History and epizootiology of Haplosporidium nelsoni (MSX), an oyster pathogen in Delaware Bay, 1957–1980

JOURNAL

OF INVERTEBRATE

PATHOLOGY

40,

118-

History and Epizootiology Oyster Pathogen SUSAN E.FoRD' Oyster Sciences,

Research Nelson

Laboratory, Hall-Zoology,

141 (1982)

of Haplosporidium nelsoni (MSX), an in Delaware Bay, 1957-1980 AND HAROLD H. HASKIN

New Jersey Agricultural Experiment Rutgers The State University, P.O.

Received November

Station, Box 1059,

18, 1981; accepted February

and Department Piscataway,

New

of Biological Jersey 08854

18, 1982

Between 1957 and 1959, a previously unknown sporozoan parasite, now designated as Haplonelsoni (formerly Minchinia nelsoni), or MSX, killed 90-95% of the oysters in lower Delaware Bay. Native oysters have been studied for more than 20 years since then to determine long-term disease and mortality patterns resulting from this host-parasite association. Development of resistance to MSX-kill in native oysters has reduced disease mortality to about half the original level, even though the pathogen continues to be very active in the bay. Since the initial epizootic, MSX levels have fluctuated in a cyclic pattern with peaks every 6 to 8 years. Periods of low disease pressure follow very cold winters, while average or above average winter temperatures correlate with high MSX activity. During peak years, every oyster in the lower bay may become infected. Although the parasite is salinity limited, salinities in the lower bay, the area from which oysters are marketed, are nearly always favorable for MSX, and fluctuations in river flow have almost no effect on MSX in this region. Infection periods recur each summer. Some oysters die soon after becoming infected; others survive through winter, but die in spring as the pathogen compounds normal overwinter stresses. Many survivors are able to suppress or rid themselves of infections when temperatures approach 20°C in late spring. Resistance to MSX-kill in native oysters is not correlated with an ability to prevent infection, but with restriction of parasites to localized, nonlethal lesions. The persistence of “hot spots” for infection in areas where oysters are sparse, the lack of spores in infected oysters, and faiiure to transmit the disease experimentally lead to the hypothesis that an alternate or reservoir host produces infective stages of MSX. KEY WORDS: Haplosporidyum nelsoni; oyster, pathogen of; Crassostrea virginica; Delaware Bay, New Jersey. sporidium

INTRODUCTION

The eastern American oyster, Crasvirginica, has been a source of rich local history and tradition, as well as a major economic resource, for many people living along the shores of Delaware Bay. Originally, these vast oyster stocks provided food only for those who lived close to the water and could obtain the shellfish themselves. In the early 19th century, however, fishermen began shipping Delaware Bay oysters to distant markets (Ingersoll, 1881; Lockwood, 1883). At first, oysters were marketed directly from natural setting areas, but by mid-19th century, the practice of “farming” oysters had started. This sostrea

1 Present address: Department of Zoology, University, Durham, N.C. 27706.

Duke 118

0022-201 l/82/040118-24$01.00/O Copyright @ 1982 by Academic Press, Inc. All rights of reproduction in any form reserved.

method, which had become standard for the fishery by the end of the century (Report of the New Jersey State Oyster Commission, 1899), and which continues to the present day, involves the transplantation of seed oysters during May and June, from natural setting areas in the upper bay to privately leased grounds in the lower bay (Fig. 1). Depending on their size, they may be marketed that fall or they may remain an additional year or more before being harvested. The lower salinities of the upper bay seed beds, which range from 9 to 18 ppt at midtide and mean river flow, provide a natural sanctuary from many pests and greatly enhance the survival of young oysters. Averaging 20 to 23 ppt, higher salinity waters of the planted grounds promote growth and fattening.

EPIZOOTIOLOGY

119

OF Haplosporidium

PLANTED

GROUND

AREAS

1 SOUTHWEST

9 ’

%

\

2

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3

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6

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ISLAND

BAR

Maurice River

6

H

PLANTED GROUNDS

5 ‘\

\

6 \

\

\

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FIG.

1. Delaware

Bay

showing

location

of major

At its peak from the late 19th to the mid20th centuries, the annual oyster harvest from the New Jersey portion of Delaware Bay averaged 1 to 2 million bushels. While the industry prospered during these years, risks involved in planting oysters were always present. The common oyster drill, Urosalpinx cinerea, was a serious problem, and heavy, but unexplained, kill from other causes was occasionally reported. Mortalities from unknown agents were local and short term, however, and posed no permanent threat to the industry (Lockwood,

New

\

/

Jersey

seed

beds

/

/

/

/

/

/--

and planted

ground

areas.

1883; Reports of the New Jersey Bureau of Shellfisheries, 1901, 1906, 1909, 1911). The first documented oyster mortality of epizootic proportions occurred in Malpeque Bay, Prince Edward Island, Canada, in 1915 (Needler, 1931). Although all evidence pointed to a highly contagious pathogen, a causative agent was never positively identified (Fraser, 1938). The disease was still present in the early 1930s when it was recognized that native Malpeque oysters had become resistant (Needler and Logie, 1947). Offspring of the survivors of the 1915

120

FORD

AND

epizootic lived while oysters imported into the bay died massively. Large-scale studies of ongoing oyster mortalities combined with investigations into their causes began in the Gulf of Mexico in the late 1940s with the work of Mackin and his colleagues (Mackin et al., 1950; Ray, 1954; Mackin, 1962). They identified and described the disease caused by the oyster pathogen Perkinsus ma&us (see Levine, 1978), formerly Labyrinthomyxa marina and originally designated as Dermocystidium marinum (see Mackin et al., 1950). P. marinus was present in Delware Bay during the mid-1950s its distribution paralleling that of imported Virginia seed oysters (Christensen, unpubl.). The disease never became the serious problem that it is in more southern waters, and it almost completely disappeared when oyster imports ceased in the late 1950s. P. marinus proliferates and kills during warm weather, and its low incidence in Delaware Bay, the northern limit of its range, is probably a consequence of relatively cool water temperatures. There was, then, no recognized, serious pathogen present when oysters in lower Delaware Bay suddenly began dying in the early spring of 1957. Kill on the New Jersey grounds was centered in an area around and west of Egg Island Bar where it ranged up to 85% (Haskin et al., 1965). Mortalities tapered off with increasing distance from this region: the offshore and western edges of the planting grounds were only lightly affected, and the seed beds suffered no mortalities at all (Fig. 2). Haskin et al. (1965a) estimated that half the oysters on the planted grounds died within a 6-week period. Seed oysters were not planted in the spring of 1957, after the kill, but plantings were made in 1958. Heavy mortalities began late that same summer and by winter, at least half the oysters had been lost. An additional 50% or more of the remaining oysters died the following spring, so that

HASKIN

total kill ranged from 75 to 85% 1 year after planting (Fig. 3). This mortality was much more widespread than in 1957, encompassing all of the lower bay and extending upbay over most of the seed beds. On the seed beds, kill decreased in an upbay direction from 70% on New Beds and Bennies to 50% at Cohansey (Fig. 3). Arnolds bed, the uppermost productive seed area, and beds in the mouths of rivers and creeks escaped most of the kill (Haskin et al., 1965a). Between 1957 and 1959, 90-95% of the oysters on the planted grounds died. Production of market oysters dropped precipitously and by 1960 reached an all-time low of about 10,000 bushels. In early 1958, this laboratory initiated a program to investigate oyster mortality throughout the bay and to provide samples for histopathological studies that might identify a cause for the mortalities. An immediate result of the program was the discovery by Leslie A. Stauber of Rutgers, of a previously undescribed multinucleated organism in both living and dead oysters from the area of highest kill. Initially called MSX for “multinucleated sphere X,” the organism has since been placed in the Haplosporida and was originally named Minchinia nelsoni (Haskin et al., 1966). Recently, in a revision of haplosporidan taxonomy, MSX has been redesignated by Sprague (1978) as Haplosporidium nelsoni. Sporulation was described by Couch et al. (1966) and linked to the plasmodial stages using the fluorescent antibody technique by Barrow and Taylor (1966). An outbreak of the parasite occurred in Chesapeake Bay in 1959, causing extensive oyster mortalities then and in subsequent years (Andrews 1966, 1968; Andrews and Wood, 1967; Andrews and Frierman 1974; Farley, 1975). MSX is present on the Eastern Shore of Virginia (Couch and Rosenfield, 1968), and in the coastal bays of New Jersey (New Jersey Oyster Research Laboratory, unpubl.). It has also been recorded in North Carolina and in Long Island Sound (Sindermann and Rosenfield, 1967),

EPIZOOTIOLOGY

OF

121

Haplosporidium

DELAWARE

BAY

OYSTER MORTALITIES SPRING 1957

FIG.

2. Oyster

mortalities

in Delaware

in Massachusetts (Krantz et al., 1972) and in Great South Bay on the south shore of Long Island (Haskin, Canzonier, and Myhre, unpubl.), but has not been the cause of widespread losses in those areas. The study described in this paper covers more than 20 years. A long study such as this has helped to eliminate much of the uncertainty of shorter ecological investiga-

Bay

(New

Jersey

portion)

in spring

1957.

tions that is caused by long-term biological cycles, and fluctuations in salinity, temperature, and other physical factors. Such a study has been particularly important in the case of the oyster-MSX relationship because it has not been possible to infect oysters under controlled conditions. Proximity, feeding, and injection techniques have consistently failed to produce

FORD AND HASKIN

122

DELAWARE

\s

BAY

OYSTER MORTALITIES 1958 - 1959

Cohansey

@I

75 - 85 %

FIG. 3. Oyster mortalities in Delaware Bay (New Jersey portion) between July 1958 and June 1959.

infections in experimental oysters (Canzonier, 1973). Limitation to field studies has made it especially critical for us to collect data under as many different natural environmental conditions as possible to compensate for minimal experimental control in our investigations.

Interpretation of field studies has been aided by an equally long-term experimental program in which laboratory-reared, imported and native Delaware Bay stocks have been exposed to natural MSX infections in trays placed on the tide flats off our Cape Shore Laboratory (Fig. 1). Close

EPIZOOTIOLOGY

monitoring of these stocks, along with experimental studies using resistant and susceptible oysters, has provided detailed information on timing of infections, on disease and mortality patterns, and on mechanisms of resistance to MSX-kill. This paper reports data collected in lower Delaware Bay where MSX is present at high levels. It describes an annual cycle of infection and mortality, and details evidence for the pathogenicity of MSX. It also describes long-term fluctuations in MSX activity and discusses physical and biological factors that influence disease patterns and levels. Finally, it offers evidence of an increase in the ability of native oysters to survive the disease caused by MSX. METHODS

From April 1958 to June 1981, we sampled 96 different groups of oysters within the New Jersey planted grounds. Some of these were natural lower bay set, but most were oysters transplanted from the seed beds for growth and conditioning before market. These oysters are called “plants.” The duration of sampling for any single group varied from 6 months to 5 years. Most were monitored for at least 1 year, but sampling beyond 2 years was frequently precluded by harvesting or by heavy mortality. Samples were collected at approximately monthly intervals from April 1958 through June 1971. Thereafter, they were taken seven or eight times per year to coincide with critical periods in the annual MSX cycle. Mortality samples consisted of a bushel of oysters and boxes collected with a 30 in. oyster dredge. Living oysters, gapers (recently dead or dying oysters still containing soft parts), and boxes (dead oysters with valves still attached, but with tissue gone) were counted. Boxes were further classilied as “new” (those with little or no fouling) and “old” (those with substantial fouling). Based on known fouling and scavenging rates before the sample date, we estimated a “recent mortality interval”

OF

Haplosporidium

123

during which the new boxes and gapers had appeared. These intervals ranged from 2 to 3 weeks during the summer to 10 weeks or more in cold weather. Using the death rate during the “recent mortality interval,” we then calculated the mortality that had occurred since the last sampling period. Mortalities occurring between sampling periods were cumulated into seasonal and annual totals. Oysters killed by drills, crabs, dredging, or mudding were identified and their mortality (predation or damage kill) was calculated separately from oysters whose cause of death could not be determined macroscopically (disease kill), including those killed by MSX. It is likely that predators kill some oysters that would otherwise die from MSX. Thus, disease mortality on natural oyster bottom is often lower than it would be if oysters were protected from predation. Caution must be exercised in comparing these figures with data obtained from tray studies in which predation is largely eliminated (Andrews, 1966; Andrews and Frierman, 1974; Haskin and Ford, 1979). Except where indicated otherwise, the terms “mortality”, ’ ’ nonpredation mortality,” and “disease mortality” are used interchangeably in this report. From each sample, 20 living oysters and up to 10 nondamaged gapers were fixed in Davidson’s solution (Shaw and Battle, 1957) for histological study. Permanent tissue slides of more than 25,000 oysters and gapers were examined. Histological preparation of the slides has been outlined by Douglass and Haskin (1976). Diagnosis was made on the basis of plasmodial stages described by Haskin et al. (1966) in a 6 pm transverse section across gill, stomach, intestine, and digestive diverticula. Information from histological examination included prevalence (percentage of sample in which MSX was positively identified), infection location and intensity, parasite appearance, oyster tissue condition, and host response as measured by blood cell aggregation (hemocytosis). Di-

124

FORD

AND

gestive gland color (evidence of feeding), shell growth, and meat quality were recorded before fixation. Abnormal gross appearance (“sick oysters”) could then be correlated with presence or absence of MSX and, when present, with its intensity and location in the oyster. According to the number of parasites found, infections were scored as follows: 0. Uninfected-no parasites found in the section. 1. Rare-l to 10 parasites in the entire section. 2. Very light-11 to 100 parasites per section. 3. Light-more than 100 parasites per section, but averaging fewer than 1 per 1000X oil immersion field. 4. Moderate-l to 5 parasites per oil immersion field. 5. Heavy-more than 5 parasites per oil immersion field. Averages were based on counts of 20 fields representing all tissues. Infection locations were rated: 1. Epithelial 2. Subepithelial, local 3. General For convenience, subepithelial, local infections will be designated as “local infections.” Parasite number and infection location scores for each oyster were then multiplied to provide a measure of infection intensity. In this system, a rare epithelial infection received a rating of 1 and a heavy, general infection, 15. Moderate or heavy general infections, which score 12 and 15 on this scale, are termed “serious” infections. Data handling. Obvious problems arise in presenting the massive amounts of data gathered during a more than 20-year study of a large population in its natural habitat. Fluctuations in environmental parameters often make interpretation of results frustrating, if not hazardous. Averages are valuable in defining general patterns and levels, but tend to mask important extremes. Presentation of individual data

HASKIN

points is prohibitive because of sheer numbers, and use of unique sets of data as examples is often restrictive and does not accurately reflect a particular general phenomenon. Our data are presented in a manner that we feel best suits each situation. Records of a single oyster group, or the average of many, may be used. Wherever possible, data are qualified with reference to extremes or to deviations from the presented pattern. RESULTS Annual

Infection

AND DISCUSSION Cycle

Infection periods. Haskin et al. (1965) and Andrews (1966) have described patterns of infection for MSX based on timed imports of susceptible oysters into epizootic regions of Delaware and Chesapeake Bays, respectively. In both areas, infective periods occur annually. For lower Delaware Bay, Haskin et al. (1965) reported that winter and spring imports first become infected in June and begin to die 6 to 8 weeks later, in August. Oysters not exposed until late summer or fall become infected then, but do not die until spring or early summer of the following year. An infective period has thus been defined for MSX in lower Delaware Bay that begins and is heaviest in June, decreases in mid-summer, then resumes with varying intensity later in the summer and may extend into November. Andrews (1966) has not reported a midsummer break in infection activity for Chesapeake Bay, but otherwise has found essentially the same pattern for MSX infection. Infection and mortality patterns. Oysters that are transplanted from upper Delaware Bay seed beds to lower bay growing grounds in May and June resemble spring imports and are exposed to a complete infection period. A planting in the spring of 1968 exhibited a representative infection pattern for oysters in lower Delaware Bay (Fig. 4). It illustrates the following description of the MSX cycle in lower Delaware Bay.

EPIZOOTIOLOGY

OF

125

Haplosporidium

JJASONDJFMAMJJ 1966

1969 EXPOSURE

FIG. 4. Representative annual MSX infection pattern showing proportion of infection types. Oysters were planted in spring 1968 at Egg Island Bar and sampled at approximately monthly intervals for a year. N = 20 for each sample.

When transplanted from the seed beds, most oysters are MSX-free or only lightly infected (Haskin and Ford, 1982). The first evidence of disease in histological sections usually appears by late July when oysters begin to show small numbers of multinucleated plasmodia localized along the basement membrane between epithelial cells of gill filaments and water tubes. The infective form of MSX, and its method of transmission are unknown, but the parasites’ initial appearance in the gill indicates that the infective form is water borne when it contacts the oyster. In the early stages of infection, plasmodia divide and proliferate rapidly along the gill epithelium, but do not penetrate the basement membrane (Farley, 1968; Myhre and Haskin, 1968). Parasite concentrations often become large enough to force the epithelial layer away from the basement membrane and cause extensive sloughing of epithelial cells, parasites, blood cells, and debris. Eventually, parasites may break through the basement membrane into the underlying connective tissue. Once inside

this barrier, they spread via the circulatory system so that the earliest systemic MSX are found in major blood vessels and in the blood sinuses surrounding the stomach and intestine. Amoeboid movement (Farley, 1967) may further their spread between connective tissue cells. For the remainder of the summer and into the fall, increasing numbers of oysters show infections and the infections become progressively heavier. As infections intensify, oysters begin to die. Mortality starts in early August, not long after general infections first appear in live oysters. It peaks in September and October, then declines with the onset of cold weather (Fig. 5). From the time of planting until winter of their first year in the lower bay, an average of 15% of the oysters die from all nonpredation causes (Fig. 6). On individual grounds, mortalities as low as 5% and as high as 35% have been recorded. Over two-thirds of the gapers examined at the peak of mortality have recognizable MSX. About 80% of gaper infections are general and nearly two-thirds are “serious” (Table 1). A September-Oc-

126

FORD

AND

HASKIN

% 90 60 40: i 4 L

1964

/ i

20.

.’

JAMES PLANTS

RIVER

/-’ 1’

z

. Monthly

.

I

Prevalence

1964

1965

4

Mortality

INFECTION

1966 YE4R5

1967

1969

TYPE

1969

OF EXPOSURE

FIG. 5. MSX prevalence and monthly and cumulative nonpredation mortality for James River, Virginia, oysters transplanted to Delaware Bay in June 1%4. Arrows mark beginning of infection periods. N = 20 for each prevalence sample.

tober lag in the rate of increase of prevalence (Fig. 4) probably reflects a situation in which diseased oysters are dying faster than infections are developing in the population. Later in the fall, mortality slows, but parasites continue to proliferate in their hosts. In this situation, many infections become widespread and intense without being immediately lethal. As a result, prevalence and intensity peak by early December as water temperatures approach 5°C. During most of the past 23 years, winter prevalences ha,ve averaged 40 to 60%. Individual plantings, however, have shown prevalences as low as 5% or as high as 100%. On the average, half of the infections have become general by this time, many having moderate to heavy parasite concentrations.

Even when disease levels are highest, however, some infections remain local or epithelial. In fact, some oysters which we diagnose as having no MSX may well have very light, local infections missed in the sectioning. Consequently, it is probable that observed prevaIences of 80 to 90% actually represent 100% infection levels and that in some years, virtually every oyster in lower Delaware Bay becomes infected. Activity of both host and parasite slows as water temperature falls below 5°C and little prevalence change occurs over the winter. There is, however, a consistent decrease in the proportion of general infections, notably the “serious” ones, from November to late February. This is partly because many oysters enter the winter period with intense general infections, and for

EPIZOOTIOLOGY Seasonal

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Spring

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Mortslity Total

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Hnphporidium

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Pooled mortality data for seed oysters transplanted to lower Delaware Bay between 1960 First year “fall” mortalities extend from time of planting (about June 1) to mid-December; “fall” mortalities are calculated from early August. “Spring” kill comes between midand late May, and “summer” mortalities occur in June and July after the first year.

most of these individuals, death simply has been delayed. The most severely infected usually do not survive the winter and die between early December and late February while the oyster is still inactive. The loss of these oysters from the population contributes to the pronounced drop in general infections as the winter progresses (Fig. 4). Although death rates are low in December and January (Fig. 5), four out of five oysters that die during this period are infected. Eighty percent of these infections are general and nearly half are “serious” (Table 1). The overwinter period is also detrimental to MSX. The winter decline in infection intensity is not accompanied by a comparable decline in prevalence, indicating that part of the intensity drop is due to loss of parasites from oysters. Supporting this premise is the frequent occurrence of decaying parasites in late winter as well as an increase in the proportion of local infections at this time. A

high percentage of plasmodia are small, dense, perfectly spherical bodies with indistinct nuclei. Many are found, sometimes within phagocytes, being sloughed from gill and digestive system epithelia. In other instances, there is a clear progression from this plasmodial form to poorly staining globules, or to dense spheres resembling brown cell inclusions (Mackin, 1962, p. 159; Haigler, 1964). Farley (1975), finding the same overwinter disappearance of MSX in a study of the disease in Maryland, suggested that it might be due to a defense reaction by the oyster. Since oysters are in a period of greatly reduced metabolic activity at this time, it is more likely to result from other causes. Rapidly proliferating MSX contain numerous large mitochondria (Muller, 1967; Perkins, 1968), suggesting that the parasite is well equipped for aerobic metabolism, and may not tolerate the condi-

128

FORD

MSX

INFECTION

July August September October November December January February March April May June Total Mean

HASKIN

TABLE 1 CHARACTERISTICS IN GAPERS(DEAD FLANTEDGROLJNDSBETWEEN

No. gapers examined

Month

AND

53 18 43 31 42 72 68 259 335 166 45 13 1145

OYSTERQCOLLECTED

FROM

THE

1960 AND 1980

Percentage MSX prevalence

Percentage general infections

Percentage “serious” infections

45 39 63 77 74 83 81 84 64 64 73 38

67 86 89 79 77 80 78 70 58 63 61 60

38 71 59 71 55 52 35 43 23 20 24 40

70

68

37

Note. Prevalence was calculated from the total number of gapers examined. General and “serious” infections are shown as proportions of infected gapers. “Serious” infections are moderate to heavy, general infections. Means are weighted.

tions produced during long periods of anaerobiosis such as exist in the oyster during winter. Also, low temperature itself may be deleterious. The pattern for the first half of the infection cycle, as just described, is remarkably consistent from year to year. Much greater variation exists in the spring. From January through June, cumulative mortality averages about 25%, most of it occurring from mid-February to mid-April (Figs. 5, 6). Death rates during this period are highly variable and total kill actually ranges from 10 to 50%. In Delaware Bay, water temperatures rise above 5°C between mid- and late March, on the average, and it is at this time that oysters, in a generally weakened condition from their lengthy overwintering period, become active again. As they begin to pump, they are often subjected to harsh environmental conditions, including siltation and low salinities from heavy river runoff. Many are unable to cope with the stresses and die with mud or sand packed into their gills.

“Winter

kill,”

as planters call it, was a

problem long before MSX, but the disease has compounded it. Evidence that the parasite is only one of a number of stresses killing oysters at this time is provided by the decreased (although still substantial) proportion of infected gapers found in March and April (Table 1). Also, infections are lighter in spring gapers than in those collected at other seasons, suggesting that infections that might not be lethal in a more favorable environmental situation may combine with other stresses to cause mortality. In this case, even very light lesions may lower an oyster’s ability to deal successfully with other adversities. Altematively, some light gaper infections may be the remains of heavier infections that actually caused the lethal damage. Deaths of infected oysters and continued decay of parasites combine to cause a significant early spring prevalence drop. Then, as temperature nears lO”C, in mid-April, many oysters show gill lesions that resemble new summer infections in every respect. Despite this similarity, winter and spring imports have never acquired MSX before early June. Andrews (1966) made

EPIZOOTIOLOGY

parallel observations using underwater weighing of oysters exposed to MSX. He determined that late summer imports into enzootic areas of Virginia soon became sick and stopped growing, but that visible infections were rare until the following May. Thus, apparent “new” spring infections have been interpreted as late summer-fall acquisitions whose normal course of development has been halted by cold weather (Andrews, 1966; Ford, 1971). They remain subpatent, or undetectable by standard histological examination, until the following spring when warming temperatures stimulate their proliferation. More recent evidence suggests that at least some patent infections regress over the winter and then become active again in spring (Ford, 1979). From these observations, it is apparent that the first MSX kill that occurred in Delaware Bay in the spring of 1957 was the result of infections acquired late in the summer or fall of 1956. Late spring infection levels vary from year to year. In most years, particularly when winter levels are high, there is a distinct late May peak with prevalences and intensities equalling or exceeding the winter levels. Occasionally, it is very low, or even lacking entirely. The latter occurrence is usually associated with low winter disease levels. The late May prevalence peak, when it occurs, is brief. Infections begin to disappear in early June and reach a low point in July, just as a new infection cycle begins. Mortality drops sharply by May, but often rises again for a brief and variable period from mid-June to mid-August. This “second summer” mortality has averaged lo%, ranged up to 20%, but is often less than 5%. Although it occurs at the same time infections are disappearing after the May prevalence peak, and is certainly associated with MSX, this kill does not remove enough infected oysters from the population to account for the drop in prevalence. The annual repetition of this infection cycle was clearly demonstrated by oysters

OF

Haplosporidium

129

from the James River in Virginia that were planted experimentally in lower Delaware Bay in 1964 (Fig. 5). During each of 5 succeeding years, the infection pattern in these oysters was one of rising MSX levels in the fall, a winter plateau, disappearance of infections in the early spring followed by a second prevalence peak, and then a drop in early summer. Since the annual MSX cycle begins with June infections and spans parts of 2 calendar years, reference to a particular cycle in this paper will be by the year in which it started. For example, reference to MSX in 1972 will mean the 1972 cycle, beginning in July 1972 (first appearance of new infections), and ending in June 1973. Sporulution of MSX. The disappearance of infections in the June-July period has not been correlated with sporulation or the release of any other recognized infective stages. To the contrary, sporulation by MSX in Delaware Bay oysters is almost nonexistent. Fewer than a dozen cases have been seen among the many thousands of tissue slides and fresh smears examined in the course of studying natural populations and experimental stocks. Most spores have been recorded in yearling oysters and have been invariably found in the digestive gland. Mortality summary. MSX-related deaths continue after the first year, particularly in late winter, but decrease consistently in each succeeding year (Fig. 6). Pooled data from oysters planted between 1960 and 1977 show a cumulative nonpredation mortality reaching 37% by the end of the first cycle (including second summer kill in July and August). After the second year, this average has climbed to 50% and at the end of 3 years, it stands at 56%. These figures are highly variable and first year disease kill occasionally exceeds 50%, and after 3 years, may reach 90%. Added predation mortality brings first, second, and third year cumulative means to 55, 77, and 85%, respectively. It should be noted that most oysters are harvested during the fall

130

FORD

AND

and winter of their first year on the planted grounds and are not exposed long enough to incur these heavy mortalities. On the average, nonpredation losses account for about two-thirds of all mortalities, and since MSX has been positively identified in two-thirds of all undamaged gapers, we estimate that at least half of all oyster deaths in lower Delaware Bay since the early 1960s have been related to MSX. Year-to-Year

Fluctuation

in MSX Levels

Since MSX was first recognized in Delaware Bay, disease activity has followed a cyclic pattern with peaks every 6 to 8 years (Fig. 7). After the 1958-1959 epizootic, disease pressure declined sharply and from 1960 through 1962 infection levels were very low, with correspondingly low mortalities (Figs. 7, 8). In 1963 prevalence rose and remained high through the mid-1960s. A second decline toward the end of the de-

I

Systemic

Infection

0

Gtll

HASKIN

cade culminated in 1971 when oysters from Miah Maul1 to the Southwest Line, where most of that year’s seed was planted, acquired almost no MSX. The following year, however, disease activity mushroomed throughout the lower bay, producing, by December 1972, the highest prevalences to date. (Infection data for the 1958-1959 epizootic are sparse and it is possible that prevalences were higher then.) High disease levels prevailed from 1972 through 1976, after which a third decline began. By 1978, MSX activity had reached its lowest level since the early 1960s. Then in 1979 and 1980, in a pattern reminiscent of the 1972 upsurge, renewed disease pressure was sudden, and so intense that new prevalence records were again set. Representative plantings illustrate MSX infection patterns and levels, and nonpredation mortalities recorded during years of high and low disease pressure (Fig. 8).

Infection

DEEPWATER ,’ -f-i

W

Wlnler

Peak

s

Spring

Peak

z 2 50 zL

LEDGE-SOUTHWEST

LINE

FIG. 7. Comparison of MSX infections in oysters at the center (Deepwater) and at the western edge (Ledge-Southwest Line) of the planted grounds. Miah Maull plants were substituted in 1976, 1977, and 1980 because of a scarcity of plants at Deepwater. The winter prevalence peak (W) was measured between late November and late December; the spring peak (S), in late May or early June. Gill infections are usually epithelial, but may be subepithelial, local. Systemic infections include general and local infections found in tissues other than the gill, notably the visceral mass. N = 20 for each sample.

EPIZOOTIOLOGY

OF

infect 1

131

Haplosporidiunt

ion

Categories

Epilhelial Subepithelial,

LOCal

General

pL

75

Deepwater

Mixed

Plants

Deepwater

Cohansey

Plants

Cohansey-

Deepwater

1961

Shell

1966

Rock

Ptants

197

i-i

J

0

N MSX

J

M

J

J

Year

8. Annual prevalence patterns and mortalities for four grounds of seed bed transplants illustrating the effects of high and low MSX activity. Cumulative mortalities are indicated by the line graph drawn through the prevalence bars. N = 20 for each sample. FIG.

Years of high activity (1963- 1967, 19721976, and 1979- 1980), exemplified by plantings made in 1966 and 1972, were characterized by distinct winter and late spring infection peaks with prevalence maxima in the 70 to 80% range, occasionally reaching 90 to 100%. Annual mortalities in these

years generally approximated 50%, but in the peak year 1979 ranged up to 67%. Low-activity years (1960- 1962, 19691971, and 1978), represented by 1961 and 1971 plantings, had winter prevalence plateaus rarely exceeding 40%. Highest prevalences were often recorded in April, and

132

FORD AND HASKIN

a late May peak was completely missing. Annual mortality in these years was less than 25%. Variation in MSX Levels with Location in the Planted Grounds The first epizootic mortality caused by MSX was centered around Egg Island Bar and included an area known as Deepwater. Other regions had significantly less mortality (Fig. 2). Each summer since then, the first oysters to show new infections have been those closest to the center of the 1957 kill. In July, after the principal infection period, prevalences at Deepwater are clearly higher than in the surrounding areas (Figs. 1,9). In succeeding months this differential is reduced and by October, has largely disappeared-partly because prevalences

at Deepwater decrease, presumably through deaths of infected oysters, and partly because prevalences in the other areas increase. Reestablishment of the differential in December is largely due to increased prevalence at Deepwater, rather than a decline at other locations. During the high MSX years in the mid196Os, the final peak prevalences at Deepwater averaged about 30 percentage points higher than those at the Southwest Line region at the upbay edge of the planting grounds (Fig. 7). More recently, during 1972- 1976 and 1979- 1980, this differential has been largely eliminated, with equally high maximum prevalences recorded at most locations in the planted grounds. Nevertheless, the time lag in appearance of new infections at a distance from Deepwater persists.

Nx340

0% J October N-260 December N-500

FIG. 9. Chronological progression of MSX infections in newly planted oysters in lower Delaware Bay between 1966 and 1978. Prevalences at selected locations are expressed as a percentage of Deepwater prevalence during the same month. DW, Deepwater; MM, Miah Ma&; L, Ledge; R, Ridge; SWL, Southwest Line.

EPIZOOTIOLOGY

Factors Influencing Levels

MSX Patterns

and

OF

Haplosporidiltm

133

oysters support this hypothesis: spring prevalences peak as the temperature approaches 20°C and then many infections Temperature. A. Effect on seasonal disappear as it exceeds 20°C. cycle. Seasonal temperature changes are Taken together, these findings help exclearly a major influence on the annual plain the annual MSX cycle by suggesting MSX cycle. Cold weather effects on host temperature optima for both host and paraand parasite activity have already been de- site. At 5°C and below, both are relatively scribed. Elevated temperatures also play an inactive. Between about 5°C and 2O”C, the important role, particularly as they affect parasite multiplies faster than the oyster defense mechanisms against the parasite. can control it, while above 20°C resistant Myhre and Haskin (1970) compared MSX oysters can suppress or rid themselves development in two groups of laboratoryof MSX. B. Effect on year-to-year fluctuation. reared oysters. One consisted of three susceptible stocks with no history of exposure Winter temperatures are correlated with to MSX. The second group of three resis- MSX fluctuations on the planted grounds. tant stocks had ancestors well selected by When prevalences are superimposed on disease in lower Delaware Bay. Both local air temperature (Climatological Data: groups were first exposed to MSX in late New Jersey. NOAA, U.S. Department of summer. Development of infections during Commerce), averaged for the period Defall and winter was similar in all stocks with cember through March, it is immediately maximum prevalences of 73 to 80% re- apparent that each of the three low prevagardless of background. At this time, most lence periods is associated with one or infections were localized in the gill epithemore comparatively harsh winters precedlium. With rising temperatures in the ing it by a full year (Fig. 10). Conversely, spring, however, the pattern changed com- the high prevalences of the mid-1960s and pletely. Infections intensified rapidly in un- of 1972- 1976 came during periods of modselected stocks, causing significant mor- erate or high winter temperatures. talities. Those in the selected groups deIf these correlations do, in fact, indicate a creased in intensity and number, with no cause and effect relationship, they suggest concomitant mortality-a pattern described that relatively small ambient temperature for native oysters in this report. By July, differences can bring about significant MSX 87% of the unselected oysters were in- fluctuations. It is perhaps more likely that fected, while only 7% of the selected stocks the low 4-month average temperatures are had MSX. Haskin and Douglass (1971) ex- an index to shorter periods of extremely panded this finding by transferring naturally low temperature which may be the effective infected oysters from enzootic water in late controlling factor. These observations may winter to the laboratory where they were explain the lack of cyclic MSX activity in kept under various temperature regimes. the lower Chesapeake (see Comparison of Oysters held at 5°C showed little change in MSX in Delaware and Chesapeake Bays) MSX levels during the course of the ex- and the failure of the parasite to cause sigperiment; those held at 10” and 15°C dem- nificant oyster mortalities in Long Island onstrated an initial prevalence rise and then waters. In the Chesapeake, critical low no further change, while those at 18.5” to temperatures may never be reached, or 22°C showed an early infection increase may not exist long enough, to inhibit the followed by declines in both prevalence and parasite. While in Long Island waters, winters may never be mild enough to permit intensity. These investigators suggested that mechanisms of resistance to the disease significant MSX activity. are not fully effective at temperatures We have described the frequent occurrence of dead and decaying parasites in late below about 20°C. Our data from native

FORD

AND

HASKIN

TEMPERATURE 1 MSX PREVALENCE

66

56

60

62

r

64

66 MSX

66

70

72

74

76

76

60

YEAR

10. Winter temperature-MSX relationship. Temperatures were measured at Millville, 12 miles north of the mouth of the Maurice River and are an average of daily mean air temperatures for December through March of the corresponding MSX year. Except where noted, prevalences were averaged for all samples collected during the year from paired Deepwater and Southwest Line grounds during the first year after planting. Post-first year plants were included in 1958, 1959, 1962, 1963, 1965, 1967, and 1977; December and May samples only were used for 1958 and 1959; and Deepwater plants only were used for 1958 and 1960- 1965. N = 18 for 1958 and 40 for 1959. Thereafter each prevalence point represents 100 to 380 oysters. FIG.

winter histological samples and have ascribed it to the adverse effects of winter conditions on MSX. However, the reappearance and proliferation of parasites within the same host when temperatures rise in spring argues against a direct and permanent low-temperature elimination of MSX from oysters as the cause of cyclic fluctuations in prevalence. An alternate hypothesis relating cold winters to MSX

fluctuations sion.

is presented later in the discus-

Salinity. Salinity is the dominant control on MSX in the upper Delaware estuary, particularly where mean salinity is below 15 ppt (Haskin and Ford, 1982). In the lower bay, association of high disease activity with high salinity drought conditions in the mid-1960s (Fig. 7) also suggested a connection between disease cycles and salinity

EPIZOOTIOLOGY

OF

Haplosporidium

13s

there. However, the high MSX levels be- kill oysters, MSX cannot successfully comtween 1972 and 1976, a period of high Del- plete its life cycle, rarely sporulates, and aware River flows and correspondingly cannot produce infective particles within less than average salinities, seriously this host, and that the source of the paraweakened this linkage. In fact, year-to-year sites that infect oysters is some other ordisease fluctuations in the lower bay are not ganism. correlated with river flow. Numerous oyster associates have been Salinities in the lower bay do fluctuate examined histologically and in proximity seasonally, though. In spring, average experiments (Sprague, 1961; Oyster Reminimum values are about 17 ppt, with ex- search Laboratory, unpubl.), but until the tremes as low as 12 ppt recorded very present none has proved to be another host briefly in some years. Fall maximum for MSX. Hillman (1978) has recently desalinities average 24 ppt with occasional scribed a haplosporidan parasite infecting extremes above 30 ppt. In addition, sa- three species of Teredo from Barnegat Bay, linities vary within the planted grounds New Jersey. The spores of this parasite are from one location to another. Deepwater, indistinguishable, under the light microwith the highest MSX levels, also has the scope, from MSX spores. Plasmodial, prehighest salinity. But at 22.5 ppt, it averages spore, and spore stages of what is probably less than 3 ppt higher than Ridge or Souththe same organism have been found in west Line. Thus, both seasonal and re- Teredo navalis in Delaware Bay (Oyster gional variations within the planted grounds Research Laboratory, unpubl.). The possiare relatively small and do not fall outside ble role of this parasite of shipworms in the the range favorable to MSX (Haskin and association of MSX and oysters is now Ford, 1982). In fact, salinities in the planted under joint study by Hillman and this labogrounds seem to be ideally suited to the ratory. Our analysis fails to implicate either tempathogen. perature or salinity as directly affecting the Experimental studies have shown that established MSX infections disappear from MSX-oyster relationship in a manner that oysters transferred from high to low salinity would result in the year-to-year disease water (Sprague et al., 1969; Haskin, un- fluctuations observed in Delaware Bay. Furthermore, these fluctuations do not corpubl.). However, in both cases, salinities associated with parasite loss were well relate with changes in the abundance or distribution of the oyster population or with below those encountered in the planted its level of resistance to disease kill. These grounds. The possibility that consistent small differences may affect disease levels facts, together with the probability of a recannot be discounted, but salinity fluctuaservoir host, lead us to postulate that cyclic tions apparently have far less influence on MSX levels may reflect cyclic abundance of MSX patterns in the lower bay than do such a host. By this hypothesis, the immediate cause temperature cycles. of MSX fluctuations in the lower bay would Reservoir host control of MXS cycles-an hypothesis. Failure to transmit MSX ex- be variations in the intensity and duration perimentally from oyster to oyster com- of the infective period, i.e., in the number of infective particles produced and the bined with the evidence that the parasite rarely produces spores in oysters in Dela- length of time they are present and water ware bay suggests an alternate host for borne. High winter and spring prevalences MSX. These and other pieces of evidence suggest that large numbers of infective parhave been discussed by Andrews (1%6) and ticles have been available throughout the Farley (1967) in their analyses of MSX in entire June to November infective period. Chesapeake Bay. All indications are that Lower winter prevalences followed by very despite its capacity to proliferate in and to low spring prevalences may indicate low

136

FORD

AND

production of infective material confined to an early summer period. In this scheme, winter temperatures would work directly on the reservoir host, rather than on the parasite, to influence MSX fluctuations. Thus, cold winters might reduce the reservoir population and diminish the amount of infective material produced the following summer. Such a reservoir host linkage might then explain the delay of a year between an extremely cold winter and a subsequent low infective period. Infective particle concentration may also play a key role in prevalence differentials among areas within the planted grounds. The pattern of decreasing prevalence in an outward direction from the center of the planted grounds (Fig. 9) suggests a source of infective particles in the Deepwater area or perhaps farther toward the mouth of the bay. As they are initially produced, these infective stages would be concentrated enough to infect virtually all oysters in the immediate area. Distribution over the planted grounds from this source could account for the delay in appearance of new infections at a distance from the source, and dilution might result in the infection of fewer oysters. If the initial concentration is great enough, however, even the periphery of the planted grounds might contain enough material to infect nearly every oyster. Such a situation could have produced the very high prevalences found throughout the planted grounds between 1972 and 1976, and in 1979 and 1980. Resistance to MSX. The elimination of MSX-susceptible oysters from a population will strongly influence disease levels. Selective MSX mortality increases the proportion of resistant oysters, both in the generation under selection and in their progeny (Haskin and Ford, 1979). We examined groups of seed bed transplants to assess the effect of prior selection by MSX on mortality, prevalence and infection intensity in Delaware Bay native oysters. A. Mortality. Decreased mortality in native oysters was noted soon after the first

HASKIN

epizootic (Haskin and Ford, 1979). Twoyear kill for planted oysters in 1957-1959 was 90-95%. Since 1960, it has averaged 50%, but has fluctuated with disease activity rather than exhibiting a steady decline. Lowered death rates, even when infection levels are high, have been a major factor in the continued existence of an oyster industry in Delaware Bay. Mortality patterns in oysters exposed to MSX over a number of years indicate that, although deaths after the first year are occasionally heavy, even in well-selected stocks (Fig. 5), oysters undergoing second or third infection cycles generally have lower mortality rates than oysters experiencing their first heavy MSX pressure (Fig. 6). The above evidence for resistance to MSX-kill in native oysters under natural conditions parallels our findings of markedly reduced deaths in laboratory-reared progeny of oysters that have survived several generations of rigorous experimental selection. Under the experimental conditions, native oysters have nearly three times as many survivors after 3 years exposure to MSX than do unselected imports (Haskin and Ford, 1979). B. Prevalence and infection intensity. Both prevalence and infection intensity are measures of disease, but they differ in sensitivity as indicators of stress that MSX places on the oyster. For instance, two groups of oysters may have identical prevalences, but infections in one may be mostly general, while those in the second, mostly localized. The greater stress on the first group is clearly shown by a comparison of infections parameters in living and in dead oysters (Table 2). On the average, gaper prevalence is nearly 1.5 times higher than in living oysters. Epithelial and local infections are no more prevalent in dead than in live oysters, while general and “serious” infections are approximately 3 and 3.5 times more frequent, respectively, in gapers. Also, oysters noted as “sick” during gross examination, and subsequently found to have MSX, have 2 times more

EPIZOOTIOLOGY

OF

137

Haplosporidium

TABLE 2 OF INFECTION PARAMETERS BETWEEN LIVE OYSTERS AND GAPERS COLLECTED FROM GROUPS UNDERGOING THEIR FIRST YEAR’S Exposuaa TO MSX ON THE PLANTED GROUNDS, 1960- 1980

COMPARIXIN

Live oysters 4533

Number examined:

Prevalence Epithelial infections Subepithelial infections General infections “Serious” infections

Gapers 704

No.

%

No.

%

22%

49

500

71

1.4

848

19

60

9

0.5

591

13

82

12

0.9

767

17

358

51

3.0

347

8

189

27

3.4

Nore. “Serious” infections are moderate to heavy, general infections. All proportions number of live oysters or gapers examined.

general, and 4.5 time more “serious” infections than do oysters infected but not noted as “sick.” These observations lead to the conclusion that infections are rarely lethal until they develop into general infections, and that infection intensity is a better measure of how oysters handle the parasite than is prevalence. With this background, we can examine changes in prevalence and infection intensity in natural populations as a consequence of MSX selection. Prevalence in first year plants between 1960 and 1980 shows considerable year-to-year fluctuation (Fig. 11). In contrast there are indications that the proportion of “serious” infections decreased from early to mid-1960s. For the most part, “serious” infections now represent a distinctly smaller proportion of the total number of infections than they did earlier in the MSX epizootic. When the same parameters in first year plants are compared to those in oysters undergoing their second, third, or fourth year of exposure and selection in the lower bay, a similar pattern emerges (Fig. 12). Little difference can be seen in prevalence levels between selected and unselected groups. When infection intensity is considered, on the other hand, it is evident that in September and May, when water temperatures are around 2O”C, oysters that have experi-

Gapers/live

are based on the total

enced previous selection have only about half as many “serious” infections. These data also support the hypothesis that resistant mechanisms are more effective at high than at low temperatures, for this difference is not present in December and March when the temperature is near 5°C.

FIG. 11. Comparison of MSX prevalence with the proportion of “serious” (moderate to heavy, general) infections in live oysters. Oysters were sampled 5 to 10 times during their fast year of exposure. In each year after 1966, one group at Deepwater or Miah Maull was paired with a group at Ledge-Southwest Line to eliminate variation due to planting ground location. Seed oysters were not planted in 1962, 1963, and 1%5. New plants were not available in Deepwater or Miah MauU in 1%7 and 1977. Numbers above bars indicate ovsters examined in the case of mevalence and total infections in the case of “serious” infections. -

138

FORD

AND

HASKIN

viduals that cannot prevent infections from becoming widespread, intense, and lethal. Comparison Delaware

FIG. 12. Comparison of siasonal MSX prevalence and proportion of “serious” (moderate to heavy, general) infections between seed bed transplants undergoing their first year of heavy MSX selection and those that have already experienced one or more years of selection. Data were collected between 1964 and 1976. Plantings were paired by year and planting location to minimize variability due to these factors. Numbers above bars indicate oysters examined in the case of prevalence and total infections in the case of serious infections.

These comparisons show that survivors of selective MSX mortality in the native population have as many infections as unselected oysters, but that these infections are less intense. Two earlier observations help in the interpretation of these data: (1) When oysters are exposed to MSX, not all infections progress invariably from local gill lesions into widespread systemic infections. Even at times of highest prevalences, some oysters have only light, localized infections. (2) Intense general infections are far more likely to be lethal than are light, local ones. Taken together, these findings indicate that within a susceptible population there exists a wide range of host environments with varying suitability for MSX development. An oyster with the ability to localize parasites has a clear survival advantage, and an increased fraction of the population with this capacity is a consequence of selective mortality of those indi-

of MSX Patterns in and Chesapeake Bays

Andrews (1966) and Farley (1975) have reported seasonal MSX patterns in the Chesapeake similar to that which we have described for Delaware Bay, with highest prevalences in winter and again in late spring. Susceptible James River seed, in experimental trays, has been regularly exposed to MSX in the high salinity (15-25 ppt) portion of both Delaware and Chesapeake Bays (Andrews and Frierman, 1974; Haskin and Ford, 1979) and provides a good comparison of disease conditions in the two locations. During the early 196Os, James River oysters exposed at the Virginia Institute of Marine Science (VIMS) on the lower York River had first season (June to November) kill of about 30%. The same stock, exposed during the same period on the tidal flats at our Cape Shore Laboratory in Delaware Bay lost 60-65%. Greatest mortalities in both areas occurred during the mid-1960s drought when peak kill in the Chesapeake location reached 62%, while in the Delaware it climbed up to 85%. Subsequently, as the drought ended, Chesapeake losses were 45-55%, while those in the Delaware ranged between 50 and 75%. These tests, in fully enzootic areas of both estuaries, demonstrate consistently higher MSX kill at the Delaware Bay location. They suggest, therefore, that MSX pressure may be higher in Delaware Bay than in Chesapeake Bay. It is important, however, to remember that disease levels do vary, even within regions of high MSX activity, and it may not be valid to extrapolate measurements at a single location to the entire bay. One of the striking differences between the two locations is the comparatively steady disease pressure in the lower Chesapeake in contrast with its cyclic pattern in Delaware Bay. Between 1960 and 1963, and again from 1967 to 1971, max-

EPIZOOTIOLOGY

imum prevalences in the Chesapeake were 40- 50%. During the drought, from 19641966, they ranged from 60 to 80%, while during 2 very wet years, in 1972 and 1973, MSX activity was markedly reduced (Andrews and Frierman, 1974). Thus, the two major changes in disease pressure in the lower Chesapeake (as measured at VIMS) have been closely associated with salinity, a situation which clearly contrasts with lower Delaware Bay. One interesting parallel can be drawn between the two estuaries. In Delaware Bay, Deepwater persists as a “hot spot” and possible source of MSX infective material, despite a scarcity of oysters in the area. This is similar to the situation described by Andrews and Wood (1967) in which apparently isolated oysters in lower Chesapeake Bay regularly became infected with MSX. Both findings strengthen the argument for a reservoir host’s providing infective material. MSX-Past

and Future

This study of the interactions between a dominant member of an important invertebrate community and its serious, previously unknown pathogen, has yielded much valuable information. It is clear that MSX is well established in Delaware Bay and that the intensity of its attack on oysters has not diminished. The survival of oysters under attack depends in large measure on their ability to defend against the pathogen by localizing the parasite and preventing infections from becoming intense and lethal. Since the first disease outbreak killed 90% of the oysters in the lower bay in the late 1950s natural selection has increased the proportion of disease resistant oysters to the point where mortalities are cut approximately in half. There is still much to be learned about MSX that could prove useful in its control. Experimental work would be accelerated immeasurably by the availability of a systern for controlled transmission, or infec-

OF

139

Haplosporidium

tion. This in turn will probably depend on more complete information on the life cycle and perhaps resolution of the question of a reservoir host. The comparative physiology and biochemistry of susceptible versus highly resistant oysters available from our selection and breeding program will undoubtedly yield information on the underlying mechanisms by which selected oysters can contain the disease. Such knowledge would carry promise of more effective management of interactions between oysters and their parasites. ACKNOWLEDGMENTS The work reported here spans 23 years. It has involved the dedicated efforts of many people based at our field laboratory in Bivalve, New Jersey. The continuity and integrity of the field sampling has depended largely on the team work of Donald E. Kunkle, Senior Laboratory Biologist, and the late William A. Richards, Boat Captain. In addition to the senior author, two histopathologists, John Myhre and Daniel O’Connor, prepared and examined slides. Undergraduate assistants too numerous to mention have assisted in collection and work-up of samples. We deeply appreciate the succession of state and federal administrators who have had the vision and insight to provide funds for this long-term project. Support to the junior author has been nearly continuous: in the early years by contract with the Bureau of Commercial Fisheries of the U. S. Fish and Wildlife Service, and, since it inception, by grants-in-aid under P. L. 88-309. by the National Marine Fisheries Service and the Division of Fish, Game and Shellfisheries of the State of New Jersey. Finally, we want to thank the many oystermen who permitted us to sample their grounds and without whose support and cooperation, this study would not have been possible. This is New Jersey Agricultural Experiment Station Publication D-325041-82, supported by State funds and by Public Law 88309 funds.

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Minchinia 23-36.

nelsoni.

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Nat.

Shellfish.

ASSOC..

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ANDREWS, J.. D., AND FRIERMAN, M. 1974. Epizootiology of Minchinia nelsoni in susceptible wild oysters in Virginia, 1959 to 1971. J. Invert&r. Puthol., 24, 127-140. ANDREWS, J. D., AND WOOD, J. L. 1967. Oyster mortality studies in Virginia. VI. History and distribution of Minchinia nelsoni, a pathogen of oysters, in Virginia. Chesapeake Sci., 8, l- 13. BARROW, J. H., Jr., AND TAYLOR, B. C. 1966. Fluorescent-antibody studies of Haplosporidian parasites of oysters in Chesapeake and Delaware Bays. Science, 153, 1531-1533. CANZONIER, W. J. 1973. Tissue grafts in the American oyster, Crassostrea virginica. Proc. Nat. Shellfish. Assoc. 64, 92-101. COUCH, J. A., FARLEY, C. A., AND ROSENFIELD, A. 1966. Sporulation of Minchinia nelsoni (Haplosporida, Haplosporidiidae) in Crassostrea virginica (Gmelin). Science, 153, 1529- 1531. COUCH, J. A., AND ROSENFIELD, A. 1%8. Epizootiology of Minchinia costalis and Minchinia nelsoni in oysters introduced into Chincoteague Bay, Virginia. Proc. Nat. Shell&h. Assoc., S&51-59. DOUGLASS, R. W., AND HASKIN, H. H. 1976. Oyster-MSX interactions: Alterations in hemolymph enzyme activity in Crassostrea virginica during the course of Minchinia nelsoni disease development. J. Znvertebr. Pathol., 27, 317-323. FARLEY, C. A. 1%7. A proposed life cycle of Minchiniu nelsoni (Haplosporida, Haplosporidiidae) in the American oyster Crassostrea virginico. J. Protozool., 15, 616-625. FARLEY, C. A. 1968. Minchinia nelsoni (Haplosporida) disease syndrome in the American oyster Crassostrea virginica. J. Protozool., 15, 585-599. FARLEY, C. A. 1975. Epizootic and enzootic aspects of Minchinia nelsoni (Haplosporida) disease in Maryland oysters. J. Protozool., 22, 418-427. FORD, S. E. 1971. MSX - Ten years in the lower Delaware Bay. Proc. Nat. Shellfish. Assoc., 61, 3. FORD, S. E. 1979. Chronic infection of Minchinia nelsoni (MSX) in Delaware Bay oysters. Proc. Nut. Shellfish Assoc., 69, 193- 194. FRASER, R. 1938. “Pathological Studies of Malpeque Disease,” Fisheries Res. Board of Canada, MS report of the Biological Station No. 144. HAIGLER, S. A. 1964. A Histochemical and Cytological Study of the Brown Cells Found in the Auricular Pericardial Gland and Other Tissues of the Oyster, Crassostrea virginica (Gmelim), M.S. Thesis, University of Delaware, Newark. HASKIN, H. H., CANZONIER, W. J., AND MYHRE, J. L. 1965. The history of MSX on Delaware Bay oyster grounds 1957-65. Amer. Malcol. Union Bull., 32, 20-21. HASKIN, H. H., AND DOUGLASS, W. R. 1971. Experimental approaches to oyster-MSX interactions. Proc. Nat. ShellJish. Assoc., 61, 4.

HASKIN, H. H., AND FORD, S. E. 1979. Development of resistance to Minchinia nelsoni (MSX) mortality in laboratory-reared and native oyster stocks in Delaware Bay. Mar. Fisher. Rev., 41, 54-63. HASKIN, H. H., AND FORD, S. I. 1982. Huplosporidium nelsoni on Delaware Bay seed oyster beds: A host-parasite relationship along a salinity gradient. J. Znvertebr. Pathol., in press. HASKIN, H. H., STAUBER, L. A., AND MACKIN, J. A. 1966. Minchinia nelsoni sp. (Haplosporida, Haplosporidiidae): Causative agent of the Delaware Bay oyster epizootic. Science, 153, 1414-1416. HILLMAN, R. E. 1978. The occurrence of Minchinia sp. (Haplosporida, Haplosporidiidae) in species of the molluscan borer Teredo, from Bamegat Bay, New Jersey. J. Znvertebr. Pathol., 31, 265-266. INGERSOLL, E. 1881. “The History and Present Condition of the Fishery Industries.” Section X (Monograph B). “A Report on the Oyster-Industry of the United States.” U. S. Gov. Printing Of&e, Washington, D.C. KRANTZ, E. L., BUCHANAN, L. R., FARLEY, C. A., AND CARR, A. H. 1972. Minchiniu nelsoni in oysters from Massachusetts waters. Proc. Nat. Shellfish. Assoc., 62, 83-88. LEVINE, N. D., 1978. Perkinsus gen. n. and other new taxa in the protozoan phylum Apicomplexa. J. Parasitol., 64, 549. LOCKWOOD, S. 1883. The American oyster: Its natural history and the oyster industry in New Jersey. In “Fifth Annual Report of the Bureau of Statistics of the Bureau of Labor and Industries of the State of New Jersey, pp. 217-350. Trenton, New Jersey. MACKIN, J. G., OWEN, H. M., AND COLLIER, A. 1950. Preliminary note on tlie occurrence of a new protistan parasite, Dermocystidium marinum n. sp. in Crnssostrea virginica (Gmelin). Science, 111, 328-329. MACKIN, J. G. 1962. Oyster disease caused by Dermocystidium marinum and other microorganisms in Louisiana. Zn “Studies on Oysters in Relation to the Oil Industry” (J. G. Mackin and S. H. Hopkins, eds.), Publ. Inst. Mar. Sci., Vol. 7, pp. 132-299. MULLER, E. 1967. Electron microscope studies of Minchinia nelsoni. Proc. Nat. Shellfish-Assoc., 57,3. MYHRE, J. L., AND HASKIN, H. H. 1968. Some observations on the development of early Minchinia nelsoni infections in Crassostrea virginica and some aspects of the host-parasite relationship. Proc. Nat. Shellfish Assoc., 58, 6. MYHRE, J. L., AND HASKIN, H. H. 1970. MSX iufections in resistant and susceptible oyster stocks. Proc. Nat. Shellfish. Assoc., 60, 9. NEEDLER, A. W. H. 1931. The oysters of Malpeque Bay. Biol. Bd. Canada. Bull., No. 22. NEEDLER, A. W. H., AND LOGIE, R. R. 1947. Serious mortalities in Prince Edward Island oysters caused

EPIZOOTIOLOGY

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5, 41, 73-89.

New Jersey Bureau of Shell Fisheries, Reports of 1901, 1906, 1909, 1911. Trenton, New Jersey. New Jersey State Oyster Commission, Report of, 1899. Trenton, New Jersey. PERKINS, F. 0. 1%8. Fine structure of the oyster pathogen. Minchinia nefsoni (Haplosporida, Haplosporidiidae). J. Invertebr. Pathol., 10, 287-307. RAY, S. M. 1954. “Biological Studies of Dermocystidium marinum, a Fungus Parasite of Oysters,” Rice Institute Pamphlet, Special Issue, November, pp. 1-114. SHAW, B. L., AND BATTLE, H. 1. 1957. Gross and

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