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

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

Randomized, blinded, crossover experimental study.

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

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.

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.

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