We may need to reconsider the treatment of eye infections in the light of research that found that horses produce more tears than previously thought.
Tears protect the eye by keeping the cornea moist. The tear film also carries nutrients to the cornea, which has no blood supply of its own.
The same route is also used to deliver medication to the cornea.
How many tears does a healthy horse produce? How quickly is the tear film replaced? There have been few studies into this, yet it is important to know, as it will affect the concentration of drugs applied to the eye - and so may affect their efficacy.
Dr Thomas Chen and Dr Daniel A. Ward looked at the rate of production of tears in normal horses' eyes, in a study carried out at the College of Veterinary Medicine, University of Tennessee, Knoxville. A full report of the research, entitled "Tear volume, turnover rate, and flow rate in ophthalmologically normal horses", has been published in the American journal of Veterinary Research.
The researchers instilled fluorescein dye onto the eye and then collected small samples of the tear film, measured the fluorescein concentration, and noted how it changed with time.
They used two mathematical approaches to analyse the results. One method, which was considered the more accurate, suggested that the rate of tear production was 33.6L /min. The mean tear film volume was estimated to be 233.74 L.
The results showed that the tear film was likely to be completely replaced within a matter of 7 minutes.
This suggests that current treatment protocols may not be adequate, and that drugs may need to be administered more frequently.
For example, effective treatment of corneal ulcers infected with Pseudomonas aeruginosa may require high concentrations of antimicrobial medication to be maintained in contact with the cornea.
Based on their estimate of tear production, Chen and Ward calculate that to treat such infections with tobramycin, it may be necessary to administer the antibiotic at 30-45 minute intervals to keep the concentration in the tear film above the minimum level required to inhibit the growth of 90% of the organisms.
The study was performed on healthy eyes. Diseased eyes would not necessarily respond in the same way. If anything, the rate of tear production might be expected to be higher.
The authors recommend that similar studies be carried out on horses with corneal disease to discover the rate of tear production in those animals. However, until that is done, they question whether other systems of administration - such as more frequent dosing regimens or constant indwelling infusions might be necessary to ensure that adequate levels of medication are maintained in the tear film.
For more details see:
Tear volume, turnover rate, and flow rate in ophthalmologically normal horses.
T Chen, DA Ward.
American Journal of Veterinary Research June (2010) 71, 671-676