Serum testosterone levels in young normal horses

Concentration of serum testosterone in XY sex reversed horses

The XY Sex Reversal Syndrome of the horse is a condition associated with female or intersexual development in genetic males. In our previous study, 38 sex reversed XY mares were classified according to behavior, gross clinical phenotype, gonadal status, and H-Y phenotype. Four classes were described, ranging from potentially fertile female (Class I) to virilized intersex (Class IV). In the present study, serum testosterone concentrations were measured in 29 sex-reversed XY mares, 3 normal mares and 3 normal stallions. Serums were obtained during the breeding season (March-August), and were stored at -70 C until assayed. Serum testosterone concentrations in the normal XX mares ranged from nondetectable to 0.41 ng/ml; in normal XY stallions, from 1.04 ng/ml to 2.4 ng/ml; and in XY mares, from nondetectable to 5.4 ng/ml. Sex reversed mares previously assigned to Class I or II had serum testosterone concentrations ranging from nondetectable to 0.22 ng/ml. Serum testosterone concentrations in XY mares were correlated with sex phenotype and behavior. Although the range of steroid concentrations among XY mares may be quantified more accurately with increased sampling, serum testosterone concentrations can be used currently as an added parameter for study of the sex reversed condition.

Progressive myotonia in foals resembling human dystrophia myotonica

A severe and progressive neuromuscular disorder accompanied by clinical, electrophysiological, and pathological features resembling human dystrophia myotonica was observed in three foals. This disorder was apparent as early as 1 month of age and involved progressive skeletal muscle dysfunction, initially characterized by proximal muscle hypertrophy and hypertonicity with subsequent muscle stiffness, weakness, and atrophy. Multisystem involvement was manifested in one case by testicular hypoplasia, early cataract formation, and borderline glucose intolerance. Prolonged dimpling of these large rear-limb muscles was elicited by percussion. Myotonic discharges were identified by electromyography. Percussion dimpling and the typical myotonic discharges persisted after neuromuscular blockade. Distinctive histologic muscle changes included ring fibers, sarcoplasmic mass formation, variation in fiber diameter size, and internally positioned nuclei.