Bacterial disease of Shrimp
Rickettsial Infections
This infection is not recorded yet from Indian waters, systemic rickettsial infections were reported from cultured P. monodon from Malaysia and Singapore. In P. monodon, the rickettsia occurred within large cytoplasmic vacuoles where it formed-microcolonies of 19 to 33 f.lm in diameter. In heavy infections, cells with rickettsial inclusions were widespread in mesodermally and ectodermally derived tissues, but absent in endodermally derived tissues such as midgut, hepatopancreas and caeca. Experimental treatment using medicated feeds containing 1.5 to 2. 0 kg of oxytetracycline per 1000 kg offered was found to be successful in reducing monalities.
Vibrio sp. were found to constitute the predominant normal microflora of the culturable species of shrimps. Due to their rich presence in the shrimp's microflora, researchers have found Vibrio sp. as frequent and opponunistic pathogen of the shrimps. The opponunistic pathogenic Vibrio sp. establish lethal influence as a result of other primary conditions such as other infectious diseases, nutritional disorders, extreme environmental stress and wounds. However there are a few Vibrio sp. which are true pathogens.
Luminescent Vibrio in hatcheries
In hatcheries, larval mortalities associated with luminescence are reported in epizootic proponions in P. monad Oil and P. merguiellsis. The causative bacteria are strains of Vibrio parahaemolyticus, Vibrio alginolyticus, V. harveyi and V.spiendidus. The affected larvae refuse to feed. Scanning Electron Microscopy (SEM) studies indicate that the vibrios colonise specifically the feeding appendages and oral cavity. Rigorous management and sanitation helps to control the infestation. The separation of mother shrimps and their faecal matters from the eggs has to be done as soon as possible after spawning. Anemia nauplii being used as live feed should be rinsed before introducing into the hatchery during feeding. Chlorination and other forms of water treatment such as ultraviolet irradiation and filtration should be done to reduce the initial load of the rearing water. The affected shrimps are treated using antibiotics such as chloramphenicol, sodium nitrostyrenate and the nitrofurans (furazolidone, nitrofurazone and prefuran)
In India two species of Vibrio are found to be pathogenic to the shrimps. These include, Vibrio anguillarum and Vibrio alginolyticus. In shrimps moralities were reported to begin towards the end of the growout period as the shrimp reach marketable size of 25 to 35 g or 2 to 3 months after stocking PL-20. Dead shrimps can be harvested in the morning from the pond edges. It is estimated that at harvest 5 to 30 percent of the
production only could be attained. Most of the surviving shrimps also exhibit stunted growth. Indian white prawn, P.indicus cultured in brackishwater ponds are affected by Vibrio anguillarum. Blackening or whitening of the basal pan of the antenna, the oviduct and edges of the abdominal shells are the symptoms. Frequent water exchange, feeding with compounded diets containing antibiotics chloramphenicol help to control the disease. In addition to the above furacin at 1 mg per liter of water, terramycin at 40 mg per kg of biomass for 10 to 15 days through feed or feeding tylosin or tiamutin at 100 mg active ingredient per kg offced for two weeks help to contain the sickness.
Tiger prawn, Penaeus monodon cultured in ponds are affected by V. alginolyticus. Septicaemic conditions followed by loss of reflexes and cuticular fouling by epibionts are the symptoms. The gills are often brown in colour. Early signs include, body reddening, extended gill covers and slight melanized erosions of the uropods, pleopods and periopods. Affected shrimps reveal empty stomachs and midguts and in some cases white watery liquid oozes out. Reducing the biomass (by panial harvest) and increasing the water exchange help to contain this disease. For the subsequent production cycle, it is advisable to dry the pond bottom until the bottom soil cracks. If excessive detritus is noticed the same has to be physically removed. Quicklime (CaO) is normally applied at the rate of 0.5 kg/m' of pond bottom. The treatment pattern is much the same as that of Vibrio anguillarum infections.
Brown spot shell disease / burned spot disease / rust disease / shell disease / black spot disease
The above-referred names are synonyms of bacterial disease caused by a group of bacteria. In majority of cases, Vibrio, Pseudomonas and Beneckea have been known to cause this disease.
The disease is recorded from the freshwater prawns as well as from Penaeus sp. cultured in India. Providing better water quality, removal of infected and dead prawns, reducing the stock and adequate nutrition help to control the disease. Feeding terramycin incorporated feed at 0.45 mg per kg of feed for two weeks, bath treatment using 0.05 to 1. 0 mg of malachite green per litre of water are suggested.
Fungal diseases, diagnosis and management:
Fungal diseases have been reported to cause extensive morality ranging from 20 to 100 percent. Several fungi are known to be shrimp pathogens. Three groups of fungi commonly infect the larval stages of shrimps while another one attacks the juvenile or larger shrimps. The common fungus affecting the larval shrimps is Lagenidium. Apart from this species, Siropodium and Haliphthoros also affect the larvae. These fungi
generally require a thin cuticle which is noticed only in shrimp larvae. The most common fungi affecting the larger shrimp belongs to Fusarium sp. Environmental factor such as low salinity prevailing in the monsoon season is found to precipitate fungal infections in the hatchery as well as growout systems.
Fungal infections in shrimp larvae:
In the case of larval fungal infections (larval mycosis), it is interesting to note that the infection starts from a fungal spore which attaches itself to the egg of the shrimp and then germinates. The germling (mycelium) then grows as the larva of shrimp grows, ramifies through the body wall of the larva and develops rapidly inside it replacing the muscles and soft tissues of the larva. Ultimately the entire body of the larva becomes a mess of mycelia of fungus. P. monodon, P. indicus and P. merguiensis (banana shrimp) are affected by the fungi Lagenidium and Fusarium sp. To prevent this disease in the hatchery, the inflow water has to be thoroughly filtered. Chemical and ultraviolet irradiation of inflow water is also effective. Application of malachite green at 0.001 to 0.006 mg per litre of water and treflan at 0.01 mg per litre are also found to be effective.
Mycosis of adult shrimps:
Although the exact extent to which the mycosis of adult shrimps caused by Fusarium sp. affect the shrimp aquaculture is not known, it is certainly considered as a potential threat. Almost all the culturable shrimps are known to be affected. The Fusarium sp. may be identified by the presence of canoe-shaped microconidia and also due to the presence of cotton wool like growth.
The fungus gains entry into the body through the already eroded areas or cracks on the cuticle. Preventation and treatment courses are much like that of larval mycosis.
Protozoan diseases, diagnosis and management:
Protozoan parasites and commensals of shrimp are found to occur externally or internally. The externally occurring ones are considered harmless unless they are present in large numbers. Those present internally can cause disease and are representatives of Microsporidia, Haplosporidia and Gregarina.
Cotton shrimp disease/milky disease of shrimps:
The cotton/milky shrimp disease is caused by the protozoan parasite belonging to Microsporidian group. Almost all the culturable shrimps are affected. The muscie tissue becomes milky. The microsporidians are abundant in the infected shrimp and cause the white appearance. No eggs are found in milky shrimps and it is inferred that the microsporidians infection render the shrimp incapable of reproduction. Microsporidians are present in the affected shrimp in the form of spores which are microscopic. These spores are transmitted horizontally. Providing better water quality in the hatcheries and growout ponds and following strict farm husbandry practices prevent this disease. Although no satisfactory treatment is evolved yet, experimental results indicated the usage of 0.0075 mg of malachite green per litre of water in static condition for the Post Larvae and addition of commercial bleach to the culture system are successful.
Ciliate infestation:
The Ciliates, Zoothamnium, Epistylis and Vorticella and suctoreans, Ephelota gemmipara and Acineta may invade all the life stages of the shrimps and cause respiratory and locomotory difficulties when present in large numbers on the gills and shell. In the pond grown shrimps, the ciliates may form a fuzzy mat on the shell as a result of the deterioration of the culture water. Ciliate infestation can be prevented by avoiding heavy silt, high nutrient load, turbidity, and low oxygen tension. Affected shrimps can be treated with baths of chloroquin diphosphate or formalin to remove the ciliates.
Gregarine disease:
Gregarines are common parasites of the digestive tract of shrimps. Their presence in large numbers in the gut interferes with particle filtration to the hepatopancreatic ducts or through the gut resulting in large scale mortalities. Providing better water quality conditions prevents this disease.
DISEASES INFLUENCED BY DIETS:
Pond grown shrimps are more often subjected to nutritional disorders than their younger stages in hatchery or nursery rearing period primarily because of the culture facility. In the case of unfed shrimps, they lose their normal full and robust appearance. The shell becomes thin and flexible as it covers the underlying tissue such as tail meat that becomes greatly resorbed due to lack of nutrients. The moulting is curtailed and in
due course of time the shell and gill become darker.
Chronic soft shell syndrome:
This disease occurs among the juveniles and adults. The affected shrimp is characterized by a persistently soft and thin exoskeleton, weakness and greater susceptibility to cannibalism. Inadequate amounts of nutrients such as calcium and potassium is known to create this sickness. Dietary and environmental manipulation prevents the occurrence of this disease. This occurs among the juveniles to adults. Affected shrimps have bluish exoskeleton which is also soft and thin. The shrimps also become lethargic. Low levels of the carotenoid astaxanthin in the diet, poor soil and water qauality are the causative factors. Reducing the stocking density, feeding with high quality feed and frequent Water exchange in the culture system prevent this disease.
DISEASES OF UNKNOWN AETIOLOGY
Inflammation and melanization:
Instances of tissue darkening is observed in shrimp farming. The blood· cells congregate in particular tissue areas (inflammation) where damage has occurred and this is followed by pigment (melanin) deposition. An invasion by infective agent, injury or presence of toxins causes this defect.
Gills are prone to darkening due to their fragile nature and their function as collecting site for elimination of the body's waste products. Gills darken readily upon exposure to toxic metals or chemicals and also as a result of infection by fungi like Fusarium sp.
Cramped shrimp: Shrimps exposed to a variety of cuIture conditions develop cramped nature. The tail is drawn under the body and becomes rigid to the point where it cannot be straightened. The cause and remedy are not yet studied in detail.
VI. DISEASE INSPECTION AND CERTIFICATION
The rapid growth of shrimp farming in recent years has led to increase in the live transport ofthe shrimp young ones from one region to another. Such large movements inevitably pose a potential risk of introduction of hitherto unknown pathogens. With the improvised culture techniques, the chances of spreading of the disease is increased. The spread of viral pathogens in shrimps world over and Epizooti Ulcerative Syndrome
(EUS) in the case of the fish in Asia can be taken as the best example in this regard. Transfer and introduction of different stages of shrimps has to be controlled by suitable. regulations. This will reduce the risk of transferring pathogens from one place ot another or from an imported stock to the native stock. Regular inspections on the health status and sanitary conditions of the shrimp farms are to be carried out by trained personnel. The International Office ofEpizootics and Animal Health Problems in Aquaculture, of late has dealt with crustacean pathogens through the Fish Diseases Commission (de Kinkelin, 1992).
The listed shrimp pathogens include: MBV, BP, BMNV and IHHNV. The approach for health control in aquaculture involves:
(i) assessment of health status of animals in a production site based upon inspections and standardised sampling procedures followed by laboratory examinations conducted according to the OlE codes.
(ii) constraints of restocking open water and farming facilities only with products having a health status higher than or equal to that of animals already living in the considered areas,
(iii) eradication of disease when possible, by slaughtering of infected stocks, disinfection and restocking with pathogen free animals, and
(iv) notification by every member country of its particular requirements, besides those provided by the code, for importation of aquaculture animals and animal products.
CONCLUSION
A number of diseases of shrimps cultured has been enumerated in the preceding discussion. The diseases pose threat to obtain maximum production. In many instances the poor water quality conditions of the culture system only predispose the candidate species towards diseases. Maintenance ofbest water quality could therefore be described as 'health maintenance' and be given top priority in shrimp farming. This will reduce the harmful effects of chemoprophylaxis as well as chemotherapy in farming activities.
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