Rickettsia-like Infection of Pandalid Shrimp
Category 2 (In Canada and of Regional Concern)
Common, generally accepted names of the organism or disease agent
Rickettsia-like infection of prawns in British Columbia, Stained prawn disease.
Scientific name or taxonomic affiliation
Disease most prevalent in Howe Sound but has been observed in one prawn from Sabine Channel in the Strait of Georgia, British Columbia.
Impact on the host
The organisms predominantly infect the fixed macrophages between the tubules on the outer surface of the hepatopancreas resulting in an intense focal melanotic response. These compact melanotic whorls of cells then migrate towards the cuticle and are supposedly shed with the moult. The development of the gonad and the morphology of secondary sexual characteristics were abnormal in infected prawns. In the laboratory, the organisms are transmitted orally (feeding on infected prawns), horizontally in the water (exposure to screened effluent from infected prawns), and can survive freezing at about -15 °C for at least 10 days. Prawns from an unaffected area that were fed diseased prawns in the laboratory began to die of the disease about 2 months after exposure. The cumulative mortality over 4 months was 84% in comparison to 14% for unexposed controls.
Gross Observations: Black pepper-like stippling on the surface of the digestive gland occurs in early stages of infection. A dark discolouration of the cuticle, especially along the ventral surfaces of the cephalothorax and along the edges of cuticular segments is characteristic of the later stages of the disease.
Histology: Melanotic whorls of cells with a few central cells in the less advanced whorls containing basophilic stippling reminiscent of intracellular Rickettsia-like organisms. These tissue lesions are most evident on the surface of the digestive gland and throughout the connective tissue in the ventral half of the cephalothorax. The Rickettsia-like organism is evident in tissue sections stained with the modified Macchiavello's stain as described by Clarke (1981) with further modifications of using a counterstain of 0.1% methylene blue in distilled water without phenol and by staining deparaffinized histological sections instead of air-dried tissue smears.
Electron Microscopy: Colonies of Rickettsia-like organisms occur within the cytoplasm of haemocytes and are usually associated with melanotic encapsulation. The organisms are monomorphic; spherical in shape; possess an indistinct cell wall with an inner plasma membrane, granular cytoplasm, and filamentous central bodies; and reproduce by binary fission.
Methods of control
No known treatment. Infected prawns should not be released or discarded (even after freezing) in marine waters.
Bower, S.M., G.R. Meyer and J.A. Boutillier. 1996. Stained prawn disease (SPD) of Pandalus platyceros in British Columbia, Canada, caused by a rickettsial infection. Diseases of Aquatic Organisms 24: 41-54.
Clark, G. (ed.). 1981. Staining Procedures. Fourth Edition. Williams and Wilkins, Baltimore, p. 413-414.
Bower, S.M., Meyer, G.R. (1998): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Rickettsia-like Infection of Pandalid Shrimp.
Date last revised: August 1998
Comments to Susan Bower
- Date modified: