Retrospective evaluation of leflunomide as second-line immunosuppressive therapy in dogs with meningoencephalomyelitis of unknown origin: 18 cases (2018-2025).

Abstract

BACKGROUND: Meningoencephalomyelitis of unknown origin (MUO) comprises a group of noninfectious inflammatory diseases affecting the canine central nervous system. To date, no consensus exists regarding the optimal treatment. Leflunomide has been used as a second-line immunosuppressant, but clinical outcome data remain limited. HYPOTHESIS/OBJECTIVES: To evaluate the clinical use, efficacy, and adverse effects of leflunomide with glucocorticoids in dogs with suspected MUO that required adjunctive therapy after initial glucocorticoid monotherapy. ANIMALS: Eighteen client-owned dogs with a presumptive diagnosis of MUO treated with leflunomide. METHODS: Retrospective case series (2018-2025). Data included clinical signs, neurologic assessments, imaging and cerebrospinal fluid results, laboratory findings, treatment protocols, adverse effects, and survival. Treatment response was classified as complete, partial, or none using the neurodisability scale. Data analyses included descriptive statistics, Kaplan-Meier survival analysis, Cox proportional hazards models, and group comparisons applying Fisher's exact and Kruskal-Wallis tests. RESULTS: The response rate to leflunomide was 66% (22% complete, 44% partial), and the estimated median survival following leflunomide initiation was 1277 days (95% CI, 194 days-not reached due to a low event rate), with 65% of dogs surviving at 1 year. Adverse effects occurred in 39% of dogs, most commonly mild signs of gastrointestinal disease or dysfunction (28%). CONCLUSIONS AND CLINICAL IMPORTANCE: Leflunomide appears to be a well-tolerated and potentially effective second-line treatment for dogs with suspected MUO.