. or other sites under study should be sampled regularly.
Whenever possible examine fresh material. Fish should be freshly killed, without anaesthetic, and kept moist throughout examination.
Fish should be obtained live, if possible, and killed immediately prior to examination.
There are several good reasons:
Parasites are more easily recognised and identified.
Parasites, especially ectoparasites, may leave the host or die after death.
Collection of blood parasites is nearly impossible after death.
Decomposition starts immediately after death and internal parasites may be destroyed by hosts enzymes.
Handle fish as little as possible.
Kill fish by cutting though cranium or though spinal cord immediately behind the head.
Fish should be kept WET at all times during examination.
If the fish is already dead, refrigerate, but keep moist. Do not freeze as most small parasite become unrecognisable and only large helminths and crustacea can be recovered. If examination is to be delayed, fix in 10% Formal saline and slit open the body cavity to allow fixative to penetrate internal organs.
It is essential to examine skin and gills for ectoparasitic protozoa immediately after death as these may die or leave the host within a short time, e.g. flagellates.
By Dr. C. SOMMERVILLE
SCOTLAND
Kill fish quickly by cutting through the spinal cord with a sharp scalpel in the region immediately posterior to the gills.
Blood can be collected at this stage from the heart of major vessels using a Pasteur pipette. Place a few drops on a slide and allow to clot. A smear can also be made, fixed in methanolfor 10 minutes and stained later.
Examination of skin
Take «scrapings» for HP examination (several, if fish is large). Scrape with a sharp scalpel in an anterior to posterior derection and place mucus and epithelial cells on a slide in a drop of water. Avoid scraping scales as these reduce the visibility of small protozoa. Thin preparations are essential. Spread scrapings thinly and cover with a coverslip. Examine under HP.
NB. Scrapings should be made along the dorsum in an anterior to posterior direction including head, from the fins and from any discoloured areas or lesions.
Examine the entire fish under low power using a stereo microcope. Be sure to examine under fins as well as other areas. Large metazoan parasites and Argulus can be seen in this way.
Examination of gills
Remove operculum and examine inside.
Remove a whole gill and place on a slide or in a petri dish (add water if necessary and examine under low power on the stereo microscope. Separate the primary lamellae with needles to observe large monogenea and crustacea. Examine any lesions in detail.
Cut off lamellae and remove gill arch. Place lamellae on a slide and spread thinly-chop if necessary and cover with coverslip. Examine under High Power.
Other organs
Make incision along ventrum from vent of head. Remove abdominal wall to expose
viscera.
Examine visceral surfaces, abdominal cavity and pericardial cavity carefully under low
power using stereo microscope. Examine any abnormalities or cysts, spots etc. in detail
under Hight Power.
Remove alimentary canal and associated organs by cutting across oesophagus and around
anus. Divide alimentary canal into stomach, pyloric caeca, fore,-mid and hind-intestine,
and rectum. Examine surface and scrape contents onto a slide. Examine contents under
High Power.
Compress sections of alimentary canal between slides and examine under High Power.
Dissect and make squash preparations from heart, liver, gallbladder, spleen, kidney, gonads, urinary bladder and swim bladder.
Dissect out eyes and open nares. Examine under low Power and High Power for hel
minths.
Squash lens and examine for eye flukes. Dissect out the separate tissues of the eyes care
fully to determine the location as the site is helpful for identification.
Remove skin and slice muscle for helminth larvae and protozoan cysts. Squash muscle bet ween slides or glass plates.
Open cranial cavity, examine and make smear of brain tissue.
