Hepatic progenitor cell activation in liver disease in dog and cat. K19 immunohistochemistry of liver sections from different types of liver disease in dog and cat. HPCs are activated in acute and chronic hepatitis and in biliary disease. The extent and location of the ductular reaction depends on type and severity of disease. Canine extrahepatic cholestasis and feline neutrophilic cholangitis were selected as representative biliary diseases. The lower panel shows K19 positive hepatocellular carcinoma in dog and cat. [6,11, Unpublished observations section: Valtolina et al., Van Sprundel et al.].