Case report of bacterial infections in a redclaw crayfish (Cherax quadricarinatus) hatchery

Redclaw crayfish, Cherax quadricarinatus, is a native tropical freshwater crayfish in northern Australia and has become an important aquacultured animal. Viruses, bacteria and parasites have been found in redclaw crayfish with some of the pathogenic bacteria causing massive losses in the redclaw hatcheries. The variety of bacteria cultured from redclaw larvae in the hatchery was determined using the commercial API and Biolog systems and confirmed with 16S rRNA, rpoD and aroA PCRs. These techniques were also used to find a candidate bacterium to be used as a probiotic. With histopathology, Gram-negative bacteraemia presented in the hepatopancreatic tissue of redclaw crayfish accompanied with inflammatory cell infiltration. Aeromonas hydrophila was identified with both bacterial identification systems. A. hydrophila caused high mortality in the stage 2 larvae in the hatchery. The use of three commercially available probiotics containing Bacillus species and locally-isolated candidate bacteria (Acinetobacter genospecies 6, Acinetobacter grimontii and Chryseobacterium balustinum) did not inhibit the development of the A. hydrophila - associated bacteraemic disease. In this study, A. hydrophila was resistant to penicillin, chloramphenicol/florfenicol and tilmicosin. The evidence of an infection by bacteria common in freshwater that enter after hatching and affected predominantly the hepatopancreas of the larvae indicated the necessity of at least attempting to control the most obvious sources of the bacteria. This directly led to changes in hatchery management regarding better control of the microbiological quality of the incoming water.