First occurrence of Aphanomyces astaci epidemic infection in cultured yabby Cherax destructor (Clark, 1936) in Norther Italy.

In the winter of 2009 an epizootic mortality occurred in cultured yabby Cherax destructor on a semi-intensive crayfish farm in Veneto region (Northern Italy). The death of crayfish of all ages and sizes was observed over one month with a 70‐100% cumulative mortality. During the following winter season in 2011 a new severe episode of the disease affected the surviving yabbies. Moribund and dead crayfish were found in ponds often with loss of appendages. The clinical signs observed were lethargy, reduced food intake, poor limb coordination with crayfish lying on their dorsal surface unable to upright themselves. Affected crayfish were collected in both episodes (total number =107) for laboratory evaluation. Histological, bacteriological and bio‐molecular analysis were performed. Gross examination showed erosive and melanised ulcerative lesions in the intersternal soft abdominal cuticle, the joints of pereiopods, uropodes and telson. Temnocephalid ectocommensal flatworms and their eggs were frequently observed. Histological examination showed infiltration of aseptate branching hyphae underneath epicuticular ulcers in the abdominal segments. Hyphae in the cuticle and in the hypodermis were surrounded by deposits of melanin and haemocyte infiltration. Subepidermal granulomas and cuticular bacterial ulcers with heavy melanisation were present. Granulomas were found also in the green gland and in the hepatopancreas. Bacteriological analysis of the haemolymph of affected specimens showed polymicrobism: Aeromonas hydrophila, A. sobria, Hafnia alvei, Citrobacter brakii, and Shewanella putrefaciens were isolated in 2010, Pseudomonas anguilliseptica and Aeromonas hydrophila were isolated in 2011. Bio-molecular examination of melanised abdominal cuticle was performed by PCR (Oidtmann et al., 2006). Positive samples (2 positive/2 specimens analyzed in 2010; 12 positive/17 specimens analyzed in 2011) were sequenced and appeared homologous to published sequences of Aphanomyces astaci (GU320248.1). Under field conditions, Cherax destructor seems to exhibit incomplete resistance to crayfish plague as previously shown experimentally by Unestam (1976). In these episodes (in 2010 and 2011) the disease course was influenced by stressful environmental conditions (e.g. high stocking density during late summer and autumn months with aquatic plant cover reduction and low temperature ≤ 7°C in winter) which also promoted secondary bacterial infections.