Withering syndrome in farmed red abalone Haliotis rufescens: Thermal induction and association with a gastrointestinal Rickettsiales-like prokaryote

Withering syndrome (WS) is a chronic wasting disease responsible for mass m ortality in wild populations of black abalone Haliotis cracherodii. The eti ology of WS is uncertain with limited evidence for the role of a gastrointe stinal Rickettsiales-like prokaryote (RLP). We documented for the first rim e the occurrence of animals with clinical signs of WS and associated morpho logical changes in another haliotid species, the red abalone H. rufeseens. In this study, 60 juvenile red abalone (8 cm) were randomly selected from a farmed population raised at 14 degrees C that was known to have low-intens ity RLP infections bur lacked clinical signs of WS. The abalone were held i n triplicate containers receiving water of approximately 14.7 degrees C (Co ntrol, Co) or 18.5 degrees C (elevated temperature, ET) and were fed equall y for 220 d. Survival was 100% (30/30) for the Co group and 67% (20/30) for the ET group. The ET group animals had higher RLP infection intensities an d showed more clinical signs (mantle retraction. lower weight gain. lower c ondition index) and morphological changes (digestive gland degeneration) as sociated with WS. In trials conducted immediately before termination of the experiment, ET group animals fed at half the rare of Co group animals. Amo ng ET group animals, the intensity of RLP infections in the posterior porti on of the esophagus was positively correlated with WS clinical signs and mo rphological changes, whereas no correlations were present among Co group an imals. During 1997-1998 and in conjunction with elevated seawater temperatu res associated with El Nino, several abalone farms in California experience d a dramatic increase in the proportion or red abalone showing signs of WS. Examination of 66 red abalone from five commercial farms revealed that ani mals with more advanced RLP infections had more severe WS clinical signs an d associated morphological changes. Collectively, these data demonstrate th at RLP infection plays a key role in the etiology of WS in red abalone and that warm water enhances the severity of the pathogenic effects of RLP infe ction.