Measurement of Tear Production in English Angora and Dutch Rabbits

The purpose of this study was to establish normal values for tear production tests in different breeds of domestic rabbits. Healthy adult rabbits (n = 60; 120 eyes) of 2 different breeds (English angora and Dutch; n = 15 of each sex and breed) were used in this study. Tear production was measured by using the 1-min Schirmer tear test (STT), phenol red thread test (PRTT), and endodontic absorbent paper point tear test (EAPTT). In addition, horizontal palpebral fissure length was evaluated as a measure of ocular adnexal dimensions. Tear production (mean ± 1 SD) in English angora rabbits was 5.4 ± 1.6 mm/min according to the STT, 25.0 ± 2.7 mm in 15 s for the PRTT, and 18.8 ± 2.1 mm/min by the EAPTT; in Dutch rabbits, these values were 4.6 ± 1.2 mm/min, 23.6 ± 2.3 mm in 15 s, and 16.9 ± 1.7 mm/min, respectively. Only the EAPTT revealed a significant difference in tear production between English Angora and Dutch rabbits. These results provide reference values for tear production in English Angora and Dutch rabbits according to 3 different quantitative tear film assessment methods.Abbreviations: EAPTT, endodontic absorbent paper tear test; HPFL, horizontal palpebral fissure length; PRTT, phenol red thread test; STT, Schirmer tear testDomestic rabbits (Oryctolagus cuniculus) are popular pets and commonly referred to veterinarians for routine health care and solving health problems. Rabbits are also common laboratory research animals, because they are tractable and inexpensive to maintain. Ocular lesions of all segments of the eyes have been reported in wild, pet, and research animals.² Rabbits have prominent, laterally placed eyes, allowing them a large field of vision, which is typical of prey species. They have an upper eyelid, a larger and thinner lower eyelid, and a third eyelid (or nictitating membrane). Rabbits normally blink infrequently, averaging 10 to 12 times hourly.¹²The tear film is as vital to the normal function of the eyes as any anatomic component and is essential in the maintenance of corneal clarity.^10,13 The aqueous layer is responsible for lubricating, oxygenating, and nourishing the cornea; flushing debris from the corneal surface; providing various antibacterial properties. The mucin layer allows the aqueous tear fluid to spread evenly over the cornea and allows the adherence of the tear film to the corneal surface.⁵ The lipid layer of tears contains waxy esters, sterols, triglycerides, cholesterol, and some polar lipids. This lipid mixture coats the surface of the tear film and retards tear evaporation.¹³Quantitative tests for the evaluation of tear film include the Schirmer tear test (STT), phenol red tear test (PRTT), and endodontic absorbent paper point tear test (EAPTT). The STT is the most commonly used, standard method for tear film assessment. The results of various ophthalmic diagnostic tests have been reported for New Zealand white rabbits^1,3,11 but not for any other breed. The purpose of the current study was to establish normal values for tear production in 2 breeds of domestic rabbits, English angora and Dutch, according to the STT, PRTT, and EAPTT.