Oyster Diseases in Chesapeake Bay

J. D. ANDREWS

Introduction

Oyster disease studies have beenpursued at the Virginia Institute ofMarine Science (VIMS) from 1950 to1977. The first decade was devoted toDermocystidium marinum which caus­es a warm-season wasting disease.Zoospores of the pathogen exhibit or­ganelles collectively called the apicalcomplex found only in protozoa of thesubphylum Apicomplexa (Perkins,1976). After 10 years of monitoringmortalities and parasites in ChesapeakeBay, a new disease caused by the hap­losporidan Minchinia nelsoni appearedin 1959 (Andrews and Wood, 1967). Itwas discovered 2 years earlier in Dela­ware Bay (Haskin et aI., 1966). A re­cent overview and bibliography isgiven by Kern (1976).

Field methods developed in the1950's were applied to the new disease.These consisted of monitoring im-

ABSTRACT-Three major diseases ofoysters have been monitored in Virginia es­tuaries for 2-3 decades. Dermocystidiummarinum, causing a warm-season wastingdisease, was discovered in Virginia in 1950and continues actively to kill oysters wherebeds or populations are found in high­salinity waters (>15%0). This diseasespreads by close proximity ofdying oystersto other oysters, hence each isolated bedmust be sampled in early fall annually todocument activity of the pathogen. Controlinvolves avoiding infected seed oysters,cleaning beds of all oysters after harvest,and isolation of new beds.

A new pathogen, Minchinia nelsoni(MSX), caused catastrophic oyster mor­talities in 1959-60, and oyster plantingceased thereafter in a large area of high­salinity (>15%o) waters in lower Chesa­peake Bay. A third pathogen, Minchiniacostalis (Sea Side Organism or SSO), wasfound almost simultaneously on Seaside ofVirginia in high-salinity waters ( >30 %o).

January-February 1979

ported Jots of disease-free oysters inlegged trays on natural oyster beds.Disease-free oysters were obtainedfrom low-salinity waters « 15 %0) inthe James River seed area of Virginia.Legged trays lined with I-inch meshhardware netting prevented smotheringand predation which were the majorinterfering problems on natural bot­toms.

In 1959, a disease caused by Min­chinia costalis, a pathogen closely re­lated to M. nelsoni, was discovered onSeaside of Eastern Shore, Virginia, inhigh-salinity waters (Andrews et aI.,1962). This disease has a well-definedseasonal pattern of activity, and the

J. D. Andrews is with the Virginia Institute ofMarine Science and School of Marine Science,The College of William and Mary, GloucesterPoint, VA 23062. This paper is Contribution No.898 of the Virginia Institute of Marine Science.

Both these haplosporidan parasites kill na­tive susceptible oysters at rates of 20-50percent annually. Strains resistant to MSXwere selected from survivors by laboratorybreeding.

SSO appears to be an endemic pathogenthat causes confined periods ofinfection andmortality. Sporulation and infection occurregularly each May-June associated withoyster deaths. A long incubation period of8months with hidden or subclinical infectionscharacterizes the disease. SSO is confinedto high-salinity waters along the seacoastfrom Cape Henry to Long Island Sound.MSX is a highly infectious pathogen thatappears to be new by importation or adventofa virulent strain. Infections occur during5 warm months (June-October) and deathsoccur throughout the year. Direct transmis­sion has not been achieved in the laboratoryfor either haplosporidan.

Transmission of the diseases and life cy­cles are still important objectives after 18years of studies.

pathogen achieves sporulation regu­larly in May-June each year (Andrewsand Castagna, 1978). The life cyclesof these two haplosporidans areobscure, and artificial infections havenot been achieved. A comparison ofepizootiological traits is made for cluesto sources of infection which are a per­sistent mystery. Hypotheses on timingof activities and infective sources arederived from these comparisons.Studies of the two diseases in Chin­coteague Bay were made by Couch andRosenfield (1968).

The chronological history of oysterdiseases in Chesapeake Bay, themethods evolved for studying mor­talities and prevalences, and clues tolife cycles derived from epizootiologi­cal studies are offered for comparisonwith those of the protozoan parasitesdescribed by colleagues from Europeand Australia at this symposium.

Methods of Monitoring Diseases

Most diseases and parasites in estu­arine environments are dependent uponwater movements in one way or anotherfor dispersion. Examples in Virginiainclude Nematopsis, Bucephalus, Pin­notheres (pea crabs) in oysters, andsacculinids in mud crabs (Xanthidae).Most of these exhibit more intensiveinfestations or infections near sourcesof infective stages. Dermocystidiummarinum may be found in verylocalized centers of infection such aspiers, public oyster beds, and bridges.Dispersal in tidal waters becomes anexceedingly difficult dilution problemfor diseases that require dosages ofmany infective particles to establish in­fections. Dermocystidium marinum isin this category.

The spotty distribution of D. mari­num disease has always required annualsurveys to establish the activity of thedisease in particular beds. Thioglycol­late tests (Ray, 1952) of all gapers, andsamples of live oysters from Augustthrough October from public and pri­vate beds were necessary. Tray lots ofknown age, source, and history weremonitored to establish patterns of infec­tion, spread, over-wintering, and salin­ity tolerance (Andrews and Hewatt,1957). The advantages of monitoring

45

trays over sampling beds of oysters alsoinclude choice of strains (susceptibles,resistants, etc.) and elimination of pre­dation and smothering. The timing oftransplanting between disease-free anddisease-prevalent areas can be fixed ad­vantageously by use of trays with dis­crete populations.

The tray method of monitoring be­came important in studying Minchinianelsoni because natural and plantedbeds were lacking after 1960 in thedisease-infested areas. Those few oys­ters that survived or set thereafter wereselected by the disease in unknown de­gree thereby distorting disease patterns.The haplosporidans exhibited muchmore uniform distributions without thepatchiness and proximity effects ofother oyster parasites. Hence, a pair oftrays of oysters gave prevalence andmortality data that applied to ratherlarge areas. The tendency of MSX tofluctuate up and down Chesapeake Bayby large distances from one year toanother must still be reckoned with byjudicious locations of trays. Suscepti­ble control oysters were al ways used forthese stations monitoring disease activ­ity levels. Typically, live samples weretaken every month from these trays inmajor oyster-growing estuaries.

The legged trays used in Virginiahold about I bushel of oysters and lotsof 500 are initiated usually. Fouling ofthe I-inch mesh liners is intensive andrequires monthly examinations in coldseasons and more frequent ones inwarm periods. Oysters are double­counted at each visit and gapers andboxes (empty shells) removed. Thetrays are located on oyster beds (oftenbarren) beside a marker stake. Samplesof 25 live oysters are processed intopermanent stained sections on slides.Except for D. marinum, infections areall diagnosed from permanent slides.Blue crabs or spider crabs are kept insome trays of older oysters to keepdown fouling by sea squirts andsponges. Small fishes (blennies,gobies, clingfish) scavenge gaper meatsin the trays (scuba observations).

Each group of tray oysters is treatedas a distinct population of known his­tory and origin without mixing untilnumbers of oysters fall below 100 and

46

no longer give reliable mortality esti­mates. Monthly mortalities are calcu­lated for each 15- to 30-day period ofobservation based on the number aliveat the beginning of the interval. Annualmortalities are calculated using instan­taneous rates. Prevalences are given asnumber of cases per 25 live oysters, orin percentages. Incidence of infectionsper unit of time is difficult to obtain as ismorbidity (cases to deaths). At theVIMS Pier, diseases are monitored sea­sonally by weighing individuallymarked oysters underwater weekly forshell growth (Andrews, 1963).

History of OysterDisease Studies in

Lower Chesapeake Bay

Dermocystidium marinumEra-1950's

Disease studies began in Virginiawith the discovery of D. marinum inoysters in 1950. The thioglycollateassay technique (Ray, 1952) permittedepizootiological studies of this diseasein the 1950's when microtechniquefacilities were lacking or inadequate. Itis not known when D. marinum becameendemic in Chesapeake Bay. Increasedmortality rates about 1940 may havereflected its introduction. ChesapeakeBay oysters are more susceptible to thedisease than South Carolina nati ve oys­ters (Andrews and McHugh, 1956).

The 1930 winter mortality of oystersin Mobjack Bay (Prytherch I), discov­ered in March, was not caused by D.marinum for it does not kill oysters inlate winter. The timing of this mortalitywas confirmed by Dumont2 (and reportdated April 1930) who participated inthe investigation. Neither was it causedby Nematopsis as proposed by Pry­therch (see footnote I) for these para­sites have not been demonstrated to killoysters in Virginia (Feng, 1958).Prytherch's proposal to limit Nematop­sis in oysters by reducing mud crab

1Prytherch, H. F. \93\. Report of the investiga­tion on the mortality of oysters and decline ofoyster production in Virginia waters. Mimeogr.rep., 12 p. U.S. Dep. Commer., Bur. Fish.2Dumont, W. H., Bureau of CommercialFisheries, U.S. Fish and Wildlife Service.

populations was largely effected in themid-1960's by importation of the sac­culinid parasite Loxothylacus panopaeiinto Chesapeake Bay from the Gulf ofMexico in oyster shipments (Van Engelet aI., 1966).

In the 1950's, D. marinum was theprimary cause of oyster deaths in Vir­ginia waters during the waml season.Losses to smothering and predationwere large on planted private oysterbeds with marginal bottom textures.Yields seldom exceeded I bushel foreach bushel of seed oysters planted, andoften in mesohal ine areas (10%0 to25 %0) yields were only one-half bushel.With initial seed-oyster counts of 1,000to 2 ,000 per bushel, total losses in 2 or 3years of culture were 65 to 85 percent ofthe number planted. Dermocystidiummarinum caused a major part of theselosses in most high-salinity areas.Seed-oyster plantings were usually held3 years before marketing, which accen­tuated losses.

Continuous culture on public andprivate grounds that were intermixed inoyster-growing areas assured con­tinuity of D. marinum infections. Theonly known reservoir of infective parti­cles is in live oysters. Recruitment onpublic beds and repeated plantings onprivate beds insured that some old in­fected oysters were present to spreadthe disease when they died. Most infec­tions occur from dying oysters in nearproximity «15 m) (Andrews, 1965).Dermocystidium marinum spreads veryslowly into new areas or new bedswithout residual infected oysters.Thorough cleaning of beds and fallow­ing or isolation are the chief controlmethods. All stages found in oysters areinfective (trophozoites and sporangia)as well as zoospores released in seawa­ter.

A scarcity of oysters (hosts) hascaused a decline in abundance of D.marinum in lower Chesapeake Baysince 1960 when private plantingsceased. It persists as a constant threat,in areas where oyster populations arebuilt up, by living in oysters on pilingsand in fringe areas of salinity toleranceof the disease (12-15 %0) where naturalrecruitment of oysters occurs. Over­wintering occurs as cryptic stages in

Marine Fisheries Review

live oysters. The parasite requires high.temperatures (>20°C) for multiplica­tion and oysters tend to discharge it infall at cool temperatures. Low salinitiesalone inhibit acti vity but do not eradi­cate the parasite from oysters. Persis­tent low salinities or absence of oysters,as in Mobjack Bay, will eliminate thedisease in a few years.

The earliest studies were made withSea-Rac3 (Chesapeake Corporation,West Point, Va.) off-bottom trays sus­pended from catwalks at VIMS piers.These trays were examined daily toprocure gapers (dead oysters) for diag­noses (Hewatt and Andrews, 1954).The close proximity of trays at the piersinsured maximum rates of spread of D.marinum. Trays were comparable to in­fested beds of planted oysters in diseaseactivity. Many beds of oysters weresampled in the 1950's to confirm thatD. marinum was killing oysters on pub­lic and private grounds (Andrews andHewatt, 1957). Since VIMS pier withits pilings was a continuous reservoir ofD. marinum infections due to recruit­ment of oysters, it was necessary toplace oysters on abandoned oystergrounds to decrease the interference ofthis pathogen in Minchinia studies.

MSX-A New Pathogen Outof Control, 1959 to 1977

The appearance of Minchinia nelsoniin 1959-60 changed the whole industryof oyster culture in Chesapeake Bay(Andrews, 1966, 1968). No longerwere planters able to tolerate losses asthey had with D. marinum kills. Onlytrial plantings were made after 1960 inhigh-salinity waters (> 15%0). MSX, asM. nelsoni was called, replaced anddisplaced D. marinum as the majorcause of oyster mortalities. The scarcityof oysters prevented D. marinum fromspreading actively except in localizedpockets. MSX killed oysters at annualmortality rates of 50-60 percent andwith peak death rates of 20-25 percentmonthly.

Delaware Bay disease, caused by M.nelsoni. spread throughou t lower

3Reference to trade names or commercial firmsdoes not imply endorsement by the NationalMarine Fisheries Service, NOAA.

January-February 1979

Chesapeake Bay in I year (1960). Itinfected over long distances from aMobjack Bay focus in 1959. The dis­tribution of MSX infections may shifttens of miles upstream in one warmseason depending on salinities and un­known factors of in fection. Thesechanges reflect level of disease activitymore than absence of infections in bor­derline areas. MSX tends to exhibitrather uniform levels of infections andkills over wide areas with appropriatesalinity levels (about 15 to 25%0). Pres­ence or absence of infected oysters andlocal abundance of oysters have no dis­cernible effect on MSX activity. Prox­imity of oysters to each other is not afactor as in D. marinum. These patternsof uniform infection and mortalitymake monitoring of the disease rela­tively easy (Andrews and Frierman,1974). The lack of planted beds forceduse of tray monitoring after the first 2 or3 years. Source of oyster stocks,amount of selection by MSX, and localstrains of oysters became much moreimportant than location of experiments.Fortunately, this pathogen also requiresmesohaline waters, hence disease-freestocks were obtainable from low­salinity areas of the James River. Theimportance of this limiting salinityparameter becomes evident from theproblems of French scientists studyingMarteilia refringens on Brittany coastscharacterized by high salinities in alloyster-growing estuaries (Grizel et aI.,1974). If controls are not adequate, thetiming of mortalities may appear er­ratic, and periods of infectivity aredifficult to determine. Many years ofsampling have confirmed the disease­free status of James River control oys­ters. Long periods of incubation (latentor subclinical cases) for MSX presentdifficult problems of insuring absenceof disease in controls unless long-termmonitoring is pursued.

Intensi ve mortal ities of oysterscaused by MSX made possible selec­tion of resistant strains by laboratorybreeding of survivors. These resistantoysters did not usually show clinicalinfections in Chesapeake Bay, and mor­tality in trays was reduced to about 10percent annually. To utilize thesestrains in Virginia requires hatchery

production of seed oysters which is notyet economically feasible. It is impor­tant to recognize the potential of geneticresistance to haplosporidan diseaseswhich may be used in other cir­cumstances and areas where hatcheriesare utilized.

Discovery of SSO (Minchiniacostalis)-an Endemic Pathogenon Seaside-Epizootiology

The discovery of SSO (Wood andAndrews, 1962) provided an opportun­ity to study an endemic haplosporidanwith stabilized patterns of activity. Itsrestriction to near-oceanic coastalwaters with high salinities provided in­sights into the salinity tolerances ofhaplosporidans. Concurrent infectionsof the two Minchinia diseases in Sea­side bays provided opportunities tostudy seasonal progressions and regres­sions.

The climax of SSO enzootics occursin May-June each year with 20-50 per­cent mortalities compressed into abouta I-month period (Andrews and Cas­tagna, (978). There is an annual lifecycle with new infections occurringduring May-June deaths or shortlythereafter. Infections are hidden untilthe following March, usually a periodof 8 months. Prevalences of 30-40 per­cent infections are typical in mid-Maybefore deaths begin. Remission of lightplasmodial cases occurs in May-Juneand sporulation occurs in nearly all oys­ters that die. Old infections are rare by IJuly and new ones remain hidden. Oys­ters imported for exposure after 1 Au­gust do not acquire infections. Feedingand injecting spores did not achieve in­fections. Successive enzootic kills up to5 years have been observed in indi­vidual lots of oysters, but declining inintensity. Sporulation is often incom­plete in gapers.

Numerous examples of SSO activityon Seaside of Eastern Shore are givenby Andrews and Castagna (1978).An example of persistent SSO mortalityin a particular tray of oysters over a3-year period is given in Figure I. TrayS46 contained susceptible James Riverseed oysters imported 15 May 1964.Three successive enzootics of SSO oc­curred with no interference by MSX.

47

Figure I. -Three consecutive SSG enzootics on Seaside in a lot of susceptibleJames River seed oysters. MSX activity was minimal in this lot with slightmortalities in early fall.

'0

The May-June mortalities are typical intiming and intensities of death rates.Infections tended to decline as selectionoccurred, hence death rates were lowerin successive years. This suggests thatresistant strains can be obtained against

drews and Frierman, 1974). Diseasedistribution tends to be erratic in areadue to fluctuating salinities and infec­tion pressures. The prime infectionperiod is early summer (late May tomid-July). Incubation takes 4-5 weeks.Deaths begin in 6-8 weeks, usually by IAugust each year. Death rates of 20-25percent monthly in late summer and fallare followed by declines to <5 percentmonthly as cold weather occurs. A latewinter kill (March typically) of )0-20percent occurs of which abou t two­thirds is attributed to MSX as shown bygapers. The last of the early-summerinfection cases die in June-july of thesecond year with very intensi ve plas­modial infections .

Oysters imported after I August andbefore I November acquire subclinicalinfections that are hidden or localized .Typically these infections remain sub­clinical until April or May of the fol­lowing year, after which they developrapidly and cause deaths in June-July.These deaths are mixed with some lag­ging cases from earlier infections inspring imports.

Prevalences are high (often >50 per­cent) in May before deaths begin.These "late summer" infections fail tooccur in some years for unexplainedreasons. In years of intensive infectiono

S 46 SWASH BAY

A

SSO as already has been achieved forMSX-caused disease.

Patterns of MSX Activity

Patterns of infectivity and mortalitycaused by MSX are well-known (An-

_ 1966 2nd(28%)

'- -~I

M A M

Isf ENZOOTIC­

1965 (38%)

{ SSo~/~ ~1966 j,

IMSX I

I19673rd __ '(25%)

_IMPORTED 15 MAY 1964

/,e_o ....

w.... 20<ta:

r~ 15wo

20

10

2'

2'

a:w0.

....ZWua:w0.

r....z~ 10

Figure 2 (left) and Figure 3 (right). -MSX kills of late-summer and spring imports of susceptible James River seed oysters. Notethat earlier import lot shows mortality beginning a month earlier than spring-import group. Prevalences of MSX in percentagesare given above arrows for samples of 25 oysters.

2 SEPT. 29 OCT..

o 4.-J .-J

o

12

t16

t

A

Y-92 0 16

t tY-93 8

t

I 25....z0 EARLY SUMMER::E

20 1977 INFECTIONSa:w0-

....Z 15w'-'a:w0- 10

'"W....<la:I imports....<l " _.-w 00

M A M J1977

24

t

MSX

I A

50

t

/II 1

I 1I 1

I 1~' 1

1Y90-1

(58 %~ \

\\

\/

64t

72

t

A i M

1977

Y-90

Y-91

F I M

LATE SUMMER1976 INFECTIONS

Imports_Sept 1976/ _

- ,/

r....~ 30:>a:~ 25

....z~ 20a:w0.

Z "

w....~ 10

:I:....<tWo

48 Marine Fisheries Review

Table 1.-Prevalences of MSX in susceptible oysters in trays at Glouces­ter Point, Va., York River, 197!>-n.

1 Year of import for spring lots (March to March) and year aher import for lalilots.'Closed tray 13 May 1976.

in Table I.A comparison of timing of deaths is

given in Figures 2 and 3 for late-sum­mer and spring imports of susceptibleoysters. Oysters imported in September1976 began dying in late May with apeak death rate in early July and a vaguesecond peak in September. About 60

64

53'7

53

46

65

55

54

34

6644

Mortality'1st year

(%)

12a8

20324

20a

48a

2816a4

72502464a

16128

16

Prevalences(%J

8 Mar. 1977

1 Sept. 1976

26 Aug. 1975

13 June 1977

6 Mar. 1975 8 July 197528 June 1976

6 Mar. 1975 24 June 197626 Aug. 1975 6 Apr. 1976

13 May 197613 Jan. 197610 Aug. 1976

4 Mar. 1976 24 June 197629 July 1976

4 Mar. 1976 26 June 197620 Sept. 197616 Dec. 19762 Sept. 197629 Oct. 197618 May 1977

22 June 197729 July 1977

24 Sept. 1976 18 May 19778Mar.1977 19MaY1977

13 July 197712 Sept. 197713 July 1977

12 Sept. 19774 Nov. 1977

29 July 1977 4812 Sept. 1977 16

Date of import Date sampled

YB3

Y94

Y86

Y88

YB4YB5

Trayno.

Y90

Y93

Y91Y92

Y87

portrays monthly death rates and preva­lences in a pair of duplicate tray-oysterlots. Total warm-season mortalities aregiven under each tray number designa­tion. No D. marinum was involved inthese lots and deaths were caused al­most entirely by MSX. Typical preva­lences and annual mortalities are given

pressure, these late infections may ap­pear clinically in November and De­cember, but most mortality is still de­layed until June-July.

Early summer infections in marginalareas of low salinities ( < 15 %0) are de­layed in becoming clinical untilNovember or later (Andrews4 ). Theseinfections persist through the winterand are actively discharged by oystersabout 1 May when salinities are<10%0. Unless sampling is done, theseinhibited infections may never beknown, for oysters are not killed. How­ever, transplanting oysters with sub­clinical infections to high-salinity wa­ters results in patent infections in aweek or two in the warm season anddelayed patency in the cold season.

In the high salinities (>30%0) of Sea­side, MSX infected oysters erraticallyby year and locality, and then oftenregressed with few or no deaths. Thetiming of infections was typical but re­gression occurred in late summer andspring (Andrews and Castagna, 1978).

Examples of recent MSX mortalitiesare given in Figures 2-5. Each graph

4 Andrews, 1. D. 1965. Fluctuations of MSX(Minchinia nelsoni) in the James River and sea­sonal effects of salinity and temperature. Unpubl.manuscr.

16t

28

tY-BB

(WRECK SHOAL J Y BB

(43%)

Import( __e_ _",-

oJ;,~M~~'~A~~M~-;=:;:::~:L-'----:A-,r-S"---TI ----:o-rl-N-T1~~1976

45

40

'5

'0

30'""'Q.

:I:....zo,. 35

....z

"'<..>f5 25Q.

"'....'"'":I:....'""'o 10

oNaA1975

12t

(HORSEHEAD ROCK) YB3­(53%)

YB3 ---

,/

(RAINBOW ROCK) YB4_/(51 %) /

///

I

YB4 ___

Imporl

\M i A I M ' J

Figure 4 (left) and Figure 5 (right). - Trays of susceptible oysters in pairs reveal about 50 percent mortalities from MSX in 1975and 1976. Typical timings of initiation and of peaks of mortalities are shown by these lots of oysters.

Y-B7 a 48 ~~(67%1t t 1\

a -+-t I \

(RAINBOW ROCK) Y B7 ,~(5B%)51 \

I \I \I \I \I \

\Vi

\\

40

:I:....z 350,.:5 30Q.

....Z

25"'<..>

'"~20

~

"' 15....'"'":I: '0....:30

January-February /979 49

Figure 6.-0ne resistant and two susceptible lots of oysters responded tointensive MSX activity in 1977. Time of import affected time of first deaths insusceptible oysters.

MAR 6 import DI :(52%) ----:,

percent of the oysters died. March 1977imports began dying in early July, amonth earlier than the typical I Augustinitiation of deaths. The year 1977 wasone of very intensive MSX activity.The September 1976 imports were sub­jected to an intensi ve additional infec­tion period in June 1977 which elevateddeath rates to 30 percent per month andaccelerated deaths in comparison withthose in spring imports in Figure 3.Usually there is a 2-month period be­tween initiation of deaths (I June)caused by infections acquired in latesummer and those from early-summerexposure (I August). Note that highdeath rates persisted into August andSeptember in the latter group.

Additional pairs of trays of oystersimported in March 1975 and March1976 are shown in Figures 4 and 5.These are typical years with deaths be­ginning I August and about 50 percentdead during the first warm season ofexposure to MSX. There appear to beslightly higher death rates with origin ofsusceptible seed oysters further up theJames River. Wreck Shoal oysters hada few deaths from MSX on the seed bedin 1976 which is unusual. RainbowRock and Horsehead Rock are seedbeds progressively further up the riverfrom Wreck Shoal.

The feasibility of breeding strains ofoysters resistant to MSX, by selectionof survivors as brood stock, is shown inFigure 6. This experiment was intended

Comparison of DiseasesCaused by Minchinia costalis

and M. nelsoni

Seaside disease (SSO) and DelawareBay disease (MSX) are active fromChesapeake Bay to Long Island Soundalthough MSX has been reported fromNorth Carolina and Massachusetts.SSO is confined to high-salinity coastalwaters whereas MSX penetrates far upthe Delaware and Chesapeake Bays insome years and has been continuouslyactive in the lower bays for 21 and 19years respectively.

A comparison of epizootiology anddevelopmental stages of the two dis­eases is presented. Minchinia costa/isis considered the more adapted patho­gen and presented as the norm for lifecycles of haplosporidans. Seaside dis­ease differs from Delaware Bay diseaseas follows.

I) SSO is restricted to high-salinitywaters along the seacoast, that is>30%0 usually with a lower limit ofabout 25%0. The name "costalis,"meaning rib or coast, reflects this limit­ed distribution. Oysters imported toVIMS from Seaside in April have ex­hibited regression of SSO (salinities<20%0). A few cases of SSO have de­veloped in disease-free, imported, sus­ceptible oysters in Bayside of EasternShore creeks with salinities of about25%0. Seaside seed oysters with SSOinfections were regularly importedcommercially to Bayside creeks prior tothe advent of MSX.

In contrast, MSX does not thrive inthis environment. It requires salinitiesof 15 to 25 %0 for full epizootic acti vity.Oysters frequently acquire early sum­mer infections on Seaside but few dieand regression occurs. MSX infectionsand acti vity are highly variable by yearsand bays on Seaside. This is not a resultof resistance to SSO or MSX, inheritedor acquired, for it occurs in susceptibleJames River stocks.

2) Mortality from SSO is limited toabout I month (late May to late Junetypically) with very high monthly deathrates. Monthly death rates of 50 percentfor the peak 15 days in the first half ofJune are not uncommon and 90 percentrates have been recorded. The short­term, sharp mortalities occur regularly

oA

RESISTANT STRAIND3'-. , _.(BOlo) .....,.-.- " ......

........ - ....... I .................... ,--...... ....

as a study of interactions of D. mari­num and MSX in susceptible and resis­tant populations of oysters. Oystersfrom tray lots infected with D. mari­num in the fall of 1976 were added tolots D 1 and D3 with 02 as a control.Low temperatures in September toNovember) 976 permitted most oystersto discard D. marinum infections be­fore winter temperatures prevailed.Consequently, the disease was very lateinfecting experimental oysters in 1977and only a few deaths in late October in01 can be attributed to D. marinum.

The disparate curves and seasonalmortalities between 0 I and 02 (nativesusceptibles) and 03 (lab-bred selectedresistant strain) fully demonstrate thevalue of genetic resistance to MSX inenzootic waters (Figure 6). These threetrays were placed 50 feet apart in a rowoff VIMS at Gloucester Point, Va.They were exposed to exactly the sameconditions, except 0 I was importedfrom James River in March and 02 inlate May. The 3-week earlier initiationof MSX-caused deaths in 01 presum­ably means that infections occurred inearly May. This is an earlier infectionperiod than had been demonstrated pre­viously. The resistant-oyster lot (03)contained oysters of the same size rangeas the susceptibles, but they had beenexposed to MSX since the previous fallof 1976 without deaths. That is, therehad been no culling of oysters from thegroup by MSX or any other agent.

A M1977

M

35

:I:....z 300::E

lk:w 25Q.

....Zw 20ulk:WQ.

~ 15

w....<t 10lk:

:I:....<t 5w0

0

50 Marine Fisheries Review

in May-June in all Seaside bays fromCape Charles to Cape Henlopen, in­cluding Chincoteague Bay. Nearly alldeaths are associated with sporulationsand very intensive infections.

MSX kills around the year withpeaks in late summer (August-Septem­ber) and early summer (June-July) ofthe following year depending on time ofoyster import and infection. Peak deathrates do not usually exceed 25 to 30percent monthly. Deaths are caused byplasmodial infections of great variabil­ity in timing and intensity. There islittle synchrony in development ofcases and of deaths.

3) Infections of SSO are initiatedonly during enzootics or shortly there­after (June-July). Oysters exposed afterI August do not exhibit SSO kills until22 months later, that is, after exposureto a May-June enzootic. James Riverseed oysters can be grown to marketsize in this time without losses to SSO.

Infections of MSX occur for 5months (late May through October), al­though late-summer and fall infectionsfail to occur in some years. MSX infec­tions also occur during the period ofmortalities in the warm season but errat­ically.

4) SSO sporulates regularly in May­June enzootics. Sporulation results inimmediate deaths of oysters. Most gap­ers and some live oysters exhibit sporu­lation. Maturation of spores is variableby years, but it often occurs in gapersand sometimes in live oysters. If sporu­lation is not achieved, regression ofplasmodia often occurs, but not aftersporulation begins.

MSX sporulates rarely and in youngsusceptible oysters most commonly.The effect is damaging but not neces­sarily lethal immediately. Consequent­Iy, cases of sporulation have beenfound in nearly every month of theyear, and more often in live oysters thanin gapers in 18 years of routine slidereading. In this long series of slides,spore cases have predominated in Juneas they do for SSO. The occurrence ofsporulation has been one case per 2,000plasmodial cases diagnosed.

5) All plasmodia sporulate syn­chronously in SSO throughout the con­nective tisuses of all organs. Infectionsare intensive at this time and tissues are

January-February 1979

riddled with sporonts and sporocysts. Atypical disease syndrome of clusteredcells and large sporulation stages givesa curdled appearance to tissues which isrecognizable at low magnification( 100 X). Epithelia are not involved usu­ally, and the oyster becomes a sack ofwhitish sporocysts.

MSX sporulation is confined to epi­thelia of digestive tubules. One to sev­eral sporonts and sporocysts are visiblein each 7 j.Lm section of all tubules. Thisimplies migration of plasmodia to thetubule epithelia. Numbers of plasmodialeft in connective tissues seem low andthese do not sporulate there. Sporocystscommonly bulge into the lumen of thetubules. All stages of sporulation maybe seen at one time but mature sporesare often scarce in live oysters.

6) The incubation period is long (8months) and relatively fixed in durationin SSO. Between June-July infectionsand appearance of early plasmodia inMarch, only occasional uninucleatehaplosporidan cells in epithelia of di­gestive tubules are seen. Infected oys­ters do not grow new shell in spring.

Incubation period for MSX variesfrom about 4 weeks to 10 months. Inpart, this is related to time of exposureof oysters. Salinities <15%0 retard in­fections. Presumably, infection pres­sure is involved in rapidity of diseasedevelopment, for in years of intensivelosses, deaths often begin early (July).Late-summer infections are always de­layed, but may appear from Novemberto May, again related to intensity ofinfective pressure as implied by preva­lences and death rates.

7) All stages of SSO average smallerin size than those of MSX with sporesaveraging about 4J.Lffi and 8j.Lm long,respectively, and sporocysts rangingfrom 10 to 20 j.Lm and 30 to 50 J.Lffi indiameter, respecti vely. Plasmodia areextremely variable in size and difficultto distinguish between the two patho­gens. Rapid multiplication of SSOtends to produce many <5 j.Lm plas­modia with few nuclei whereas MSXcells are often 10 j.Lm or larger wi thmany nuclei. Overlapping sizes occur.

Notes on Life Cycles

The prime questions about life cyclesare: 1) What is the source and the cellu-

lar form of infective particles? 2) Isthere an alternate host? 3) What is theexplanation of the hidden infections?and 4) How may infections be initiatedartificially? Farley (1967) proposed alife cycle for M. nelsoni, but thosequestions have not been resolved satis­factorily.

The most important evidence bearingon life cycles from epizootiologicalstudies is determination of infectiveperiods from field studies. SSO infectsin June-july during or immediatelyafter enzootics in May-June. It has a

relatively short infection period com­pared with the 5 months for MSX. Thelong incubation periods tend to confuseand obscure the timing of infections.Both pathogens produce infections dur­ing periods of oyster mortality. Any

stage of the pathogen being dischargedby live or dying oysters could be infec­tive. There is little evidence of dis­charge of spores of SSO by live oysters.Gapers killed by SSO have an abun­dance of sporonts or spores. Generaldisintegration of connective tissues isrequired to release them, and this doesnot happen until oysters become gapersas evidenced by stained-slide prepara­tions.

MSX does not produce enoughspores in oysters to sustain widespreadinfections. Hence, there is a wide­spread belief in an alternate host, al­though none have been demonstratedfor haplosporidans. Failure of infectionexperiments led Pixell-Goodrich(1915) to conclude that M. chitonis inchi tons required another host. Barrow(1965), working with M. pickfordi infreshwater snails, claimed directtransmission, but his experimentalsnails were survivors of an earlier en­zootic and may have been infected ear­lier in nature. Again, a long incubationperiod may have been involved.

The scarcity of spores has been ahandicap in attempts to produce MSXinfections. Feeding spores in aquariaand inoculations into the mantle cavity

have been tried sporadically wheneverspores were available without success.This was tried at VIMS with both MSXand SSO in the mid- and late-1960'swith fresh spores directly from gapers.But MSX spores became very rareagain until 1976.

51

Table 2.-Chronological occurrence of spores of Minchinia nelsoni.

Totals 44 cases in 16 years

cases of sporulation in susceptible spatin June 1967 following exposure toMSX in September 1966.

Occurrence ofSporulation of MSX

The occurrence and rarity of MSXsporulation in Virginia deserves somedocumentation (Table 2). The first casewas in a gaper found 3 November 1960.Except for 1966 and 1967, only I or 2cases were encountered each year inabout 10,000 oysters processed « Icase of sporulation per 2 ,000 cases ofMSX). Of 44 cases of sporulationfound in 16 years before 1976, approx­imately one-third were in resistant oys­ters and two-thirds in susceptibles.Many more resistant oysters were pro­cessed into slides. No selection or spe­cial effort to obtain sporulation caseswas made. Oysters were preservedprimarily for prevalence data. In1966-67, 15 of 19 spore cases occurredin live oysters in June-july and the other4 in gapers in mid-winter. Sporulation

is localized in the digestive tubuleepithelia and is not necessarily dis­abling or lethal. Hence, deaths dependmore upon progression of systemic in­fections and spore cases are erratic intiming. It appears that the live oysters inJune had been carrying spores sinceJanuary-February of 1967 at least.

Infection experiments with MSXwere repeated in the fall of 1976. Spor­ulation was common in the Rappahan­nock River spat but mature spores werescarce. No success was achieved withinoculations or feedings of tissueminces, and extra spores remained un­changed (none open or shriveled) in thebottoms of bowls for over a month.Two-month incubation periods were al­lowed before experimental oysterswere sacrificed.

A comparison of life cycles of MSXand SSO reveals long incubation peri­ods in both pathogens. The failure ofhaplosporidan infection experimentsmay be linked to these incubationperiods. Seasonal imports of disease-

GGGGLLGGGL3LGLGGGGLL2LLLLGLLLLLLGLLLLLLLLLL

14G30L

Live orgaper

15R29S

Source of Resistant oroysters susceptible

James R. SJames R. SJames R. SJames R. 5James R. SJames R. 5Eg9 Is. RPotomac R. 5Long Is. Sound 5Mobjack Bay RLong Is. Sound SJames R. SHampton Bar RJames R. 5Hampton Bar R1965 set RJames R. S1965 set R1964 set R1964 set R1965 set R1964 set RJames R. SPiankatank 5James R. S1966 set R1967 set RLong Is. S1968 set S5easide native SMd. 1969 set 51969 set RJames R. S1970 set RMd. 1970 set 55easide natives 55easide natives S5easide natives SJames R. 5James R. 5James R. S

Trayno.

60JJ6Y17MJ9MJ11Y23P2AP4AP5AP6P5AY25P18Y31P18P22Y32P25P10P6P28P10Y67P27MJ16P32P40P53P57S72P66P64MJ22P80P81S86S86B45Y79J37MJ26

3 Nov. 196020 Mar. 196113 Nov. 19634 Sept. 1964

18 Nov. 196420 Oct. 196510 June 196616 June 196620 June 196620 June 1966

1 July 19661 July 1966

15 Dec. 19664 Jan. 1967

17 Jan. 196716 Feb. 196723 Feb. 196713 June 196721 June 196727 June 196729 June 196712 july 196726 July 196730 Aug. 196722 Sept. 196718 Jan. 196818 June 1968

8 Oct. 19698 Oct. 1969

31 July 197017 Feb. 197130 June 197131 Aug. 197128 Sept. 197128 Sept. 1971

1 June 197217 july 197224 May 197322 May 197429 Oct. 1975

6 Nov. 1975

Datesampled

Sporulationin Susceptible Spat

An unexpected and isolated event in1976 provided a new view of sporula­tion of MSX in oysters. On 20 Sep­tember 1976, a tray containing thou­sands of about I-inch spat was found tohave about 40 percent mortality. These4-month-old spat had been reared in thelaboratory at VIMS in May and held ina pond (a sanctuary from MSX) untiltransferred to the York River on 8 July1976. There were no deaths on 23 Au­gust 1976. Fresh smears of digestivetubule tissues on 21 September 1976revealed 16 cases of advanced sporula­tion in 74 unselected live spat (21.6percent). A live sample taken at thesame time for slide material had 88 per­cent MSX infections (43 in 49 oysters)and 39 percent were in sporulation.This was unprecedented in 18 years ofmonitoring oysters for MSX, both na­tive and lab-bred stocks. None of 75other lots of oysters in trays in the samevicinity (some only 50 feet away) ex­hibited sporulation or anything un­usual, although MSX activity was in­tensive in susceptible oysters. '

MSX had infected the spat and de­veloped to sporulation in 10 weeks. Asecond batch of the same lot of suscep­tible spat was imported from the pondto the York River on 16 August 1976.These did not die or show MSX infec­tions through the fall and winter untilJanuary 1977, but died of MSX inJune-july of 1977 without sporulations.This was typical timing for late­summer infections, and it shows thatthe 8 July lot acquired infections in theYork River after importation.

This experience suggests that sporu­lation ofMSX is more likely to occur inyoung, susceptible oysters. The parentsof the spat lot were susceptible oystersfrom the Rappahannock River beingused as controls for MSX activity. Thisis evidently what happened in Mary­land in the drought years of 1965 to1966 (Couch et aI., 1966) when farmore sporulation cases occurred therethan had ever been encountered in Del­aware Bay and lower Chesapeake Baybefore 1976. That is, the oysters wereyoung susceptibles although not spat.Also Myhre's (1972) experiments withsusceptible and resistant spat in Dela­ware Bay are pertinent. He found four

52 Marine Fisheries Review

free oysters to enzootic areas has per­mitted definition of infection periodsand incubation periods, but the causesand criteria for acti vating these phe­nomena are still obscure. The patternsin SSO suggest an annual cycle with along incubation period being normal.Perhaps MSX in acclimated hostswould also have an annual cycle withsporulation at the time of enzootic kills.It seems to be epizootic and out of con­trol on the mid-Atlantic coast with highvirulence and consequently disruptedlife-cycle patterns.

What kind of infection source couldinfect thousands of tiny spat confinedand crowded in one tray in the openwaters of York River almost simulta­neously? Obviously the infecti ve parti­cles had to be abundant and ubiquitousin the area, even if only a few are re­quired to establish an infection. Annualinfections of MSX have been persistentin Chesapeake Bay although late­summer ones have failed some years.There was no time for fouling or­ganisms to build up on the tray. Sinceeach spat must filter the infective parti­cles out of the water, it seems hardlylikely that an alternate host, such as ablue crab defecating on the tray, was apotential source. If there is an alternatehost, the infective particles are water­borne for long distances and the dilu­tion factor must be staggering.

Direct Infections VSAlternate Hosts

The rapid spread of MSX from alocalized center in Mobjack Bay in1959 to all high-salinity areas of lowerChesapeake Bay in 1960 suggests di­rect transmission from these dying,infected oysters. The alternative expla­nation for such a rapid expansion ofdistribution almost requires equallyrapid spread of an alternate host. Thisimplies a host new to Chesapeake Bayor an endemic species newly parasit­ized by MSX without serious reductionof numbers. There has been no evi­dence of a newly imported exoticspecies to fill this role. Is it reasonableto assume that mutation of MSX in anendemic host, not requiring oysters asal ternate hosts, provided the impetusfor epizootics in Delaware Bay andChesapeake Bay oysters? A strain ofMinchinia nelsoni was present in Vir-

January-February /979

glnla as early as 1953 but with lowvirulence (Andrews, 1968). From pres­ent evidence, I conclude that MSX wasintroduced with exotic oysters, first intoDelaware Bay, and that it spread 2years later into ~hesapeake Bay. IfMSX was introduced in oysters, it isunlikely that an alternate or other host isinvolved. Failure to achieve artificialinfections seems to be the major im­petus for speculations about otherhosts. Scarcity of oysters in high­salinity areas that could provide infec­tive particles is another problem to beexplained. The ready achievement ofresistant strains of oysters to MSX inlaboratory and field populationssuggests that oysters can adapt to hap­losporidans in a few decades if muta­tions and exotic strains are exceptedand excluded. Direct studies of viru­lence and life cycle are precluded untilartificial infections can be attained.Availability of spores makes SSO thepreferred species to use in infection ex­periments. Manipulation of environ­mental factors should reveal whatcriteria favor sporulation of SSO.

[Note added in proof. The nameDermocystidium marinum has beenchanged to Perkins us marinus byLevine ( 1978) since preparation of thismanuscript. ]

Literature CitedAndrews, J. D. 1963. Measurement of shell

growth in oysters by weighing in water. Proc.Natl. Shellfish Assoc. 52: I-II .

____ . 1965. Infection experiments in na­ture with Dermocystidium marinum inChesapeake Bay. Chesapeake Sci. 6:60-67.

____ . 1966. Oyster mortality studies inVirginia. V. Epizootiology of MSX, a protis­tan pathogen of oysters. Ecology 47: 19-31.

1968. Oyster mortality studies inVirginia. VII. Review of epizootiology andorigin of Minchinia nelsoni. Proc. Natl.Shellfish. Assoc. 58:23-36.

____ , and M. Castagna. 1978. Epizooti­ology of Minchinia costatis in susceptible oys­ters in Seaside Bays of Virginia's EasternShore. 1959-1976. J. Invertebr. Pathol.32: 124-138.

____ , and M. Frierrnan. 1974. Epizooti­ology of Minchinia nelsoni in susceptible wildoysters in Virginia, 1959 to 1971. J. Invertebr.Pathol. 24:127-140.

____ , and W. G. Hewatt. 1957. Oystermortality studies in Virginia II. The fungusdisease caused by Dermocystidium marinum inoysters of Chesapeake Bay. Ecol. Monogr.27: 1-25.

____ , and J. L. McHugh. 1956. The sur­vival and growth of South Carolina seed oys­ters in Virginia waters. Proc. Natl. Shellfish.Assoc. 47:3-17.

____ , and J. L. Wood. 1967. Oyster mor­tality studies in Virginia. VI. History and dis­tribution of Minchinia nelsoni, a pathogen ofoysters, in Virginia. Chesapeake Sci. 8: 1-13.

____ , and H. D. Hoese.1962. Oyster mortality studies in Virginia: III.Epizootiology of a disease caused by Haplo­sporidium costale, Wood and Andrews. J. In­sect Pathol. 4:327-343.

Barrow, J. H., J r. 1965. Observations on Min­chinia pickfordae (Barrow 1961) found insnails of the Great Lakes region. Trans. Am.Microsc. Soc. 84:587-593.

Couch, J. A., C. A. Farley, and A. Rosenfield.1966. Sporulation of Minchinia nelsoni (Hap­losporida, Haplosporididae) in the Americanoyster Crassostrea virginica. Science (Wash.,D.C.) 153: 1529-1531.

____ , and A. Rosenfield. 1968. Epizooti­ology of Minchinia costalis and Minchinia nel­soni in oysters introduced into ChincoteagueBay, Virginia. Proc. Natl. Shellfish. Assoc.58:51-59.

Farley, C. A. 1967. A proposed life cycle ofMinchinia nelsoni (Haplosporida, Haplospo­ridiidae) in the American oyster Crassostreavirginica. 1. Protozool. 14:616-625.

Feng. S. Y. 1958. Observations on distributionand elimination of spores of Nematopsis os­trearum in oysters. Proc. Natl. Shellfish. As­soc. 48: 162-173.

Grizel, H., M. Comps, 1. R. Bonami, F. Cousse­rans, J. L. Duthoit, and M. A. Le Pennec.1974. Recherche sur I'agent de la maladie de laglande digestive de Ostrea edlllis Linne. Sci.Peche 240:7-30.

Haskin, H. H., L. A. Stauber, and J. A. Mackin.1966. Minchinia nelsoni n. sp. (Haplosporida,Haplosporidiidae): causative agent of the Del­aware Bay oyster epizootic. Science (Wash.,D.C.) 153: 1414-1416.

Hewatt, W. G., and 1. D. Andrews. 1954. Oystermortality studies in Virginia. I. Mortalities ofoysters in trays at Gloucester Point, YorkRiver. Tex. J. Sci. 6:121-133.

Kern, F. G. 1976. Minchinia nelsoni (MSX) dis­ease of the American oyster. Mar. Fish. Rev.38( 10):22-24.

Levine, N. D. 1978. Perkinsus gen. n. and othernew taxa in the protozoan phylum Apicom­plexa. J. Parasitol. 64:549.

Myhre, J. L. 1972. Minchinia nelsoni (MSX)infections in resistant and susceptible oysterstocks. In H. Haskin (editor), Disease resistantoyster program. A report to the NationalMarine Fisheries Service. Rutgers-The StateUniversity, New Brunswick, N.J., 20 p.

Perkins, F. O. 1976. Dermocystidium marinuminfection in oysters. Mar. Fish. Rev.38(/0):19-21.

Pixell-Goodrich, H. L. M. 1915. Minchinia: ahaplosporid ian. Proc. Zool. Soc. Lond.1915:445-457.

Ray, S. M. 1952. A culture technique for thediagnosis of infections with Dermocystidillmmarinum Mackin, Owen, and Collier in oys­ters. Science (Wash., D.C.) 116:360-361.

Van Engel, W. A., W. A. Dillon, D. Zwerner,and D. Eldridge. 1966. Loxothylacus panopaei

(Cirripedia, Sacculinidae) and introducedparasite on a xanthid crab in Chesapeake Bay.Crustaceana 10: II 0-1 12.

Wood, J. L., and 1. D. Andrews. 1962. Haplo­sporidium costale (Sporozoa) associated with adisease of Virginia oysters. Science (Wash.,D.C.) 136:710-711.

53