Final answer:
Gamma-Glutamyl Transferase (GGT) is the blood marker that increases in hepatocellular damage and is considered a sensitive marker for cholestatic damage in horses and ruminants.
Step-by-step explanation:
The blood marker that exhibits an increase in hepatocellular damage, especially hepatic necrosis, in horses and ruminants is Gamma-Glutamyl Transferase (GGT). GGT is reasonably specific to the liver and can be elevated with minor levels of liver dysfunction, making it a more sensitive marker for cholestatic damage than alkaline phosphatase (ALP). It is also a useful marker in identifying the cause of an isolated elevation in ALP and is raised in acute and chronic alcohol toxicity.