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Measurement of tear production using the phenol red thread test in the common mynah (Acridotheres tristis).

Abstract: To evaluate tear production in the common mynah (Acridotheres tristis) using the phenol red thread test (PRTT) and to make a comparison of measurements with the PRTT placed in the fornices of lower and upper eyelids, tear production of both eyes in 22 healthy adult captive mynah birds was evaluated. After positioning of threads in the fornices of upper and lower eyelids, the PRTT values of the birds were 17.5 [+ or -] 3.1 mm/15 s and 19.2 [+ or -] 2.5 mm/15 s, respectively. A significant difference was found between PRTT values for upper eyelids and lower eyelids (P = 0.01). This study provides novel data for normal reference ranges of PRTT values in healthy common mynah birds and shows that a difference is found depending on where the PRTT thread is placed.

Key words: eye, tear production, PRTT, phenol red thread test, avian, common mynah, A cridotheres tristis

Clinical Brief

The common mynah (Acridotheres tristis) is a family member of Sturnidae (starlings). They are stocky, brown birds with a glossy black head and throat; yellow bill; bare yellow skin behind the eyes; and yellow legs. These birds are found in southern Asia from southeastern Iran through Afghanistan, Pakistan, India, Sri Lanka, southern China, and Vietnam. (1)

The mynah is commonly maintained as a pet in Iran, but there is a lack of knowledge about normal ophthalmologic parameters in this species. It is thus important to establish reference values for ophthalmic tests routinely used, including the measurement of tear production.

The Schirmer tear test (STT) is the most commonly used method to evaluate aqueous tear production in veterinary medicine. However, because of the 5-mm width of the filter paper strip and the small globe size of the eyes of birds such as the myna, its use is problematic in such species. Another quantitative tear film test is the phenol red thread test (PRTT). The shorter time required to perform the PRTT and the small size of the thread may make this test particularly suitable for use in birds and other small animals. (2) The PRTT is performed by placing a 75-mm-long cotton thread impregnated with pH-sensitive phenol red dye in the ventral fornix of the eye for 15 seconds. The cotton thread is much less irritating to the corneal surface and eyelids than the filter paper strips used in the STT. The PRTT provides measurements of the residual tear volume in the lacrimal lake and of basal tearing. (3)

The PRTT has been documented in several avian species (3-5) and in small mammals (6-8) as a practical and reliable method for quantitative tear film evaluation. The purpose of this study was to establish a reference range for the PRTT and to compare the values obtained when the thread was placed in the fornices of upper and lower eyelids in clinically normal common mynah birds.

Materials and Methods

This study was approved by the Iran Society for Prevention of Cruelty to Animals in accordance with Iranian ethical codes for studies on laboratory animals. The study population consisted of 22 healthy adult common mynahs. All birds weighed between 83 and 120 g (mean [+ or -] SD; 100 [+ or -] 8 g). All animals were housed indoors in separate labeled cages with constant temperature (24[degrees]C) and humidity in an air-conditioned room and were fed a mixture of fruits and grains. A consistent 24-hour lighting cycle (12 hours of light and 12 hours of dark) was maintained. A full physical examination was performed on all birds, which were subject to a full ophthalmic examination using direct (Direct Ophthalmoscope, Welch Allyn, Skaneateles Falls, NY, USA) and indirect ophthalmoscopy (Binocular Indirect Ophthalmoscope, Welch Allyn) and slit lamp biomicroscopy (PSL Portable Slit Lamp, Reichert Inc. Munich, Germany). All animals were selected based on normal results of physical and ophthalmic examinations including fluorescein staining and slit lamp biomicroscopy.

Birds were gently restrained physically without sedation during the procedure. Any variation in absorptive capability of the threads was minimized by using only phenol red threads from one manufacturer and a single lot number (lot number 9143L, Zone-Quick; Menicon America Inc, San Mateo, CA, USA). To limit interobserver variation, one investigator (S.M.R.) performed all tests. First, the lower eyelid of each mynah was gently everted, and the folded end of the cotton thread was placed into the ventral lower conjunctival fornix by using forceps. After 15 seconds the thread was removed, and the red portion of the thread was immediately measured in millimeters using a digital vernier caliper (IP54, 0-150 mm, 0.01 mm resolution; Guanglu, Guilin, China). The PRTT thread was then inserted under the upper eyelid with the same technique. An interval of 10 minutes was allowed to elapse between lower and upper eyelid measurements. All PRTT measurements were performed between 8 and 10 am to minimize the possible variations associated with diurnal changes. Birds were not disturbed by the procedures used, and no ocular discomfort was observed in any bird over the 6 hours after measurement.

Statistical analysis was performed using a statistical software program (SPSS for Microsoft Windows 17.0, SPSS Inc, Chicago, IL, USA). Normality was tested by a 1-sample Kolmogorov-Smirnov test. A paired samples t test was used to compare results of the PRTT obtained from upper eyelid and lower eyelid of right and left eyes. Means and SDs were calculated for right and left eyes separately in each bird and for all eyes combined. Pearson correlation was used to evaluate the relationship between mean PRTTs and body weight. A value of P < 0.05 was considered statistically significant.


All continuous numeric data obtained for PRTTs in the population used in this investigation were normally distributed according to the 1-sample Kolmogorov-Smirnov test (P > 0.3). All data were expressed as millimeters per 15 seconds. The mean [+ or -] SD PRTT values for the upper eyelid of the left eye (OS) and right eye (OD) were 16.6 [+ or -] 3.8 and 18.4 [+ or -] 3.6 mm/15 s, respectively. The mean [+ or -] SD PRTT values for the lower eyelid of OS and OD were 19.5 [+ or -] 2.4 and 18.9 [+ or -] 3.3 mm/15 s, respectively. No differences were observed between the values obtained from the PRTT placed under the lower eyelid for OS and OD (P = 0.35). As there was no statistically significant difference between OS and OD, the data for both eyes were averaged for further analysis. The mean [+ or -] SD PRTT values for upper eyelids and lower eyelids of the study population were 17.5 [+ or -] 3.1 mm/15 s (range, 11.5-25.0 mm/15 s) and 19.2 [+ or -] 2.5 mm/15 s (range, 14.5-24.5 mm/15 s), respectively. A significant difference was found between PRTT values for upper eyelids and lower eyelids (P = 0.01). Pearson correlation revealed that a linear relationship did not exist between mean PRTTs and the body weights of birds in the study populations (P = 0.7).


Results of the present study provided information on ranges of values for PRTT in healthy common mynah birds. Values for the PRTT test when used in the lower conjunctival fornix have been published for Amazon parrots (Amazona species) (12.5 [+ or -] 5 mm/15 s) (3) and screech owls (Megascops asio) (15 mm [+ or -] 4.3 mm/15 s). (5) A range of 1 to 25 mm/15 s has been reported for Amazon parrots. Holt et al (4) measured the PRTT values of different species of psittaciform birds by placement of the thread in the superior conjunctival fornix (OS = 20.1 [+ or -] 3.9 mm/15 s, OD = 19.8 [+ or -] 4.3 mm/15 s). A direct comparison between PRTT values of mentioned studies and our study is difficult because of differences in anatomy of the lacrimal glands and conjunctival sac and size of different bird species. (3,4) Nevertheless, the mean [+ or -] SD PRTT values of common mynahs in our study were greater than those reported for Amazon parrots and screech owls and lower than PRTT values of other psittaciform birds.

In the current study, we noted a significant difference between tear measurements when the PRTT thread was placed under the upper eyelid margin and in the lower conjunctival fornix. The exact cause for this variation is unknown; however, this difference could be caused by the larger conjunctival sac of the lower eyelid compared with the upper eyelid margin and the anatomy of the drainage system of tears in bird eyes. (9,10) Although the values were statistically different, a difference of less than 2 mm/15 s is unlikely to be clinically significant.

In the normal avian eye, Meibomian glands are absent. The nictitating membrane is well developed and actively mobile. There is no gland of the nictitating membrane proper; however, a lacrimal gland is present inferotemporal to the globe of the eye, and a Harderian gland sits adjacent to the posterior sclera, near the base of the nictitans but not part of it. (2) The tears drain into the conjunctival sac on the bulbar surface of the lower lid and then exit via the inferior and superior nasolacrimal puncta at the medial canthus. (10) Holt et al (4) noted that the placement of the cotton thread in the superior conjunctival sac might be expected to reflect a smaller portion of the residual tear volume than if the thread was placed inferiorly. Here we hypothesize that the thread in the lower conjunctival fornix is absorbing tear fluid from the lacrimal lake formed in the inferior conjunctival fornix, whereas the same pooling effect is not seen when the thread is placed under the upper lid.

Despite the actively mobile nictitans membrane in common mynah, difficulty placing threads or extrusion of threads was not observed during this study.

Placement of phenol thread and Schirmer strips in the lower conjunctival fornix is more commonly used to evaluate tear production in animals, and here we also suggest using the lower conjunctival fornix technique to evaluate tear secretion in the common mynah bird. Whatever technique is used, the key feature is that the same placement method be used whenever tear production is measured. This has not been the case in previous papers on avian patients, and to our knowledge, no studies have evaluated whether there is a difference in PRTT or STT measurements from upper and lower fornix in any mammal species.


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(8.) Rajaei SM, Sadjadi R, Sabzevari A, Ghaffari MS. Results of phenol red thread test in clinically normal Syrian hamsters (Mesocricetus auratus). Vet Ophthalmol. 2013;16(6):436-439.

(9.) Bayon A, Almela RM, Talavera J. Avian ophthalmology. Eur J Companion Anim Pract. 2007; 17(3):113.

(10.) Doneley B. Clinical anatomy and physiology. In: Doneley B, ed. Avian Medicine and Surgery in Practice: Companion and Aviary Birds. London, UK: Manson Publishing; 2011:7-39.

Seyed mehdi Rajaei, DVM, DVSc, Manely Ansari mood, DVM, DVSc, Hamid Khorram, DVM, DVSc, Masoud Selk ghaffari, DVM, PhD, and David L. Williams, MA, VetMB, PhD, CertVOphthal, CertWEL, FHEA, FRCVS

From the Department of Clinical Sciences, Faculty of Specialized Veterinary Sciences, Science and Research Branch, Islamic Azad University. Tehran, Iran (Ansari mood, Khorram); the Department of Clinical Sciences, Faculty of Veterinary Medicine, Karaj Branch, Islamic Azad University, Alborz, Iran (Rajaei, Selk ghaffari); and the Department of Veterinary Medicine, Madingley Road. Cambridge UK CB3 0ES (Williams).
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Article Details
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Title Annotation:Clinical Brief
Author:Rajaei, Seyed mehdi; Ansari mood, Manely; Khorram, Hamid; Selk ghaffari, Masoud; Williams, David L.
Publication:Journal of Avian Medicine and Surgery
Article Type:Clinical report
Date:Jun 1, 2015
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