Marine Bivalve Mollusks asReservoirs of Viral Finfish Pathogens:

Significance to Marine and Anadromous Finfish Aquaculture

THEODORE R. MEYERS

Introduction

Epidemiologists have long knownthat bivalve mollusks are capable ofharboring disease agents significant tohuman health. The mollusks ac­complish this phenomenon by filterfeeding, whereby seawater is pumpedin through the gills which entraplarger planktonic food organismswithin the cilia and mucus of therespiratory epithelium (Purchon,1977). As a result, these mollusks alsotake in smaller incidental particles in­cluding bacteria and viruses suspend­ed in ambient seawater or adsorbedonto the surfaces of organic and in­organic solids.

Consequently, human pathogensincluding coliform and Vibrio speciesof bacteria as well as polio andhepatitis viruses have been detected inbivalve species from clean and organi­cally polluted waters (Ross, 1956;Mitchell et aI., 1966; Hamblet et al.,1967; Liu et aI., 1967; Lovelace et aI.,1968; Kampelmacher et al., 1972;Ayres, 1975; Thompson and Vander­zant, 1976). Disease outbreaks causedby some of these agents have occurredin the human populace following in-

ABSTRACT-Filter-feeding bivalvemollusks can bioaccumulate variousdisease agents significant in the manage­ment ofpublic health. Recent evidence hasindicated that marine bivalves can harborviruses that are virulent for salmonidspecies. Examples offinfish viruses occur­ring in shellfISh tissues and those that havepotential of occurring are presented.

14

gestion of poorly cooked or rawshellfish from contaminated areas(Mason and McLean, 1962; Liu et aI.,1967; Sakazaki, 1969.

Recent evidence has shown thatmarine shellfish are potential reser­voirs for certain finfish pathogensthrough the same bioaccumulationprocess of filter feeding. This infor­mation has some significance whenconsidering finfish disease epi­zootiology in both marine andfreshwater finfish rearing systemshaving bivalve mollusks directly in thewater supply or immediately down­stream. Any epizootic occurringwithin a finfish population can resultin the release of high concentrationsof the pathogen into ambient waterfrom dead or dying fish. As an exam­ple, infectious hematopoietic necrosisvirus (IHNV) in hatchery effluentsduring acute disease outbreaks hasbeen reported at levels as high as 400p.f.u./ml (Leong and Turner, 1979).

Recorded epizootics in feral finfishpopulations, though uncommonwhen fish are normally dispersed,become more evident when fish areconcentrated in smaller areas as oc­curs during periods of spawning or

Participation of shellfish in theepizootiology of endemic and/or exoticfinfISh diseases is possible and should beconsidered for future management offishhealth. To reduce potential introductionsoffinfISh disease agents which may be in­cidental in shellfish tissues, certification ofbivalve stocks is advised before movementinto new waters.

smolt outmigration. As much as 1,600p.f.u./ml of IHNV has been reportedin fresh water near spawning sockeyesalmon, Oncorhynchus nerka,(Mulcahy et aI., 1983). Nearbyshellfish beds filtering water con­taminated by diseased finfish, wouldbe capable of entrapping the causativedisease agent. Filter feeding shellfishcan accumulate viruses and bacteriawithin their tissues at much higherconcentrations than present in sur­rounding seatwater (Mitchell et aI.,1966).

The actual concentration in thetissues is dependent upon: The rate atwhich bivalves pump water throughthe gills; the concentration andphysical characteristics of the microbein the ambient seawater; and seawatertemperature, salinity, and turbidity(Galtsoff, 1964; Hamblet et aI., 1967;Liu et aI., 1967). Eventually, inciden­tal contaminants are depurated or ex­pelled from the tissues back into theseawater. This virus or contaminantloss occurs over a period of time andis called the depuration rate. This rateis enhanced by exposure of theshellfish to waters free of the contami­nant and is proportional to the degreeof tissue contamination.

Other factors influencing thedepuration rate are the same as thosedescribed for bioaccumulation (Liu etaI., 1967). Consequently, depuration

Theodore R. Meyers is Assistant Professor ofFisheries, School of Fisheries and Science,University of Alaska, Juneau, 11120 GlacierHighway, Juneau, AI( 99801. Views or opinionsexpressed or implied are those of the author anddo not necessarily reflect the position of the Na­tional Marine Fisheries Service, NOAA.

Marine Fisheries Review

and bioaccumulation rates for a givencontaminant vary among species ofbivalve and within a species depend­ing upon animal size and environmen­tal parameters influencing animalphysiology. Hard clams, Mercenariamercenaria can depurate human poliovirus type I within 96 hours if placedin clean seawater at l3-15°C (Liu etaI., 1967). However, after 40-60 daysthe European flat oyster, Ostreaedulis, still cannot completelydepurate certain strains of molluscanlPN viruses, some of which arepathogenic to finfish (Hill and Alder­man, 1977; Hill et aI., 1982).

Whether a depurated disease agentremains infectious for its natural hostdepends upon its stability within theenvironment outside normal hosttissues. It is highly probable that bedsof filter feeding shellfish con­taminated with a stable, viable finfishpathogen could disseminate water­borne infectious particles to other fin­fish hosts during depuration.

The following discussion presentsexamples of finfish pathogens actual­ly isolated from bivalve mollusks andothers which have potential for bioac­cumulation in shellfish tissues. Theseagents are all viruses which can infectsalmonid fishes and at least one cen­trarchid species. However, other fin­fish species may also be susceptible tosome of these viral agents. There arebacterial agents (Vibrio sp., etc.) com­mon to bivalve tissues which arepathogenic for both finfish and othershellfish, but these will not be includ­ed herein (Tubiash et aI., 1965, 1970;Lovelace et aI., 1968; DiSalvo et aI.,1978; Leibovitz, 1978; Garland et aI.,1983).

Viral Finfish PathogensIsolated From Bivalves

13pz Reovirus

A new serotype of reovirus wasisolated from juvenile Americanoysters, Crassostrea virginica, held inrearing facilities of two differentLong Island, N.Y., shellfish hatch­eries (Meyers, 1979; Meyers andHirai, 1980). The virus, designated as

46(3)

13pz, is very stable, remaining viablein both seawater or oyster tissues forlonger than 60 days (Meyers, 1980). Itdoes not replicate in oyster tissues butis infectious for various fish cell linesand for at least two finfish species.

The virus produces a 44 percentmortality in bluegill, Lepomismacrochirus, fingerlings causingsevere necrotic hepatitis (Meyers,1980). It also produces chronic self­limiting granulomatous hepatitis andoccasional pancreatitis in adult andfingerling rainbow trout, Salmogairdneri (Meyers, 1983). Thus, forrainbow trout, 13pz is weakly virulentfor the age classes tested, but its effecton sac fry or on other salmonidspecies has not been examined. Insalmonid aquaculture, this virus maynot be significant as a primarypathogen but would contribute toundesirable stress and lowered hostresistance to other biological or en­vironmental pressures affecting fishsurvival.

JOV-l

A second reovirus-like agent, la­beled JOV-l, has been isolated fromJapanese oysters, C. gigas, reared in aJapanese hatchery (Nagabayashi andMori I). Little information is yetavailable, but apparently the virus isinfectious for lO-week-old rainbowtrout, producing a 40 percent mortali­ty in infected fish. Fifteen-week-oldtrout are reported refractory toclinical disease.

IPN MoUuscabirnaviruses

Infectious pancreatic necrosis virus(IPNV) was first islated from brooktrout, Salvelinus jontinalis (Snieszkoet aI., 1957), and has traditionallycaused disease in juvenile salmonids.However, other nonsalmonid strainsof lPNV have recently been isolatedfrom many other freshwater andmarine finfish species as reviewed byHill (1982). Many of these newisolates are not responsible for clinical

'Nagabayashi T., and S. Mori. 1983. XVI An­nual Meeting of the Society for InvertebratePathology, Cornell Univ., Ithaca, N.Y. Un­pub!. abstr.

disease in their respective hosts andare variable in their infectivity andvirulence for rainbow trout (Hill,1982). Included among these non­salmonid IPN viruses are strainswhich have been isolated from severalspecies of marine bivalves, two speciesof gastropod, and one decapodcrustacean (Hill, 1976a, b; Hill,1982). Such isolates are nearly allbiochemically, biophysically, andserologically (three exceptions repre­sent different serotypes) indistin­guishable from reference strains ofIPNV in fish. At least eight of theseshellfish isolates are pathogenic inrainbow trout fry producing typicalclinical signs of IPN disease and frymortality (Hill, 1982). Definitiveevidence is still lacking to substantiatewhether such molluscan IPN virusescan replicate in shellfish tissues or aremerely bioaccumualted contaminants.Regardless of this final determina­tion, the confirmed pathogenicity ofthese viruses for rainbow trout andtheir prolonged stability in shellfishtissues (~50 days; Hill and Alder­man, 1977) make them significantdisease agents to be considered in themanagement of anadromous salmo­nid health.

Viral Finfish PathogensHaving Potential To

Occur in Bivalves

There are other viral finfishpathogens which have prolongedviability within the natural environ­ment and could potentially ac­cumulate in nearby mollusk tissues ifreleased from clinically diseased orcarrier fish.

CSV

The chum salmon virus (CSV) isanother recently discovered reovirussimilar in many respects to the 13pzagent. However, CSV is a differentvirus (WintonZ

) which was isolated

'Winton, J. R. 1984. Department ofMicrobiology, Oregon State University MarineScience Center, Newport, Oreg. Unpub!. data.

15

from spawning chum salmon, On­corhynchus keta, caught in Japan.This virus can infect chum, chinook,0. tshawytscha; and kokanee, 0.nerka, salmon fingerlings, producinga self-limiting necrotic hepatitis (Win­ton et al., 1981). As with the 13p2agent, CSV remains viable for monthsoutside fish host cells. Thus, it couldpotentially accumulate in filter­feeding bivalve reservoirs near dis­eased fish hosts. This virus could besignificant in salmonid husbandry if itcaused similar host debilitation asdiscussed for the 13P2 agent.

IHNV

Infectious hematopoietic necrosisvirus (IHNV) is an extremely virulentpathogen of juvenile salmonids inwestern North America. Consequent­ly, this rhabdovirus is particularly im­portant to Pacific salmon, Oncorhyn­chus spp., and steelhead, S. gairdneri,husbandry in the Pacific Northwestand Alaska (Grischkowsky, 1981;Groberg et al., 1982, Groberg, 1983;Rohovec, 1983). Whether IHNVcould accumuJate in filter feedingmollusks is speculative, but physicallyand biologically possible.

It is clear that IHN virus can re­main infectious for several months infreshwater rivers (Wedemeyer et al.,1978; Mulcahy et al., 1983), and re­cent laboratory studies indicate de­tectable levels of virus after 27 and 22days in estuarine and saltwaters,respectively (Toranzo and Hetrick1982). It becomes conceivable thathigh concentrations of IHNV shed byinfected spawning salmon or out­migrating clinically diseased smoltscould accumulate in marine bivalvemollusks located directly below shortstream systems. Shellfish could thenparticipate in waterborne transmis­sion of the virus for a period of timeduring the depuration process.Although considerable dilution ofvirus concentrations would be involv­ed, evidence suggests that low levelsof infectious particles may be suffi­cient for transmission of the disease(Mulcahy et aI., 1983).

16

Salmonid IPNPiscibirnaviruses

Although infectious for salmonids,IPN molluscan birnaviruses are lessvirulent than the true salmonidstrains. However, these true salmonidstrains are also capable of survivingfor long periods of time in fresh,estuarine, and saltwaters (Ahne, 1982;Wedemeyer et al., 1978; Toranzo andHetrick, 1982). Consequently, theyare stable enough in the environmentto make possible their bioaccumula­tion and later dissemination by filterfeeding mollusks.

Conclusions

Marine bivalve mollusks are un­avoidable in estuarine and salt waterswhich may be used in the rearing ofanadromous fishes or which may bedirectly downstream of freshwater eggand fry facilities operating on shortwatersheds. No less abundant are themany freshwater species of filterfeeding clams and mussels whichpopulate certain stream and lake bot­toms around the world. The establish­ed occurrence of certain viral finfishpathogens in common species ofmarine bivalve mollusks indicates thatshellfish could serve two roles in theepizootiology of these disease agents,and possibly others: 1) Shellfish maybe seasonal reservoirs for someendemic finfish pathogens originatingfrom finfish epizootics and/or fromsubclinically diseased "carrier fish"; 2)shellfish could introduce exotic finfishdiseases when transported to otherwaters for commercial or experimen­tal purposes from areas where suchagents may be endemic.

In terms of practical fisherymanagement, there is little that couldbe done about involvement of marinebivalve mollusks in endemic finfishdisease epizootiology. However, reali­zation of their potential as diseasereservoirs would add a new dimensionto the future investigation of certainfinfish disease outbreaks.

Conversely, there are obvious pro­phylactic measures for reducing thepossible dissemination of exotic fin­fish diseases by shellfish introductions

into "new" waters. Specific inver­tebrate diseases, particularly those af­fecting commercially importantmarine bivalve mollusks, are becom­ing increasingly important regardingtheir geographic containment andcontrol. This concept is emphasizedby a growing recognition for the im­portance of health certification of im­ported and exported shellfish species(Rosenfield and Kern, 1978; Elston,1981; Elston3). Consequently, if cer­tification procedures are exercised forshellfish pathogens, some additionaleffort made in screening mollusktissues for incidental viral finfishpathogens would provide significantprotection against the introduction ofthese agents by movement of shellfishstocks.

Literature Cited

Ahne, w. 1982. Comparative studies on thestability of four fish-pathogenic viruses(VHSV, PFR, SVCV, IPNV). Zentralbl. Vet.Med. B 29:457-476.

Ayres, P. A. 1975. The quantitative bacteri­ology of some commercial bivalve shellfishentering British markets. J. Hyg., Cam.74:431-440.

DiSalvo, L. H., J. Blecka, and R. Zebal. 1978.Vibrio anguillarum and larval mortality in aCalifornia coastal shellfish hatchery. Appl.Environ. Microbiol. 35:219-221.

Elston, R. 1981. Discussion of shellfish cer­tification issues. Aquaculture 7(4): 28-33.

Galtsoff, P. S. 1964. The American oyster,Crassoslrea virginica Gmelin. U.S. Fish.Wildl. Serv., Fish. Bull. 64:1-4.

Garland, C. D., G. V. Nash, C. E. Sumner,and T. A. McMeekin. 1983. Bacterialpathogens of oyster larvae (Crassoslrea gigas)in a Tasmanian hatchery. Aust. J. Mar.Freshwater Res. 34:483-487.

Grischkowsky, R. S. 1981. Infectious hema­topoietic necrosis virus. In R. A. Dieterich(editor), Alaskan wildlife diseases, p.339-348. Univ. Alaska, Fairbanks.

Groberg, W. J., J r. 1983. The status of viralfish diseases in the Columbia River Basin.Proc. Viral Dis. Salmonid Fish ColumbiaRiver Basin Workshop. Bonneville PowerAdmin., Portland, Oreg.

____, R. P. Hedrick, and J. L. Fryer.1982. Viral diseases of salmonid fish inOregon. In B. R. Melteff and R. A. Neve(editors), Proc. North Pac. Aquaculture

3Elston, R. Pathology and health certificationcriteria of the Japanese scallop, Palinopeclenyessoensis: A case history and model approach.Manuscr. Battelle, Pacific Northwest Division,Marine Research Laboratory, 439 West SequimBay Rd., Sequim, WA 98382.

Marine Fisheries Review

Symp., Alaska Sea Grant Rep. 82-2, p.345-357.

Hamblet, F. E., W. F. Hill, Jr., E. W. Akin,and W. H. Benton. 1%7. Effect of turbidityon the rate of accumulation and eliminationof poliovirus type I by the eastern oyster(Crassostrea virginica). In Proc. Gulf S. Atl.Shellfish Sanit. Res. Conf., N.H.S., p. 13-23.

Hill, B. J. 1976a. Properties of a virusisolated from the bivalve mollusc Tel/inatenuis (da Costa). In L. A. Page (editor),Wildlife diseases, p. 445-452. Plenum Press,N.Y.

____. 1976b. Molluscan viruses: Theiroccurrence, culture and relationships. InProc. 1st Int. Colloq. Invertebr. PathoI. , p.25-29. Kingston, Ont.

_-,-__.1982. Infectious pancreatic necrosisvirus and its virulence. In R. J. Roberts(editor), Microbial diseases of fish, p. 91-114.Academic Press, N.Y.

-----:::-:-__, and D. J. Alderman. 1977.Observations on the experimental infection ofOstrea edu/is with two molluscan viruses.Haliotis 8:297-299.

_---,--,-__, K. Way, and D. J. Alderman.1982. Further investigations into thepathogenicity of lPN-like viruses for oysters.In Proc. III Int. Colloq. Invertebr. Pathol.,p.273-274.

Kampelmacher, E. H., L. M. vanNoorleJansen, D. A. Mossel, and F. J. Groen. 1972.A survey of the occurrence of Vibrioparahaemo/yticus and V. a/gino/yticus onmussels and oysters and in estuarine waters inthe Netherlands. J. Appl. Bacteriol.35:431-438.

Leibovitz, L. 1978. A study of vibriosis ata Long Island shellfish hatchery. New YorkSea Grant Reprint Series. NYSG-PR-79-Q2.N.Y. Sea Grant Inst., Albany.

Leong, J., and S. Turner. 1979. Isolation ofwaterborne infectious hematopoietic necrosisvirus. Fish Health News 8:vi-viii.

46(3)

Liu, O. c., H. R. Seraichekas, and B. L.Murphy. 1%7. Viral depuration of the north­ern quahaug. Appl. Microbiol. 15:307-315.

Lovelace, T. E., H. Tubiash, and R. R.Colwell. 1968. Quantitative and qualitativecommensal bacterial flora of Crassostreavirginica in Chesapeake Bay. Proc. Natl.Shellfish. Assoc. 58:82-87.

Mason, J. 0., and W. R. McLean. 1962.Infectious hepatitis traced to the consumptionof raw oysters. Am. J. Hyg. 75:90-111.

Meyers, T. R. 1979. A reo-like virus isolatedfrom juvenile American oysters (Crassostreavirginica). J. Gen. Virol. 43:203-212.

_---::-__. 1980. Experimental pathogenicityof reovirus I3p, for juvenile Americanoysters Crassostrea virginica (Gmelin) andbluegill fingerlings Lepomis macrochirus(Rafinesque). J. Fish Dis. 3:187-201.

_-----:----;-_. 1983. Serological and histo­pathological responses of rainbow trout,Sa/mo gairdneri Richardson, to experimentalinfection with the I3p, reovirus. J. Fish Dis.6:277-292.

____, and K. Hirai. 1980. Morphology ofa reo-like virus from juvenile Americanoysters (Crassostrea virginica). J. Gen. Virol.46:249-253.

Mitchell, J. R., M. W. Presnell, E. W. Akin,J. M. Cummins, and O. C. Liu. 1966. Ac­cumulation and elimination of poliovirus bythe eastern oyster. Am. J. Epidemiol.86:40-50.

Mulcahy, D., R. J. Pascho, and C. K. Jenes.1983. Detection of infectious haematopoieticnecrosis virus in river water and demonstra­tion of waterborne transmission. J. Fish Dis.6:321-330.

Purchon, R. D. 1977. The biology of themollusca, 2nd ed. Pergamon Press, Oxf., 596p.

Rohovec, J. S. 1983. Current fish disease con­trol policies affecting the Columbia RiverBasin. Proc. Viral Dis. Salmonid Fish Col-

umbia River Basin Workshop. BonnevillePower Admin., Portland, Oreg.

Rosenfield, A., and F. G. Kern. 1978.Molluscan imports and the potential for in­troduction of disease oganisms. In R. Mann(editor), Exotic species in mariculture, p.165-183. MIT Press, Cam., Mass.

Ross, B. 1956. Hepatitis epidemic conveyed byoysters. Sven. Larkartidn 53:989-1003.

Sakazaki, R. 1969. Halophilic Vibrio infections.In H. Riemann (editor), Food-borne infec­tions and intoxications, p. 115. Acad. Press,N.Y.

Snieszko, S. F., E. M. Wood, and W. T.Yasutake. 1957. Infectious pancreaticnecrosis in trout. Am. Med. Assoc. Arch.PathoI. 63:229-233.

Thompson, C. A., and C. Vanderzant. 1976.Relationship of Vibrio parahemo/yticus inoysters, water and sediment, andbacteriological and environmental indices. J.Food Sci. 41:117-122.

Toranzo, A. E., and F. M. Hetrick. 1982. Com­parative stability of two salmonid viruses andpoliovirus in fresh, estuarine and marinewaters. J. Fish Dis. 5:223-231.

Tubiash, H. S., P. E. Chanley, and E. Leifson.1965. Bacillary necrosis, a disease of larvaland juvenile bivalve molluscs. J. Bacteriol.90: 1036-1044.

-c::-::-:-:---:-' R. R. Colwell, and R. Sakazaki.1970. Marine vibrios associated with bacillarynecrosis, a disease of larval and juvenilebivalve mollusks. J. Bacteriol. 103:271-272.

Wedemeyer, G. A., N. C. Nelson, and C. A.Smith. 1978. Survival of the salmonid virusesinfectious hematopoietic necrosis (IHNV)and infectious pancreatic necrosis (IPNV) inozonated, chlorinated and untreated water. J.Fish Res. Board Can. 35:875-879.

Winton, J. R., C. N. Lannon, J. L. Fryer,and T. Kimura. 1981. Isolation of a newreovirus from chum salmon in Japan. FishPathol. 15: 155-162.

17