Veterinary Ophthalmology (2014) 17, 4, 305–308

DOI:10.1111/vop.12161

BRIEF COMMUNICATION

Corneal sensitivity in healthy bovine calves Kyle L. Tofflemire,* Elizabeth M. Whitley,† Renee D. Dewell,‡ Stacie A. Gould,‡ Rachel A. Allbaugh,* Gil Ben-Shlomo,* Annette M. O’Connor‡ and Robert D. Whitley* *Department of Veterinary Clinical Sciences, College of Veterinary Medicine, Iowa State University, Ames, IA, 50011, USA; †Department of Veterinary Pathology, College of Veterinary Medicine, Iowa State University, Ames, IA, 50011, USA; and ‡Department of Biomedical Sciences, College of Veterinary Medicine, Iowa State University, Ames, IA, 50011, USA

Address communications to: R. David Whitley Tel.: 515 294 2199 Fax: 515 294 7520 e-mail: [email protected]

Objective To evaluate corneal sensitivity as measured by the corneal touch threshold in healthy bovine calves. Animals Studied Twelve clinically normal male calves with predominantly Holstein genetics and a median age of 76.5 days (range, 67–92 days). Procedures Corneal touch threshold (CTT) of the central cornea was measured in both eyes of each calf using a Cochet-Bonnet aesthesiometer. Results The mean  standard deviation corneal touch threshold of all eyes was 1.33  1.1 g/mm2 (range, 0.62–66.15 g/mm2), corresponding to a filament length of 34.56  8.02 mm (range, 14–47.5 mm). There was no significant difference between fellow eyes. Conclusions Cochet-Bonnet aesthesiometry was well tolerated in all 12 calves using a modified head restraint. Calves in this study may have a relatively sensitive central cornea compared to adult cattle and some other species; however, wide variation among individuals and eyes may be possible. Studies utilizing larger calf populations are necessary to establish reference ranges. Key Words: Aesthesiometry, bovine, calf, cornea, corneal sensitivity, corneal touch threshold

INTRODUCTION

Corneal sensitivity has been investigated in various animal species utilizing corneal aesthesiometry. The CochetBonnet aesthesiometer measures the amount of pressure necessary to elicit a blink reflex when applied to the corneal surface. The current design of the Cochet-Bonnet aesthesiometer consists of a 0.12-mm-diameter nylon filament with an adjustable length of 60–5 mm. When applied to the corneal surface, the filament length range corresponds to a pressure of 0.4–15.9 g/mm2 (5–180 mg/ 0.0113 mm2). Thus, the applied pressure is inversely proportional to the length of the filament. The corneal touch threshold (CTT) is defined as the pressure at which the majority of applied stimuli results in a blink reflex. Factors that influence CTT include sickness,1 diabetes mellitus,2 intraocular pressure,3 and skull conformation.4,5 Age also affects CTT in some species, with foals and crias having greater corneal sensitivity than adult horses and © 2014 American College of Veterinary Ophthalmologists

alpacas, respectively.1,6 Only one report of CTT in adult cattle is available,7 and data have not been published on CTT in calves. The purpose of this study was to determine the CTT in a group of healthy male calves. MATERIAL AND METHODS

The study was part of a larger ocular study8 and was approved by the Iowa State University (ISU) Institutional Biosafety Committee (IBC#11-D-0017-A) and the Institutional Animal Care and Use Committee (IACUC 8-117187-B). Corneal touch threshold was measured in each eye of 12 male calves (Bos taurus) with predominantly Holstein genetics. The median age of the calves was 76.5 days (range, 67–92 days). Complete physical examination was within normal limits on all calves prior to ophthalmic evaluation. None of the calves had ocular disease as determined by rebound tonometry, fluorescein staining, slit-lamp

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biomicroscopy, and binocular indirect ophthalmoscopy, all of which were performed immediately after corneal aesthesiometry. A Cochet-Bonnet aesthesiometer (Luneau; Western Ophthalmics, Lynnwood, WA, USA) with a new 0.12mm-diameter nylon filament was used to determine the CTT of the central cornea of each eye. Beginning with the longest filament length (60 mm), the Cochet-Bonnet aesthesiometer was oriented perpendicularly to the central cornea and advanced until contacting the corneal surface, resulting in a slight bend in the filament. The corneal surface was contacted up to five times; when at least three of five contacts resulted in a blink reflex, the CTT was recorded. If less than three of five contacts elicited a blink reflex, the filament length was shortened by 5-mm increments and the process was repeated. The same investigator (KLT) performed all measurements. For aesthesiometry, calves were restrained using a portable modified head restraint placed on the front of each calf’s pen (Fig. 1). Calves were housed indoors at 68–70 °F (20–21 °C) with approximately 65% humidity for the duration of the study. Statistical analysis of data was performed using GraphPad Prism software (GraphPad Software, Inc., La Jolla, CA). A Wilcoxon signed-rank test was used to evaluate for differences between fellow eyes. RESULTS

The mean  standard deviation corneal touch threshold of all eyes was 1.33  1.1 g/mm2 (range, 0.62–6.15 g/mm2), corresponding to a filament length of 34.56  8.02 mm (range, 14–47.5 mm). There was no significant difference between fellow eyes (P = 0.76; Fig. 2). However, the value obtained from the left eye of one calf was possibly a high magnitude outlier (applied pressure = 6.15 g/mm2). Table 1 compares data with and without this value.

Figure 1. Photograph of the portable modified head restraint, which allowed evaluation with the head in a normal physiologic position and minimal manipulation of the eyelids.

Figure 2. Pressure necessary to reach CTT in normal male calves using a Cochet-Bonnet aesthesiometer. Data are not statistically different between fellow eyes (error bars indicate 95% confidence interval). Table 1. Central corneal touch threshold of male calves in g/mm2

Right eye Mean Standard deviation Range Left eye Mean Standard deviation Range All eyes Mean Standard deviation Range

With outlier eye

Without outlier eye

1.18 0.46 0.62–2.01

1.18 0.46 0.62–2.01

1.48 1.51 0.71–6.15

1.06 0.34 0.71–1.81

1.33 1.10 0.62–6.15

1.12 0.40 0.62–2.01

DISCUSSION

Cochet-Bonnet aesthesiometry was well tolerated and did not appear to cause undue stress in the calves used in this study. The modified head restraint was easily applied and allowed rapid performance of corneal aesthesiometry. The central cornea was evaluated because this region has been shown to be most sensitive in other species, likely due to high nerve fiber density and overlap.1,4–6 Notably, our findings suggest the central cornea of calves is relatively sensitive when compared to several other species,2,4,5 approaching that of cria, sheep, and horses.6,7 However, the range of CTT values in the calves we studied encompass previously published mean CTT values for several other species, possibly suggesting wide variability among calves and eyes. A summary of published data on corneal sensitivity in various species is provided in Table 2. As Wieser et al. recently explained, filament length and pressure measuring units (g/mm2 and mg/ 0.0113 mm2) are not directly proportional and changes to the filament composition affect the pressure applied at a given filament length. As a result, data must be compared with pressure measuring units.7 However, caution should be exercised when interpreting data across studies, even in

© 2014 American College of Veterinary Ophthalmologists, Veterinary Ophthalmology, 17, 305–308

corneal sensitivity in healthy calves 307 Table 2. Summary of central corneal touch threshold data in various animals; listed in order from highest to lowest corneal sensitivity

Cria* Sheep* Horse* Calf* Goats* Normoglycemic dog* Cow* Cat Alpaca* Diabetic dog* Brachycephalic cat Guinea pig* Pigmented rabbit† Dolichocephalic dog† Albino rabbit† Mesaticephalic dog† Brachycephalic dog† *

Pressure (g/mm2)

Reference

0.85 1.13 1.23 1.33 1.44 1.6 1.67 1.77 2.8 2.8 3.64 3.7 3.98‡ 6.55‡ 7.7‡ 8.23‡ 12.48‡

6 7 7 This study 7 2 7 5 6 2 5 11 12 4 12 4 4 2

Cochet-Bonnet aesthesiometer (range of pressure 0.4–15.9 g/mm ). Cochet-Bonnet aesthesiometer (range of pressure 0.96–17.68 g/mm2). ‡ Converted to g/mm2 from previously published data.

instrument, user, or documentation error. However, because none of these issues were recognized during the study, it is possible that this extreme CTT value represents a normal finding in calves, consistent with wide inter- or intra-individual variability. A larger study population would be necessary to determine whether similar variability exists in other calves. In light of the relative paucity of new material in bovine ophthalmology, we describe the only available information on corneal sensitivity in calves. Although data are not available on CTT in female calves, our results suggest calves may have a relatively sensitive central cornea compared to adult cattle and some other species. However, studies utilizing larger animal populations, and female calves, are necessary to establish reference ranges. FUNDING

This report was supported by a grant from the Iowa Healthy Livestock Advisory Council.



CONFLICT OF INTEREST

The authors declare no conflict of interest. pressure measuring units, as measurement technique and environmental conditions, such as temperature and humidity, may affect aesthesiometry results.9 Previous studies suggest immature individuals, including foals and cria, have increased corneal sensitivity compared to adults of the same species.1,6,10 Consistent with those studies, calves in our study had a lower CTT than adult cattle evaluated in a prior report.7 In that Austrian study, Austrian Spotted Mountain and Holstein cattle ranging from 1 to 9 years of age were evaluated under environmental conditions (temperature, humidity) similar to ours, suggesting age and/or breed contribute to corneal sensitivity in cattle. There are several limitations with this study. The study population, which was not a random sample of the bovine population, and size, which was dictated by a hypothesistesting study,8 limit conclusions that can be drawn about corneal sensitivity in the general bovine population. Corneal sensitivity data were not critical for the hypothesistesting study and, although more calves were used in that study, logistical issues precluded aesthesiometry in these additional animals. Data on female calves were not obtained for similar reasons. The left eye of one calf in our study had an extremely high CTT (6.15 g/mm2), while CTT of this calf’s right eye was within the range of values obtained among other calves in this study (1.63 g/mm2). Due to this asymmetry and the fact that this calf was the fourth youngest in our study population of twelve animals (at 72 days of age), age alone is unlikely to sufficiently explain the extreme CTT value. Alternative explanations for this CTT value may include local, systemic, or neurologic illness, and

REFERENCES 1. Brooks DE, Clark CK, Lester GD. Cochet-Bonnet aesthesiometer-determined corneal sensitivity in neonatal foals and adult horses. Veterinary Ophthalmology 2000; 3: 133–137. 2. Good K, Maggs D, Hollingsworth S et al. Corneal sensitivity in dogs with diabetes mellitus. American Journal of Veterinary Research 2003; 64: 7–11. 3. Blocker T, Hoffman A, Schaeffer D et al. Corneal sensitivity and aqueous tear production in dogs undergoing evisceration and intraocular prosthesis placement. Veterinary Ophthalmology 2007; 10: 147–154. 4. Barrett P, Scagliotti R, Merideth R et al. Absolute corneal sensitivity and corneal trigeminal nerve anatomy in normal dogs. Progress in Veterinary and Comparative Ophthalmology 1991; 1: 245–254. 5. Blocker T, van der Woerdt A. A comparison of corneal sensitivity between brachycephalic and domestic short-haired cats. Veterinary Ophthalmology 2001; 4: 127–130. 6. Rankin AJ, Hosking KG, Roush JK. Corneal sensitivity in healthy, immature, and adult alpacas. Veterinary Ophthalmology 2012; 15: 31–35. 7. Wieser B, Tichy A, Nell B. Correlation between corneal sensitivity and quantity of reflex tearing in cows, horses, goats, sheep, dogs, cats, rabbits, and guinea pigs. Veterinary Ophthalmology 2013; 16: 251–262. 8. Gould S, Dewell R, Tofflemire KL et al. Randomized blinded challenge study to assess association between Moraxella bovoculi and infectious bovine keratoconjunctivitis in dairy calves. Veterinary Microbiology 2013; 164: 108–115. 9. Boberg-Ans J. On the corneal sensitivity. Acta Ophthalmologica 1956; 34: 149–162. 10. Acosta MC, Alfaro M, Borras F et al. Influence of age, gender and iris color on mechanical and chemical sensitivity of the cornea and conjunctiva. Experimental Eye Research 2006; 83: 932–938.

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11. Trost K, Skalicky M, Nell B. Schirmer tear test, phenol red thread test, eye blink frequency and corneal sensitivity in the guinea pig. Veterinary Ophthalmology 2007; 10: 143–146.

12. Millodot M, Lim C, Ruskell C. A comparison of the corneal sensitivity and nerve density in albino and pigmented rabbits. Ophthalmic Research 1978; 10: 307–311.

© 2014 American College of Veterinary Ophthalmologists, Veterinary Ophthalmology, 17, 305–308

Corneal sensitivity in healthy bovine calves.

To evaluate corneal sensitivity as measured by the corneal touch threshold in healthy bovine calves...
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