WHITE SPOT DISEASE (WSD) IN PACIFIC WHITE SHRIMP (LITOPENAEUS VANNAMEI) IN THAILAND: EPIDEMIOLOGY, DISEASE ASSOCIATED RISK FACTORS AND MOLECULAR TYPING

Abstract

Prevalence of white spot disease (WSD) in an intensive shrimp culture area located in Chanthaburi province, Thailand during 2009-2014 was observed. Retrospective data of 1,952 WSD cases were analyzed for the association between WSD occurrence and climate factors negative binomial regression (NBR) models. A high number of WSD cases were found between October to February, while a less number of cases were reported during March to June, and the lowest numbers were reported in May. The multivariate NBR model indicated significant associations between an increased number of WSD cases with decreased atmospheric temperature and more variation of daily atmospheric temperature. Case-control study using logistic regression model was also used to identify the risk of WSD occurrence at farm-level in this area. Results of an interview survey of 157 intensive shrimp farms showed that farms sharing inlet water, culturing shrimp year round and with a single owner operating more than one farm were identified as WSD risk factors. The analysis also showed WSD risks to be reduced at farms that applied lime to disinfect pond bottoms and use of probiotics feeding supplementation. A total of 137 white spot syndrome virus (WSSV) samples causing disease in pond during 2007-2014 were collected from eastern and southern Thailand. The variations in their genome were analyzed using Polymerase chain reaction (PCR) targeting the 5 variable loci, including Indel-I, Indel-II and Variable number tandem repeats (VNTRs) located in ORF75, 125 and 94. Analysis of Indel-I and Indel-II showed the newly 3 WSSV genotypes identified in Thailand. These genotypes were related to WSSV from Vietnam, India, Brazil and Saudi Arabia. Analysis of the VNTRs showed high degree of variation, which at least 33 genotypes were detected. The similarity of WSSV genome in several WSSV isolated collected during 2007-2014 suggested that WSSV genome is now stable. The results from this study may be used as database for further epidemiological study of WSSV.