Viral Infections in Pearl Oysters
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Category
Category 3 (Host Not in Canada)
Common, generally accepted names of the organism or disease agent
Akoya oyster disease caused by "Akoya virus" and probably other viral infections.
Scientific name or taxonomic affiliation
"Akoya virus" (small viral particles) was named after the virus associated with extensive mortalities in cultured Japanese akoya pearl oysters Pinctada fucata martensii (Miyazaki et al. 1999). Miyazaki et al. (1998) and Renault (2016) reported that the size was similar to that of the viruses in the family Picornaviridae and resembled the virus from the scallop Pecten novaezelandiae described by Hine and Wesney (1997) . However, Wada (2003) was critical of the identification of the causative agent because: the putative "Akoya virus" was isolated from diseased pearl oyster using fish cells; only Dr. T. Miyazaki and co-workers have successfully isolated the virus; and they have not provided any genetic data nor antibodies for peer confirmation of the identity of the pathogen. In the black lipped pearl oyster Pinctada margaritifrea from French Polynesia, Comps et al. (1999, 2001) reported virus-like particles that also resembled the virus from the scallop Pecten novaezelandiae described by Hine and Wesney (1997) and the picornalike virus associated with granulocytomas in mussels Mytilus edulis from Denmark reported by Rasmussen (1986). The relationship between the "Akoya virus" and the morphologically similar virus from P. margaritifrea is not known (Renault 2016).
Suzuki et al. (1998) isolated a birnavirus in the genus Aquabirnavirus from symptomatic pearl oysters (Pinctada fucata) from the Uwa Sea, Ehime Prefecture, Japan. This marine birnavirus was isolated on finfish cell lines (CHSE-214 and RSBK-2) and was detected and identified using a 2 step (nested) PCR (Suzuki et al. 1998; Kitamura et al. 2000, 2002). However, the virulence of this isolate against P. fucata was weak suggesting that this marine birnavirus is an opportunistic pathogen (Kitamura et al. 2002). Birnavirus infections have also been reported in clams.
Norton et al. (1993) reported a virus reminiscent of viruses in the family Papillomaviridae (referred to as Papovaviridae prior to 1999)in the golden lipped pearl oyster Pinctada maxima from the Torres Strait, northern Australia. Other bivalves including Crassostrea virginica and Mya arenaria were reported to be infected by papova-like viruses.
Geographic distribution
The "Akoya virus" was reported from western regions of Japan (Miyazaki et al. 1999). Virus-like particles associated with pathology have also been detected in Pinctada fucata from China (Miyazaki et al. 1999), Pinctada maxima from Torres Strait, Australia (Norton et al. 1993) and Pinctada margaritifera in French Polynesia (Comps et al. 1999, 2001).
Host species
Pinctada fucata, Pinctada fucata martensii (spat and adults), Pinctada margaritifera and Pinctada maxima. Crassostrea gigas and Chlamys nobilis cultured in close proximity to diseased P. fucata martensii in Japan also showed mass mortalities, muscular damage and the "Akoya virus" was isolated from them (Miyazaki et al. 1998).
Impact on the host
Infection with the "Akoya virus" was associated with mass mortalities (over 50% of the annual production in 1996 and 1997) among farmed Japanese pearl oysters, P. fucata, from 1994 to 1999 (Miyazaki et al. 1999). The marine birnavirus (MABV) isolated from symptomatic P. fucata by Suzuki et al. (1998) and determined to have weak pathogenicity was speculated to be activated under stressors such as the surgical operation to insert an artificial pearl nuclei, significant temperature fluctuations, etc. (Kitamura et al. 2000).
The exact nature and the role of virus-like particles observed in Pinctada margaritifera in French Polynesia are unknown because a virus has not been purified nor chemically characterized from diseases pearl oysters. Also, it has not been demonstrated that the "Akoya virus" infects P. margaritifera (Comps et al. 2001).
The virus reported by Norton et al. (1993) was detected in 2 Pinctada maxima with histological lesions in the labial palps out of a sample of 50 young adult (2 to 3 years old) wild caught pearl oysters. It is not known if infection causes mortalities among wild stocks. However, it is possible that this apparent virus may become more pathogenic to P. maxima under intensive culture conditions or cross-infect other hosts and produce virulent disease (Norton et al. 1993). This disease has similarities to viral gametocytic hypertrophy in C. virginica and the papova-like viral infection in the gill epithelium of M. arenaria (Farley 1976, 1978; Harshbarger et al. 1977 (1979); Norton et al. 1993; Elston 1997).
Diagnostic techniques
Gross observations
Poor growth as indicated by the edge of the shell and sluggish closing of the valves when Pinctada fucata martensii was disturbed by touching the lip of the mantle lobe. Atrophy of the adductor muscle, mantle lobes (which were transparent) and the body accompanied by yellowish to brown discolouration (Miyazaki et al. 1998, 1999). Diseased P. margaritifera in French Polynesia had an unusual secretion of mucus and displayed grossly visible abscesses in the adductor muscle (Comps et al. 1999, 2001).
Histology
In Pinctada fucata martensii from Japan, necrosis (including karyopyknosis, karyorrhexis and marginal hyperchromatosis) and degeneration (atrophy and vacuolization) of muscle fibres were reported. Haemocyte (many agranulocytes and granulocytes) infiltration into the adductor muscle and musculature of the foot, mantle lobe, heart and gills was observed. In severe cases, the perimysium (a fibrous tissue that encloses the adductor muscle) also contained many haemocytes and necrotic cells. The Periodic Acid Schiff (PAS) reaction indicated decreased levels of glycogen in the severely damages muscle fibres. No Feulgen-positive inclusions were observed suggesting that the putative virus had RNA (Miyazaki et al. 1999). In pearl oysters that survived the infection, necrotized muscle fibres are replaced by dense fibrous tissues that develop between the thinned muscle fibres which remained in the lesion (Miyazaki et al. 2000).
In diseased Pinctada margaritifera in French Polynesia, lesions in the adductor muscle were similar to those described in P. fucata. At the periphery of the lesion, the muscle was infiltrated by haemocytes with a progressive increase in haemocyte concentrations and alteration of muscle structure towards the centre. The centre of the lesion was occupied by a granulomatous tissue consisting of granulocytes, macrophages and cellular debris. Internal areas of dense connective tissue including muscle fibre debris in some pearl oysters suggested the occurrence of a healing process (Comps et al. 1999, 2001).
In diseased Pinctada maxima from Torres Strait, Australia, lesions were confined to the ciliated columnar epithelium of the palps inner surface. Infected cells had massive hypertrophy of the nucleus (up to 7X) and little to no cytoplasm. In affected nuclei, the nuclear chromatin was emarginated and the central area contained an amorphous eosinophilic mass (with haematoxylin and eosin stain) that was Feulgen-positive. In many cases the central mass (called an inclusion) was separated from the peripheral nuclear chromatin by a clear zone (Norton et al. 1993).
Electron microscopy
In Pinctada fucata martensii from Japan, the sarcoplasm of necrotic muscle fibres of the adductor muscle contained membranous inclusion bodies within electron-dense debris. The inclusion bodies contained many round, non-enveloped, viral particles (25 to 33 nm in diameter). In addition, the infected muscle fibres had thinned or fragmented myofibrils, vacuoles in the sarcoplasm, mitochondria with destroyed cristae, expanded reticula and almost no glycogen granules (Miyazaki et al. 1999). In the adductor muscle, healed lesion contained cells that actively produced many microfibrils within the cytoplasm (Miyazaki et al. 2000).
The virus-like particle from diseased P. margaritifera in French Polynesia had a diameter of 40 nm with a membrane like envelope coating an electron dense core about 35 nm in diameter (Comps et al. 1999). Within the altered muscle of P. margaritifera, numerous interstitial or glial cells containing electron-dense and membrane bound ovoid bodies appeared in association with muscle fibres and connective tissue which could be related to the process of regeneration of muscle tissue (Comps et al. 2001).
In diseased Pinctada maxima from Torres Strait, Australia, non-enveloped icosahedral virus-like particles (about 60 nm in diameter) were observed in the central mass (inclusion) of hypertrophied nuclei (Norton et al. 1993).
Immunological assay
A standard indirect fluorescent antibody technique (IFAT) using the anti-Y-6 virion antibody as the primary antibody was used as described by Kitamura et al. (2000). This assay was employed to determine the organs in which the viral specific antigens were expressed.
DNA probes
Two step polymerase chain reaction (PCR) assays were performed on samples from Pinctada fucata from the Uwa Sea, Ehime Prefecture, Japan as described by Kitamura et al. (2000). Nucleotide sequencing was conducted on a few PCR positive samples and resulting sequences were compared to that of strains isolated from P. fucata by Suzuki et al. (1998), isolates from the clam Sinonovacula (=Sinonovacura) constricta and a marine fish from south west Japan. Molecular comparison between the isolates indicated that all were highly homologous in the VP2/NS junction leading to the conclusion that the virus isolated from P. fucata belonged to the marine birnavirus (MABV) group (Kitamura et al. 2000).
Culture
Miyazaki et al. (1998, 1999) indicated that they could culture the "Akoya virus" in finfish cell lines (EK-1 from eel kidney and EPC from epithelioma papilosum cyprini). Akoya pearl oysters experimentally inoculated into the lip of the mantle lobe with the cultured virus showed pathological signs of the disease, marked mortalities and the virus was isolated from the adductor muscle of the moribund and dead oysters (Miyazaki et al. 1998).
Suzuki et al. (1998) and Kitamura et al. (2000. 2002) isolated a birnavirus in the genus Aquabirnavirus from Pinctada fucata on finfish cell lines (CHSE-214 and RSBK-2).
Methods of control
No known methods of prevention or control. Because the causative agent(s) is/are still unknown, and because the pearl oysters are cultured in open sea, there is no efficient control method available (Wada 2003). Miyazaki et al. (1999) proposed that the "Akoya virus" were harboured by juvenile oysters during the winter. These infected oysters then became diseased on the return of optimum water temperatures (above 25°C) and served as a source for horizontal transmission of the infection and resulting mass mortalities. Anecdotal information suggested that short durations (less than 1 month) of high temperatures (above 25°C) during the summer resulted in lower mortality of oysters with "Akoya virus" infections (Miyazaki et al. 2000). Thus, moving the pearl oysters into areas where the water temperature is lower during summer to autumn months may reduce the mortality from this disease. Norton et al. (1993) indicated that the difficulty of detecting the viral infection in P. maxima from Torres Strait, Australia would significantly reduce the efficacy of health certification testing of oysters for translocation. Transplantation of the pearl oysters from areas where the existence of a virus disease is suspected, should be prohibited (Wada 2003).
Miyazaki et al. (2000) reported that an anti-viral, recombinant feline interferon-ω (rFeIFN-ω) administered by injection was efficacious in preventing mortality of P. fucata with "Akoya virus" infection for at least 30 days following challenge via inoculation with cultured viruses. Miyazaki et al. (2002) determined that this treatment was effective because haemocytes of P. fucata possessed receptors for binding the rFeIFN-ω, a mammalian (feline) interferon produced in silkworms by a recombinant baculovirus. However, prophylactic rFeIFN-ω administration was not completely effective in preventing mortality especially when viral invasions occurred several days after rFeIFN-ω administration (Miyazaki et al. 2000).
References
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Citation Information
Bower, S.M. (2022): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Viral Infections in Pearl Oysters.
Date last revised: December 2022
Comments to Susan Bower
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