Abstract

Fish lice, ectoparasites of the genus Argulus, are branchurian crustaceans that can significantly impact fish health by causing mechanical damage to cutaneous barriers and increasing susceptibility to other infections.^1,2,3 While many treatments have been reported in various species, none are yet reported in elasmobranchs.⁴ In this study, the safety and efficacy of milbemycin oxime and lufenuron in the commercial product Sentinel® were evaluated in freshwater stingrays for treatment of Argulus sp. Doses of both compounds used in this study were lower and administered over a shorter exposure time than those reported to cause morbidity or mortality in teleosts.^1,5,6 Seven juvenile Magdalena river stingrays (Potamotrygon magdalenae) and ten juvenile smooth back river stingrays (Potamotrygon orbignyi), accessioned as a result of a confiscation from illegal trade, had severe infestations of ectoparasites that were identified morphologically as Argulus sp. Treatment was pursued, however, ultimately all animals had to be euthanized due to lack of long-term housing options at any North American institution. No animals were euthanized solely for this study. Animals were treated topically with milbemycin oxime and lufenuron at (0.015 mg/L and 0.30 mg/L, respectively) in a six-hour immersion treatment once weekly for two treatments. They were visually examined for skin lesions as well as behavior and appetite daily by animal care staff, and the only abnormality noted was a 5 mm diameter epidermal abrasion 1 day after the first treatment that was likely due to conspecific trauma. A subset of animals were euthanized and necropsied on days 0, 1, 35, and 70 post-second treatment. Four stingrays (two P. magdalenae and two P. orbignyi) were euthanized on day 0, and the same number were euthanized on day 1 to assess acute effects. On day 35, six stingrays were euthanized and necropsied to evaluate chronic effects. Within eight hours of the first treatment, Argulus sp. load in all study animals had markedly decreased. There were no Argulus sp. detected at the time of the second treatment. Complete gross and histologic evaluation was completed for all animals. No gross abnormalities were detected with the exception of thin body condition in some animals; no Arugulus sp. were noted. Rare branchial monogeneans were present in two individuals (one on day 1 and one on day 35). The only histologic lesions of significance, hepatocellular depletion (7/14) and skeletal muscle atrophy (6/14), were attributed to poor nutritional state at the time of acquisition. No histologic evidence of toxicity was detected. On day 70, the final three animals (one P. magdalenae and two P. orbignyi) were euthanized. No Argulus sp. were evident grossly, and skin scrapes and gill clips were negative for parasites. The commercial formulation of milbemycin oxime and lufenuron in Sentinel®, used as an immersion treatment at 0.015 mg/L and 0.30 mg/L, respectively, for a six-hour immersion once weekly for two treatments, effectively eradicated Aruglus sp. in a population of juvenile P. magdalenae and P. orbignyi, and did not cause mortality or clinical, gross or histologic evidence of acute or chronic toxicity.

Acknowledgements

The animals used in this study were originally confiscated by the government in illegal trade and were housed and eventually relinquished to John G. Shedd Aquarium. During their time at the aquarium, they were treated for ectoparasites but unfortunately had to be euthanized due to the lack of housing options in a North American zoological institution. No animal was euthanized only for the purposes of this study. The authors wish to thank Tiffany Adams, Kurt Hettiger, Maureen Koneval, George Parsons, Dan Lorbeske, Erica Hornbrook, Lauren Czudak, Bernadette Maciol, Sarah McMillan, Rachel Parchem, and Eric Turner of the John G. Shedd Aquarium for their animal care and technical assistance as well as the University of Illinois Veterinary Diagnostic Laboratory Histology Laboratory for slide preparation.

* Presenting author
+ Student presenter

Literature Cited

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