Abstract

Fish lice (Argulus spp.) are obligate external parasitic crustaceans of freshwater and saltwater fishes. Clinical signs are primarily related to the feeding process and include local erythema and ulceration, increased mucous production, skin irritation ("flashing"), anemia, lethargy, and inappetance.^5,6 Mortality rates are normally low but may be elevated in juvenile fish, or due to secondary infection or immunosuppression.^1-5,6 Transmission occurs by direct contact or via fomites.^5,7 The life cycle is direct and eggs are laid on the substrate.^5,6,8

At the National Aquarium, Baltimore, an outbreak of fish lice was documented in a 30,000-gallon tropical freshwater exhibit within a large walk-through aviary. The system comprised three connected display tanks and a waterfall. The system water was treated with biological and mechanical filtration and ozone disinfection, and water temperature was 82–84°F. The animal population included a variety of Australian turtles, teleosts, elasmobranchs, and crustaceans.

The parasites were first identified in a desert mogurnda (Mogurnda larapintae) that was found dead in May 2011. Gross necropsy showed a light load of Argulus sp., and visual exams confirmed the presence of Argulus sp. on other fish in the system. Isolation and biosecurity measures were implemented pending treatment. Over the following weeks, parasite loads increased, particularly in more sedentary species such as giant gudgeon (Oxyeleotris selheimi), sleepy cod (Oxyeleotris lineolatus), and grunters (Hephaestus spp., Leiopotherapon unicolor, Syncomistes butleri). Six species of catfish in the exhibit were unaffected. Additional mortalities associated with elevated parasite burdens occurred in giant gudgeons (n = 2) and sleepy cod (n = 3).

Common antiparasitics, such as formalin and organophosphates, could not be used due to possible toxicity to some exhibit animals and potential exposure of staff and visitors. The chitin inhibitor diflubenzuron (Dimilin 25W, Uniroyal Chemical, Naugatuck, CT) was dosed at 0.01 mg/L as a long-term bath. The drug was re-dosed every seven days after a 50% water change for a total of six weeks. To accelerate the parasite life cycle, water temperature was increased to 86°F. No parasites were observed after three weeks of treatment, and there has been no recurrence in the eight months since treatment. No adverse effects on water quality parameters, water clarity, or non-target species were observed, and other systems remained unaffected.

The source of the outbreak may have been barramundi (Lates calcarifer), which were introduced 12 weeks prior to the incident. These had been in holding for more than five years but may have been subclinical carriers. Species identification of the parasite might help determine the source. Effective treatment of this large exhibit would not have been possible without coordinated efforts by animal husbandry, veterinary, life-support, and water chemistry personnel.

Acknowledgments

The authors wish to thank Dr. Araceli Lucio-Forster and Dr. Paul Bowser from Cornell University College of Veterinary Medicine, for assistance with parasite identification and publication.

References

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