Abstract

In 1993, an epizootic occurred in captive Penaeus monodon spawners at a research facility in northern Queensland, Australia. The spawners exhibited lethargy, failure to feed, redness of the carapace and pleiopods, and an increased mortality rate. A reliable bioassay of 0.45 mu m filtered, cell-free extracts of infected tissue produced mortalities approaching 100% in inoculated prawns. On average, the infected prawns became dark red by Day 6, produced red faeces by Day 10 and first mortalities by Day 13. Red faeces was a feature of this disease that has not been previously reported. When the survival curves of a dietary route were compared to the inoculation route, there was a lag period of 21 d. After subtraction of 21 d, the survival curves were statistically the same (G = 1.284, df = 1, p > 0.05). Pathological changes were found in the subcuticular epithelium and underlying muscle, depleted haematopoietic tissue, lymphoidal organ, hepatopancreas and gut. Increased eosinophilic refractile material was observed in the subcuticular epithelium and basement membrane and in the capsule surrounding the hepatopancreas. There were extensive areas of haemocytic infiltration and melanisation of the subcuticular epithelium with haemocytic replacement and necrosis of the underlying muscle. There were large areas of necrosis in the hepatopancreas, as cells with pyknotic nuclei sloughed into the lumen. Small (20 nm) icosahedral virions were observed in gut cells with transmission electron microscopy, and partial characterisation indicated that it was a nonenveloped DNA virus, similar to a parvovirus.