Normal Range for the Schirmer Tear Test and Intraocular Pressure in Healthy Latvian Darkhead Lambs and Ewes

SIMPLE SUMMARY: Sheep are susceptible to various ophthalmic conditions, such as congenital, acquired, and infectious diseases. To obtain an accurate diagnosis and avoid misinterpretation, a comprehensive ophthalmological examination that includes diagnostic tests, such as the Schirmer tear test (STT) and intraocular pressure (IOP) measurement in particular species and breeds, is essential. Decreased STT can illustrate dry eye syndrome (keratoconjunctivitis sicca), but increased STT can characterize ocular discomfort and pain. Tonometry, at the same time, is useful to diagnose intraocular inflammation—uveitis. Normal STT and IOP values have been described in the literature for small animals such as cats and dogs; however, they are poorly described for small ruminants such as sheep. As far as we know, there are no reports of IOP values in lambs and no reports on STT and tonometry in Latvian Darkhead sheep. Therefore, this study aimed to improve data and determine the reference range for STT and tonometry values in clinically normal Latvian Darkhead lambs and ewes and will provide specific diagnostic values for Latvian Darkhead sheep. ABSTRACT: A complete ophthalmological examination, including basic diagnostic tests such as the Schirmer tear test (STT) and intraocular pressure (IOP) measurement performed by tonometry in particular species and breeds, is essential for obtaining a clear diagnostic result and avoiding diagnostic misinterpretations. STT and IOP values have been poorly described for sheep. Therefore, this study aimed to determine the normal range for STT and tonometry values in clinically normal Latvian Darkhead lambs and ewes. Both eyes of 100 sheep (200 eyes)—50 lambs (1–3 months old) and 50 ewes (1–8 years old)—underwent complete ophthalmic examinations, including STT and IOP evaluation. The mean ± standard deviation of STT values for both eyes in lambs and ewes were 13.12 ± 3.91 mm/min and 13.68 ± 4.09 mm/min, respectively. The IOP in lambs and ewes was 14.04 ± 3.68 mmHg and 19.16 ± 3.24 mmHg, respectively. Furthermore, the suggested reference range for the STT was 12.00–14.23 mm/min in lambs and 12.52–14.84 mm/min in ewes, while the reference IOP range was determined to be 13.00–15.08 mmHg in lambs and 18.24–20.08 mmHg in ewes. No statistically significant differences in STT and IOP values were observed for both eyes. However, the IOP value for both eyes was statistically significantly higher in ewes compared to lambs (p < 0.01). This study provides reference values for the STT and IOP in healthy Latvian Darkhead lambs and ewes.