Effect of eyelid manipulation and manual jugular compression on intraocular pressure measurement in dogs

Heidi E. Klein Departments of Veterinary Clinical Sciences, School of Veterinary Medicine, Purdue University, West Lafayette, IN 47907.

Search for other papers by Heidi E. Klein in
Current site
Google Scholar
PubMed
Close
 DVM, MS
,
Sheryl G. Krohne Departments of Veterinary Clinical Sciences, School of Veterinary Medicine, Purdue University, West Lafayette, IN 47907.

Search for other papers by Sheryl G. Krohne in
Current site
Google Scholar
PubMed
Close
 DVM, MS, DACVO
,
George E. Moore Departments of Veterinary Clinical Sciences, School of Veterinary Medicine, Purdue University, West Lafayette, IN 47907.
Comparative Pathobiology, School of Veterinary Medicine, Purdue University, West Lafayette, IN 47907.

Search for other papers by George E. Moore in
Current site
Google Scholar
PubMed
Close
 DVM, PhD, DACVIM
,
Ahmed S. Mohamed Comparative Pathobiology, School of Veterinary Medicine, Purdue University, West Lafayette, IN 47907.

Search for other papers by Ahmed S. Mohamed in
Current site
Google Scholar
PubMed
Close
 BVSc, MSc
, and
Jean Stiles Departments of Veterinary Clinical Sciences, School of Veterinary Medicine, Purdue University, West Lafayette, IN 47907.

Search for other papers by Jean Stiles in
Current site
Google Scholar
PubMed
Close
 DVM, MS, DACVO

Abstract

Objective—To determine the effect of eyelid manipulation and manual jugular compression on intraocular pressure (IOP) measurement in clinically normal dogs.

Design—Randomized clinical trial.

Animals—30 dogs (57 eyes) without diseases or medications that affect IOP.

Procedures—An applanation tonometer was used to measure IOP during eyelid manipulation or jugular compression. Six manipulations were used in each eye, including minimal eyelid manipulation, maximal dorsoventral extension of the eyelids, lateral eyelid extension, manual compression of the ipsilateral jugular vein, manual compression of both jugular veins, and lateral eyelid extension with manual compression of both jugular veins. Skull type and position of globe in the orbit were recorded.

Results—The 2 manipulations that caused the greatest significant increase in mean IOP were lateral eyelid extension with compression of both jugular veins (difference from baseline IOP, 17.6 mm Hg; 95% confidence interval [CI], 15.7 to 19.5 mm Hg) and lateral eyelid extension alone (16.5 mm Hg; 95% CI, 14.6 to 18.4 mm Hg). Dorsoventral eyelid extension (6.42 mm Hg; 95% CI, 4.5 to 8.3 mm Hg) and compression of both jugular veins alone (3.0 mm Hg; 95% CI, 1.1 to 5.0 mm Hg) significantly increased mean IOP, compared with baseline. Compression of the ipsilateral jugular vein increased mean IOP (0.3 mm Hg; 95% CI, −1.6 to 2.2 mm Hg) from baseline, but not significantly.

Conclusions and Clinical Relevance—Traction on the eyelids or pressure on both jugular veins can significantly increase IOP values as measured by use of applanation tonometry in clinically normal dogs.

Abstract

Objective—To determine the effect of eyelid manipulation and manual jugular compression on intraocular pressure (IOP) measurement in clinically normal dogs.

Design—Randomized clinical trial.

Animals—30 dogs (57 eyes) without diseases or medications that affect IOP.

Procedures—An applanation tonometer was used to measure IOP during eyelid manipulation or jugular compression. Six manipulations were used in each eye, including minimal eyelid manipulation, maximal dorsoventral extension of the eyelids, lateral eyelid extension, manual compression of the ipsilateral jugular vein, manual compression of both jugular veins, and lateral eyelid extension with manual compression of both jugular veins. Skull type and position of globe in the orbit were recorded.

Results—The 2 manipulations that caused the greatest significant increase in mean IOP were lateral eyelid extension with compression of both jugular veins (difference from baseline IOP, 17.6 mm Hg; 95% confidence interval [CI], 15.7 to 19.5 mm Hg) and lateral eyelid extension alone (16.5 mm Hg; 95% CI, 14.6 to 18.4 mm Hg). Dorsoventral eyelid extension (6.42 mm Hg; 95% CI, 4.5 to 8.3 mm Hg) and compression of both jugular veins alone (3.0 mm Hg; 95% CI, 1.1 to 5.0 mm Hg) significantly increased mean IOP, compared with baseline. Compression of the ipsilateral jugular vein increased mean IOP (0.3 mm Hg; 95% CI, −1.6 to 2.2 mm Hg) from baseline, but not significantly.

Conclusions and Clinical Relevance—Traction on the eyelids or pressure on both jugular veins can significantly increase IOP values as measured by use of applanation tonometry in clinically normal dogs.

Intraocular pressure is maintained by the constant production and drainage of aqueous humor. Accurate IOP measurement is part of the diagnosis of glaucoma and uveitis and is critical for monitoring the response to treatment. Prior studies1,2 in dogs have shown that body position and neck pressure applied with a collar can affect IOP. Anecdotally, excessive neck and head restraint, eyelid retraction, inadvertent digital pressure on the globe during measurement, and other inappropriate positioning can cause erroneous IOP measurements.3

In the authors' experience, a large percentage of dogs that are referred with an erroneous diagnosis of glaucoma based on high IOP measurements are brachycephalic dogs or dogs with prominent eyes. The prominence of eyes is related to the position of the globe relative to the orbital rim and is not strictly dependent on skull type. For example, all brachycephalic dogs have globes rostral to the orbital rim, but not all dogs with globes rostral to the orbital rim are brachycephalic. Greyhounds are dolichocephalic dogs and have globes that protrude rostral to the orbital rim, and other dolichocephalic dogs, such as the Borzoi, have globes caudal to the orbital rim.

The purpose of the study reported here was to determine the effect of eyelid manipulation and manual jugular compression on IOP measurement in clinically normal dogs. Our hypothesis was that eyelid manipulation and jugular vein pressure would significantly increase IOP measurement value and that skull type and position of globe in orbit would modify this increase.

Materials and Methods

Animals and clinical examination—Thirty dogs that were owned by persons affiliated with Purdue University Veterinary Teaching Hospital or were patients of the hospital between March and July 2009 were included in the study. The study was approved by the Purdue Animal Care and Use Committee, and owners signed a consent form prior to inclusion of their dogs. A complete slit-lamp biomicroscopic examination and indirect ophthalmoscopy with the eyes dilated were performed on all subjects subsequent to data acquisition. Eyes were excluded from the study if there were corneal abnormalities or any disease that could affect IOP or if there was any medication being administered that could affect IOP. In previous studies,4–6 IOP in clinically normal dogs ranged from 7 to 29 mm Hg. In the authors' clinical experience, IOP typically does not exceed 25 mm Hg in dogs without eye disease. Given this, dogs were excluded from the study if either eye had a baseline IOP > 25 mm Hg.

A scalpel blade handle was rested on the orbital rim to determine whether the globe protruded rostral to the orbital rim, extended to the orbital rim, or was caudal to the orbital rim. Skull type was recorded as brachycephalic, mesaticephalic, or dolichocephalic.

Study protocol—Intraocular pressure was measured in 1 or both eyes during 6 randomized manipulations of eyelid and body restraint by use of an applanation tonometer.a The 6 manipulations were baseline with minimal eyelid restraint and no pressure on the jugular veins, maximal extension of the eyelids ventrally and dorsally with no pressure on the jugular veins, extension of the eyelids laterally with no pressure on the jugular veins, minimal eyelid restraint with manual pressure on the jugular vein ipsilateral to the eye being measured, minimal eyelid restraint with manual pressure on both jugular veins, and extension of eyelids laterally with manual pressure on both jugular veins (Figure 1). One investigator (HEK) acquired IOP measurements and manipulated eyelids. One assistant restrained all animals and applied jugular vein pressure. Dorsoventral eyelid extension was enough to make the eyelids taut. When the eyelids were extended laterally, the palpebral fissure was approximately 5 mm wide, which was wide enough to allow IOP measurement. Application of jugular pressure caused distension of the jugular veins, and pressure was applied for ≥ 1 minute prior to IOP measurement.

Figure 1—
Figure 1—

Eyelid manipulations performed on dogs while IOP was measured by use of an applanation tonometer. A—Baseline with minimal eyelid restraint. B—Maximal extension of the eyelids ventrally and dorsally. C—Extension of the eyelids laterally.

Citation: Journal of the American Veterinary Medical Association 238, 10; 10.2460/javma.238.10.1292

Proparacaine hydrochloride 0.5% ophthalmic solution was applied to the eye to be measured at least 1 minute prior to measurement and reapplied before measurement if > 15 minutes had elapsed since the previous application of proparacaine.7 Proparacaine that was designated for use in this study was stored at 4°C and discarded after being opened for 2 weeks. The same tonometer was used throughout the study and was calibrated daily. Manipulation order was randomized with a minimum of 5 minutes between each manipulation. In each manipulation, IOP of 1 or both eyes was measured until 3 measurements with < 5% variance were acquired for each eye; these 3 measurements (first, second, and third reading) were recorded. It was randomly determined which eye was measured first when both eyes were measured. To minimize stress, no attempt was made to place dogs into consistent body position for IOP measurement; body position was not recorded and included sternal recumbency, sitting, and standing positions. All measurements were acquired in < 1 hour for each dog between 8 am and 5 pm.

Statistical analysis—Quantitative variables were assumed to be normally distributed, considering the large number of observations and according to the central limit theorem. Analysis of variance for repeated measures was used to evaluate statistical difference of mean IOP between different levels of categorical variables and their interactions. Mean IOPs for different levels of categorical variables shown to be significant in the ANOVA model were compared by use of a Tukey Studentized range test. Data are expressed as mean ± SD unless otherwise indicated. A multivariate generalized linear model procedure for repeated measures was used to evaluate significance of the effect of the manipulation, position of the globe in orbit, skull type, age, order of IOP measurement within each manipulation (first, second, or third), eye (left vs right), order of eye measured, and age groups (≤ 7 years vs > 7 years) on the IOP by use of standard statistical software.b For all tests, values of P < 0.05 were considered significant.

Results

Of the 30 dogs (57 eyes) in the study, 23 were purebred dogs, with 17 breeds represented, and 7 were mixed-breed dogs. The mean age was 4.7 ± 2.9 years (median, 4.2 years [range, 0.48 to 12.01 years]). There were 13 brachycephalic, 15 mesaticephalic, and 2 dolichocephalic dogs. Two dogs had globes caudal to the orbital rim, 8 had globes that reached the orbital rim, and 20 had globes that protruded rostral to the orbital rim.

The manipulation, skull type, position of globe in orbit, and age were found to be significant (P < 0.001) variables when IOP was the dependent variable. The order of IOP measurement within each manipulation (P = 0.18), the order of eye measured (P = 0.66), and eye (P = 0.93) were not significant variables.

The mean IOP was 14.0 ± 3.9 mm Hg in the baseline manipulation. Intraocular pressure with ipsilateral jugular pressure was 14.2 ± 4.2 mm Hg and was not significantly different from baseline. A significant increase in mean IOP from baseline resulted from bilateral jugular pressure, dorsoventral eyelid extension, lateral eyelid extension, and lateral eyelid extension with bilateral jugular pressure (Table 1). The median and mean IOPs for manipulations that involved lateral extension of the eyelids were > 25 mm Hg (Figure 2). The IOP in certain dogs was as high as 51 mm Hg in the dorsoventral eyelid extension and 69 mm Hg in the lateral eyelid extension manipulation.

Figure 2—
Figure 2—

Box-and-whisker plots of IOP during the 6 manipulations performed on 30 dogs (57 eyes). For each box, the horizontal line represents the median and the upper and lower boundaries represent the 75th and 25th percentiles, respectively. Whiskers represent the minimum and maximum values, and circles represent outliers. Base = Baseline. BiJ = Bilateral jugular pressure. DV = Dorsoventral eyelid extension. IpJ = Ipsilateral jugular pressure. L = Lateral eyelid extension. LBiJ = Lateral eyelid extension combined with bilateral jugular pressure.

Citation: Journal of the American Veterinary Medical Association 238, 10; 10.2460/javma.238.10.1292

Table 1—

Mean ± SD IOP and difference from baseline IOP (minimal eyelid manipulation) for each manipulation in 30 dogs (57 eyes).

ManipulationIOPRangeDifference (95% CI) from baseline
Baseline14.0 ± 3.96–24
Ipsilateral jugular14.2 ± 4.25–290.3 (−1.6 to 2.2)
Bilateral jugular*17.0 ± 5.97–373.0 (1.1 to 5.0)
Dorsoventral*20.4 ± 7.46–516.4 (4.5 to 8.3)
Lateral extension*30.4 ± 12.19–6916.5 (14.6 to 18.4)
Bilateral jugular and lateral extension*31.6 ± 12.68–6917.6 (15.7 to 19.5)

All values are reported in mm Hg.

Significantly (P < 0.05) different from baseline as assessed by use of the Tukey Studentized range test.

— = Not applicable.

Dogs with globes rostral to the orbital rim had a higher mean baseline IOP (15.2 ± 3.2 mm Hg) than did dogs with globes at (11.7 ± 4.3 mm Hg) or caudal to the orbital rim (9.8 ± 2.7 mm Hg). Additionally, eyelid manipulation or jugular compression modified this effect and magnified it (Figure 3). Brachycephalic dogs similarly had a higher mean baseline IOP (16.4 ± 2.9 mm Hg) than did mesaticephalic (11.8 ± 3.7 mm Hg) and dolichocephalic dogs (13.5 ± 1.0 mm Hg), and the effect of the manipulations was exaggerated in the brachycephalic dogs (Figure 4).

Figure 3—
Figure 3—

Box-and-whisker plots of IOP during the 6 manipulations performed on the same dogs as in Figure 2, grouped by position of globe in orbit. C = Entire globe is caudal to the orbital rim. O = Globe reaches the orbital rim. R = Globe protrudes rostral to the orbital rim. See Figure 2 for remainder of key.

Citation: Journal of the American Veterinary Medical Association 238, 10; 10.2460/javma.238.10.1292

Figure 4—
Figure 4—

Box-and-whisker plots of IOP during the 6 manipulations performed on the same dogs as in Figure 2, grouped by skull type. B = Brachycephalic dogs. D = Dolichocephalic dogs. M = Mesaticephalic dogs. See Figure 2 for remainder of key.

Citation: Journal of the American Veterinary Medical Association 238, 10; 10.2460/javma.238.10.1292

Dogs ≤ 7 years of age had a significantly higher mean baseline IOP (14.8 ± 3.8 mm Hg; 95% CI, 14.2 to 15.5 mm Hg) than did dogs > 7 years of age (10.7 ± 2.4 mm Hg; 95% CI, 9.86 to 11.47 mm Hg), as found previously.6 Additionally, the IOP of younger dogs was higher than that of older dogs in each manipulation.

Discussion

Dorsoventral eyelid manipulation, lateral eyelid manipulation, and pressure on both jugular veins caused a significant increase in IOP measurement from baseline. Dogs with globes rostral to the orbital rim and brachycephalic dogs were particularly susceptible to these increases. Increases in IOP measurement from eyelid manipulation and jugular pressure may explain the inaccurate diagnosis of glaucoma in some referral patients.

The results of this study agree with similar findings in human medicine. Gandhi et al8 found that attempted eyelid closure during IOP measurement caused significant increases in IOP, which is consistent with the finding in the present study that eyelid manipulation led to increased IOP measurement. Tight neckties in 1 study9 led to increased IOP measurement, which corresponds to bilateral jugular pressure in the present study; however, in another study,10 it was found that a tight necktie did not significantly alter IOP measurement. One theorized cause of the increased IOP attributable to neck pressure is resistance to outflow of aqueous humor because of increased pressure in the episcleral vasculature from jugular pressure and resultant venous stasis.9,11 Another related theory for the cause in IOP increase is that jugular pressure leads to increased choroidal blood volume caused by venous stasis.12

Brachycephalic dogs and dogs with globes that protrude rostral to the orbital rim are likely more susceptible to increases in IOP from restraint techniques because of the greater exposure of the eye relative to the eyelid. When the eyelids are manipulated laterally in these dogs, the globe may be pushed caudally; because they have a shallower orbit, more pressure may be placed on the eye, causing an increase in IOP. In dogs with other skull types, more of the eye is protected by the orbit.

Although the applanation tonometer has been found to be accurate in dogs, care must be taken by the examiner to ensure accurate readings.13,14 Eyelid restraint should be minimal; even excessive tension on the skin by the restrainer may produce eyelid tension. Keeping the dog as calm as possible can help to prevent excessive restraint. To be sure that there is no pressure on the jugular veins, collars and leashes should be removed or at least not used during the restraint. Additionally, in the authors' experience, the best way to hold the head is to place one hand under the chin and the other behind the head. If a wide variety of IOP measurements are obtained, the examiner should reevaluate the technique being used and attempt to obtain several measurements that are close in value.

ABBREVIATION

CI

Confidence interval IOP Intraocular pressure

a.

Tono-Pen Veterinary Tonometer, Medtronic Solan, Jacksonville, Fla.

b.

SAS, version 9.1, SAS Institute Inc, Cary, NC.

References

  • 1.

    Pauli AMBentley EDiehl KA, et al. Effects of the application of neck pressure by a collar or harness on intraocular pressure in dogs. J Am Anim Hosp Assoc 2006; 42:207211.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 2.

    Broadwater JJSchorling JJHerring IP, et al. Effect of body position on intraocular pressure in dogs without glaucoma. Am J Vet Res 2008; 69:527530.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 3.

    Ollivier FJPlummer CEBarrie KP. Ophthalmic examination and diagnostics, part 1: the eye examination and diagnostic procedures. In: Gelatt KN, ed. Veterinary ophthalmology. 4th ed. Ames, Iowa: Blackwell Publishing, 2007;438483.

    • Search Google Scholar
    • Export Citation
  • 4.

    Knollinger AMLa Croix NCBarrett PM, et al. Evaluation of a rebound tonometer for measuring intraocular pressure in dogs and horses. J Am Vet Med Assoc 2005; 277:244248.

    • Search Google Scholar
    • Export Citation
  • 5.

    Miller PEPickett JPMajors LJ, et al. Clinical comparison of the Mackay-Marg and Tono-Pen applanation tonometers in the dog. Prog Vet Comp Ophthalmol 1991; 1:171176.

    • Search Google Scholar
    • Export Citation
  • 6.

    Gelatt KNMacKay EO. Distribution of intraocular pressure in dogs. Vet Ophthalmol 1998; 1:109114.

  • 7.

    Herring IPBobofchak MALandry MP, et al. Duration of effect and effect of multiple doses of topical ophthalmic 0.5% proparacaine hydrochloride in clinically normal dogs. Am J Vet Res 2005; 66:7780.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 8.

    Gandhi PDGurses-Ozden RLiebman JM, et al. Attempted eyelid closure affects intraocular pressure measurement. Am J Ophthalmol 2001; 131:417420.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 9.

    Teng CGurses-Ozden RLiebman JM, et al. Effect of a tight necktie on intraocular pressure. Br J Ophthalmol 2003; 87:946948.

  • 10.

    Theelen TMeulendijks CFGeurts DE, et al. Impact factors on intraocular pressure measurements in healthy subjects. Br J Ophthalmol 2004; 88:15101511.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 11.

    Bigger JF. Glaucoma with elevated episcleral venous pressure. South Med J 1975; 68:14441448.

  • 12.

    Schuman JSMassicotte ECConnololy S, et al. Increased intraocular pressure and visual field defects in high resistance wind instrument players. Ophthalmology 2000; 107:127133.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 13.

    Priehs DRGum GGWhitley RD, et al. Evaluation of three applanation tonometers in dogs. Am J Vet Res 1990; 51:15471550.

  • 14.

    Dziezyc JMillichamp NJSmith WB. Comparison of applanation tonometers in dogs and horses. J Am Vet Med Assoc 1992; 201:430433.

  • Figure 1—

    Eyelid manipulations performed on dogs while IOP was measured by use of an applanation tonometer. A—Baseline with minimal eyelid restraint. B—Maximal extension of the eyelids ventrally and dorsally. C—Extension of the eyelids laterally.

  • Figure 2—

    Box-and-whisker plots of IOP during the 6 manipulations performed on 30 dogs (57 eyes). For each box, the horizontal line represents the median and the upper and lower boundaries represent the 75th and 25th percentiles, respectively. Whiskers represent the minimum and maximum values, and circles represent outliers. Base = Baseline. BiJ = Bilateral jugular pressure. DV = Dorsoventral eyelid extension. IpJ = Ipsilateral jugular pressure. L = Lateral eyelid extension. LBiJ = Lateral eyelid extension combined with bilateral jugular pressure.

  • Figure 3—

    Box-and-whisker plots of IOP during the 6 manipulations performed on the same dogs as in Figure 2, grouped by position of globe in orbit. C = Entire globe is caudal to the orbital rim. O = Globe reaches the orbital rim. R = Globe protrudes rostral to the orbital rim. See Figure 2 for remainder of key.

  • Figure 4—

    Box-and-whisker plots of IOP during the 6 manipulations performed on the same dogs as in Figure 2, grouped by skull type. B = Brachycephalic dogs. D = Dolichocephalic dogs. M = Mesaticephalic dogs. See Figure 2 for remainder of key.

  • 1.

    Pauli AMBentley EDiehl KA, et al. Effects of the application of neck pressure by a collar or harness on intraocular pressure in dogs. J Am Anim Hosp Assoc 2006; 42:207211.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 2.

    Broadwater JJSchorling JJHerring IP, et al. Effect of body position on intraocular pressure in dogs without glaucoma. Am J Vet Res 2008; 69:527530.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 3.

    Ollivier FJPlummer CEBarrie KP. Ophthalmic examination and diagnostics, part 1: the eye examination and diagnostic procedures. In: Gelatt KN, ed. Veterinary ophthalmology. 4th ed. Ames, Iowa: Blackwell Publishing, 2007;438483.

    • Search Google Scholar
    • Export Citation
  • 4.

    Knollinger AMLa Croix NCBarrett PM, et al. Evaluation of a rebound tonometer for measuring intraocular pressure in dogs and horses. J Am Vet Med Assoc 2005; 277:244248.

    • Search Google Scholar
    • Export Citation
  • 5.

    Miller PEPickett JPMajors LJ, et al. Clinical comparison of the Mackay-Marg and Tono-Pen applanation tonometers in the dog. Prog Vet Comp Ophthalmol 1991; 1:171176.

    • Search Google Scholar
    • Export Citation
  • 6.

    Gelatt KNMacKay EO. Distribution of intraocular pressure in dogs. Vet Ophthalmol 1998; 1:109114.

  • 7.

    Herring IPBobofchak MALandry MP, et al. Duration of effect and effect of multiple doses of topical ophthalmic 0.5% proparacaine hydrochloride in clinically normal dogs. Am J Vet Res 2005; 66:7780.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 8.

    Gandhi PDGurses-Ozden RLiebman JM, et al. Attempted eyelid closure affects intraocular pressure measurement. Am J Ophthalmol 2001; 131:417420.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 9.

    Teng CGurses-Ozden RLiebman JM, et al. Effect of a tight necktie on intraocular pressure. Br J Ophthalmol 2003; 87:946948.

  • 10.

    Theelen TMeulendijks CFGeurts DE, et al. Impact factors on intraocular pressure measurements in healthy subjects. Br J Ophthalmol 2004; 88:15101511.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 11.

    Bigger JF. Glaucoma with elevated episcleral venous pressure. South Med J 1975; 68:14441448.

  • 12.

    Schuman JSMassicotte ECConnololy S, et al. Increased intraocular pressure and visual field defects in high resistance wind instrument players. Ophthalmology 2000; 107:127133.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 13.

    Priehs DRGum GGWhitley RD, et al. Evaluation of three applanation tonometers in dogs. Am J Vet Res 1990; 51:15471550.

  • 14.

    Dziezyc JMillichamp NJSmith WB. Comparison of applanation tonometers in dogs and horses. J Am Vet Med Assoc 1992; 201:430433.

Advertisement