Antinociceptive effects of subcutaneous hydromorphone in bearded dragons (Pogona vitticeps)

Abstract

Objective: To investigate the efficacy of subcutaneous hydromorphone in bearded dragons by evaluating changes in thermal quantitative sensory testing.

Study design: Randomized, blinded, crossover experimental study.

Animals: A group of six client-owned, healthy adult bearded dragons.

Methods: Thermal threshold (TT) testing was conducted using a ramped stimulus (2 °C second^-1) applied to the medial thigh until a response (leg kick, escape, attention to device) was observed or a maximum cut-off temperature (60 °C) was reached. Cold thermal latency (CTL) was tested by applying a 0 °C probe on the medial thigh until a behavioral response (same as TT) was elicited or the cut-off time (60 seconds) was reached. For baseline, both tests were repeated three times with a 20 minute rest interval between each test. Later, a randomized subcutaneous injection of hydromorphone (0.5 mg kg^-1) or equal volume of saline was given in the dorsal axillary region. TT and CTL were determined at 0, 2, 4, 8, 12, and 24 hours after injection by a single blinded evaluator. The procedure was repeated 4 weeks later with the alternative treatment. Data were analyzed using mixed-effect models on ranks. Repeatability and test-retest reliability were measured using the coefficient of variation (CV) and Spearman's ⍴. Significance was set at p ≤ 0.05.

Results: Hydromorphone significantly increased TT compared with placebo at all time points (p ≤ 0.002) with good repeatability (CV 28.5%). The highest median (interquartile range) TT of 60 (60-60) °C was recorded 2 and 4 hours after injection. CTL time was longer overall after hydromorphone injection (p < 0.001), regardless of time (p = 0.257). TT and CTL showed low test-retest reliability (⍴ = 0.21 and 0.38, respectively). No skin damage was observed.

Conclusions and clinical relevance: Hydromorphone (0.05 mg kg^-1, subcutaneously) provides antinociception in bearded dragons for 24 hours.

Keywords: analgesia; bearded dragon; cold thermal latency; hydromorphone; thermal threshold.