Clinical white spot disease status in Penaeus monodon during the middle of the culture period - its epidemiological significance

Abstract

First paragraph: White spot disease (WSD) is arguably the most serious disease affecting penaeid shrimp, not just in Asia but globally. Since its first report in Japan in 1993 (Nakano, Koube, Umezawa, Momoyama, Hiroaka, Inouye & Oseko 1994; Momoyama, Hiraoka, Inouye, Kimura & Nakano 1995), the disease is now rampant in most of Asia (Wongteerasupaya, Vickers, Sriurairatana, Nash, Akarajamorn, Boonsaeng, Panyim, Tassanakajon, Withyachumnarnkul & Flegel 1995; Wang, Tan, Kou & Chen 1997; Mohan, Shankar, Kulkarni & Sudha 1998; Park, Lee, Lee & Lee 1998) and Central and South America (Global Aquaculture Alliance 1999a,b). In Asia alone, the loss of farmed shrimp production attributed to WSD is estimated at US $ 4-6 billion (Lightner 2003). Realizing the devastating impact of the disease on the shrimp farming industry, over the past 15 years both farmers and scientific communities have been working towards identifying ways to prevent and predict the disease outbreaks. Several factors, including stocking of WSSV infected post-larvae (PL), stress because of higher pH and un-ionized ammonia and lower water temperature have been identified as the major risk factors for WSD outbreaks in shrimp farms (Flegel & AldaySanz 1998; Mushiake, Shimizu, Satoh, Mori, Arimoto, Ohsumi & Imaizumi 1999; Corsin, Turnbull, Mohan, Hao & Morgan 2005). Detection of white spot syndrome virus (WSSV), the necessary causative agent of WSD in shrimp, is paramount for management of shrimp farms. Sensitive diagnostic techniques have been developed and they have been included into pond side kits such as Shrimple (EnBio Tec Laboratories Co. Ltd) and Rapidot (Shankar 2006) to assess the level of WSSV infection in larvae, grow-out shrimp and in brood stocks, so that effective managerial measures can be intervened to prevent the disease outbreaks. It is well known that WSSV can be detected at stocking, and that the association with crop outcome is most probably system specific. It is also known that WSSV can be found during the production cycle at a low level without leading to disease outbreaks (Tsai, Kou, Liu, Liu, Chang, Peng, Hsu, Wang & Lo 1999), and that finding histopathological signs of WSSV in moribund and dead shrimp is a good indication that the crop will fail (Mohan, Corsin, Thakur, Padiyar, Madhusudhan, Turnbull, Hao & Morgan 2002a). Nevertheless, it is still not known whether the histopathological signs of WSSV in a representative sample of shrimp collected during production allow a prediction of the occurrence of WSD outbreaks or of poor crop performance. In this study, we examined the association between the WSD status half way through the crop (i.e. at 6th week) and WSD status at the harvest and at the time of stocking and with the crop outcomes in terms of yield per hectare, survival, average shrimp weight and days of culture.