A Cross‐Sectional Study of the Association between Risk Factors and Hemorrhagic Disease of Grass Carp in Ponds in Southern China

Abstract

A cross-sectional survey of 215 Grass Carp Ctenopharyngodon idella ponds was conducted in southern China between May 2010 and November 2011. An in-depth questionnaire was developed to evaluate a series of biosecurity practices, environmental factors, and management factors at the farm level. Fish samples with clinical hemorrhagic signs were also collected from each pond to assess the clinical disease of Grass Carp reovirus by using reverse transcription (RT) PCR assay. The association between the incidence of Grass Carp hemorrhagic disease (GCHD) and risk factors was analyzed using logistic regression. Of the 215 ponds, 144 showed GCHD-positive responses to RT-PCR assay. In addition, survey results revealed that inferior environmental conditions occurred in most ponds with an incidence of GCHD; such conditions included a thick mud layer; no cleansing and restoration practices (CRPs) before culture; and poor water quality (i.e., high ammonia nitrogen and nitrite concentrations and low transparency). Logistic regression model results suggested that decreased risks were associated with fry vaccination, a safe water source, and deepening of the water level, whereas increased risk factors mainly included no CRPs, excessive rearing density, disease history, and inferior water quality. Presently, control efforts are restricted to immunization of Grass Carp as the best management option for farms. Deepening the water levels and improving water sources can also effectively reduce the incidence of GCHD by diluting the pond rearing densities.