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DESCRIPTION OF A TREATMENT FOR THE CONTROL OF TAURA SYNDROME VIRUS IN Litopenaeus vannamei UNDER INTENSIVE
CULTURE
Carlos A. Ching1 and Chalor Limsuwan2 1 Nicovita Technical Manager – Alicorp SAA, e-mail: [email protected] 2 Ph.D. Professor, Kasetsart University, Thailand, e-mail: [email protected] Abstract A procedure to reduce the impact of Taura virus (TV) infection in Litopenaeus vannamei is described herein. The treatment is based on the prevention of shrimp molting by interrupting water exchange, no or reduced feeding, maintaining a minimum pH of 8.0, and avoiding the use of chemicals in the culture water. This paper shows the results of such treatment in an intensive culture in Naozhou island, Province of Guangdong, China. Introduction Taura syndrome virus (TSV) first appeared in Ecuador in 1992, resulting in high mortality rates. Since then, numerous prevention efforts have been attempted. TSV is now broad spread among most world’s shrimp-producing countries, and this has encouraged efforts in the area of genetic improvement to produce TSV-tolerant Litopenaeus vannamei with successful results in Thailand and the US. Nevertheless, nearly no practical protocols have been reported for the treatment of sick shrimp once TS mortality has started. Disease description Taura syndrome has been reported in all shrimp cultivation stages, but attacks are more intense during the first month, when the cuticle of juveniles is affected by lesions in the form of melanosis, semi-empty intestines, and expanded chromatophores (Figure 1).
Figure 1. Juvenile showing typical Taura syndrome lesions
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The histopathological changes caused by TSV include multi-focal necrosis areas in the sub-cuticular epithelium, the connective tissue, and the adjacent striated muscle. Affected cells frequently show nuclear picnosis and karyorrhexis (Figure 2). Due to damage in the sub-cuticular region, infected animals lose the ability of molting successfully, thus they die in the process. Only if these tissues recover, individuals can survive the molting process. Figure 2. Histological comparison between a normal area and an area affected by Taura syndrome virus in the cuticle of Litopenaeus vannamei. Taura syndrome treatment Once TS mortality is detected, action should be taken to prevent shrimp from molting, including lowering feeding rates, maintaining water pH above 8.0, avoiding water exchange, and providing maximum aeration to promote good water quality. Given that feed intake can induce molting, lowering the daily ration to a level just enough to maintain shrimp alive is recommended. In some cases of extremely high mortality, all feed rations in the day must be avoided. Low pH levels (<8.0) are one other factor promoting shrimp molting. The application of calcium carbonate or hydrated limestone can maintain a minimum pH of 8.0 to control mortality. It has also been noticed that during strong rains shrimp molt more frequently due to decreased pH levels, resulting in potential high mortality in the event of a TSV attack. The disease is also related with salinity, since low water salinity levels are associated with high mortality rates due to decreased mineral availability during the molting process. Dead shrimp should be removed daily in order to avoid water quality deterioration and disease propagation due to canibalism. Using chemicals (fertilizers, antibiotics) in the pond should also be avoided during a TSV infection.
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Healthy tissue with a normal cuticle
Affected cuticle that cannot absorb the minerals required for molting.
Picnosis
Karyorrhexis
10 um
The treatment described herein was tested in an intensive shrimp culture in Naozhou Dao - Guangdong, China, in 2009, obtaining significant differences in the survival rates between the treated and the control ponds (Table 1). Table 1. Summary comparing average survival rates between TSV-treated (TSV-T) and non-treated (Control) ponds
Area (ha)
Density (shrimp/m²)
Harvest (kg)
Yield (kg/ha)
Wt. (g)
Survival (%) 2
FCR Time (days)
Control1 0.34 146.8 1,622 4,770 13.0 25.4 2.54 87 TSV-T1 0.75 147.0 8,654 11,540 15.7 50.3 1.26 78
1: Results from 3 ponds (N=3) per treatment. 2: TSV mortality was increased due to low pH levels caused by excess rain associated with PARMA typhoon a few days prior to harvest. The treatment was continued until mortality ceased. After the critical stage (4 - 5 days), feeding rates were increased and water exchange was resumed. Recovered animals showed discrete spots in the cuticle where TSV necrosis existed (Figure 3). After a couple of molts, these animals recovered completely. Figure 2. Shrimp recovering from a TSV attack. Notice the spotted cuticle after the first molt post-infection Conclusion Most shrimp-producing countries have not yet developed shrimp families resistant to TSV. The TSV treatment reported herein can prove to be extremely valuable for improved performance in those cultures with non-TSV resistant animals. This treatment, which is being able to decrease the impact of TS in intensively-cultured shrimp in Thailand and China, can contribute to improve shrimp culture sustainability.
Editors: Carlos Ching [email protected] Máximo Quispe [email protected]
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Cuticle of a shrimp recovered from TSV
