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Abstracts
Workgroup: Frontiers in Disease Research
Studies On Life Cycle Stages Of The Oyster Parasite Haplosporidium nelsoni (MSX)
Principal Investigator(s):
Susan E. Ford, Rutgers University, Haskin Shellfish Research Laboratory, susan@hsrl.rutgers.edu
Co-Investigator(s):
Robert D. Barber, and Kathryn A. Ashton-Alcox, Rutgers University, Haskin Shellfish Research Laboratory
Funding Period: April 1, 1991 - December 31, 1992
Life cycle studies and transmission experiments on Haplosporidium nelsoni began soon after the parasite was first identified as the cause of epizootic mortalities of oysters, Crassostrea virginica, in the late1950s and early 1960s. One outcome of these studies was that the spore stage, presumed to be a necessary part of the transmission process, was extremely rare in oysters. The general conclusion of these investigations was that an intermediate or alternate host was probably producing the stages that infected oysters, that the oyster might not be a natural host for H. nelsoni, and that the supply of infective particles might be totally independent of the supply of oysters.
In this project, we developed information on the significance of sporulation of H. nelsoni in young oysters and on the findings of ingested haplosporidan spores in the digestive tract lumina of oysters by intensive sampling of spat and yearling oysters in Delaware Bay during 1991 and 1992 and by examining archived tissue sections for the presence of haplosporidan spores in digestive tract lumina. To introduce new ideas into investigations of the life cycle of H. nelsoni, we convened a workshop of experts with knowledge about parasite life cycles, the distribution of other spore or cyst-forming marine organisms, model epizootics, and nearshore ocean circulation, as well as scientists with direct knowledge of the oyster parasite .
Spores of H. nelsoni were found from mid May through the third week in July. Over all sampling dates, about 40% of the spat that had advanced plasmodial infections also had spores, although peak prevalence of spores in advanced infections was 75-100%, suggesting that eventually, all advanced infections progress to the spore stage. The highest number of spores in an individual spat was 1.1 x 106, but the overall mean was 1.6 x 105 per individual. The high frequency of sporulation in spat differs greatly from that in older oysters, in which far fewer than 1% of infected individuals have ever been found with spores. Spat are much smaller than adult oysters, with consequently different physiological properties, including a higher metabolic rate; however, whether any of these conditions helps to support sporulation is presently not known.
Ingested haplosporidan spores were found at all locations examined in Delaware Bay, as well as in several other US east coast locations where H. nelsoni-infected oysters have been found. They predominated from May through October when they were present in 20% to 40% of the oysters examined. We extrapolated our findings to estimate that there must be several hundred spores per liter in the water filtered by the oysters. Although the spores were present during the infection period for H. nelsoni, their frequency showed a weak, negative correlation (r = -0.55; p < 0.02; N = 17 years) with H. nelsoni prevalence the following year, suggesting that if they are a stage in the life cycle of that parasite, they are not directly infective to oysters, but may infect an alternate host.
The conclusions of the workshop on the "Life Cycle and Transmission of H. nelsoni " are summarized below. Although not all are new, they were refined by the involvement of outside experts and are intended to stimulate future research:
1) It is likely that an alternate or intermediate host, or both, exists for H. nelsoni, but we should not give up on looking at the possibility of direct transmission via spores produced in oyster spat (< 1-yr old).
2) It is not surprising that spores would be produced in only one life stage (spat) of the host (oyster); similar occurrences are known in mosquitoes.
3) Because spores are so infrequent in adult oysters, the oyster may be an abnormal/ adventitious host. In this case, an alternate (normal) host would exist that would most likely be very similar to the oyster (i.e., a sessile bivalve) and the seasonal infection cycle would probably also be similar.
4) If an intermediate host exists, it is likely to be quite different from the oyster and possibly one that is itself highly mobile or is dispersed by water currents (i.e., zooplankton, including larval forms). The parasite must have some mechanism to maintain itself near potential hosts (oyster or other similar estuarine species) in the estuary. The potential host is not likely to be a commercially valuable fish species because these have been examined extensively for parasites. Small non-commercial species are candidates, but haplosporidans have never been found in a vertebrate host.
5) An H. nelsoni spore produced in another host might not exactly resemble that produced in oysters, but would be in the classified in same phylum. (Myxobolus cerebralisi, the agent of whirling disease in salmon, for instance, forms two distinct spores in two different hosts: a myxosporean spore in the salmon and a triactinomyxon spore in tubificid worms.) Spore size should not be considered an immutable criterion for differentiating between species.
6) Potential intermediate hosts should match the geographical distribution of H. nelsoni and should be producing spores just before oysters become infected. Such intermediate hosts may experience a rapid die-off at this time; also, sporulation may be extensive in the tissues, causing discoloration that may be evident macroscopically or in fresh squashes.
7) Spores could be looked for in water and sediment samples, employing centrifugation and sieving to concentrate appropriate-size particles. Using oysters themselves as trapping and concentrating mechanisms (i.e., search of gut contents) may be the best way.
8) The viability of H. nelsoni spores in the environment may not be great because ultrastructurally the attachment of the spore lid does not appear to be very strong. On the other hand, dye exclusion studies suggest that they could be long-lived. Also, spores that settled to the bottom would likely be attacked by bacteria and bottom feeders. Conversely, ingested spores might survive the digestive process or even be made infective in the gut of a vector species, which might also transport them.
9) Previous transmission studies with H. nelsoni and H. costale spores may have been compromised by lack of freshness of spores. Experience in other in other host-parasite systems indicates that fresh spores are needed for transmission.
10) A detailed, ultrastructural comparison of H. nelsoni stages in spat and adult oysters during the sporulation period may provide clues as to why spores are so rarely formed in adults. Comparisons should also be made of physiological differences (e.g., enzyme activities or concentrations of metabolic substrates/products).
11) The use of haplosporidans that regularly produce spores (H. costale or M. teredinis) as models would be a valuable avenue of research.
12) A combination of settling and filtration of water before passing it over oysters would help determine infective particle characteristics.
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