what is the first-line treatment for keratoconjunctivitis sicca in dogs and how do I monitor response?

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Topical cyclosporine A (CsA) 0.2% ointment applied twice daily is first-line treatment for immune-mediated keratoconjunctivitis sicca (KCS) in dogs, combined with a topical ocular lubricant applied as needed for palliative comfort.156 CsA works by blocking calcineurin-mediated interleukin-2 production, suppressing T-helper and cytotoxic T-cell infiltration of the lacrimal gland, resulting in regression of chronic corneal neovascularization and pigmentation and healing of indolent corneal ulcers.56 CsA is commercially available as Optimmune (0.2% ointment, Merck Animal Health).5 Topical lubricants are used continuously and palliatively alongside immunosuppressive therapy rather than as a substitute for it.1 For dogs that do not respond adequately to CsA, topical tacrolimus applied twice daily is the established alternative, as both are calcineurin inhibitors with similar mechanisms of action.25 Topical pimecrolimus twice daily has also demonstrated superior therapeutic effect compared to CsA in non-responders.5 A 100 microgram subconjunctival injection of liposome-encapsulated sirolimus every 2 weeks showed similar efficacy and safety to daily CsA or tacrolimus over 14 weeks and represents an emerging alternative for dogs requiring less frequent dosing.11 CsA can cause ocular hypersensitivity in some patients and is less effective in advanced KCS.5 Chronic topical immunosuppressive therapy carries risks of corneal and conjunctival infections and corneal squamous cell carcinoma.5 Monitor treatment response with the Schirmer Tear Test 1 (STT-1) and Tear Film Break-Up Time (TFBUT). STT-1 values and TFBUT are strongly correlated (r = 0.924), so improvement in one reflects improvement in the other.1 Normal TFBUT is ≥20 seconds; a TFBUT <5 seconds indicates an unstable precorneal tear film.1 For dogs treated with the aqueous calcineurin inhibitor SCY-641 twice daily, STT-1 values were significantly higher than placebo by day 14 and continued to improve through day 56, sugg