Safety and efficacy of oral mirtazapine in New Zealand White rabbits (Oryctolagus cuniculus)

One of the most common presenting complaints in rabbits is decreased food intake and/or fecal output. This can occur as a clinical consequence of pain, anesthesia and systemic or gastrointestinal disease. In conjunction with diagnosing and treating the underlying condition, veterinary care should aim to normalize food intake and fecal output. Mirtazapine is a tetracyclic antidepressant that has appetite stimulating effects in other species. However, studies in rabbits are currently lacking. Nine six-month old, intact (4 male, 5 female) New Zealand White rabbits (Oryctolagus cuniculus) were used in a randomized, blinded, controlled, complete cross-over experiment. Each rabbit received a low-dose of mirtazapine (1 mg/kg), high-dose (3 mg/kg) and control treatment orally once a day for a total of 2 sequential days with a 5-day washout period between treatment groups. Fecal output, food intake and body weight were measured before, during and after treatment. Fecal output was 25% higher (95% CI: 6.3%-46.9%, P= 0.01) on treatment days following high-dose mirtazapine treatment. Body weight was 1.5% higher (95% CI: 0. 5% to 2.6%, P= 0.006) for all groups on the 2 days following treatment compared to baseline. Body weight was reduced by 2.1% (95% CI:-3.6% to-0.6%, P= 0.011) the week following high-dose mirtazapine treatment. Mirtazapine did not have a statistically significant effect on food intake. Oral administration of mirtazapine at 3 mg/kg once a day resulted in increased fecal output but not a concurrent increase in food intake. Overall, mirtazapine is well tolerated and may be a useful treatment to increase fecal output in rabbits. However, due to a mild reduction in weight following treatment, clinical monitoring is warranted.