Some horses display stallion-like behaviour despite appearing to have been castrated. Before embarking on exploratory abdominal surgery, how do you distinguish between those with a retained testis (cryptorchid or “rig”) and those that just behave badly?
One technique is to assess resting serum testosterone concentrations to identify horses with functioning testicular tissue.
Recently published research suggests that testosterone levels vary through the year and that this should be borne in mind when interpreting the results. Apparently, spring is the best time to use a serum testosterone assay to confirm the presence of a suspected retained testicle.
Researchers at the Gluck Equine Center, University of Kentucky, and the University of California Davis (UCD), examined data from blood samples submitted from suspected cryptorchids to the clinical endocrinology laboratory at UCD.
Serum from 179 suspected cryptorchids with serum testosterone greater than 100pg/ml were included in the study. In the UCD laboratory, testosterone levels lower than 50pg/ml are interpreted as evidence of absence of testicular tissue, while levels >100pg/ml confirm the presence of testicular tissue.
The research team found that serum testosterone concentration varied with season, being higher in spring than at other times of the year and lower in fall compared with summer and winter. Concentrations of testosterone reached a peak in May and were lowest in November.
They also noticed a significant association between age and testosterone concentration – with testosterone levels being lower in cryptorchids less than two years old. Testosterone levels also declined in horses older than nine years old.
They advise that borderline low testosterone concentrations found late in the year might still be positive – so a further test should be performed the following spring if the animal continues to display stallion-like behaviour.