Abstract

Monogenetic trematode infestations are common in captive marine aquariums and have caused significant morbidity and mortality in a variety of fish species. The most common of these marine parasites is Neobenedenia melleni. This organism has been a persistent disease problem with fish at The Living Seas Pavilion at Epcot since the 1980's. This fish ectoparasite is capable of infecting a wide variety of fish species and has greatly restricted the species of fish that can be maintained in the system. The Living Seas is the world's largest closed marine system containing more than 6 million gallons of synthetic seawater and is home to a variety of marine teleosts, elasmobranchs, sea turtles, bottlenose dolphins and manatees.

Effective medical options for large marine systems have historically been limited to copper sulfate and organophosphates. These drugs can induce a variety of side effects and their use has been associated with several elasmobranch mortalities. Praziquantel is an antihelmintic commonly used to treat cestodes and trematodes in mammals. Medicated dips and baths in praziquantel have been shown to be safe and effective for a variety of fish and elasmobranchs.

In conjunction with plans to upgrade and renovate the existing life support system at the Living Seas, a long-term parasite control program was formulated. During the process of developing a trematode control plan, several unanswered questions regarding the parasite and its treatment were considered. In order to answer some of these queries, several objectives were created.

These included:

 Determine the levels of trematode parasitism in the system and continue to monitor levels before, during and after the control measures had been initiated.

 Determine the efficacy of praziquantel as a water treatment in a large multispecies marine aquarium.

 Measure the chemical stability of praziquantel in synthetic seawater.

 Evaluate the life cycle of the parasite and determine which environmental factors could be adjusted to help control infection rates.

Trematode ova in the water and adult parasites on the fish were used to help determine system infection rates and the efficacy of the control program. Mesh screens (0.1 micron) were placed on the skimmers and used to capture trematode ova in the water. The number of ova on these screens were counted and used to help estimate egg concentrations in the water. In order to determine the incidence of trematode infection, 5 fish were caught each week and given a freshwater dip for 3 minutes. Parasites that fell off the fish during this dip were counted and used to help estimate rates of infection.

Praziquantel was added to the system at a concentration of 2 ppm. The powdered praziquantel was dissolved into the water by mechanically expelling the material through a fine mesh beneath the water surface. HPLC was used to measure praziquantel levels daily to determine chemical stability in the system. During the treatment period, activated charcoal was removed from the system and the ozone unit was shut down. Sand and gravel filters were kept on-line throughout the treatment.

The exact life cycle of Neobenedenia melleni has not been well described. Clinical observations at several large aquariums have shown seasonal variation in the levels of infection. It is suspected that a variety of environmental cues are used by the parasite to regulate life cycle stages. While it is known that some ova can lie dormant for long periods of time, it is unclear how long this dormancy can last and what may trigger the hatching process. Initial studies at the Living Seas indicate that incubation rates can be shortened by increases in water temperature. Studies are also underway to determine the maximum length of dormancy.

Water treatment with praziquantel at a concentration of 2 ppm proved to be both safe and efficacious. Elasmobranchs, teleost fish, sea turtles, marine mammals and humans were all exposed to the medicated water and demonstrated no detectable effects. Prior to water treatment, adult parasites on the fish were numerous and often associated with morbidity and mortality. After the treatment, no trematodes could be found on the examined fish. This included healthy fish that were caught for fresh water dips and necropsy samples from fish that died from other reasons. Prior to treatment, more than 400 trematode ova were identified each time the screens were evaluated. After treatment, these levels fell to zero. After treatment, no trematodes could be found on any of the fish for more than 12 weeks. At that time, juvenile trematodes were found on healthy spadefish as part of our long term monitoring system.

During the initial praziquantel water treatment, praziquantel concentrations remained stable for 18 days and remained above l ppm for more than three weeks. During the second treatment, praziquantel levels fell more rapidly and by 5 days, the concentrations were less that 1 ppm.

Acknowledgements

The authors are indebted to the efforts of several aquarium interns who helped with various aspects of this project. These interns include Allen McDowell, Kendra Portada & Aaron Grissom. We would also like to thank the numerous aquarists at The Living Seas Pavilion at Epcot, who were critical to these studies and who provide continued energy to our ongoing battle with this disease problem.