QPX, a Thraustochytrid-like Disease of Clams
Category 2 (In Canada and of Regional Concern)
Common, generally accepted names of the organism or disease agent
Quahog Parasite Unknown (QPX), Chytrid-like disease.
Scientific name or taxonomic affiliation
Organism of uncertain taxonomic position tentatively identified as an unusual member of the phylum Labyrinthulomycota, (= Labrinthomorpha) possibly in the family Thraustochytriidae (Maas et al. 1999). This affiliation was confirmed by analysis of the gene sequence of the18S rDNA component of the small-subunit ribosomal DNA (Ragan et al. 2000). Maas et al. (1999) suggested that QPX was a primitive member of the phylum because of the absence of typical sagenogenetosomes (sagenogens) and ectoplasmic nets in the clam host. However, Kleinschuster et al. (1998) reported the development of an ectoplasmic net in cultured QPX that had been transferred to sterile seawater. Members of the phylum Labyrinthulomycota have been placed in the monophyletic assemblage known as stramenophiles.
Initially reported from wild and hatchery stocks in the Gulf of St. Lawrence, Canada (Drinnan and Henderson 1963, Whyte et al. 1994). Currently reported in New Brunswick, Nova Scotia and Prince Edward Island, Canada and in Massachusetts, Connecticut, New Jersey, New York and Virginia, USA. In Virginia, QPX was not found in Mercenaria mercenaria from Chesapeake Bay but was present in cultured M. mercenaria from 3 coastal embayments (Ragone Calvo et al. 1998). In Connecticut, QPX was not considered to pose a threat to the hard clam (M. mercenaria) industry because of low prevalence (0.3% in 2358 clams from 77 different samples from along the shoreline monitored over a period of about 8 years) (Sunila 2006). Lyons et al. (2007) found no latitudinal gradient in QPX prevalence or frequency over its geographic distribution and suggested that local factors were important in determining its distribution. A morphologically similar parasite was reported from Ruditapes decussatus from Portugal (Azevedo and Corral 1997).
Mercenaria mercenaria and Mercenaria mercenaria variety notata. Results of investigations by Ford et al. (2002), Ragone Calvo and Burreson (2002), Ragone Calvo et al. (2007), and Smolowitz et al. (2008) indicate that different strains of M. mercenaria varied in their susceptibility to infection, disease and mortalities caused by QPX. Mercenaria mercenaria stocks/strains from southern pasts of the USA Atlantic coast (e.g., Florida) were more susceptible than those from more northern areas (e.g., Massachusetts and New York).
Impact on the host
QPX disease in Mercenaria mercenaria (quahog or hard clam) disrupts the connective tissue throughout the body and is associated with gross lesions (swellings and nodules) in the mantle of infected quahogs. Necrotic haemocytes indicate the possible production of a toxic substance with lytic activity. The in vitro cytotoxic affects of QPX on haemocytes from M. mercenaria matched the pathogenicity of QPX in quahogs (Perrigault and Allam 2008). Aggregations of QPX vegetative stages are usually surrounded by clear zones (mucoid substance). The mucoid material produced by QPX resists host antimicrobial activity that occurs in filter-sterilized M. mercenaria plasma (Perrigault et al. 2008a and b) and may prevent phagocytosis by quahog haemocytes. Thus, the mucoid secretions of QPX may represent an important virulence factor (Anderson et al. 2003).
Smolowitz et al. (1998) reported that QPX-infected M. mercenaria grew more slowly and had a lower condition index than uninfected quahogs. QPX appears to be most prevalent in cultured M. mercenaria, in quahogs being held in the hatchery or nursery, or occasionally in densely set natural populations. QPX was suggested to be the primary cause of significant wild quahog stock mortalities in New Brunswick in 1959 (Drinnan and Henderson 1963) but no mortalities attributed to QPX have occurred in wild stocks in Atlantic Canada since that time (Bacon et al. 1999, MacCallum and McGladdery 2000). QPX was associated with 80-90% mortalities in juvenile M. mercenaria (up to 30 mm in shell length) in a nursery and up to 100% in hatchery broodstock on Prince Edward Island (Whyte et al. 1994, Bacon et al. 1999). And QPX has caused severe mortality (80 to 95% in some instances) in aquacultured M. mercenaria stocks in Massachusetts (Fraser 1996, Smolowitz and Leavitt 1997, Smolowitz et al. 1998, Walton et al. 2008b), New Jersey (Ford et al. 2002) and the eastern coast of Virginia (Ragone Calvo and Burreson 2002). In the summer of 2002, QPX, emerged as a severe threat to the M. mercenaria fishery, causing significant quahog mortalities in New York waters. However, QPX has also been observed in apparently healthy wild adult populations from Atlantic Canada and at some locations in Virginia (McGladdery et al. 1993, Ragone Calvo et al. 1997, 1998).
While many aspects of the basic biology and epizootiology of QPX disease are still unknown, observations to date suggest that genetic variability in the host and/or in the QPX pathogen could be responsible for differences in susceptibility toward the infection and in the presentation of the disease (Ragone Calvo and Burreson 2002; Ragone Calvo et al. 2003a and b, 2007; Camara et al. 2004; Dahl et al. 2008). Laboratory studies indicate that QPX has a direct life cycle and QPX was directly transmitted between quahogs within 3 months of exposure (Smolowitz et al.2001). QPX appears to be widely distributed in the marine environment on the east coast of North America. It has been detected in almost all different types of environmental samples (water, sediment, algae, invertebrates, detritus) and in marine aggregates (i.e., marine snow) especially from coastal areas experiencing repeated disease outbreaks (Lyons et al. 2005, Liu et al. 2008, Gast et al. 2008a and b). Liu et al. (2008) suggested that sediments represent a natural reservoir for the parasite. Also, the occurrence of QPX-laden aggregates (i.e., marine snow) suggests a means for the spread and survival of pathogens between epizootics and provides a specific target for environmental monitoring of QPX (Lyons et al. 2005).
Mortality caused by QPX is usually most severe in the spring and summer months in M. mercenaria at least one year old in the eastern USA. For example, Liu et al. (2008) reported that the prevalence of QPX disease in quahogs in Raritan Bay, New York, increased during the spring and summer, peaked in August and declined in the fall. However, from a compilation of published reports, Lyons et al. (2007) found that QPX infections occurred throughout the year with no discernable seasonal trends in the prevalence or frequency of disease.
Gross Observations: Swellings and round yellow-tan nodules (1-5 mm in diameter) in the mantle, often at the mantle edge and close to or directly adjacent to the siphon or adductor muscle. The gills can also be infected. Other non-specific signs of infection include decrease in new shell growth, swollen, retracted, tan-coloured mantle edges, mucus and sand granules caught between the swollen mantle and shell edges and a high degree of chipping of the shell edge in quahogs from sandy locations. No gross signs of QPX infection, including lack of nodules in the mantle, have been observed in infected M. mercenaria in Canada (MacCallum and McGladdery 2000).
Squash Preparations: Vegetative cells within the nodules that may contain up to 40 daughter cells.
Histology: Abscesses or necrotic lesions containing various stages of vegetative and spore-like stages, commonly within halos of translucent (mucoid), non-staining, tissue usually observed in the mantle, gills and gonad. QPX has less frequently been detected in the connective tissue of the foot, labial palps, digestive gland, kidney, heart and adductor muscle. Dove et al. (2004) reported significantly larger numbers of parasites and higher biomass of QPX in visceral infections than in infections found only in the mantle. Three basic vegetative forms occur in the tissues: 1) thallus (trophozoite, single nucleated organism, 2-10 µm in diameter), 2) sporangium formed from a large thallus undergoing endosporulation - appears to lack a well defined membrane bound nucleus (10-15 µm in diameter) and 3) mature sporangia (18 to 25 µm in diameter) containing 20 to 40 endospores (immature thalli, 1.5 to 2 µm in diameter) each with a basophilic cell wall (Smolowitz et al. 1998, Ragone Calvo et al. 1998, Dove et al. 2004). All stages have a basophilic cell wall that varies in thickness and staining intensity and are usually surrounded by a cell free zone up to 8 µm thick with mucoid staining properties. When numerous parasites are present in a focal lesion, large lucent (mucoid) areas free of host cells were formed (Ragone Calvo et al. 1998). In some cases, the cell free zones showed evidence of stellar, possibly cytoplasmic, extrusions (mucofilamentous net) from the vegetative stages. Phagocytic multinucleate giant inflammatory cells of various sizes and containing 3 to 25 nuclei within the cytoplasm and haemocyte encapsulation of QPX occur as part of the quahog response to infection (Smolowitz et al. 1998) The haemocytic response was often associated with moribund looking QPX (Ragone Calvo et al. 1998).
Electron Microscopy: Maas et al. (1999) suggested that QPX is a more primitive member of the phylum Labyrinthulomycota because of the absence of typical sagenogenetosomes and ectoplasmic nets characteristic of most Labyrinthulomycota. Sagenogenetosomes were not detected by Kleinchuster et al.(1998) nor Smolowitz et al. (1998). However, Whyte et al. (1994) illustrated a sagenogenetosome-like structure near the surface of a thallus. Nevertheless, scale-like laminated cell walls characteristic of the phylum were observed on some endospores and occasional thalli. Also, sporangia containing numerous nuclei, which occur as a result of mitosis and are difficult to see via light microscopy, were obvious in electron micrographs.
Small subunit ribosomal DNA (SSU rDNA) sequences for several isolates of QPX have been deposited in GenBank (Ragone Calvo et al. 1998, Maas et al. 1999, Ragan et al. 2000 and Stokes et al. 2002). Polymerase chain reaction (PCR) primers specifically designed to amplify a 665 base pair segment of the SSU rDNA of QPX was able to detect QPX in infected quahog genomic DNA and did not amplify DNA of uninfected M. mercenaria nor DNA of other species of thraustochytrids that were tested (Stokes et al. 2001, 2002). This PCR assay was sufficiently sensitive to detect 20 fg QPX genomic DNA and 1 fg cloned QPX SSU rDNA. Field validation indicated that the PCR assay was equivalent to histological diagnosis if initially negative PCR products were reamplified (Stokes et al. 2002). The limited sensitivity of the PCR assay for this parasite may be attributed to the focal nature of infection (i.e., QPX is usually localised in the tissues of M. mercenaria and not dispersed throughout its host) thereby increasing the possibilities of sampling error. The PCR primers identified by Stokes et al. (2002) were used in a nested PCR protocol with products analysed by denaturing gradient gel electrophoresis (DGGE) (Lyons et al. 2005; Gast et al. 2006, 2008a). The PCR identification of QPX proved effective at detecting QPX and provided new and important information on potential exposure of quahogs to environmental sources of pathogenic organisms. However, this assay targeted the relatively conserved SSU rDNA region, limiting the differential specificity of the test because of the potential for cross-reaction with unidentified, related species. Subsequently, Lyons et al. (2006) developed a real-time quantitative polymerase chain reaction (qPCR) assay using primers designed to amplify a transcript identified from QPX differential display analysis (GenBank accession number DV942140). Application of the assay demonstrated positive qPCR results from naturally contaminated environmental samples including marine aggregates (i.e., marine snow), quahog pseudofeces, haemocyte aggregation nodules from infected quahogs and should provide a valuable tool for characterizing QPX parasite abundances in coastal environments and for improving quahog disease diagnostics (Lyons et al. 2006, Liu et al. 2008, Fitzsimons-Diaz et al. 2008).
A cocktail of two DNA probes for QPX was used to detect the parasite by in situ hybridisation (ISH) and these probes did not hybridize with tissues of M. mercenaria nor with several other species of thraustochytrids that were tested (Stokes et al. 2002, Gast et al. 2008a). The ISH assay has been utilized to detect QPX in marine aggregates (i.e., marine snow), collected from coastal embayments in Cape Cod, Massachusetts, USA where QPX outbreaks have occurred (Lyons et al. 2005). Qian et al. (2007) were not able to detect significant molecular genetic variations between isolates of QPX from an outbreak in Raritan Bay, New York, from the original outbreak in Massachusetts and from another outbreaks in Massachusetts when they assessed regions of the ribosomal DNA (small subunit or 18S rDNA, ITS1, 5.8S rDNA and ITS2) and four regions of mitochondrial genes (fragments of coxI, cob, nad1 and nad7).
Culture: QPX extracted from mantle tissue of infected M. mercenaria into sterile seawater containing antimicrobics were cultured in modified Eagle Minimum Essential Medium (MEM) supplemented with 10% heat-inactivated foetal bovine serum and antibiotics (Kleinschuster et al. 1998). All forms that were observed in infected M. mercenaria, developed in the cultures. As QPX proliferated in the culture media, it produced abundant mucoid material that bound the individuals together. Optimal culture conditions were a temperature of 22 °C, salinity of 28 ppt and pH 7 to 8 (Brothers et al. 2000). Upon transfer of the culture forms to sterile seawater, clusters of endospores adhered to the bottom of the culture flask and developed ectoplasmic nets. Within 1 to 4 days, zoospores formed from the adherent endospores (Kleinschuster et al. 1998). Cultures of QPX examined by Brothers et al. (2000) did not produce zoospores on transfer to seawater. Ragan et al. (2000) indicated that MEM culture with subsequent confirmation by microscopy was more sensitive in detecting QPX than standard histological methods. However, MEM will support the growth of a range of morphologically similar marine thraustochytrids and thus, additional assays such as PCR (see above) must be applied to the cultured organisms to confirm identification. In experimental trials, QPX from cultures and washed free of mucous were not infectious to quahogs by injection or bath exposure (Smolowitz et al. 2001) but infection could be achieved by using QPX cultured in sea water with macerated quahog tissue (Smolowitz et al. 2008). However, Dahl and Allam (2007) and Dahl et al. (2008) were able to infect M mercenaria in the laboratory by injecting QPX from laboratory maintained cultures into the pericardial cavity (83% infected 2 weeks post injection) and into the pallial cavity (18% infected 31 weeks post injection). Buggé and Allam (2005) developed a fluorometric microplate technique for the in vitro measurement of proliferation and viability of QPX. Perrigault et al. (2008a and b) used in vitro procedures to demonstrate that the plasma of M. mercenaria is able to alter the growth of QPX with plasma from QPX challenged quahogs and quahogs from New York having higher anti-QPX activity than plasma of quahogs from Florida. Alternately, QPX had variable in vitro cytotoxic affects on haemocytes from M. mercenaria which matched well with their in vivo pathogenicity of the quahog stocks assayed (Perrigault and Allam 2008).
Further Research: Techniques to identify the aetiologic agent to species and provide more specific diagnosis is needed. Differences in pathological presentation between infections of clams in Canada and those along the eastern United States, also requires more research.
Methods of control
Avoid importing stocks carrying this organism into areas currently free of QPX. However, QPX appears to be a ubiquitous member of the normal marine and bivalve flora on the east coast of North America and M. mercenaria disadvantaged in some way (e.g., unfavourable genotype-environmental interactions or stocks imported from southern locations) may be more susceptible to infection. Thus, QPX may be an opportunistic facultative parasite not dependant on a parasitic way of life. However, Perrigault et al.(2008d) reported that both salinity and temperature affect defence parameters and disease progression in M. mercenaria with higher QPX related mortalities occurring at higher salinities and higher temperatures. QPX may be limited to areas with salinities above 25 ppt (Ragone Calvo 1998).
Stress on cultured M. mercenaria associated with high planting densities and poor husbandry are believed to increase the risk of QPX disease problems (Ragone Calvo and Burreson 2002, Walton et al. 2008a). Lyons et al. (2007) reported a higher prevalence of QPX in M. mercenaria from cultured (farmed) beds than in wild populations with the highest prevalence in quahogs of intermediate size (20 to 55 mm in shell length). It may be possible to keep the incidence of disease under control through good plot husbandry and the removal of infected and dying quahogs (Gast et al. 2008a and b). In an extreme case with very high quahog mortalities associated with QPX, over 1 million quahogs were removed from culture beds in Wellfleet Harbour, Massachusetts during the winter which apparently prevented the spread of QPX disease (Walton et al. 2008b). Reduction of the stocking density was reported as effective in reducing mortalities to a negligible level. However, Kraeuter et al. (1998) reported no significant difference in the prevalence of QPX in juvenile M. mercenaria (<10 mm shell length) after 4 months at densities of up to 860 quahogs per square meter on intertidal and subtidal sites in New Jersey. Also, Ford et al. (2002) noted that although there was a significant trend towards higher QPX levels at higher planting densities, the considerable variability in the data made it difficult to determine the effect of density with a high degree of confidence. The dynamics of infection and pathogenicity under different holding and handling conditions require more investigation to manage QPX proliferation in cultured M. mercenaria (MacCallum and McGladdery 2000).
Ford et al. (1997) concluded that hatchery produced seed are an unlikely source of QPX, based on results of extensive surveys of M. mercenaria from 13 different hatcheries in six states of the USA over a 3 year period. Because strains of M. mercenaria seem to vary in susceptibility to QPX disease, growers in enzootic areas should consider the geographic origin of quahog seed as an important component of their QPX disease avoidance/management strategies (Ford et al. 2002; Ragone Calvo and Burreson 2002; Ragone Calvo et al. 2003a and b, 2007; Dahl et al. 2008; Walton et al. 2008a). Dahl and Allam (2008) and Dahl et al. (2010) indicated that the higher susceptibility of southern strains of M. mercenaria to QPX could not be attributed to cooler (winter) water temperatures as an aggravating factor and that aquaculture practices such as the selection of fast-growing stocks may exacerbate QPX disease problems. However, the identification of genes and other molecular markers in M. mercenaria that are associated with resistance to QPX could be used as biomarkers for the selection of quahogs for the development of strains resistant to QPX (Perrigault et al. 2008 a and c). Dahl et al. (2010) provided evidence for the utilization of local wild broodstock to enhance the resistance of cultured strains of M. mercenaria to QPX.
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Bower, S.M. (2010): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: QPX, a Thraustochytrid-like Disease of Clams.
Date last revised: April 2010
Comments to Susan Bower
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