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Epidemiology of infectious diseases in cultured Litopenaeus vannamei shrimp, with emphasis on white spot disease

More:  Institute 
Reference no: 3E060792
Period: October 2004 till December 2006
Status: Completed

Taxonomic term: Litopenaeus vannamei (Boone, 1931) [WoRMS]

Institute  Top 
  • Katholieke Universiteit Leuven; Departement Biologie; Afdeling Dierenecologie en -systematiek; Aquatische Ecologie en Evolutiebiologie, more, co-ordinator

Shrimp farming represents the third largest economical activity of Ecuador, preceded only by the petroleum and banana industries. However, in the past two decades, this Ecuadorian industry was hit by disease outbreaks on several occasions. White Spot Disease (WSD) drastically decreased the country’s production output from 120,000 ton Pacific white shrimp (Penaeus vannamei) in 1998 to just 37,000 ton in 1999. To allow a global and sustainable development of the industry, it is a priority to develop prevention and control methods for diseases, and to reduce the risk of introductions of new pathogens. To contribute to this goal, the determinants associated with shrimp diseases of economic importance, such as white spot disease (WSD), need to be characterized, in particular those related to environmental factors. The aim of this thesis was twofold: (a) obtain a better understanding of WSD determinants associated with the environment, using an epidemiological approach, and (b) design a preventive control strategy for shrimp diseases.

To improve the knowledge of the WSD epidemics, risk factors were studied at national level (Ecuador) and at pond level. It was found that at the country level WSD is influenced by climate. Three epidemiological surveys in Ecuadorian farms showed an apparent association between lower temperatures and increased mortality rates in commercial ponds. Histopathological observations of shrimp infected with White Spot Syndrome Virus (WSSV) showed lymphoid organ spheroids and cells with pyknotic and karyorrhectic nuclei in the haematopoietic tissue, lymphoid organ, connective tissue, heart and antennal gland. Occasionally, pyknotic cells were encapsulated without apparent injury to the adjacent tissue and without melanization. Several times during the present research, the risk of disease outbreaks was higher in larger ponds than in smaller ponds. This was attributed rather to a larger population size than to pond size. The risk was related to an increment in the size of the shrimp groups, the bigger ones representing higher levels of cannibalism. In conclusion, climate – in particular temperature – and population size were found to play an essential role in WSD outbreaks.

Two strategies for disease control were studied. A first prevention strategy consisted in parcelling the ponds using nylon nets, diminishing the risk of large groupings. Higher survival and yield were obtained in small areas compared with medium and large areas. It was found that differences in yield between small and large commercial areas were more pronounced at higher densities. As such, parcelling can be useful to diminish the impact of WSD - or any other infectious disease transmitted by contact - in ponds stocked at high density. At low stocking densities, this strategy does not produce any significant effect.

A second prevention strategy focuses on the preventive control of any disease, with special emphasis on the regional level. An online operational alert system (SAEMA) for the Gulf of Guayaquil - the largest shrimp farming region of Ecuador - was developed, based on production surveillance. An alert index containing standardized anomalies of production and management indexes (AIPM) was established, based on harvest data. This alert system calculates deviations from historical averages. AIPM-drops present a 2-month forward warning concerning drops of survival, and the field data confirm that AIPM can be used as a valid alert index. Four alert levels can be displayed at three levels: regional, farm and pond level. The system has been operating successfully in real time during six months (form May to October 2006) in 19 shrimp farms. SAEMA is an important step forward in the elaboration of a complete early warning system for shrimp epidemics and represents a useful contribution to the control of diseases.

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