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Dermo is the common name for an oyster disease caused by the protozoan parasite Perkinsus marinus. Perkinsus marinus is a single-celled organism that has many different life stages. In the picture below, taken by LCF students in the summer of 1999, P. marinus cells can be seen as perfectly round spheres that are dark purple in color (bluish color in the middle and a darker color on the outside).
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Although this organism can survive in freezing waters, its infectious stages only live in warmer temperatures. Dermo is found most often in the months from May through October when the water is between 25 and 30 degrees Celsius (77 and 86 degrees Fahrenheit). Although Dermo prefers high salinity ranges, it can be found in water with salinity levels anywhere between 8 and 32ppt.
Salinity is a measure of how much salt is in the water. Full strength seawater at the mouth of the Chesapeake Bay has a salinity of 32 ppt, or parts per thousand. This means that there are 32 grams of salt in 1000 ml of water. (This also equals 3.2%. Although 3.2% salt does not sound like a lot, when you accidentally swallow some seawater at the beach while boogie boarding, it certainly tastes salty!) The Chesapeake Bay becomes less and less salty as you go up the bay, because fresh water mixes with the seawater to dilute it. Summers in the bay are the saltiest, because the hot weather evaporates the water, leaving the salt behind. Since Dermo prefers high salinity and warm temperatures, this means that the summer months are the beginning of a growth period for the Dermo!
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Oysters become infected with Perkinsus marinus through the filter feeding process, when the protozoan first attacks the lining of the digestive system. As an oyster becomes more highly infected, its rate of growth slows, the amount of soft tissues in the oyster's body decreases, adductor muscle function becomes impaired, and reproduction is inhibited. Oysters infected with Perkinsus marinus usually do not die for one to two years. Therefore, many oysters with Dermo survive to reach harvest size. Some oysters show a resistance to the Dermo disease. Although these oysters become infected with the disease, they are able to survive with the parasite in their systems and some eventually overcome the disease.
Dermo was first found in the Chesapeake Bay in the early 1950's. Since that time it has contributed to a major decline in the number of oysters harvested in the Chesapeake Bay.
Changes in climate from year to year affect the prevalence and distribution of Dermo. In years with hot, dry summers Dermo prevalence increases and Dermo is more widely distributed because of the increased temperature and salinity (due to the reduction of fresh water flowing into the Bay). For example, in the late 1980's there were several hot, dry summers. During this time Dermo became a major threat to Chesapeake Bay oysters. In 1993 and 1994 heavy spring rains helped to reduce the prevalence of Dermo in the Bay by lowering the salinity. In 1995, salinity levels returned to normal and Dermo returned to many previously infected areas.
At the Living Classrooms Foundation, student scientists are testing the oysters in the Patapsco River Oyster Sanctuary for Dermo using Ray's Fluid Thioglycolate Assay procedure. Here is a step-by-step guide to what they do with pictures of some of the students and staff in the Bioscience lab at the Living Classrooms Foundation Weinberg Education Center.
1. Four times a year (once each season), students and staff collect oysters from the Oyster Sanctuary which is located around Ft. Carroll in the Patapsco River. The oysters are collected using a dredge from the Foundation's skipjack, S.V. Sigsbee. In order to have enough information to make the data valid, 30 oysters must be collected and tested for Dermo. After 30 oysters have been collected, they are brought back to the Weinberg Education Center where they are placed in a saltwater aquarium overnight. This allows the oysters time to adjust to their new environment before they are prepared for the Dermo test, and provides a warm environment for the Dermo cells to grow.
2. Students who are participating in the Living Bay Online program dissect each oyster in order to remove its rectum.
3. Each rectum is placed into 1.0 ml of thioglycolate/antibiotic media in a sterile microcentrifuge tube. Thioglycolate is a cell support media which allows the P. marinus to grow. The samples are incubated at room temperature in the dark for 7 days.
4. At the end of the incubation period, each tissue sample is removed from the tube with tweezers and placed on a separate slide containing 1 drop of diluted Lugols Iodine.
5. Students pull the sample into small pieces with two probes and add one more drop of diluted Lugols Iodine.
6. Students cover each slide with a coverslip, incubate for 20 minutes, and examine the slides under the light microscope at 10x. P. marinus cells can be seen as perfectly round spheres that are dark purple in color (bluish color in the middle and a darker color on the outside).
7. Students count the number of Dermo cells in each sample and rate the level of infection of each oyster as follows:
| uninfected - | 0 dermo cells in entire sample |
| very light - | 1-10 cells in entire sample |
| light - | 10-30 cells in entire sample or 1-2 cells in each viewing field |
| medium - | 30-100 cells in entire sample or 2-5 in each viewing field |
| heavy - | >100 cells in entire sample or >20 cells
in each viewing field (also may appear jet black in cases of heavy infection) |
8. Finally, students determine "Dermo prevalence" for the sample by totaling the number of infected oysters and dividing by the total number of oysters tested.
After a year of testing, we have determined that Dermo is still present in the Patapsco River oysters, but that the level of infection is relatively low. Since the salinity levels at Fort Carroll varied over the year between 7.5 and 17 ppt, we knew it was possible that Dermo might exist there. We did indeed find Dermo!
Don't let the graph fool you though - although the number of oysters infected increases throughout the year, the intensity of the infection is still low. A Dermo intensity of 1 or 2 is a light infection, and might be like a touch of the flu to you or me.
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Living Classrooms will continue testing in the years to come, as the ability to further monitor the health of the Patapsco River oysters is of obvious advantage. As Living Classrooms continues to restore, map, and assess the population growth of Patapsco River oysters, information on disease of these oysters will be essential. This information may also aid recent efforts to assess the oyster populations of protected beds in the Chesapeake.
The Dermo testing that we have done would not have been possible
without the help of many. The Shellfish Division of the Department of Natural
Resources (www.dnr.state.md.us) originally
surveyed and mapped the oyster bed at Fort Carroll, developed the restoration
plan with LCF, and processed the closure of the bed to become a sanctuary.
Oxford
laboratories (www.dnr.state.md.us/education/education.html)
generously demonstrated techniques, provided practice in scaling Dermo, and
served as a resource for questions and troubleshooting.
The National Academy
of Sciences (www4.nas.edu/nas/nashome.nsf)
has aided our efforts by sharing protocol details and hints for student scaling
adaptation.
And finally, this work would not have been possible without the crew of the
skipjack Sigsbee, who diligently collected oysters and water quality
data four times in 1999. Finally, Living Classrooms Foundation would like to
thank all the students and teachers that helped in any way with this project.
Although it is still widely used, the Ray's Fluid Thioglycolate medium method described above is labor intensive, time consuming, relatively insensitive, and not necessarily specific to P. marinus. There are other techniques that often prove more accurate to scientists:
In the first method, DNA probes are used to specifically tag and identify any P. marinus present in the oyster sample. PCR (polymerase chain reaction) amplification of the P. marinus DNA followed by gel electrophoresis techniques allows the Dermo DNA to be detected. It is an extremely accurate method, because it can identify the presence of even very small amounts of the protozoan DNA.
The second method is by labeling Perkinsus cells with an antibody probe. These probes can be very specific as well, and are then viewed with florescence under the microscope.
Because Dermo is transmitted from one oyster to another, one of the major ways scientists are avoiding new outbreaks of Dermo is by making sure infected oysters are not transplanted to uninfected oyster bars.
A second method for avoiding outbreaks is to allow areas that are already affected by Dermo to lie fallow from seeding with baby oysters until the infected oysters have all died and the bar can be seeded without the presence of Dermo.
Leffler, M., 1998. Restoring oysters to U.S. coastal waters: A national Commitment. Sea Grant Oyster Disease Research Program, National Sea Grant College Program
Brooks, William K. The Oyster.
Hedeen, Robert A. The Oyster: The life and lore of the celebrated bivalve.
Kennedy, Victor S., Roger I.E. Newell, and Albert F. Eble, Editors. The Eastern Oyster: Crassostrea virginica.
Wheeler, Timothy B. Bay oysters in peril again. The Baltimore Sun, September 21,1995.
Oyster diseases linked to pollution: Toxic chemicals tied to Dermo. CBF News. Vol. 17 No. 3, November 1992.
Jensen, W.P. and Jack Travelstead. Disease, not management, led to oyster declines. Bay Journal, Vol. 2 No. 9, December 1992.
Galtsoff, P.S. 1964. The American Oyster Crassostrea virginica Gmelin. Fish. Bull. 64:1-480
