The prevalence of elevated gamma-glutamyltransferase and sorbitol dehydrogenase activity in racing Thoroughbreds and their associations with viral infection.

Abstract

In racehorses, serum gamma-glutamyltransferase (GGT) activity is positively correlated with cumulative days in training and, when ≥100IU/L, has been associated with poor performance. The prevalence of increased GGT activity in North American Thoroughbreds and its aetiopathogenesis are unknown. Four emerging viruses, pegivirus E (PgV E; equine pegivirus), hepacivirus A (HcV A; equine hepacivirus), pegivirus D (PgV D; Theiler's disease virus), and equine parvovirus-hepatitis (EqPV-H) have been identified in horses with clinical and subclinical hepatopathy. Available prevalence data indicate these viruses may commonly infect racehorses and contribute to increased liver enzyme activity in this population. To investigate the association between viral infection and increased liver enzyme activity in racing Thoroughbreds. Cross-sectional study. Prerace blood samples were collected from 802 Thoroughbreds and tested for GGT and sorbitol dehydrogenase (SDH) activity, and the presence of PgV E, HcV A, PgV D and EqPV-H nucleic acid. Increased SDH and/or GGT were detected in 56.2% of the 802 serum samples. The infection prevalence and relative risk (RR) of having concurrently increased liver enzyme activity were: PgV E=18.2% (RR=0.820, 95% CI=0.662-0.978, P=0.03), HcV A=2.5% (RR=1.132, 95% CI=0.719-1.466, P=0.6), PgV D=0.5% (RR=0.875, 95% CI=0.165-1.598, P>0.9), EqPV-H=2.9% (RR=0.916, 95% CI=0.564-1.266, P=0.7). Longitudinal samples were not tested. While viral infection was common among Thoroughbreds in this study, infection did not explain the high prevalence of increased liver enzyme activity. In fact, PgV E infection was associated with a reduced risk of having increased liver enzyme activity, indicating PgV E is unlikely to be a cause of hepatitis in horses. Importantly, like GGT, increased SDH activity was highly prevalent in this study, and provides additional evidence that hepatocellular injury was occurring in these horses.