Note: In NO way am I a medical professional!
Just my opinion, most livers are tough to kill. And, there's typically signs of liver damage before it gets really bad. I'm not sure about kava, but I'd imagine a weakened liver would have signs in the beginning.
For example:
Yellowing of the eyes and finger nails. Like, the white part of the eyeball gets this yellow look to it. It's because the liver isn't cleaning out the bilirubin in blood efficiently. Bilirubin is what's left of old red blood cells after the body breaks them down. They're getting replaced all the time!
Brownish urine. Sort of like tea colored. The human body is of excellent design. If the liver isn't working properly, the kidneys take on the roll of bilirubin removal. This creates a brownish color to the urine. The kidneys DO remove some throughout the day, but they have to double up on the removal as the liver starts to slack off.
Light colored stools. No, not the kind of stools around your minibar..lol. The kind you dread coming on when there's nothing but a public bathroom available. :/ Anyhow, I think it has something to do with the liver not creating bile. And bilirubin is removed through bile in the digestive tract, and also some in urine.