Intracellular Bacterial Disease of Scallops

On this page

• Category
• Common, generally accepted names of the organism or disease agent
• Scientific name or taxonomic affiliation
• Geographic distribution
• Host species
• Impact on the host
• Diagnostic techniques
• Methods of control
• References
• Citation information

Category

Category 2 (in Canada and of regional concern)

Common, generally accepted names of the organism or disease agent

Intracellular bacterial disease.

Scientific name or taxonomic affiliation

Members in the genus Francisella are Gram-negative, nonmotile, pleomorphic coccobacilli (size c. 0.2–1.7 µm) and are generally known to be facultative intracellular parasites of a range of animals around the world (Sjöstedt 2005, Brevik et al. 2011).  Francisella halioticida, was originally described from farmed abalone (Haliotis gigantea) in Japan (Kamaishi et al. 2010, Brevik et al. 2011) and subsequently reported in farmed Yesso scallops (Mizuhopecten (=Patinopecten) yessoensis) in British Columbia, Canada (Meyer at al. 2017). The gross signs of disease and histopathology were like those associated with an unidentified intracellular prokaryote detected in farmed M. yessoensis from British Columbia during the late 1980s and 1990s (Bower et al. 1992). Recent studies indicated there are 2 phenotypically and genetically different types of F. halioticida; the isolate in Yesso scallops from Japan designated as the 'J-scallop type' to distinguish it from strains from abalone called the 'abalone type' (Kawahara et al. 2021).

Geographic distribution

In scallops, F. halioticida was detected and isolated from adductor muscle lesions of farmed M. yessoensis from British Columbia, Canada (Meyer et al. 2017) and from southern Hokkaido, Japan (Kawahara et al. 2018). Francisella species have also been identified as 1 of the predominant types of bacteria present in the adductor muscle of diseased M. yessoensis from a commercial scallop farm in Dalian, Liaoning Province, China (Yu et al. 2019).

Host species

Francisella halioticida was described from abalone (Haliotis gigantea and Haliotis discus discus) in Shimane Prefecture, Japan (Kamaishi et al. 2010, Brevik et al. 2011) and detected and isolated from Yesso scallops (M. yessoensis) from the west coast of Canada (Meyer et al. 2017) and southern Japan (Kawahara et al. 2018). More recently, F. halioticida was detected in mussels (Mytilus edulis, Mytilus galloprovincialis and their hybrids) in Normandy and northern Brittany, France (Charles et al. 2020). Charles et al. (2020) indicated that there was no recent aquatic species introductions into the French mussel culture areas from areas in Canada and Japan where F. halioticida was found. However, in early 1987, a hatchery in Brittany imported M. yessoensis for several aquaculture trials, which could be considered as a potential hypothetical connection (Charles et al. 2020). Cano et al. (2020) detected DNA segments of Francisella spp. in the blue mussel (M. edulis) from the United Kingdom, smooth clam (Callista chione) from Italy and Peruvian scallops (Argopecten purpuratus) from Chile.

Impact on the host

Although the cause of intracellular bacterial disease in farmed M. yessoensis on the west coast of Canada was unknown, Bower and Meyer (1991, 1994) and Bower et al. (1992) recognized the disease as causing pinkish-orange pustules in the soft body tissues (especially the adductor muscle). In addition, the disease was associated with shell damage (concholin deposits along the edge of the shell), poor growth and high mortalities. Almost 25 years later, Meyer et al. (2017) hypothesized that a very similar (if not the same) disease was caused by F. halioticida, a pathogen described as the causative agent of mass mortalities in farmed giant abalone (Haliotis gigantiea) in Japan (Kamaishi et al. 2010, Brevik et al. 2011). Soon after, Kawahara et al. (2018) reported that F. halioticida was the most probable cause of adductor muscle lesions in farmed M. yessoensis in Japan. Kawahara et al. (2018) also indicated that the occurrence of orange/pinkish abscess lesions associated with bacterial infections were known from northern Japan since the 1970s.

Parallel, independent laboratory experiments in Canada and Japan revealed that M. yessoensis challenged with F. halioticida via bath exposure resulted in high mortality and histological lesions characterized by massive haemocyte infiltration. The presence of F. halioticida was confirmed using PCR, and F. halioticida was re-isolated from a portion of dead and surviving specimens. Francisella halioticida was not detected in the control groups nor was any histopathology observed. These results fulfilled Koch's classic criteria for establishing disease causation and provided conclusive evidence that F. halioticida causes adductor muscle lesions and high mortality in M. yessoensis (Kawahara et al. 2019).

The disease caused by F. halioticida in M. yessoensis is characterized by the presence of visible lesions in the adductor muscle and histopathology consisting of focal granuloma-like lesions as well as multifocal or diffuse areas of intense haemocyte infiltration, primarily observed in the adductor muscle but can also occur in the connective tissue of all other organs (Meyer et al. 2017, Kawahara et al. 2018). Mortality rates as high as 85% have been reported with this disease; however, adductor muscle lesions and detection of F. halioticida was recorded from several culture sites in British Columbia where the prevalence of infection was very low (approx. 1%) and the disease did not appear to be having any significant impact (G. Meyer unpublished data). Recent studies have revealed the presence of 2 different strain types of F. halioticida (Kawahara et al. 2021); however, both strains are pathogenic in M. yessoensis (Kawahara et al. 2019).

Note that very similar looking disease conditions reported in Atlantic sea scallops, Placopecten magellanticus, harvested from the north-west Atlantic coast of the U.S.A. were associated with rod-shaped bacteria that stained acid-fast positive and Gram-positive and were identified as a Mycobacterium sp. using PCR methodology (Grimm et al. 2016). Grimm et al. (2016) also indicated that the a non-acid fast staining bacterium isolated from orange/pink nodular lesions in P. magellanticus from the Damarascotta area of Maine in October 2008 and May 2009 were found to have 99.5% DNA homology with Williamsia maris. Likewise in Japan, F. halioticida was not detected in adductor muscle lesions from all M. yessoensis (Kawahara et al. 2018). Thus, it appears that other bacterial species can cause similar looking lesions in the tissues of scallops.

Diagnostic techniques

Gross observations

Gross signs of the disease include pinkish-orange coloured lesions in the adductor muscle that often coincide with the presence of concholin deposits and shell erosion or damage.

Histology

Lesions occur in the connective tissues of all organs and vary in structure from irregular patches of intense haemocyte infiltration to form focal granuloma-like lesions, often with a core of necrosis, to encapsulated patches of haemocytes that usually contain necrotic cells. However, the tiny Gram negative coccobacilli are difficult to detect, requiring oil immersion lens (1000 x magnification) and the number of discernable bacteria is usually quite low relative to the size of the lesions (Meyer et al. 2017, Kawahara et al. 2018).

Electron microscopy

Depending on the stage of infection, F. halioticida -like prokaryotes can be observed within a few haemocytes associated with the lesions.

DNA probes

Two PCR assays have been developed for detection of F. halioticida:

1. primer pair Megai-60 and Megai-480r published by Kamaishi et al. (2010) which amplifies a 423 bp fragment of the 16S rDNA gene
2. primer pair Fh-rpoB/F and Fh-rpoB/R, which amplifies a 907 bp fragment of the DNA-directed RNA polymerase beta subunit (rpoB) gene published by Brevik et al. (2011).

Further details concerning PCR procedures and sequencing are detailed in Meyer et al. (2017) and Kawahara et al. (2018, 2019).

In situ hybridisation (ISH) protocols were based upon the procedures and probes published by Kamaishi et al. (2010). ISH gave strong positive signals in the central part of lesions with the accumulated necrotic cells in the adductor muscle of diseased M. yessoensis and less intense signals in peripheral regions of the lesions (Meyer et al. 2017; Kawahara et al. 2018, 2019).

In diseased M. yessoensis, ISH was used to determine that the adductor muscle was the most commonly infected organ followed by the digestive gland, gonad and heart, and in a few specimens in the mantle and gills (Meyer et al. 2017). In contrast to standard histopathology staining techniques, histological tissue sections stained using ISH procedures provide an enhanced method for visualization of F. halioticida infections (Meyer et al. 2017, Kawahara et al. 2018). However, the ISH probe cross-reacted (strong positive reaction) with Rickettsia-like prokaryotes (Meyer et al. 2017). Positive ISH results obtained from archived histology samples of farmed M. yessoensis from British Columbia collected during the early 1990s suggest that F. halioticida is not necessarily a new disease and has likely been present on the west coast of Canada for the past 30 years (Meyer et al. 2017). However, not all M. yessoensis from Japan with lesions in the adductor muscle tested positive for F. halioticida using PCR, suggesting the possibility that another pathogen may have been causing the M. yessoensis disease in Canada during the 1990s.

Culture

Material from lesions was inoculated on modified Eugon agar (MEA) prepared according to Kamaishi et al. (2010), which was supplemented with ampicillin and polymyxin. Plates were incubated at 15 °C for 12-17 days and resulting colonies were subcultures on MEA without antibiotics and incubated at 20 °C for 8 days. The identity of the resulting isolates were determined by molecular analysis using previously published primers designed to amplify the bacterial 16S rDNA gene and DNA–directed RNA polymerase beta unit (rpoB) for F. halioticida (Kawahara et al. 2018). Initially, the growth of the bacteria can be quite slow and require up to 15 days incubation at 15 °C to become visible. Colonies are typically white to slightly yellow in colour, have a smooth surface and tend to coalesce and be very sticky. A Gram-stained smear of the resulting colonies should reveal Gram negative coccobacilli approx. 0.5 to 1 µm in diameter. Nevertheless, the bacterial isolates should be further tested using PCR and sequence analysis to confirm the species identification.

Methods of control

No known methods of prevention or control. Initial detection and investigations into lesions occurring in M. yessoensis from 6 grow-out locations in southern British Columbia, Canada experiencing poor growth and mortalities in 1989 lead to the suggestion that the disease appeared to be related to sub-optimal culture conditions Getchell et al. (2016). Very little is known about what triggers an F. halioticida outbreak, but similar to other diseases; stressors, such as adverse environmental conditions, poor rearing conditions and handling etc., are precursors that can exacerbate infections and ultimately contribute to poor growth, lesions and mortalities.

Given the apparent broad host range of F. halioticida, biological characterisation and comparisons of isolates from various hosts will be required to establish preventive measures (Kawahara et al. 2018).

References

Bower, S.M. and G.R. Meyer. 1991. Disease of Japanese scallops (Patinopecten yessoensis) caused by an intracellular bacterium. Abstract. Journal of Shellfish Research 10(2): 513.

Bower, S.M. and G.R. Meyer. 1994. Causes of mortalities among cultured Japanese scallops, Patinopecten yessoensis, in British Columbia, Canada. In: Bourne, N.F., B.L. Bunting and L.D. Townsend (eds), Proceedings of the 9th International Pectinid Workshop, Nanaimo, B.C., Canada, April 22-27, 1993. Canadian Technical Report of Fisheries and Aquatic Science 1994: 85-94.

Bower, S.M., J. Blackbourn, G.R. Meyer and D.J.H. Nishimura. 1992. Diseases of cultured Japanese scallops (Patinopecten yessoensis) in British Columbia, Canada. Aquaculture 107: 201-210.

Brevik, Ø.J., K.F. Ottem, T. Kamaishi, K. Watanabe and A. Nylund. 2011. Francisella halioticida sp. nov., a pathogen of farmed giant abalone (Haliotis gigantea) in Japan. Journal of Applied Microbiology 111: 1044-1056.

Cano, I., D. Ryder, S.C. Webb, B.J. Jones, C.L. Brosnahan, N. Carrasco, B. Bodinier, D. Furones, T. Pretto, F. Carella, B. Chollet, I. Arzul, D. Cheslett, E. Collins, K.B. Lohrmann, A.L. Valdivia, G. Ward, M.J. Carballal, A. Villalba, I. Marigómez, S. Mortensen, K. Christison, W.C. Kevin, E. Bustos, L. Christie, M. Green and S.W. Feist. 2020. Cosmopolitan distribution of Endozoicomonas-like organisms and other intracellular microcolonies of bacteria causing infection in marine mollusks. Frontiers in Microbiology 11 (2778): Article 577481, 22 pp.

Charles, M., A. Villalba, G. Meyer, S. Trancart, C. Lagy, I. Bernard and M. Houssin. 2020. First detection of Francisella halioticida in mussels Mytilus spp. experiencing mortalities in France. Diseases of Aquatic Organisms 140: 203-208.

Getchell, R.G., R.M. Smolowitz, S.E. McGladdery and S.M. Bower. 2016. Diseases and parasites of scallops, In: Shumway, S.E., G.J. Parsons (eds.) Scallops: Biology, Ecology, Aquaculture, and Fisheries. Elsevier Science, Oxford, pp. 425-468 (see pg. 437).

Grimm, C., C. Huntsberger, K. Markey, S. Inglis and R. Smolowitz. 2016. Identification of a Mycobacterium sp. as the causative agent of orange nodular lesions in the Atlantic sea scallop Placopecten magellanicus. Diseases of Aquatic Organisms 118: 247-258.

Kamaishi, T., S. Miwa, E. Goto, T. Matsuyama and N. Oseko. 2010. Mass mortality of giant abalone Haliotis gigantea caused by a Francisella sp. bacterium. Diseases of Aquatic Organisms 89: 145-154.

Kawahara, M., M. Kanamori, G.R. Meyer, T. Yoshinaga and N. Itoh. 2018. Francisella halioticida, identified as the most probable cause of adductor muscle lesions in Yesso scallops Patinopecten yessoensis cultured in Southern Hokkaido, Japan. Fish Pathology 53 (2): 78-85.

Kawahara, M., G.R. Meyer, G.J. Lowe, E. Kim, M.P. Polinski, T. Yoshinaga and N. Itoh. 2019. Parallel studies confirm Francisella halioticida causes mortality in Yesso scallops Patinopecten yessoensis. Diseases of Aquatic Organisms 135: 127-134.

Kawahara, M., K. Yoshitake, T. Yoshinaga, and N. Itoh. 2021. Francisellosis of Yesso scallops Mizuhopecten yessoensis in Japan is caused by a novel type of Francisella halioticida. Diseases of Aquatic Organisms 144: 9-19.

Meyer, G.R., G.J. Lowe, S.R. Gilmore and S.M. Bower. 2017. Disease and mortality among Yesso scallops Patinopecten yessoensis putatively caused by infection with Francisella halioticida. Diseases of Aquatic Organisms 125: 79-84.

Sjöstedt, A.B. 2005. Family III. Francisellaceae fam.nov. In: Brenner, D.J., N.R. Krieg, J.T. Staley, G.M. Garrity and others (eds). Bergey's manual® of systematic bacteriology, Vol 2. The Proteobacteria: Part B, the Gammaproteobacteria. Springer US, Boston, MA, p 199−210.

Yu, Z., C. Liu, Q. Fu, G. Lu, S. Han, L. Wang and L. Song. 2019. The differences of bacterial communities in the tissues between healthy and diseased Yesso scallop (Patinopecten yessoensis). AMB Express 9: 148, 13 pp.

Citation information

Bower, S.M., Meyer, G.R. (2021): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Intracellular Bacterial Disease of Scallops.

Date last revised: March 2021

Comments to Susan Bower