Bonamia exitiosa (Bonamiasis of New Zealand Dredge Oysters)
Category 3 (Host Not in Canada)
Common, generally accepted names of the organism or disease agent
Microcell disease, Bonamiasis, Haemocyte disease of dredge oysters.
Scientific name or taxonomic affiliation
Bonamia exitiosa (=exitiosus) (Berthe and Hine 2003) with close resemblance to Bonamia ostreae a pathogen of European flat oysters Ostrea edulis. Apparently, B. exitiosa and Bonamia roughleyi are genetically very similar (Abollo et al. 2008, Hill et al. 2010). Hill et al. (2010) grouped these two named species as the B. exitiosa/B. roughleyi clade and indicated that justification for drawing species boundaries among the primarily austral microcells with affinities to B. exitiosa and B. roughleyi remains elusive.
Foveaux Strait and other locations around South Island, New Zealand including Golden Bay, Tasman Bay, the Marlborough Sounds and Wellington Harbour (Hine 1997). More recently, B. exitiosa was detected in Ostrea edulis from the Galician coast (NW Spain) and the Manfredonia Gulf, Italy (Adriatic Sea) including concurrent infections with Bonamia ostreae (Abollo et al. 2008, Narcisi et al. 2010). Similar parasites were reported from Ostrea angasi in Australia (Hine and Jones 1994); Ostrea puelchana in San Antonio Bay, Argentina (Kroeck and Montes 2005); Ostrea equestris in North Carolina, USA (Carnegie et al.2005); and Cassostrea ariakensis experimentally introduced into North Carolina, USA (Burreson et al. 2004). Bonamia sp. was also reported from Ostrea chilensis from Chile (Kern 1993, Campalans et al. 2000). Based on genetic (parsimony) analysis, White et al. (2008) suggested that a single Bonamia species which resembles B. exitiosa may occur in New Zealand, Australia, Argentina and North Carolina. Ostrea aupouria from New Zealand and Ostrea stentina from Tunisia were infected with Bonamia spp. determined to be closely related to B. exitiosa and Bonamia roughleyi based on the genetic sequences of the small subunit (SSU) and internal transcribed space (ITS) rDNA genes (Hill et al. 2008, 2010).
Ostrea chilensis (=Tiostrea chilensis, =Tiostrea lutaria), Ostrea edulis and probably Ostrea angasi based on the similarity of the molecular sequence of the 18S-ITS-1 region of the SSU rRNA operon (Corbeil et al. 2006b). However, the pathology associated with infection in O. chilensisand O. angasi is very different (Hine 1996b). In Spain, B. exitiosa DNA was detected in tissues of Crassostrea gigas highlighting the possibility of this parasite to survive extracellularly and in other non-typical hosts (Lynch et al. 2010).
Impact on the host
Like Bonamia ostreae, this intrahaemocytic protozoan quickly becomes systemic with overwhelming numbers of parasites coinciding with the death of the oyster. In New Zealand, large-scale mortalities of native dredge oyster (91% between 1975 and 1992 in Foveaux Strait (Doonan et al. 1994, Cranfield et al. 2005)) have been attributed to this parasite. The fishery was closed in 1993 with severe economic impact on local communities. Investigations have shown that B. exitiosa was present in Foveaux Strait oysters in 1964, and a long standing association between host and parasite is suggested by a consistent annual pattern of infection (Hine 1991a,b). There is a seasonal variation in infection with the highest prevalence occurring in the austral autumn (April). Analysis of the epizootic event from 1986-1992 lead Cranfield et al. (2005) to propose that stressors increased the susceptibility of the oysters to an enzootic parasite.
On entry into the host, probably via the gut, B. exitiosa are phagocytosed by haemocytes but are not killed by these cells. Following parasite growth and division the host cell lyses, releasing less than 20 B. exitiosa which are in turn phagocytosed. Increased haemocyte production results in a haemocytosis at the expense of gametogenesis. lnfected haemocytes are initially observed in connective tissue, but as infection progresses they can be found in all tissues, with egress via the gonads, kidney, digestive tract, and gill, either by tissue leakage, haemocyte diapedesis, or host decomposition (Hine 1991a). Kidney and gonad infections occur mainly in April and May. Death may be caused by exhaustion of energy reserves as a result of increasing haemocyte production, rather than a toxic effect produced by the parasite (Hine 1997).
Stressors such as exposure to extreme temperatures (7 or 26 °C) and salinity (40 ‰), starvation (prolonged holding in filtered sea water), handling (vigorous stirring four time per day), or heavy infection with an apicomplexan can affect the disease dynamics of B. exitiosa in O. chilensis (Hine et al. 2002, Hine 2002). Co-habitation with infected oysters in holding tanks seems to promote the spread of infection (Hine et al. 2002).
Mass mortalities due to bonamiasis have not been observed since 1992, and oyster stocks are now recovering, but the parasite is still widespread at low intensity, similar to 1964 levels (Hine and Jones 1994). Lack of mortalities among oysters with low levels of infection may be due to host-parasite kinetics, rather than a change in parasite pathogenicity (Hine 1996a). Bonamia exitiosa transmission is direct and horizontal, and therefore high densities of oysters in closely spaced beds, and lack of resistance to infection, favour the development of epizootics. However, epizootics decrease stock densities markedly, and are more likely to kill off susceptible oysters rather than those with resistance or tolerance. Thus conditions that do not favour direct horizontal transmission, such as low density and increased resistance, are created, and lower parasite burdens do not cause disease or mortality (Hine 1997).
Smear: Make acetone- (or methanol-) fixed impression smears from heart tissue (preferably ventricle since the auricles contain an abundance of serous cells which make detection of the parasite difficult). Stain with Wright, Wright-Giemsa or equivalent stain (e.g. Hemacolor, Merck; Diff-QuiK, Baxter). Examine for 2-5 µm spherical or ovoid organisms with a central nucleus (fried egg appearance) within or outside the haemocytes. Note: The organisms are enlarged by this method compared to those in fresh or histological preparations. Diggles et al. (2003) determined that the examination of stained heart imprints appears to be the most time and cost effective method for screening large numbers of oysters in situations where reduced sensitivity may be tolerated but high specificity (reduced diagnosis of false positive results) is required.
Histology: Examine haematoxylin and eosin stained tissue cross-sections for haemocyte inclusions in the 2-3 µm size-range. B. exitiosa is distributed systemically in advanced infections. In early infections, B. exitiosa is often observed within haemocytes in focal infiltrations in the connective tissue of the gill and mantle, and in the vascular sinuses around the stomach and intestine. Histology appears most useful in epidemiological studies where detection of physiological state, other associated disease agents or pathological lesions is required (Diggles et al. 2003). Light microscopy can not be used to distinguish between species of Bonamia (Diggles et al. 2003). However, in Ostrea edulis infected with both B. exitiosa and B. ostreae, Abollo et al. (2008) found B. exitiosa to be larger (mean diameter = 2.8 µm, SE = 0.07 µm, range: 2–5 µm, N = 73) with a central nucleus (sometimes subcentral but rarely peripheral), compared to the smaller B. ostreae (mean diameter = 1.6 µm, SE = 0.04 µm, range: 1–2.5 µm, N = 55) with a peripheral nucleus and scant cytoplasm.
Electron Microscopy: The ultrastructure of B. exitiosa resembles other haplosporidians in the possession of haplosporosomes, haplosporogenesis, persistence of mitotic microtubules during interphase and of the nuclear envelope during mitosis, and occurrence of a diplokaryotic or multi-nucleate plasmodial stage. Unlike other haplosporidians, a stage containing a large vacuole derived from enlargement of one or more mitochondria has been observed in B. exitiosa. Plasmodial forms of B. exitiosa are distinguished from other developmental stages of this parasite by their size (4.0-4.5 µm), irregular cellular and nuclear outline, amorphous cytoplasmic inclusions (multi-vesicular bodies), and arrays of Golgi-like smooth endoplasmic reticulum. Forms of intermediate cytoplasmic density were more electron dense than the plasmodial forms and slightly smaller in diameter (3.0-3.5 µm). Haplosporosomes are formed from Golgi/nuclear material complexes and are similar in construction and structure to some viruses. Hine and Wesney (1992, 1994a) hypothesized that the haplosporosomes may represent an early viral element in eukaryotic cells. Dense forms can be used to differentiate between B. exitiosa and B. ostreae. Dense forms of B. exitiosa are less dense, slightly larger in size (3.0 ± 0.3 µm mean diameter n = 61 in comparison to B. ostreae with a mean diameter of 2.4 ± 0.5 µm, n = 64), have more haplosporosomes, mitochondrial profiles and lipoid bodies per ultrastructure section, and have smaller tubulovesicular mitochondria than B. ostreae. In addition, dense forms of B. ostreae lack nuclear membrane-bound Golgi/nuclear cup complexes and a vacuolated stage (Hine et al. 2001). Although Hine et al. (2001) presented ultrastructural differences between B. ostreae and B. exitiosa, Narcisi et al. (2010) found that the ultrastructural characteristics of B. exitiosa occurring in Italy were so variable that they cannot be used to definitively identify a Bonamia species.
DNA Probes: The sequence of the small subunit (SSU) ribosomal DNA gene of B. exitiosa has strong identity with that of B. ostreae but is sufficiently divergent to show polymorphism by restriction fragment length polymorphism (RFLP) analysis (by digesting polymerase chain reaction (PCR) product (amplified with primers BO and BOAS as described by Cochennec et al. (2000)) with Bgl1 (Promega)). The B. ostreae profile consisted of 2 bands of 120 and 180 base pairs (bp) while the B. exitiosa profile consisted of a unique band of 304 bp (Hine et al. 2001). Although the in situ hybridisation (ISH) assay outlined by Cochennec et al.(2000) also detected other species of haplosporidians (e.g., Haplosporidian nelsoni, see Carnegie and Cochennec-Laureau 2004), it appeared to be a better technique than PCR for screening small numbers of oysters when high sensitivity is required and when strict control over fixation and screening is possible (Diggles et al. 2003). For reliable ISH results, Diggles et al. (2003) recommended that samples not be fixed for longer than 48 hr in formalin (10% formalin in sea water) nor be transferred to 70% ethanol for prolonged periods of time after fixation. The optimisation of PCR for the specific detection of B. exitiosa will provide a sensitive diagnostic technique but other visual confirmation methods must be employed to rule out the possibility of false positives (Diggles et al. 2003). Recently, a real-time TaqMan PCR assay was developed for the detection of Bonamia spp. that was more sensitive than histology, was comparable to conventional PCR in sensitivity but produced more rapid results with a low risk of sample cross-contamination, and could be optimised to determine the intensity of infection (Corbeil et al. 2006a).
Methods of control
To date there are no known eradication or control procedures. Until the method(s) of transmission and host specificity of this parasite has been fully investigated, the movement of oysters out of endemic areas should be avoided. Hine (1996a) indicated that B. exitiosa can be transmitted directly from oyster to oyster and therefore disease spread is related to stocking density. Also, the apparent reduction in pathogenicity and/or selection for B. exitiosa tolerant stocks may explain the 20 to 30 year cycles of large mortalities experienced by O. chilensis in Foveaux Strait, New Zealand. Cranfield et al. (2005) proposed that mechanical disturbances of oysters by increasingly intense dredging and resulting modification of the benthic habitat was a major source of stress that correlated with the epizootic. They indicated that the recovery of the oyster population was closely linked to the regeneration of habitat. They also suggested that the fishery could be improved by mitigating mechanical disturbances during dredging by use of lighter dredges and less damaging towing strategies as well as pursuing rotational fishing strategies that allow benthic habitat to regenerate from undisturbed areas.
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Bower, S.M. (2011): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Bonamia exitiosa (Bonamiasis of New Zealand Dredge Oysters)
Date last revised: May 2011
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