Fenbendazole Toxicity in Sharks
American Association of Zoo Veterinarians Conference 2007
Gwen E. Myers1, DVM; Michael M. Garner2, DVM, DACVP; Michael T. Barrie1, DVM; Christopher E. Yach3, DVM; Gerald L. Pribyl4, DVM
1The Columbus Zoo and Aquarium, Powell, OH, USA; 2Northwest ZooPath, Monroe, WA, USA; 3West Flamingo Animal Hospital, Las Vegas, NV, USA; 4Gentle Doctor Animal Hospital, Las Vegas, NV, USA


Fenbendazole is a benzimidazole anthelmintic that is used in an extra-label manner for the treatment of intestinal parasitism in many species. There are increasing reports, both published and anecdotal, of fenbendazole toxicity in several avian, reptile, and mammalian species.1,3-6 Current recommendations include fenbendazole administration as part of the quarantine protocol for some elasmobranchs.3

Fenbendazole dosing recommendations for most elasmobranchs, including zebra sharks (Stegostoma fasciatum), blacknose shark (Carcharhinus acronotus), sand tiger shark (Carcharias taurus), nurse shark (Ginglymostoma cirratum), and lemon shark (Negaprion brevirostris) are 25 mg/kg PO 3 times/wk for 3 wk.3 While some specimens tolerate this dose without obvious detriment, others become acutely ill. Clinical signs can be apparent as early as 24 h, and death within 1 wk. Signs of acute toxicity include anorexia, lethargy, necrotic margins to tail and pectoral fins, elevated respiratory rate, and erythematous lesions on ventrum. Clinicopathologic abnormalities are also noted with the most common findings being profound leukopenia, especially granulocytopenia, and anemia, as noted in other species.5

Histologically, lesions are consistent with a radiomimetic insult—destruction of rapidly propagating cells. The most common findings include necrosis of basal cells of the gastrointestinal mucosa and hematopoietic tissue of the epigonal gland. Bacterial overgrowth in the devitalized gut mucosa leads to fatal sepsis. Fenbendazole should be used with caution in elasmobranchs.

Literature Cited

1.  Alvarado, T.P., M.M. Garner, K. Gamble, G.P. Levens, J.T. Raymond, and R. Nordhausen. 2001. Fenbendazole overdose in four Fea’s vipers (Azemiops feas). Proc. Am. Assoc. Zoo Vet. Pp. 28–9.

2.  Bonar, C. J., A. Lewandowski, and J. Schaul. 2003. Suspected fenbendazole toxicosis in 2 vulture species (Gyps africanus, Torgos tracheliotus) and marabou storks (Leptoptilos crumeniferus). J. Avian. Med. Surg. 17(1):16–19.

3.  Davis, R. 2004. Quarantine and prophylaxis for elasmobranchs. In: M. Smith, D. Warmolts, D. Thoney, and R. Hueter,(eds). Elasmobranch Husbandry Manual. Special Pub. of the Ohio Biological Survey. Pp. 143–151.

4.  Gozalo, A. S., R. Schwiebert, G. Lawson. 2006. Mortality associated with fenbendazole administration in pigeons (Columba livia). J. Am. Assoc. Lab. Anim. Sci. 45(6):63–66.

5.  Neiffer, D., D. Lvdick, K. Burks, D. Doherty. 2005. Hematologic and plasma biochemical changes associated with fenbendazole administration in Hermann’s tortoises (Testudo hermanni). J. Zoo. Wildl. Med. 36(4):1240–1242.

6.  Weber, M.A., M. Miller, D. Neiffer, S. Terrell. 2006. Presumptive fenbendazole toxicosis in North American porcupines. J. Am. Vet. Med. Assoc. 228(8):1240–1242.


Speaker Information
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Gwen E. Myers, DVM
Columbus Zoo and Aquarium
Powell, OH, USA

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