MFR PAPER 1345

Larval Nematodes Parasiticin Shellfish

THOMAS C. CHENG

Thamas C. Cheng is with the Centerfar Health Sciences, Institute forPathobiolagy, Lehigh University,Bethlehem, PA 18015. The originalinformation included in this paperwas obtained from research sup­ported by a grant (FD-00416-05)from the U. S Public Health Service.

There has been an increased interestin the parasitology of marine mollusksin recent years, especially pertaining tothose species that are of economic im­portance. From the standpoint ofshellfish biologists, this interest stemsprimarily from the possible effects ofparasites on these mollusks; however,there is another reason why such para­sites are important. Specifically, sinceseveral species of mollusks, such ascertain oysters and clams, are com­monly consumed by humans in the rawor barely cooked state, the potential oftransmission of such parasites frommollusks to humans and their estab­lishment in man must be considered.Thus, the pragmatic importance ofparasites harbored by shellfish involvesthe focal interest of shellfish biologistsas well as that of the public health­oriented parasitologist.

A third reason for the increased in­terest in parasites of mollusks rests withthe fact that zooparasites are useful asantigens in studies designed to eluci­date the immunologic responses of mol­lusks. This is especially true if the in­tent is to contlibute to our understand­ing of the process of encapsulation,commonly known as granuloma forma­tion. In such instances, the helminthsare particularly useful.

As indicated by the title of this pre­sentation, my remarks will be limited tolarval nematodes, a group of parasitesof marine mollusks that has been sadlyneglected. I shall commence by statingthat, in my opinion, the reasons whythere is a dearth of published informa-

October 1978

tion pe11aining to this category of para­sites are because I) larval nematodesare extremely difficult to identify, and2) many are tissue, rather than luminal,parasites, and consequently requirespecial procedures to isolate which arenot routine, e.g., enzyme digestion andconcentration.

Only a few species of nematodeshave been reported from commerciallyimportant marine mollusks, althoughthey have been recorded from a numberof terrestrial, freshwater, and a fewcommercially unimportant marine mol­lusks (Gerichter, 1948; Pelseneer,1928). Abbreviated accounts of theknown species in marine shellfish arepresented below. For those interested inidentification, redescriptions of thesespecies have been presented earlier(Cheng, 1967).

ECHINOCEPHALUS UNCINATUSMOLIN, 1858

Molin (1858) erected the genusEchinocephalus to include E. un­cinatus found in the spiral valves of theelasmobranch Trygon hrucco in theAdriatic Sea. The original descriptionwas based on two distinct types ofspecimens. Molin designated onespecimen, 7.0 mm long with 6 rows ofcephalic hooks, as the female, and twoothers, 24-35 mm long with ~O rows ofhooks, as males. Later, Baylis andLane (1920) considered Molin's malesto be identical with E. spinosissimus,and the female as representative of E.uncinatus.

As a result of studies by Millemann( 1963), it is now generally agreed thatthe specimens of Echinocephalusstudied by Molin (1858) and Baylis andLane (1920) actually represent the lar­v·al forms of species, the adults ofwhich are yet unknown. Also, Mil­lemann has stated that the early juvenilestages (L I and L2) of Echinocephalusspp. have 8 rows of cephalic hooks,which can only be appreciated by exam­ining en face preparations. The defini­tive number of rows of such hooks isnot attained until the L3 stage. Mil­lemann's interpretation is that Molin'smales represent the true E. uneinatusand considers E. spinosissimus as asynonym.

Larval E. uncinatus have beenreported in several species of marinemollusks. Specifically, von Linstow(1904) found it encysted in the adductormuscle of the pearl oyster, Margaritif­era vulgaris, in Ceylonese waters, andBaylis and Lane (1920) reported it inthe pelecypod Pinna sp. In addition,Hopkins (1935) reported what he be­lieved to be the larva of E. uncinatuscollected by Van Cleave in the gonad ofa sea urchin, Arbaeia punetulata, atWoods Hole, Mass., and Johnston and\'lawson (1945) reported the occur­rence of this larva in the gastropodsPolin ices conica and Katylesiascalarina from Australia. It is notedthat there is some doubt whether theselarvae are all E. uncinatus. Neverthe­less, these few reports do serve to pointout that the larvae of Eehinoeephalusspp. occur in a variety of marine inver­tebrates, among which mollusks areapparently a common group. Thedefinitive host of Eehinoeephalus spp.is usually an elasmobranch (Yamaguti,1961).

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Figure I.-Echinocepha/us pSf!udouncina/us. (A) Anterior end ofsecond-stage larva, lateral view; (B) en face view of head of second­stage larva. (Redrawn after Millemann. 1963).

Figure 2.-Scanning electron micrograph of cephalicend of third-slage larva of Echinocepha/us crassos/reai.400x.

ECHINOCEPHALUSPSEUDOUNCINATUSMILLEMANN,1951

This species (Fig. I) was describedby Millemann (1951) based on larvaefound encysted in the foot of the pinkabalone, Haliotis corrugata, collectedat San Clemente Island in southernCalifornia. In addition, it also occurs inthe southern green abalone, Haliotisfulgens.

Echinocephalus pseudouncinatuslarvae burrow into the foot of abaloneswhere they encyst in the ventral por­tion. This results in a blisterlike protru­sion on the foot. In addition, the bur­rowing motions of the larvae prior toencystment apparently weakens thefoot musculature and decreases itsefficacy as a hold-fast structure. As aresult, parasitized abalones are readilydetached from rocks.

It is noted that Millemann (1951) hasreported that only old abalones areparasitized. It remains unknownwhether this means that younger onesare refractile or if the parasite prefersolder mOllusks.

ECHINOCEPHALUSCRASSOSTREAl CHENG, 1975

The definition of this species (Fig. 2)by Cheng (1975a) was based on speci­mens of second- and third-stage larvaefrom the Japanese oyster, Crassostreagigas, collected from oyster beds off

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the coasts of Hong Kong and ThePeople's Republic of China.

The L3 , which was the most com­monly encountered stage, occurprimarily in the gonoducts of the mol­luscan host.

Although examination of routinelystained sections of parasitized oystersrevealed only slight pathologicalchanges, for example the developmentof a tunic of ,reaction elements sur­rounding the parasite-enclosinggonoduct (Cheng, 1975a), histochemi­cal studies have revealed considerablymore. It has been found that the reac­tion tunic is comprised of connectivetissue fibers, which are comprised, atleast in part, of complex carbohydratesthat are sulfated and rich in acidicgroups; tightly packed Leydig cells,which include glycogen and carboxy­lated polyanions; glycogen-containinghemocytes, which also enclose carbox­ylated poJyanions; brown cells, andmyofibers. Of these clements, only themyofibers are native to this region(Cheng, 1975b).

The presence of E. crassostreai lar­vae in the gonoductal vestibule resultsin hyperactivity and secretion of glandcells embedded in epithelial lining.Furthermore, if larvae should invadethe gonad, some damage is inflicted onadjacent molluscan gametes.

Because of the structural separationof the intragonoductal parasites fromthe periductal reaction complex, Cheng(1975b) advanced the hypothesis thatthe reaction is stimulated by moleculessecreted and/ or excreted by thenematodes and which are able to per­meate through the lining epithelium and

subtending basal lamina. I wish to em­phasize this interpretation since in­sufficient attention, in my opinion, hasbeen placed on pathogenesis resultingfrom parasite-secreted molecules in in-

vertebrate hosts. Of course, direct evi­dence for such deleterious moleculesmust rest with their isolation, charac­terization, and studying their effectsexperimentally.

Echinocephalus crassostreai servesas an example of a parasite which is ofpublic health importance. Ko et al.,(1974) demonstrated that L3 from oys­ters, upon ingestion by a cat and arhesus monkey, penetrated the stomachand intestines. This is not totally sur-

prising since the genus Echinocephalusis a member of the family Gnathos­tomatidae, which also includesGnathostoma, the larvae of whichgenus are well known to cause gastricand other types of granulomatous le­sions in humans if accidentally in­gested.

Marine Fisheries Review

SULCASCARlSSULCATA(RUDOLPHI, 1819)

Cobb (1930) described Paranisakispectinis based on a single immaturespecimen found in the visceral mass ofa scallop, Pecten, collected atBeaufort, N. C. Gutsell (1930) foundthe same nematode in Aequipectenmaximus collected from the same loca­tion. Years later, Hutton (1964), whileexamining the bay scallop, Aequipec­ten gibbus, collected off the east coastof Florida, found numerous specimensof a larval nematode which he identifiedas Cobb's worm. However, as the re­sult of finding a short, anteriorly pro­jecting caecum, Hutton transferred it tothe genus Porrocaecum. It is noted,however, that this nematode has beententatively designated as Paranisaki­opsis pectinis by Perkins et al. (1975)who reported its occurrence in the footand adductor muscles of the surf clam,Spisula solidissima, from Virginia andNorth Carolina. Since then, 1. RalphLichtenfels has informed me that thisnematode has been identified by 1. F. A.Sprent as the larva of Sulcascaris sul­cata, the adult of which is a parasite ofloggerhead tUl11es, Caretta caretta, andthe green turtle, Chelonia mydas. Thishas been confirmed by Lichtenfels.Also, Lichtenfels has informed me thatthe L3 of the nematode also occurs in thewhelk, Busycon caniculata, and themoon snail, Lunatia heros.

Earl ier, Cheng ( 1967) reported larvalS. sulcata (Fig. 3) in the adductor mus­cles of A. gibbus and A. irradians offthe coasts of North Carolina, SouthCarolina, and Florida, and later(Cheng, 1973) reported this larvalnematode in 2.3 percent of 400 speci­mens of A. irradians from off the coastof North Carolina. The larvae werecommonly, although not always, foundin the adductor muscle, causing it totake on a brownish coJor. Furthermore,the presence of more than one larva,commonly two to four, causes suf­ficient damage to the muscle that muchof the tonicity was lost. It is now knownthe brownish color associated with thisparasite is, at least in part, due to Uros­poridium spisuli, a pigmented hyper­parasite (Perkins et a!., 1975).

October /978

The occurrence of larvae of S. sul­cala in scallops and other species ofmarine mollusks is also of potentialpublic health significance since the lar­vae of certain species of related generabelonging to the family Anisakidae areknown to cause what has been desig­nated as anisakiasis in humans if acci­dentally ingested (see Jackson, 1975and Cheng, 1976 for reviews). It isnoted that Norris and Overstreet ( 1976)have reported the occurrence of the re­lated anisakid Thynnascaris in threespecies of marine mollusks, the gas­tropods Can/harus cancel/arius andThais haemasloma jloridana and thecephalopod Lol/iguncula brevis. In ad­dition, Dolgikh (1966) has reportedanother related anisakid, Thynnascarisadunca (=Contracaecum aduncum) ,in the marine gastropods Cyclonassaneritea and Nassa reticulata in theBlack Sea. Valter ( 1968) has reportedsuccess at experimentally infecting thegastropod Margarites groenlandicuswith the same parasite, and Kikuchi eta!' (1969, 1972) and Shiraki (1969)have reported Contracaecum larvae inthe cephalopod Todarodes pacificus inJapanese waters.

The life cycles of anisakids arehighly complex and still little under­stood (Cheng, 1976). The definitivehosts in the case of those larvae thathave been found in marine mollusks,including commercially importantshellfish, could be a fish, a bird, or amarine mammal. If it is a homeotherrn,then the larvae in mollusks are poten­tially infective to humans (Cheng, inpress) .

COMMENTS

As indicated by the brief review pre­sented, only a few species of nematodeshave been reported as naturally occur­ring parasites of commercially impor­tant marine shellfish, and all of thesehave been larvae, mostly L3 . This doesnot mean that nematode parasitism israre in shellfish. Cheng (1976), forexample, has noted the occurrence ofan unidentified species embedded in theLeydig tissue of Crassostrea virginicacollected in SI. Mary's Creek, SI.Mary's County, Md.

Figure 3.-Sulcascaris sulcata. (A) Lateralview of anterior portion of larva fromAequipecten gibbus; (B) en face view of an­lerior lerminal of larva. (Redrawn afler Cobb,1930.) AM, amphid; C, caecum; CC,cephalic constriction; ES, esophagus; INT,intestine; NR, nerve ring; PA, papilla; SP,submedian papilla; VEN, ventriculus.

As stated, it is suspected that thereasons no more have been reported areI) because larval nematodes aredifficult to identify, and 2) the recoveryof nematodes from mollusks requirenonroutine procedures.

Except for Echinocephalus crassos­treai in Crassostrea gigas, detailedpathological effects, both morphologi­cal and physiological, inflicted by thesehelminths on their molluscan hostshave not yet been studied. Neverthe­less, the ability of Echinocephaluspseudouncinatus and Porrocaecumpectinis to weaken the adductor mus­cles of their pelecypod hosts rendersthem of importance to the shellfish in­dustry.

Finally, since certain species ofshellfish are consumed either raw or inthe partially cooked state, the occur­rence of larval nematodes in them rep­resents potential public health hazards.The danger is real in view of what isknown about the pathogenicity of E.crassostreai to mammals and humananisakiasis.

LITERATURE CITEDBaylis, H. A., and C. Lane. 1920. A revision of

the nematode family GnathosIOl1lida~. Proc.Zool. Soc. Lond. /920:245-310.

Cheng, T. C. 1967. Marine molluscs as hOSIS for

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symbioses. With a review of known parasitesof commercially imponant species. Adv. Mar.BioI. 5,424 p.

____. 1973. Human parasites transmissi­ble by seafood-and related problems. In C.O. Chichester and H. D. Graham (editors),Microbial safety of fishery products. p. 163­189. Academic Press, N.Y.

____. 1975a. Eehinoeephalus erassostreaisp. nov., a larval nematode from the oysterCrassostrea gigas in the Orient. J. Invenebr.Pathol. 26:81-90.

____. 1975b. A structural and histochemi­cal study of the reaction complex in Cras­sostrea gigas (Mollusca) to Eehinueephaluscrassostreai (Nematoda). J. Invenebr. PathoI.26:113-119.

1976. The natural history ofanisakiasis in animals. J. Milk Food Technol.39:32-46.

____.In press. Anisakiasis.ln J. H. Steele(editor), CRC handbook series in zoonoses.Sect. C. Parasitic zoonoses. CRC Press. Cleve­land. Ohio.

Cobb, N.A. 1930. A nemic parasite of Pecten. J.Parasitol. 17: 104-105.

Dolgikh, A. 1966. Black Sea molluscs in thelife-cycle of Contraeaecum aduneum. Zool.Zh.45:454-455.

Gerichter, Ch. B. 1948. Observations on the lifehistory of lung nematodes using snails as in­termediate hosts. Am. J. Vet. Res. 9: 109-112.

Gutsell, J. S. 1930. Natural history of the bayscallop. U.S. Bur. Fish. Bull. 46:569-632.

Hopkins, S. H. 1935. A larval Echinoeephalusin a sea urchin. J. Parasitol. 21:314-315.

Hutton, R. F. 1964. A second list of parasitesfrom marine and coastal animals of Florida.T;ans. Am. Microsc. Soc. 83:439-447.

Jackson, G. J. 1975. The" new disease" stalUs ofhuman anisakiasis and Nonh American cases:a review. J. Milk Food Techol. 38:769-773.

Johnston, T. H., and P. M. Mawson. 1945.Parasitic nematodes. Repon of the British,Australian and New Zealand Antarctic Re­search Expedition, 1929-1931. Vol. 5 B(2), p.141.

Kikuchi, K., H. Hirabayashi, K. Kosugi. and S.Hayashi. 1969. Experimental studies on thedegree of pathogenicity to dog, rabbit andhuman of the larvae of Anisakis type 1 frommackerels and the larvae of a species of Con­tmcneeum from a squid. On Jpn., Engl. title.]Jpn. J. Parasitol. 18:354.___ , , ,and_____ . 1972. On the development of thelarva of Contracaecum sp. (A-type) in the in­termediate host, Todarodes pacifieus. Jpn. J.Parasitol. 21 :5.

Ko, R. C.. B. Morton, and P. S. Wong. 1974.Echinoeephalus sp. Molin, 1858 (Spiruroidea:Gnathostomatidae), an unusual nematodefrom the oyster, Cmssostrea gigns Thunberg,17'.13. Proc. 3rd Int. Congr. Parasitol., Mu­nich, Germany, p. 1731-1732.

Millemann, R. E. 195 I. Eehinoeephaluspseudouneinatus n. sp., a nematode parasite ofthe abalone. J. Parasitol. 37:435-439.

____. 1963. Studies on the taxonomy andlife history of echinocephal id worms(Nematoda: Spiruroidea) with a complete de­scription of Eehinoeephalus pseudouneina/l/s

Millemann, 1951. J. Parasitol. 49:754-764.Molin, R. 1858. Prospectus helminthum, quae in

prodromo faunae helminthological VenetiaecontinenlUr. Denkschriften. Akad. Wiss ..Math.-Naturwiss. KI. (Wein) 30:127-158.

Norris, D. E., and R_ M. Overstreet. 1976. Thepublic health implications of larval Thynnas­caris nematodes from shellfish. J. Milk FoodTechnol. 39:47-54.

Pelseneer, P. 1928. Les parasites des mollusqueset les mollusques parasites. Bull. Soc. ZooI.Fr. 53: 158-189.

Perkins, F. 0., D. E. Zwemer, and R. K. Dias.1975. The hyperparasite, Urosporidium spisulisp. n. (Haplosporea), and its effects on the surfclam industry. J. Parasitol. 61 :944-949.

Shiraki, T. 1969. Histopathological diagnosis ofthe larva migrans in the digestive tract. SaishinIgaku 24:378-389.

Valter, E. D. 1968. On the hosts of Con­(racaecum aduneum (experimental infection ufanimals with larvae of the parasite). Sed'mayaSessiya Uchenogo Soveta po Probleme"Biologicheskie Resursy Belogo Morya iVnutrennikh Vodoemov Karelii." Man, 1968goda, Tezisy Dokladov. Petrozavousk. (Engl.transl. Fish. Res. Board Can., TranI. Ser. No.2031.)

Von Linstow, O. F. B. 1904. The parasites of thepearl oyster. In W. A. Herdman (editor), Re­port to the Government of Ceylon on pearloyster fisheries of the Gulf uf Manaar, Vol. 2,p. 77-106. R. Soc., Lond.

Yamaguti, S. 1961. Systema helminthum, Vol.1Il, Pans I and II. Nematodes. IntersciencePubl., Inc., N.Y., 1261 p.

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MFR Paper 1345. From Marine Fisheries Review, Vol. 40, No. 10, October1978. Copies of this paper, in limited numbers, are available from 0822, UserServices Branch, Environmental Science Information Center, NOAA, Rockville,MD 20852. Copies of Marine Fisheries Review are available from the Superin­tendent of Documents, U.S. Government Printing Office, Washington, DC20402 for $1. 10 each.

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