Equine liver reviews highlight why test interpretation still matters

Two review articles in Veterinary Clinics of North America: Equine Practice offer a useful refresher on a familiar but still challenging clinical reality: in horses, liver disease is usually worked up through a combination of routine serum biochemistry and a narrower set of tests that assess actual hepatic function. In “Liver Metabolism and Function,” Michelle Henry Barton outlines the liver’s broad role in carbohydrate, protein, and lipid metabolism, detoxification, excretion, and nutrient storage. In the companion article, Leslie C. Sharkey focuses on how clinicians translate those functions into pathology workups, separating markers of damage or cholestasis from markers of impaired function. The issue was published as volume 22, issue 1 of the journal in April 2006. (sciencedirect.com)

That framework has held up well because equine hepatology remains a field where interpretation matters as much as any single number. The liver regulates blood glucose, fat metabolism, clotting factor and albumin production, bile formation, bilirubin handling, ammonia conversion to urea, and detoxification of drugs and toxins. Yet horses often don’t show overt signs until disease is advanced, in part because the organ has substantial reserve and regenerative capacity. Clinical signs can be nonspecific, and in acute hepatic failure, neurologic abnormalities from hepatic encephalopathy may dominate the presentation. (merckvetmanual.com)

The practical message from Sharkey’s review is that clinicians should distinguish between tests that suggest hepatocellular injury or biliary disease and tests that indicate loss of function. Current guidance still supports that approach. AAEP’s equine viral hepatitis guideline notes that hepatocellular disease tends to push AST, SDH, and GLDH earlier in the disease course, while GGT may rise later. Function-oriented abnormalities can include increased bile acids and ammonia, while traditional chemistry and coagulation findings such as bilirubin, albumin, BUN, and clotting times help round out the picture. Merck also notes an important equine nuance: fasting hyperbilirubinemia is relatively common in horses and doesn’t necessarily indicate primary liver disease, so bilirubin has to be interpreted in context. (aaep.org)

More recent research adds a prognostic layer to that older diagnostic framework. A multicenter retrospective study of 82 horses with liver dysfunction in the eastern United States evaluated whether maximal bile acids, direct bilirubin, GGT, and biopsy-reported fibrosis were associated with survival. The study reflects how clinicians increasingly want not just to identify liver disease, but to estimate outcome and guide treatment intensity, referral decisions, and conversations with pet parents. While the review articles are foundational, this newer work suggests that specific analytes may help stratify risk in a way earlier overviews could only hint at. (sciencedirect.com)

There’s also more infectious-disease context now than there was when the 2006 reviews were published. AAEP’s more recent equine viral hepatitis materials describe EqPV-H and EqHV as important considerations in horses with signs of liver disease, noting that many infected horses remain subclinical, some clear infection, and some develop persistent infection or hepatitis. That matters because it complicates interpretation: a positive viral test may not prove causation, and serial testing plus biopsy-based assessment may be needed to connect laboratory abnormalities to active hepatic disease. (aaep.org)

Why it matters: For veterinary professionals, these reviews are a reminder that equine liver cases are rarely solved by a single elevated enzyme. The clinically useful question is whether the horse has hepatocellular injury, cholestasis, impaired synthetic or clearance function, or some combination of the three. That distinction affects urgency, prognosis, monitoring, and whether the next step is supportive care, infectious workup, toxin investigation, imaging, or biopsy. It also shapes how practices explain results to pet parents, especially when a horse looks relatively stable despite concerning laboratory changes, or when bilirubin is high in a fasted horse without convincing evidence of hepatopathy. (merckvetmanual.com)

The broader industry perspective is less about a single quote and more about convergence. Across AAEP guidance, review literature, and retrospective studies, the message is consistent: serial trends, not isolated values, are often what separate noise from meaningful progression. That’s especially true in infectious hepatitis, where marker patterns can evolve over time, and in chronic disease, where loss of function may lag behind evidence of injury. (aaep.org)

What to watch: The next area to watch is whether newer prognostic studies and infectious-disease guidance translate into more standardized equine liver panels, clearer cutoff values for referral or biopsy, and stronger evidence on which combinations of enzymes and function tests best predict survival and recovery. (sciencedirect.com)