Abstract

Praziquantel has been safely and effectively used in large exhibits as an immersion treatment for Neobenedenia infection of marine teleosts.¹ Because the persistence of praziquantel in sea water is highly variable, measurement of concentrations is recommended during treatment.^1,2,3 In 2016, Neobenedenia was diagnosed in an indoor, 10⁶ L, natural sea water exhibit containing approximately 1000 fish representing approximately 100 species. Presence of Neobenedenia was monitored by examinations of live fish, necropsies, and two 64 x 32-cm, 100-µm mesh panels installed within the tank skimmer boxes. Praziquantel was delivered nine times over a two-month period, desiring to maintain concentrations above 1 mg/L between doses. The potency of bulk praziquantel powder was confirmed via third-party analysis. Ozone disinfection and protein skimming were discontinued during treatment. Praziquantel concentrations were measured, and drug additions were adjusted between 2 ppm and 5 ppm as needed to maintain the therapeutic target, but adult parasites and ova remained. During praziquantel treatment, copper sulfate was used as an additional anti-parasitic treatment due to detection of Cryptocaryon. Copper treatment was executed twice for two weeks and four weeks, respectively, at a conservative concentration of 0.06 mg/L. During the two-month praziquantel and copper treatment period, the mortality rate in the exhibit was approximately ten-fold higher than typical, and 118 fish died. Based on observations of Ellis and Watanabe, hyposalinity treatment was executed at 15 g/L for two months, followed by 22 g/L for eight months, 15 g/L for an additional two months, and remaining at 22 g/L at the time of abstract preparation.⁴ The latter 15 g/L treatment was driven by recurrent Cryptocaryon infection. No adult Neobenedenia were detected after beginning hyposalinity treatment. Surprisingly, Neobenedenia ova were detected by skimmer mesh analysis for the entire duration of hyposalinity treatment. Although the number of detected ova decreased over time, ova were still rarely detected at the time of abstract preparation. During the thirteen months of ova persistence, none contained clearly defined embryos, and attempts to monitor ova for embryonic development were unsuccessful. The mortality rate during hyposalinity treatment was similar to that seen historically during non-disease periods, with 85 deaths in a one-year period. In general, hyposalinity treatment appears to be a safe and cost-effective management tool for Neobenedenia infections for marine fish species that are tolerant of reduced salinity. It remains unclear at this time whether the prolonged presence of ova in this case reflects ongoing ova production by undetected adults, or whether ova are resistant to degradation. Based on these findings, surveillance for ova is important, and prolonged hyposalinity treatment may be needed to resolve infection. It is notable that fairly aggressive praziquantel therapy failed to eliminate infection in this case, particularly in comparison to previously successful outcomes using fewer doses at lower concentrations over a shorter period of time.

Acknowledgments

The efforts described herein were collaboratively completed by the staff of the New England Aquarium, especially the Giant Ocean Tank divers, the Environmental Quality Laboratory, and the Animal Health Department.

* Presenting author

Literature Cited

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2.  Innis C. 2012. Observations on praziquantel concentrations during experimental and clinical use in marine aquaria. AAZV 44th Annual Conference Proceedings, Oakland, CA; p. 137–139.

3.  Thomas A, Dawson MR, Ellis H, Stamper MA. 2016. Praziquantel degradation in marine aquarium water. PeerJ 4:e1857; DOI 10.7717/peerj.1857.

4.  Ellis EP, Watanabe WO. 1993. The effects of hyposalinity on eggs, juveniles and adults of the marine monogenean, Neobenedenia melleni: treatment of ecto-parasitosis in seawater-cultured tilapia. Aquaculture. 117:15–27.