Final answer:
Gamma-Glutamyl Transferase (GGT) is the blood marker that suggests longer-term liver damage in horses and ruminants. It is more sensitive and specific for indicating liver dysfunction compared to other liver enzymes like ALT and AST, especially in cases related to cholestasis and alcohol toxicity.
Step-by-step explanation:
The blood marker that demonstrates an increase in longer-term liver damage and is particularly useful in horses and ruminants is Gamma-Glutamyl Transferase (GGT). This enzyme is a sensitive marker for cholestatic damage to the liver, and elevations in GGT can be indicative of even minor or sub-clinical levels of liver dysfunction. It is useful in identifying the cause of isolated elevations in Alkaline Phosphatase (ALP) and is notably raised in instances of alcohol toxicity.
Alanine Aminotransferase (ALT) and Aspartate Aminotransferase (AST) are enzymes released into the blood with acute liver damage, but they are not specific to the liver as they are also found in red blood cells and muscle tissue. While these markers are important in the assessment of liver function, GGT is often a more reliable indicator of longer-term damage in the specific animals mentioned.