Etiological characteristics of "tail blister disease" of Australian redclaw crayfish (Cherax quadricarinatus).

In November 2019, an acute disease outbreak in Australian redclaw crayfish (Cherax quadricarinatus) occurred in a farm in Hubei, China, with a cumulative mortality rate of over 80%. One of the characteristic symptoms of the disease was blisters on the tail. This symptom is also common in diseased Procambarus clarkii every year in this country, but the causative agent has not been determined. This study analyzed the etiological characteristics of this disease. Bacterial isolation and identification combined with high-throughput sequencing analysis were conducted to obtain the microbiota characteristics in the hemolymph, hepatopancreas, and intestines. Results showed that this outbreak was caused by infection from Aeromonas hydrophila and Aeromonas veronii. The underlying cause was stress imposed on crayfish during transferring from outdoor pond to indoor pond because of temperature drops. Aeromonas infection caused remarkable changes in the structure of the microbial composition in the hemolymph, hepatopancreas, and intestines of the crayfish. The abundance of Aeromonas in the hemolymph of the sick crayfish was as high as 99.33%. In particular, KEGG metabolic pathway analysis showed that some antibiotic synthesis, enterobactin biosynthesis, and myo-inositol degradation pathways were abundant in healthy crayfish hemolymphs, which may be the mechanism of maintaining crayfish health. Conversely, inhibition of these pathways led to the disorder of microbiota structure, finally leading to the occurrence of diseases. To the knowledge of the authors, this study was the first to use high-throughput amplicon sequencing targeting the 16S rRNA gene to find the causative bacteria in aquatic animals. This protocol can provide more comprehensive and reliable evidence for pathogen identification, even if the pathogenic bacteria are anaerobes or other hard-to-culture bacteria.