Cardiorespiratory effects of a cardioselective muscarinic antagonist in anesthetized horses

Treatment of bradycardia in horses has been historically ignored because of the motility depressant effects of nonselective antimuscarinics. This study evaluated the cardiopulmonary effects of a cardioselective (M2) muscarinic antagonist, methoctramine (MET), in anesthetized horses. In a previous in vitro study, we determined that supraphysiological doses of MET were necessary to inhibit acetylcholine‐induced longitudinal jejunal smooth muscle contractions in this species. Six adult horses were allocated to two treatments in a randomized complete block design. Anesthesia was induced with xylazine/ketamine, and maintained with halothane (1% end‐tidal) and a constant infusion of xylazine (1 mg kg^?1 hour^?1) under mechanical ventilation. Invasive hemodynamic variables were monitored at baseline (approximately 45 minutes after induction) and for 120 minutes after MET or saline (control) had been injected. MET was titrated at 10‐minute intervals (10 µg kg^?1 IV) until the heart rate (HR) increased at least 30% above the baseline, or a maximum cumulative dose of 30 µg kg^?1 had been injected. A person blinded to the treatment evaluated recovery scores and monitored intestinal auscultation until 24 hours after the end of anesthesia using previously published methods. Cardiovascular parameters were analyzed by anova followed by a Dunnet's test, and nonparametrical data were analyzed by a Mann–Whitney U‐test (p < 0.05). Values were mean ± SEM unless otherwise stated. MET significantly increased HR from baseline to 120 minutes post‐injection (from 29 ± 1 to 36 ± 2 beats minute^?1 at 20 minutes). Thermodilution cardiac output (CO) and mean arterial pressure (MAP) were increased from baseline to 75 minutes post‐MET injection (from 13.9 ± 0.8 to 19.4 ± 2.0 L minute^?1 for CO at 20 minutes, and from 82 ± 3 to 103 ± 5 mm Hg for MAP at 20 minutes). Recovery characteristics and bowel auscultation scores did not differ among the groups. The return to at least 75% of the maximum auscultation score occurred at 10 (8–18) hours [median (range)] for controls and at 9 (8–12) hours for MET. It was concluded that MET increased HR and improved hemodynamic function during halothane/xylazine anesthesia with no apparent effect on return to full‐bowel motility, as assessed by auscultation. Accordingly, M2 muscarinic antagonists might be represented as a safer alternative to treat intraoperative bradycardia in horse.     

Unidentified microsporidian in western pygmy perch Edelia vittata (Nannopercidae) from Australia

Specimens of western pygmy perch, Edelia vittata Castelnau 1873, were submitted for examination because of the presence of black masses in the body wall in the abdominal area. The owner reported that approximately 10% of mature fish in one dam were affected. The masses had no apparent effect on the health of affected fish, but they were unsuitable for sale for aesthetic reasons. After necropsy and histological examination, the masses were found to be microsporidial xenomas within the abdominal cavity. Transmission electron microscopy results suggest that this microsporidian is a previously unreported species, because the melanised xenoma wall and internal structure of the spores differ from previously described species. The epidemiology of the parasite remains largely unknown and highlights the need for further surveys of the disease status and parasites of Australian native fish, especially those of interest to aquaculturists.