Laryngeal paralysis in Arabian foals associated with oral haloxon administration

Bilateral laryngeal paralysis is described in 5 Arabian and part-Arabian foals aged between 23 and 35 days. Tracheotomies resulted in complete relief of dyspnoea. Two cases showed recovery of abductor function of the right arytenoid cartilage after 3 weeks and one of these cases later recovered left abductor function. Four of the foals were autopsied at various times from one week to 6 months after the onset of respiratory obstruction. Histology of the recurrent laryngeal nerves showed active Wallerian degeneration and loss of nerve fibres in many fascicles in cases affected for one to 2 weeks. In the longer standing cases there was a severe deficiency of myelinated fibres. Histology of the dorsal and lateral cricoarytenoid muscles showed no abnormalities in the cases affected from one to 5 weeks. However, in the foal affected for 6 months there was variation in fibre diameter in the left dorsal cricoarytenoid muscle; in sections stained for myosin ATPase there was evidence of muscle fibre type grouping in both left and right dorsal cricoarytenoid muscles. It is postulated that the most likely cause of the condition was oral haloxon administration which was given to the foals at a dose rate of 1 to 2 g at fortnightly intervals, beginning at 2 days of age.