Investigation into white spots in the carapace of a moribund mud crab (Scylla serrata) from a white spot syndrome virus (WSSV) positive zone in Moreton Bay, Australia

Aug 6, 2020Australian veterinary journal

White spots on the shell of a dying mud crab from an area with white spot syndrome virus in Moreton Bay, Australia

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Abstract

A mud crab (Scylla serrata) showed multiple white spots under its carapace but tested negative for White Spot Syndrome Virus (WSSV).

Histopathological examination found no lesions indicative of white spot disease (WSD).
The white spots were identified as lighter foci in the carapace formed by concentric crystalline-like rings.
These findings suggest the white spots may result from mineral mobilization during the crab's pre-moult stage.
The cause of death may be related to complications from the moult process rather than WSSV infection.
Previous findings of WSSV in similar hosts may have been incidental to the observed symptoms.

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BACKGROUND: A freshly deceased mud crab (Scylla serrata) exhibiting multiple white spots under the carapace was found in Pumicestone Passage, northern Moreton Bay in May 2018. This crab was taken from within a biosecurity zone established due to a recent incursion of White Spot Syndrome Virus (WSSV) into populations of wild penaeids (Penaeus spp., Metapenaeus spp.) and crabs (Thalamita crenata) in the area. Because grossly visible white spots have been previously observed under the carapace of moribund S. serrata with white spot disease (WSD) in India, an investigation into the cause of death was undertaken.

CASE REPORT: The affected S. serrata was negative for WSSV DNA when gill samples were tested by real-time PCR. Histopathology found no evidence of WSD lesions in the form of basophilic hypertrophied intranuclear inclusions in any tissues of ectodermal or mesodermal origin. Histopathology of the affected carapace showed that the white spots consisted of multiple lighter coloured foci in the exocuticle formed from concentric crystalline-like rings, which extended into the endocuticle. These were interpreted as evidence of mineral mobilisation within the carapace during the pre-moult (D1 or D2) stage of the moult cycle. The cause of death in this case therefore may have been due to moult-related complications.

CONCLUSION: These observations confirm that formation of grossly visible white spots under the carapace of S. serrata are not pathognomonic for infection with WSSV. Similar observations in previous studies where WSSV was detected by PCR in this same host may have been incidental findings.

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