MANAGEMENT STRATEGIES FOR CONTROLLING DISEASES IN SHRIMP AQUACULTURE
Hassan Rostamian
Preface
Shrimp aquaculture has provided opportunity for economic and social upliftment of rural communities in the southern coastal area of our country.
Recent spread of highly Pathogenic viral diseases of shrimp, have accompanied movements of shrimp aquaculture development.
More recently outbreaks of white spot disease (WSD) which caused by the white spot syndrome virus (WSSV) in several part of Bushehr province (Helleh site) is a direct result of the ill-considered of infected shrimp in our country.
Therefore, adoption of proper shrimp health management practices, based of the principles of good aquaculture practice, is the first step towards this direction.
Abstract
This document presents the report of the management strategies for controlling diseases in shrimp aquaculture. Also included the summery of the discussion and recommendations for national and regional policies, legislation, and regulatory frameworks for reducing the risks of disease outbreaks in shrimp aquaculture.
The most important step of this document is adoption of proper shrimp health management, based on the principles of good aquaculture practice.
1- History of global disease outbreaks
At the same time as environmental concerns have increased, many diseases have proliferated to threaten the shrimp farming industry. In 1989 there existed only six world-wide shrimp viruses. By 1996 the number rose to over twenty.
The first outbreak of viruses on a large scale hit Taiwan 1987, when production was at 37 million pounds. By 1998, less than 1 million pounds were producted in Taiwan due to continued disease outbreak.
WSD outbreaks were first reported from farmed Penaeus Japonicus in Japan in 1993 and the causative agent was named penaeid rod-shaped DNA virus (PRDV) or rod-shaped nuclear virus of P.Japonicus (RV-PJ) later, outbreaks of viral disease with similar gross signs and caused by similar rod-shaped viruses were reported from elsewhere in Asia and other names were applied: hypodermal and haematopoietic necrosis baculovirus (HHNBV) in the peopleۥs Republic of Chain; white spot baculovirus (WSBV) and PmNOBIII in Taipei China; and systemic ecotodermal and mesodermal baculovirus (SEMBV) or PmNOBIII in Thailand.
The virus from the peopleۥs Republic of Chain has also been called Chinese baculovirus (CBV). Shrimp exhibiting the gross signs and histopathology of WSD have also been reported from Korea, India, The Philippines and the USA. During 1999, WSD also had a severe impact on the shrimp industries of both Central and South America.
Because of similar gross signs, ultrastructure and molecular biology, Lightner has included these virus (WSSV) complex, which is refrred to have simply as white spot virus or WSSV. In Iran the first official confirmation date of white spot disease event is on 23th June 2005 in Bushehr Province (Helleh site), but the previous outbreaks of white spot disease in Iran reported to OIE in 2003.
Shilna-Iran fishery organization (Shilat) in an interview with Mr.M.Ghodsi (The president of shrimp farms union) reported that the the first event of WSD has been happened 4 years ago in Choybdeh site (Abadan) in Khuzestan province, and the other shrimp farms in Bushehr Province already has been affected by WSD in August 2005, such as Boyrat, Mond, …
2- What Is White Spot Disease (WSD)
The WSD is caused by the white spot syndrome virus (WSSV) and affect shrimps of all age groups. The WSD outbreaks are often characterized by high and rapid mortality of infected populations, shortly after the first appearance of the clinical signs. Diseased shrimp develop anorexia, lethargy and characteristic white spots on the inside surface of the carapace. Moribund shrimp may also show a pink to red discoloration. There may be reduction in feeding levels.
White spot syndrome virus (WSSV) has a wide host range among crustaceans and is potentially lethal to most of the commercially cultivated penaeid shrimp species. Virions are rod-shaped to elliptical with a trilaminar envelope and they are large (80-120 250-380 nm). Negatively stained virions purified from shrimp haemolymph show unique, tail-like appendages. The virions are occlusion bodies. In initial reports, WSSD was described as a non-occluded baculovirus, but even while the molecular data were still limited, the preliminary WSSD DNA sequence analysis, the morphological characteristics and the general biological properties of the virus already highlighted its uniqueness. Recent data, including the genome sequence and phylogenies based on DNA Polymerase and Protein Kinase, suggest that WSSV is a member of a new virus family. The status of work on WSSV has been reviewed by Lightner, Flegel et al. Lob et al, Lo & Kou and Lo et al.
The size of the WSSV genome has been differently reported for different isolates. 305107 bp,292967 bp, and 307287 bp for viruses isolated from the peopleۥs Republic of China, Thailand and Taipei China, respectively. The sequences of these three isolates are almost identical, with the size differences being due mostly to several small insertions and one large deletion.
For two of these isolates, the analysis of the complete WSSV genome has been published. The following descriptions are based mostly on the WSSV isolate from the peopleۥs Republic of China. The WSSV genome has a total G+C content of 41%.
About 3% of the WSSV genome is made up of nine homologous regions containing 47 repeated mini-fragments that include direct repeats, a typical inverted repeat sequence is unique.
Total of 531 putative Open Reading Frames (ORFs) were identified by sequence analysis, among which 181 ORFs are likely to encode functional proteins.
Thirty-six of these 181 ORFs have been identified by screening and sequencing to encode functional proteins. Transcription of another 52 ORFs was confirmed by reverse-transcription polymerase chain reaction (RT-PCR).
For 80% of the putative 181 ORFs there is a potential polydenylation site (AAT-AAA) downstream of the ORF. Confirmation of WSD requires more detailed analysis by PCR, antibody-based assays, in-situ DNA hybridization, or transmission electron microscopy (TEM).
Using such techniques, it has been shown that WSSV from captured juvenile and brood stock shrimp or other carriers are identical or closely related. These molecular techniques have also been used to confirm infection of more than 40 penarid and non-penaeid crustacean carriers and, tentatively, an aquatic insect larval carrier. Some of these carriers have been shown to transmit WSSV to shrimp in laboratory tests.
Detection of WSSV in carrier shrimp or other crustaceans cannot be reliably accomplished by histological methods, and more sophisticated techniques are required.
3- What is the Source and Causation of White Spot Disease.
The WSSV has been established as the “necessary cause” of the WSD.
However, presence of the necessary cause alone will not lead to a WSD outbreak in a pond. In a farm situation, a number of “component cause” (risk factors) along with the “necessary cause” might become “sufficient cause” to produce WSD outbreaks.
The recent studies clearly show that WSD is not caused by any one factor Rather a number of risk factors influence the occurrence of WSD in the farm. These risk factors occur throughout the shrimp cropping cycle and in general terms full into the following categories during the different stages of the corp cycle:
• Pond preparation
• Pond filling and water preparation
• Seed quality and screening
• Water management
• Pond bottom management
• Feed management
• Disease treatment
The risk factors at each stage of the cropping cycle and their relationship to WSD outbreaks are illustrated in chart “source and causation of white spot disease”
(Please refer to Annex to see the Chart No. 1)
4- Guide lines for Controlling White Spot Disease In Shrimp Farms
4-1 Pond Preparation
Pond preparation is essential to reduce risks of shrimp disease outbreaks. The following risk factors that can significantly reduce the risk of disease outbreaks and improve shrimp production.
• Removal of bottom sludge, Particularly in ponds stocking higher densities (up to 8 numbers Per m .
• Ploughing of soil when wet (Particularly at higher stocking density farms).
• Use of lime in pond preparation.
Shrimp ponds with a history of disease outbreaks have a greater likelihood of future disease outbreaks, therefore special attention is required during pond preparation in such farms.
Farms with poor bottom soil quality, Particularly the presence of a black soil layer, will suffer crop failures.
4-1-1 Sludge removal
The sludge must be disposed away from the pond site. So that it dose not seep back into waterways, ponds, or cause other environmental problems. In farms the sludge, unless there was disease outbreak during the last crop. In such a situation extra precaution should be taken.
Sludge removal should pay attention to areas of the pond where there is a high accumulation of organic matter from pervious crop, such as feeding areas, and the side ditch in extensive farms.
If the sludge is removed properly then management of the pond becomes easier during high pH periods, due to low salinity, and plankton growth.
4-1-2 Ploughing
The main purpose of Ploughing is to expose the black soil layer underneath bottom soil to sunlight and atmospheric oxygen. By this process, the organic waste (sludge) will be oxidized. Ploughing on wet soil is particularly recommended for ponds if the planned stocking density is between 6 and 10 PL/ m and when the sludge cannot be removed properly by manual or mechanical methods.
After ploughing, dry the pond bottom for 5 to 7 days and report the procedure till no more black soil is seen. In case a heavy tractor is used for ploughing, then plogh the dry soil and then fill the pond with water to wet the soil and then again dry.
Ploughed pond bottom leads to turbid water conditions during culture period. Therefore, compaction of the bottom using heavy rollers after the whole process of pond preparation, i.e., before water intake, can avoid the turbid water condition.
4-1-3 Liming
Liming during pond preparation optimizes pH and alkalinity conditions of soil and water. The type and amount of lime to be added depends mainly on the soil pH and also on pond waters pH, which ideally should be checked before lime application. Quick lime or should be used only if the soil pH is low i.e. pH <5. If it is applied on soils of pH>5, then it may increase the water pH after filling and this high water pH condition may remain for a prolonged period even after stocking, which is not desirable. If the soil pH is more than 5, then shell lime or agricultural lime dolomite should be applied.
(Please refer to Annex to see Table no. 1)
4-2. Pond filling and water preparation
There are management practices that can be adopted to reduce risk factors associated with pond filling and preparation of water before stocking. These include:
• Water filtration (mesh of 60 holes/sq inch) reduces the risk of disease outbreak through reduced introduction of carriers to the pond.
• Disinfection of pond water can also reduce the risk of disease outbreaks in farms using higher stocking density.
• Fertilization reduces the risk of disease outbreak in lower stocking density farms.
The use of water reservoir appeared to have no significant benefit to farmers in either reducing risk of disease outbreaks or improving production. As reservoirs are normally good to improve water supply quality in shrimp farming, the findings suggest that reservoirs are not being used properly.
The proper use of reservoirs is therefore strongly recommended as disease control measure and to make water management more effective during the crop cycle.
4-2-1 Reservoir maintenance and pond filling
Reservoir pond for at last 14 days before pumping to the shrimp culture ponds to facilitate the growth of plankton in the reservoirs. This water can even be used to fill grow-out ponds just one to two days before stoking with seeds. If the water reservoir is not maintained then the grow-out pond should be filled directly with source water at least 14 days before stocking.
During water intake the cultured pond, water inlet point to avoid entry virus carriers such as crabs, wild shrimps and zooplankton and also to avoid entry of fish or crustacean, which may be predator or competitor for shrimp. The suction line should be placed at the deeper side of the reservoir pond and the foot valve of the pump should be kept at least half a foot above the pond bottom to avoid turbidity during the pumping process.
4-2-2 Fertilization of water
Inorganic fertilizers like urea or superphosphate at 30-50 Kg/ha in 3-4 dosages must be used to boost the plankton production.
Fertilizer should be first applied at least 10 days before the planned stocking date so as to obtain a good plankton bloom with green water color for stocking. It the water color remains unstable, fertilizer application should be continued even after stocking. This would avoid a risk of plankton bloom collapsing suddenly.
4-3 Seed Selection and Stocking Process
There are several important risk factor associated with shrimp seed, which include:
• Stocking of poor quality of seed (less active, more mortality during transportation and size of less than 16 mm in case of nursery reared juveniles) increases the risk of shrimp disease outbreak.
• Higher prevalence (>5% of seed population) of WSSV as determined by two-step PCR in stocked post-larvae leads to increased risk of an outbreak and lower pond production.
• Longer transport time (>6 hours) of the seed from hatchery or nursery to the pond also increases the likelihood of a subsequent disease outbreak.