Rickettsia-like Infection of Pandalid Shrimp
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Category
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
Rickettsia-like organisms.
Geographic distribution
Disease most prevalent in Howe Sound but has been observed in one prawn from Sabine Channel in the Strait of Georgia, British Columbia.
Host species
Pandalus platyceros.
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.
Diagnostic techniques
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.
Figure 1. Dorsal view of two fresh Pandalus platyceros. The upper specimen is infected with stained prawn disease showing a dark discolouration in the cuticle of the cephalothorax and along the cuticular segments of the abdomen (arrows), in comparison to the lower specimen which is healthy.
Figure 2. Dorsal view of two fresh Pandalus platyceros with the cuticle removed exposing the internal organs of the cephalothorax. The upper specimen is infected with stained prawn disease showing a black pepper-like stippling on the surface of the hepatopancreas (H) and the dark discolouration (D) along the cuticular segments of the abdomen, in comparison to the bottom specimen which is healthy.
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.
Figure 3. Histological section through melanized haemocytic encapsulations (arrows) in the epidermis and connective tissue below the cuticla in Pandalus platyceros that cause the discolouration characteristic of stained prawn disease. Haematoxylin and eosin stain.
Figure 4. Histological section through the hepatopancreas of Pandalus platyceros showing extensive aggregation of haemocytes (H) and melanotic encapsulations (ME) that cause the black pepper-like stippling on the surface of the hepatopancreas characteristic of stained prawn disease. Haematoxylin and eosin stain.
Figure 5. Histological section through a melanotic encapsulation in the hepatopancreas of Pandalus platyceros showing the Rickettsia-like microorganisms (arrow) the cause of stained prawn disease. Haematoxylin and eosin stain.
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.
Figure 6. Electron micrograph of connective tissue in the hepatopancreas of Pandalus platyceros with stained prawn disease showing Rickettsia-like microorganisms some of which are undergoing binary fission (B), within cytoplasmic vacuoles (V) of healthy haemocytes and within melanotic encapsulation (M). Uranyl acetate and lead citrate stain.
Figure 7. Electron micrograph of three Rickettsia-like microorganisms, the causative agent for stained prawn disease in Pandalus platyceros. They are spherical in shape, possess an indistinct cell wall (CW) surrounding an inner plasma membrane (PM), granular cytoplasm (GC) and filamentous central bodies. Uranyl acetate and lead citrate stain.
Methods of control
No known treatment. Infected prawns should not be released or discarded (even after freezing) in marine waters.
References
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.
Citation Information
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
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