DESCRIPTION OF A SMALL DSRNA VIRUS FROM FRESH-WATER CRAYFISH CHERAX-QUADRICARINATUS

A unique hepatopancreocyte nuclear pathology was seen during a histological survey of farmed crayfish Cherax quadricarinatus. Later, the pathology was associated with mortalities in laboratory-held juvenile populations. Affected nuclei were variably hypertrophic, had marginated and clumped chromatin and were almost fully occupied by numerous amorphous inclusions in late infections. Well-developed inclusions were intensely purple-red whilst less-developed inclusions were eosinophilic with H&E. The nucleolus remained identifiable and centrally located. Embryonic (E) cells were the only hepatopancreocyte type that did not display the changes. Crayfish from all 7 farms surveyed were infected, with a maximum prevalence of 52.3 %. The intensity was extremely low in adult crayfish. The inclusions were present in 10 of 14 crayfish families used in a heritability study. Mortalities varied greatly, with the highest mortality being 85 % by Week 8. Moribund crayfish consistently displayed a high intensity of affected nuclei. Affected nuclei were Feulgen and methyl-green negative, pyronin positive and fluoresced yellow-green with acridine orange, suggesting the pathology was associated with a double-stranded RNA virus. Nuclease digestion of the histological material confirmed the predominance of dsRNA in the inclusions. Transmission electron microscopy revealed 25 run virions in highly ordered intranuclear paracrystalline arrays, a feature apparently unique amongst the RNA viruses. The virus is morphologically and morphogenically similar to Giardia lamblia virus and is therefore named Cherax Giardiavirus-like virus.