Pond-level risk factors for White Spot disease outbreaks

Citation

Corsin F, Turnbull J, Mohan CV, Hao NV & Morgan KL (2005) Pond-level risk factors for White Spot disease outbreaks. In: Turnbull J (ed.) Diseases in Asian Aquaculture V. Diseases in Asian Aquaculture, 5. The Fifth Symposium on Diseases in Asian Aquaculture, Brisbane, Australia, 24.11.2002-28.11.2002. Manila, Phillipines: Asian Fisheries Society, pp. 75-92. http://www.fhs-afs.net/daa_v_proceedings.htm

Abstract
White spot disease (WSD) is a pandemic disease of crustaceans caused by White Spot Virus (WSSV). In the earlier stages of the pandemic, the scientific community hypothesised a number of potential risk factors for outbreaks based on information from other diseases and circumstantial evidence. As a result control strategies were recommended to farmers. Highly sensitive diagnostic methods were developed and allowed the detection of WSSV in shrimp broodstock, seed and wild animals leading to further interventions to reduce the risk of introducing the virus into ponds. Aspects of some of the hypothetical risk factors for WSD were tested through experimental trials. Field investigations were also carried out and provided further information on risk factors for outbreaks and the information generated was used to investigate WSSV transmission using mathematical modelling. In this review we summarise and discuss current knowledge of the pond level risk factors for WSD. The importance of different routes of WSSV entry to the pond has been examined. Pond preparation practices proved useful at eliminating the virus from the pond therefore reducing the risk of outbreaks. The importance of stocking WSSV positive post-larvae (PL) was also evaluated and it would appear that the prevalence rather than the mere presence or absence is more important as a predictor of WSD. WSSV presence in crabs and plankton was not associated with an increased risk of WSD in field studies. An association existed between water exchange and the occurrence of WSD cases in some farming systems. Commercial feed as a source of WSSV was also investigated. In one study a large proportion of feed samples tested were WSSV positive. Although no significant association between WSSV in feed and WSD was detected, the inclusion of WSSV-positive tissues in feed pellets should be avoided. The need for pond-level biosecurity is also discussed. Factors responsible for precipitating WSSV infection into a WSD outbreak were examined and poor shrimp health was associated with an increased risk of WSD. Similarly, stress was identified as a triggering factor for outbreaks and this was consistent with field observations that WSD cases may be preceded by higher pH and un-ionised [cb1]ammonia. Lower water temperature was associated with an increased risk of outbreaks through its effect on WSSV-induced apoptosis. Application of immunostimulants did not affect the risk for WSD whereas the use of vitamins was associated with a reduced risk of WSD in two separate epidemiological investigations. Factors affecting the epidemic within the pond are also discussed including the role of stocking density and removal of infectious shrimp. The need for further epidemiological studies for the identification of risk factors in different farming systems is discussed.