Abstract

American horseshoe crabs (Limulus polyphemus) are a commonly kept species in aquaria in the United States. Shell disease, characterized by erosion and discoloration of the shell, is a prevalent problem in horseshoe crabs, with bacteria, algae, fungi, and parasites all being identified as possible causes.^1-5 Several animals at the Georgia Aquarium have displayed shell lesions similar to what has been described in the literature and some animals also had proliferative, friable, tissue present on the book lungs and joints. The shells of two severely affected animals were scraped and the book lungs were biopsied. Fungal hyphae were present in both of the samples; these animals died prior to the initiation of any treatment. Three additional horseshoe crabs presented a few weeks later with lesions identical to those previously noted. Based on past histopathology and cytologies, a presumptive diagnosis of fungal disease was made and the decision to treat with an anti-fungal was made. There is little information regarding appropriate treatment options for shell disease or other diseases that affect horseshoe crabs.⁵ A recent study evaluated the pharmacokinetics of intravascular itraconazole in the American horseshoe crab.⁶ This study suggested that a dose of 10 mg/kg administered every 24 hours was required to reach a hemolymph level of 500 ng/ml,⁶ though the efficacy of the drug was not evaluated. Intravascular itraconazole is not currently available in the US. Therefore, voriconazole, another triazole with a broad spectrum of activity, was selected to treat the affected horseshoe crabs. Voriconazole was dosed between 7.4 mg/kg and 8.6 mg/kg with all three animals receiving the same volume; animals ranged in weight from 0.58 kg to 0.67 kg. The animals were started on daily voriconazole injections for 7 days, then decreased to every other day injections for four doses, and finally decreased to injections every third day for 6 doses. The total treatment period was 33 days. Voriconazole levels were tested prior to changing the dosing frequency, and hemolymph biochemistries were run prior to initiating treatment and at the end of treatment. Voriconazole levels varied greatly between the three horseshoe crabs. After daily injections, the hemolymph voriconazole level ranged from >10 µg/ml to 5.79 µg/ml. Following every other day dosing, values ranged from 4.51 µg/ml to 1.82 µg/ml and from 2.9 µg/ml to 1.81 µg/ml after every third day dosing. While this study is not a true pharmacokinetic study it does offer some basic information regarding voriconazole levels achieved with different dosages. In addition, it demonstrates that the horseshoe crabs appeared to tolerate voriconazole at these doses as none were noted to have adverse clinical signs such as decreased appetite, change in activity level, or significant changes in hemolymph chemistry values. The horseshoe crab shell and joint lesions did not change in gross appearance though the tissue between the joints became less friable and degenerative in appearance. The efficacy of voriconazole remains unknown at this time however, this limited case study suggests that voriconazole is tolerated by horseshoe crabs and may offer an option for treating fungal disease in these animals.

Acknowledgements

We would like to thank the veterinary technicians and animal care staff at the Georgia Aquarium, Dana Gordon from CapRx, and Dr. Cara Field.

References

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