presence of the parasite. By 1970,Comps was reporting abnormal mor-talities in the Marennes basin as-sociated with the parasite's presence.He also reported it from the Rade deBrest, in Aber Benoit, in Spain, and atArcachon. The mortalities at Marenneswere in stocks introduced from Aradon,Paimpol, and also from Arcachon.Thus it is clear that although severeoyster mortalities at that time were oc-curring only in relatively limited areas,the presence of the parasite in the majoroyster-producing areas of France andSpain had already been established.

The next few years saw a slow in-crease in the frequency of mortalitiesand by 1973 the estuary at Penze (Fig.I A) had become infected. By 1974 theMorlaix estuary was affected and by1975 it was clear that the disease waspresent in Paimpol, Binic, and in theMorbihan. Since Morbihan, togetherwith the Rade de Brest, was the majorsource of young O. edulis for trans-planting and growing, this was disas-trous in the extreme.

Meanwhile, oysters from diseasedstocks had continued to be exported to

1.3'

1C

1B

50km

s'

LACORUNA

-SANTIAGO

g'

Figure I.-Location maps of areas described in text.

lA

18

noticed in this estuary, perhaps up to 50percent in some previous years, but inthe summer of 1967 the mortality ex-ceeded 90 percent (Comps, 1970;Herrbach, 1971).

Stages of the parasite which havesince been identified as Marteilia re-jringens were recognized immediatelyin histological examination of oystersfrom the mortality site (Herrbach,1971). By the time the parasite wasrecognized, oysters from the Abers hadalready been transferred that year to theRade de Brest, to Marennes, andGalicia in Spain. Samples from theseoysters proved to be positive for the

DAVID J. ALDERMAN

David J. Alderman is with the Ministry of Ag-riculture, Fisheries and Food, Fish DiseasesLaboratory, The Nothe, Weymouth, Dorset DT48UB, United Kingdom.

Historical Background

In the late 19th century the Europeanoyster industry was a large and flourish-ing one, based mainly on the produc-tion of the flat oyster, Ostrea edulis,from natural beds. In France alone, anelaborate' 'farming" type of situationhad recently been developed. WorldWar I resulted in considerable neglectof the oyster beds and throughou tEurope the industry was just becomingreestablished when, in the early 1920' s,a large-scale mortality occurred (Orton,1924). The United Kingdom oyster in-dustry never recovered and even todayis only a fraction of its original size.

The French oyster industry did re-cover and went rapidly from strength tostrength, remaining without importantproblems until the mid-1960's. Whenproblems occurred they were in the sec-tor of the industry utilizing the intro-duced Portuguese oyster, Crassostreaangulata. Mortalities of this oyster inthe latter 1960's and early 1970's coin-cided with early experimental introduc-tions of the Japanese oyster, C. gigas,made on a limited scale from 1967 on-wards. The mortalities and the avail-ability of an alternative led to the re-placement of the Portuguese oyster, C.angulata, by the Japanese oyster, C.gigas.

Also in 1967, the first drastic mor-tality of the flat oyster, O. edulis, oc-curred in a small estuary on the north-western coast of Brittany, Aber Wrach.Occasional mortalities had been

Epizootiology ofMarteilia refringensin Europe

January-February /979 67

Table 1.-Total oyster productionin France in metric tons.

Table 2.-0yster production of Les Abers and Rade deBrest in metric tons.

Aber Wrachand Aber Benoit Rade de Brest

Year Os/rea Crassostrea Ostrea Crassostrea

1971 1,300 0 6,366 501972 1.275 0 5.572 501973 1.125 25 5.888 1251974 3.654 911975 666 376 1,341 441

the Netherlands and to Spain. At first,neither country appeared to be havingdifficulties with introduced stocks.However, by 1975, it was clear thatsome areas of Spain at least were sub-ject to severe mortalities from Aber dis-ease. In contrast, no significant losseshad been noted in the Netherlands.

Table 1 illustrates the effects of thevarious diseases on oyster production inFrance. There was a severe decline inproduction of C. angulata from 65,900tons in 1960 to below 20,000 tons in1971 when this species was alreadybeing replaced by C. gigas. This pro-duction has now reached in excess of85,000 tons. In the case of O. edulis,the figures show a slow but remorselessand accelerating decline as a direct re-sult of the spread of Aber disease from1967 onward. There was little sig-nificant reduction between 1960 and1970 when production was in the regionof 20,000 tons per annum. However,by 1971 there were the first indicationsthat Aber disease was beginning to be-come significant and by 1973 produc-tion was cut by half to 10,000 tons.Resurgence in 1974 to 14,000 tons al-most certainly represents the sale ofyoung stock to realize an asset before itcould be lost. The more indicati vefigure is seen in 1975 when total flatoyster production in France was downto 8,400 tons.

Year

19601969197019711972197319741975

68

Ostrea

21,60018.00019.10014,10014.90010,30014.1008.400

Crassostrea

65,90043.20040.30020.10053,90061.70059.50085.000

In the Abers, production of O. edulisfell from 1,300 tons to 666 tons be-tween 1971 and 1975 (Table 2) whileproduction of Crassostrea, which waszero in 197 l, had reached 370 tons by1975. In the Rade de Brest a similarpicture is found. Ostrea production fellfrom 6,366 tons in 1971 to 1,340 tons in1975, and again production of Cras-sostrea has risen from 50 tons to 441tons. In neither case has the productionof Crassostrea risen to equal the levelof Ostrea production which has beenlost. For this, economic forces arelargely to blame.

Life Cycle Considerations

Life cycle problems are discussedelsewhere in this symposium (Balouet,1979;Cahour, 1979; Grizel, 1979). Insamples received bimonthly fromCarantec in the Morlaix River during1975 an apparent sequence of parasitestages was visible. During the winterperiod (from December to February),less than 10 percent of specimensshowed the presence of M. rejringens.The stages present were mainly thosewith mature spores and refringent in-clusion bodies and were located insmall areas of the infected digestivegland. During the early spring, stageswhich Perkins (1976) has referred to asplasmodial stages became apparent inthe stomach epithelia both of animalswith visible "old" infections and thosewithout. Since the winter infectionsseemed to consist of small, Iimited fociin one or two branches of digestivegland, the problem of establishing truelevels of infection is great. Althoughthe apparent level of infection may beonly 10 percent, it is impractical to ex-pect to find every focus of infectionbecause of the limited number of sec-tions which can be made and examinedfrom a single animal. Therefore, manyapparently new infections may simplyrepresent further infection of an alreadylocally infected animal.

The number of mature and maturingparasite cells in the digestive epitheliaalso began to increase in the spring andinfection levels began to climb, eventu-ally reaching 80 percent by July. Levelsremained high until October but therewere relatively few stomach epithelial

infections during the peak of the sum-mer. An increased level of these infec-tions was measured again in the autumn(September to October) period. AfterOctober, general levels of apparent in-fection began to fall, reaching 10 per-cent in January.

The occurrence of refringent inclu-sion bodies in the gut presumably indi-cates sporangial breakdown and sporerelease. Refringent bodies were presentin the gut throughout most of the sum-mer season, but the most impressivewere the occasional animals which ap-peared to have been subject to massiveand simultaneous spore release. Thesewere the only specimens with reallysignificant tissue damage. Variableamounts of hemocyte reaction may beobserved at different stages of the dis-ease, but the massive spore release isaccompanied by breakdown of the oys-ter tissues. It seems necessary that somephysical initiator for such a mass spor!;release must exist. The tissue damagewhich accompanies this type of spor~release contrasts strongly with the ap·.parent lack of severe reaction to thepresence of enormous numbers of ma-ture parasites, and may account for thecatastrophic localized mortalitieswhich characterize the progress of Aberdisease.

An interesting example of theepizootiological problems posed byAber disease is seen in Galicia, Spain(Fig. 1B). As has been mentioned pre-viously, oysters infected with M. re-jringens had been imported into Spainas early as 1969. There is no informa-tion as to when the disease becamenaturalized in Spanish waters, but dur-ing the autumn of 1975 samples fromthe Rio de Muros showed high levels ofM. rejringens. Culture of the flat oys-ter, O. edulis, in Spain is limited to thenorthwest of Galicia and bottom cultureis practiced only at Ribadeo. Else-where, oysters are suspended either intrays or attached to ropes from rafts,such as are used in the same area formussel culture. The two sites (Wand E)marked on Fig. 1C are operated by twoseparate companies. The companyoperating the western (W) site also hasan oyster hatchery built to produce localspat, but both companies have imported

Marine Fisheries Review

large numbers of oysters for ongrowingfrom France.

In the autumn of 1975, a number ofsamples of oysters from five groups ofanimals which had originally been im-ported from France were examinedfrom the western (W) site. All of thesesamples showed levels ofM. refringensinfection between 80 and 100 percent.The sixth sample of oysters which con-sisted of hatchery spat produced in thewinter of 1975 at the nearby hatcheryalso showed 95 percent infection.Clearly, M. refringens is able to spreadto local stocks at least within the limitsof the western site raft system. Theseventh sample examined at the sametime came from the eastern (E) site setof rafts and showed zero levels of M.refringens infection. Samples takenfrom this site in the following summer,1976, again consistently showed no M.refringens.

There is, however, a complicatingfactor. Although the eastern siteshowed no levels of Abel' disease eitherin the imported French oysters or invarious other stocks, mortalities stilloccurred, particularly in the Frenchstocks. The oysters in question wereoriginally imported into Ribadeo andlaid there for some months before beingremoved to the Muros rafts. At about120 days after arrival at Muros, mor-talities commenced and ran at approxi-mately 10 percent per week throughoutthe summer. The oysters had beenexamined and found to be free of M.refringens before they left France andthey remained free of M. refringensduring the period of examination atMuros. While no indications of M. re-fringens were observed, some 30 per-cent of the animals examined showedevidence of a hematopoietic neoplasm.The cell type of this neoplasm appearsto be equivalent to that noted by Brownet a!. (1976) in Mya arenaria and des-ignated Type II. It is currently assumedthat the mortality and the neoplasm at

January-February 1979

Muros are interrelated (Alderman etal., 1977). A similar neoplasm was ob-served in O. edulis imported from theAdriatic Sea and examined fromRibadeo where no mortalities were re-ported. Subsequent reexamination ofthe material from the western site 1975investigations showed a number ofspecimens with Type II neoplastic cellsto be present. There is no positive evi-dence at present that this neoplasm isinfectious and occasional cases (vanBanning ' ) have been noted in oystersfrom Le Po in the Morbihan.

The two Spanish sites at Muros areseparated by approximately 2 km ofopen water in a wholly marine estuarywith little input of fresh water and areasonable tidal circulation. The situa-tion is thus that M. refringens is able tobecome established in the western siteand infects local oysters to very highlevels within the raft systems and pro-duces very severe mortal ities. How-ever, the parasite appears unable tocross the distance of 2 km of open seaand infect oysters on the eastern siterafts. The compl icating factor of theneoplasm, which is apparently relatedto signi ficant mortalities on the easternsite and is also present on the westernsite, makes it difficult to be certain ofthe relative importance of Marteilia in-fection in terms of the western site mor-talities.

The inabil ity of Marteilia to cross the2 km gap in Spain is indicative of someof the puzzling problems that this dis-ease is presenting. To provide a betterunderstanding of the epizootiology ofthis disease organism, controlled trayexperiments of a type carried out overmany years by Andrews (1965, 1966,1967) are necessary.

1van Banning. Paul. The Netherlands Institute forFishery Investigations. Haringkade I. Ijmuiden-1620. The etherlands. Pers. commun.

Acknowledgments

I would like to thank M. Maurin (Di-rector, I.S.T.P.M.) for providing fig-ures of oyster production in France.

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Alderman, D. J.. P van Banning, and A. Perez-Colomer 1977 Two European oysler(Os/rea edulis) mortalities associated with anabnormal haemocytic condition Aquaculture10:335-340.

Andrews, 1. D. 1965. Infection experiments innature with Dermocysridium marinum inChesapeake Bay. Chesapeake Sci. 6:60-67

1966. Oyster mortality studies inVirginia. V Epizootiology of MSX, a protis-tan pathogen of oysters. Ecology 47: 19-3 I .

____ . 1967. Interaction of two diseases ofoysters in natural waters. Proc. Natl. Shellfish.Assoc. 57:38-49.

Balouet. G. 1979. Marreilia refringens-con-siderations of the life cycle and development ofAbers disease in Osrrea edulis. In F O. Per-kins (editor), Haplosporidian and haplosporid-ian-like diseases of shellfish. Mar. Fish. Rev.41 ( 1-2):64-66.

Brown, R. S., R. E. Wolke, and S. B. Saila.1976. A preliminary report on neoplasia inferal populations of the soft shell clam, Myaarenaria: prevalence, histopathology anddiagnosis. In Proceedings of the IX Confer-ence, Society for Invertebrate Pathology, p.151-158. Printing Dep., Queens University,Kingston, Ont.

Cahour, A. 1979. Marreilia refringens and eras-sosrrea gigas. In F. O. Perkins (editor), Hap-losporidian and haplosporidian-like diseases ofshellfish. Mar. Fish. Rev. 41(1-2): 19-20.

Comps, M. 1970. Observations sur les causesd'une mortalite abnormale des huitres platesdans Ie Bassin de Marennes. Rev. Trav. Inst.Peches Marit. 34:317-326.

Grizel, H. 1979. Marreilia refringens and oysterdisease-recent observations. In F. O. Perkins(editor), Haplosporidian and haplosporidian-like diseases of shellfish. Mar. Fish. Rev.41 (1-2):38-40.

Herrbach, B. 1971. Sur une affection parasitairede la glande digestive de I'huitre plate, Osrreaedulis Linne. Rev. Trav. Inst. Peches Marit.35:79-87.

Orton, J. H. 1924. An account of investigationsinto the cause or causes of the unusual mortal-ity among oysters in English oyster beds during1920 and 1921 Fish. Invest. Ser. II. Mar.Fish. G. B. Minist. Agric. Fish. Food 6(3):1-199

Perkins, F. O. 1976. Ultrastructureofsporulationin the European flat oyster pathogen, Marreiliarefringells - taxonomic implications. J. Pro-tozool. 23:64-74.

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