Intraocular Pressure as Measured by Rebound Tonometry and Review of Ocular Lesions in Houston Toads (Anaxyrus houstonensis)
2018 Joint EAZWV/AAZV/Leibniz-IZW Conference
Christine T. Bui1, MBA, DVM; Christine M. Molter2*, DVM, DACZM; Maryanne E. Tocidlowski2, DVM, DACZM; Nicholas Millichamp3, BVetMed, PhD, DVOphthal, DECVO, MRCVS, DACVO; Philip H. Kass, DVM, PhD, DACVPM
1Gulf Coast Veterinary Specialists, Houston, TX, USA; 2Houston Zoo, Inc., Houston, TX, USA; 3Eye Care for Animals, Houston, TX, USA; 4School of Veterinary Medicine, University of California-Davis, Davis, CA, USA


Amphibians may be affected by ocular disease, such as uveitis and glaucoma, which can alter normal intraocular pressures (IOP) and require treatment. In a review of ocular lesions in critically endangered Houston toads (Anaxyrus houstonensis) housed at the Houston Zoo, cataracts and corneal hyperpigmentation were common findings. These individuals could benefit from IOP measurements to direct appropriate treatments. Intraocular pressure as measured by rebound tonometry has been reported in several amphibian species previously, but this is the first study to evaluate IOP in Houston toads and establish normal reference ranges, as they are species specific.1-3 In this study, IOP was measured by rebound tonometry (TonoVeta) in manually restrained healthy adult male and female Houston toads (n=58) in a horizontal and vertical position. Mean (+/- SD and range) intraocular pressure in all toads in a horizontal position was OD=6.5 (+/- 1.3 and 3–10 mm Hg) and OS=7.1 (+/- 1.3 and 3–10 mm Hg). This is similar to the vertical position with OD=6.2 (+/- 1.1 and 4–8 mm Hg) and OS=6.5 mm Hg (+/- 1.3 and 3–9 mm Hg). There was a significant (p<0.05) difference between the left and right eye IOP in the horizontal position, but this was not thought to be clinically important. Moreover, there were significant correlations between weight, age, and IOP. This is the first study to review ocular lesions and establish an IOP reference range for Houston toads using rebound tonometry.


a. TonoVet, Icare Oy, Helsinki, Vantaa 01510, Finland.


The authors would like to thank the Houston Zoo animal care staff, Colin Thompson, and Melissa Spradley.

Literature Cited

1.  Hahn A, Gilmour M, Payton M, D‘Agostino J, Cole G. Intraocular pressure in free-ranging anuran species in Oklahoma. J Zoo Wildl Med. 2014;45(3):686–689.

2.  Hausmann JC, Krisp A, Sladky K, Miller PE, Mans C. Measuring intraocular pressure in white‘s tree frog (Litoria caerulea) by rebound tonometry: comparing device, time of day, and manual versus chemical restraint methods. J Zoo Wildl Med. 2017;48(2):413–419.

3.  Lewin AC, Hausmann JC, Miller PE. Intraocular pressure and examination findings in three species of central and south American tree frogs (Cruziohyla craspedopus, Cruziohyla calcarifer, and Anotheca spinosa). J Zoo Wildl Med. 2017;48(3):688–693.


Speaker Information
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Christine M. Molter, DVM, DACZM
Houston Zoo, Inc.
Houston, TX, USA

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