Mikrocytos spp. of Clams
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Category
Category 1 (not reported in Canada)
Common, generally accepted names of the organism or disease agent
Mikrocytosis, Microcell disease of clams.
Scientific name or taxonomic affiliation
Two species of Mikrocytos, M. veneroïdes and M. donaxi were described from the wedge clam Donax trunculus from the Atlantic coast of France (Garcia et al. 2018). Another unidentified Mikrocytos–like parasite was detected in the Manila clam Venerupis (=Ruditapes) philippinarum on the coast of Galicia, Spain (Ramilo et al. 2014). Garcia et al. (2018) indicated that this Mikrocytos–like parasite from Spain shared a homologous 18S fragment with Mikrocytos mimicus described in the Pacific oyster Crassostrea gigas being farmed on the Norfolk coast, UK. (Hartikainen et al. 2014). Additional research into the Mikrocytos-like protists is required to understand the taxonomic relationships between species of Mikrocytos (Abbott et al. 2012). Abbott and Meyer (2014) and supported by Garcia et al. (2018) suggest that minimally new species descriptions for Mikrocytos incorporate 18S-rDNA sequence data as well as histopathological and host information. They also indicated that the inclusion of electron microscopy information in taxonomic descriptions of Mikrocytos is desirable but not feasible when the prevalence and intensity of infections are low.
Mikrocytos spp. belong to the eukaryotic supergroup Rhizaria (Burki et al. 2013). Hartikainen et al. (2014) proposed a new family (Mikrocytiidae) and new order (Mikrocytida) within the class Ascetosporea (Rhizaria: Cercozoa) for Mikrocytos spp. and Paramikrocytos canceri a sister linage that they described from the European edible crab Cancer pagurus. The best known species in Mikrocytos are parasites of oysters and are often referred to as microcells. Note that Mikrocytos spp. are not related to microcells in the genus Bonamia (e.g., B. ostreae, B. exitiosa, B. roughleyi and other Bonamia spp.) which are now known to be within the sister taxon (order) Haplosporidia of the Rhizaria (Carnegie and Cochennec-Laureau 2004; Abbott et al. 2012, 2014; Abbott and Meyer 2014; Hartikainen et al. 2014).
Geographic distribution
Mikrocytos veneroïdes was detected in Donax trunculus (wedge clams) from different wild beds along the Atlantic coast of France (Garcia et al. 2012), specifically, Oléron Island, Quiberon Bay and Douarnenez Bay (Garcia et al. 2018). Mikrocytos donaxi was detected in D. trunculus only from wild beds in Audierne Bay, on the south coast of Brittany, France (Garcia et al. 2018). Carella et al. (2019) detected a microcell-like parasites at low prevalence in focal areas of the muscle, connective tissue and intracellular in haemocytes of D. trunculus from Litorale Domitio, Campania region, Italy. With the light microscopy techniques that they used, they were not able to determine if the microcells were species of Bonamia or Mikrocytos but they speculated that the pathogen was probably a species of Mikrocytos as described from D. trunculus in France (Carella et al. 2019).
The Mikrocytos-like parasite was detected in V. philippinarum (Manila clam) from 4 sites along the coast of Galicia, Spain, specifically from natural bed located in Camariñas and from 3 farm sites located in Espasante (Ría de Ortigueira), Abanqueiro and Carril (Ría de Arousa) (Ramilo et al. 2014).
Host species
Mikrocytos veneroȉdes and M. donaxi were associated with mortality events in the wedge clam Donax trunculus in France (Garcia et al. 2018). Mikrocytos-like parasite (as well as brown ring disease) was associated with mortalities in the Manila clam Venerupis (=Ruditapes) philippinarum from Galicia, Spain (Ramilo et al. 2014).
Impact on the host
The detection of Mikrocytos spp. in clams were all associated with mortality events. In France in 2010, Mikrocytos spp. were detected in D. trunculus from 3 locations (Quiberon Bay, Oléron Island and Audierne Bay) in association with mortality rates estimated between 70 and 80% and in 2011 at 2 locations (Audierne Bay and Douarnenez Bay) with 50 to 60% mortality estimates (Garcia et al. 2018). In addition, Garcia et al. (2018) reported microcells in archived material of D. trunculus from a 50% mortality event in September 2008 in Quiberon Bay. The Mikrocytos sp. in Audierne Bay was identified as M. donaxi and M. veneroȉdes was identified at the other 3 locations (Garcia et al. 2018). However, Garcia et al. (2018) indicated that experimental infections and field surveys will be necessary to confirm the pathogenic role of M. donaxi and M. veneroȉdes in mortality events of D. trunculus.
In Galicia (NW Spain), high mortality of V. philippinarum were claimed (but not technically certified) by clam farmers in the late 2007 and early 2008 from 4 beds located in Camariñas, Espasante, Abanqueiro and Carril (Ramilo et al. 2014). Samples collected at the time of a mortality event in all 4 locations contained Mikrocytos-like parasites that ranged in prevalence from 73 to 93% in surface clams and from 3 to 33% in buried clams. Surface clams showed either lower condition index or lower weight than buried clams while differences in size (length) were not significant, However, the detection of brown ring disease signs (BRD) in clams from every bed prevented making the assumption that the microcells alone were responsible for the clam mortalities because BRD is potentially lethal for clams (Ramilo et al. 2014). Ramilo et al. (2014) also indicated that only 2 out of about 500 clams from the most productive commercial clam beds in Galicia analysed in 2009 showed microcells, but with low intensity, and since then microcells have not been detected in Galician clams by histology.
Diagnostic techniques
Gross observations
Moribund D. trunculus were characterized by a very slow closing of their valves and a limited intrapalleal fluid. Infected clams showed no specific macroscopic signs and in particular, no signs such as pustules, conchyolin deposit or necrosis and degradation of the hinge (Garcia et al. 2018). No specific macroscopic signs of microcell infection were noted in V. philippinarum.
Histology
High power (1000x oil immersion magnification) microscopic examination is required to view these tiny (1.79–3.8 μm in diameter) intracellular microcell parasites in histological preparations of infected tissue. Mikrocytos donaxi and M. veneroȉdes were found distributed throughout the tissues of D. trunculus. Infected wedge clams had several focal infections distributed in different organs, but in most cases, the infection was focal and of low intensity for any given organ. In all cases, no major haemocyte infiltration or abscess was found associated with the presence of these microcells but they were associated with adjacent tissue necrosis (Garcia et al. 2018). The Mikrocytos-like parasite in V. philippinarum was observed in the connective tissue cells of most organs including the adductor muscle, foot, mantle, gills, siphon and visceral mass. Haemocytic infiltration was observed in the tissue areas where microcells occur, which caused disorganization of muscle fibres in the adductor muscle, foot and siphon. Some of these microcells were in haemocytes and others were extracellular (Romilo et al. 2014).
Electron microscopy
Ultrastructural morphology characteristics were included in the species descriptions of M. veneroȉdes and M. donaxi (Garcia et al. 2018). Overall, the general characteristics of both species were similar to other Mikrocytos spp. including the absence of haplosporosomes and canonical mitochondria and close association with the mitochondria of the host cell. The different uninucleate stages (quiescent, endosomal and vesicular cells) described by Hine et al. (2001) for Mikrocytos mackini were observed in M. veneroȉdes but not M. donaxi possibly because of the dark colouration of M. donaxi in the examined samples (Garcia et al. 2018). Additional particular traits of M. donaxi in comparison to M. veneroȉdes was its relatively smaller size (2.52 ± 0.18 by 1.76 ± 0.13 μm compared to 2.73 ± 0.23 by 2.08 ± 0.18 μm for M. veneroȉdes) and the nucleus of M. donaxi seemed more frequent in eccentric position than in M. veneroïdes (Garcia et al. 2018). Garcia et al. (2018) also noted that in rare cases ("two parasite cells from Quiberon Bay and Audierne Bay") a host cell mitochondrion appeared to be tight against the surface (or inside) of a microcell.
DNA probes
Some of the pairs of molecular primers designed by Carnegie et al. (2003), Gagné et al. (2008), Abbott et al. (2011) and Hartikainen et al. (2014) to amplify segments of the ribosomal gene complex of Mikrocytos spp. in a polymerase chain reaction (PCR) assay were used to amplify segments of the Mikrocytos spp. in clams (Ramilo et al. 2014, Garcia et al. 2018). The resulting products were sequenced and phylogenetic analysis conducted to compare the sequences with those available in the GenBank database for other members of the family Mikrocytiidae. Garcia et al. (2018) determined that the parasites detected in D. trunculus in France were different from other described species of Mikrocytos and confirmed the existence of two species of Mikrocytos in D. trunculus in France. Ramilo et al. (2014) obtained a consensus sequence of 1670 bp of the ribosomal gene complex of the microcells from V. philippinarum in Spain which contained a section of the 18S region, the whole first internal transcribed spacer, the 5.8S region, the second internal transcribed spacer and a section of the 28S region. They determined that this sequence was divergent from those of M. mackini infecting Crassostrea gigas in the Northeastern Pacific Ocean and Mikrocytos sp. infecting Ostrea edulis in the North Atlantic Ocean (Ramilo et al. 2014).
Garcia et al. (2018) applied the in situ hybridization (ISH) technique using a digoxigenin-labelled probe, produced by PCR from the Mikrocytos genus-specific primer pairs (Msp 443F and Msp746R) developed by Gagné et al. (2008), to confirm the presence of the microcells in D. trunculus. Eleven tested D. trunculus showed specific labelling of parasite cells (with the Msp probe) in the vesicular connective tissue and muscular fibres of all organs. The infection intensity seemed higher than in the histological observations, with a diffuse distribution of the parasites in all organs (Garcia et al. 2018).
Methods of control
Bivalves from infected areas (currently or historically) should not be moved for grow-out to areas where Mikrocytos spp. have not been recorded. For some species, such as the Manila clam V. philippinarum that is farmed in many countries worldwide where frequent transfers occur for farming purposes, extra care is required to avoid spreading the Mikrocytos-like parasites to new areas (Ramilo et al. 2014). However, further investigations are necessary to confirm the lineage of Mikrocytos species, their distribution and pathogenic role and their disease epidemiology (Garcia et al. 2018).
References
Abbott, C.L. and G.R. Meyer. 2014. Review of Mikrocytos microcell parasites at the dawn of a new age of scientific discovery. Diseases of Aquatic Organisms 110: 25-32.
Abbott, C.L., S.R. Gilmore, G. Lowe, G. Meyer and S. Bower. 2011. Sequence homogeneity of internal transcribed spacer rDNA in Mikrocytos mackini and detection of Mikrocytos sp. in a new location. Diseases of Aquatic Organisms 93: 243–250.
Abbott, C.L., N. Corradi, G. Meyer, F. Burki, S.C. Johnson and P. Keeling. 2012. Multiple gene segments isolated by next-generation sequencing indicate extreme divergence of Mikrocytos mackini. Journal of Shellfish Research 31: 257. (Abstract).
Abbott, C.L., G.R. Meyer, G. Lowe, E. Kim and S.C. Johnson. 2014. Molecular taxonomy of Mikrocytos boweri sp. nov. from Olympia oysters Ostrea lurida in British Columbia, Canada. Diseases of Aquatic Organisms 110: 65-70.
Burki, F., N. Corradi, R. Sierra, J. Pawlowski, G.R. Meyer, C.L. Abbott and P.J. Keeling. 2013. Phylogenomics of the intracellular parasite Mikrocytos mackini reveals evidence for a mitosome in Rhizaria. Current Biology 23: 1541-1547.
Carella, F., N. Carrasco and G. De Vico. 2019. Baseline pathological data of the wedge clam Donax trunculus from the Tyrrhenian Sea (Mediterranean Basin). Diseases of Aquatic Organisms 133: 107-118.
Carnegie, R.B. and N. Cochennec-Laureau. 2004. Microcell parasites of oysters: recent insights and future trends. Aquatic Living Resources 17: 519-528.
Gagné, N., N. Cochennec, M. Stephenson, S. S. McGladdery, G.R. Meyer and S.M. Bower. 2008. First report of a Mikrocytos-like parasite in European oysters Ostrea edulis from Canada after transport and quarantine in France. Diseases of Aquatic Organisms 80: 27-35.
Garcia, C., I. Arzul, J.P. Joly, B. Guichard, B. Chollet, E. Omnes, C. Haond, M. Robert, C. Lupo and C. Francois. 2012. Mikrocytos like protozoans and the shellfish Donax trunculus mortality events in France. Journal of Shellfish Research 31: 273. (Abstract).
Garcia, C., C. Haond, B. Chollet, M. Nerac, E. Omnes, J.-P. Joly, C. Dubreuil, D. Serpin, A. Langlade, D. Le Gal, A. Terre-Terrillon, O. Courtois, B. Guichard and I. Arzul. 2018. Descriptions of Mikrocytos veneroïdes n. sp. and Mikrocytos donaxi n. sp. (Ascetosporea: Mikrocytida: Mikrocytiidae), detected during important mortality events of the wedge clam Donax trunculus Linnaeus (Veneroida: Donacidae), in France between 2008 and 2011. Parasites & Vectors 11: 119, 16 pp.
Hartikainen, H., Grant D. Stentiford, Kelly S. Bateman, C. Berney, Stephen W. Feist, M. Longshaw, B. Okamura, D. Stone, G. Ward, C. Wood and D. Bass. 2014. Mikrocytids are a broadly distributed and divergent radiation of parasites in aquatic invertebrates. Current Biology 24: 807-812 and Supplemental Information.
Ramilo, A., D. Iglesias, E. Abollo, M. González, S. Darriba and A. Villalba. 2014. Infection of Manila clams Ruditapes philippinarum from Galicia (NW Spain) with a Mikrocytos-like parasite. Diseases of Aquatic Organisms 110: 71-79.
Citation information
Bower, S.M. (2021): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Mikrocytos spp. of Clams.
Date last revised: February 2021
Comments to Susan Bower
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