Systemic Ivermectin Administration Reduces Aqueous Tear Production and Affects Ocular Surface Health in Clinically Healthy Dogs

Purpose This study aimed to evaluate the effects of ivermectin administration on aqueous tear production and ocular surface health in dogs and to investigate its potential association with quantitative keratoconjunctivitis sicca (KCS). Methods A total of 128 mixed‐breed dogs were included: 64 ivermectin‐treated and 64 untreated controls. Lacrimal secretion and ocular surface status were assessed using Schirmer tear test I (STT‐I), fluorescein staining, and corneal opacity scoring. Data were analyzed with nonparametric tests, with p < 0.05 considered significant. Results At 36 h after treatment, STT‐I values were significantly reduced from baseline in the ivermectin group ( p < 0.001), whereas no significant change was observed in controls ( p > 0.05). Reduction in aqueous tear production (ΔSTT‐I) was greater in ivermectin‐treated dogs (−4.38 ± 2.7 mm/min, p < 0.001) than in controls (−0.14 ± 0.9 mm/min; p < 0.001). Mean STT‐I at 36 h was lower in the treated group (6.05 ± 3.5 mm/min) compared with controls (10.08 ± 3.2 mm/min; p < 0.001). Fluorescein uptake was observed more frequently in ivermectin‐treated dogs (32.8%, p = 0.001) than in controls (7.8%, p = 0.001). KCS severity scores were significantly higher in the ivermectin group ( p < 0.05), while the control median score was 1. Conclusions Routine systemic ivermectin treatment was associated with reduced aqueous tear production. The mechanism was not evaluated but may involve altered neural control of lacrimal secretion. Even at recommended doses, ivermectin may predispose susceptible dogs to subclinical dry eye; therefore, quantitative tear testing is advisable in dogs receiving ivermectin prophylaxis.