What's Viscum?

My Siamese boy suffered chronic kidney failure. He was 4 years old. It's
now one year later and he's doing much better. His latest blood tests
confirm this.

My question is really to what Viscum complex does to a cat? The most I
have is that viscum is a species of mistletoe.

He's currently on a diet of Royal Canin "Renal S/O" supplemented by 3 ml
of "Renes / Viscum comp. PLV" per week. I understand that the Renal
food is low in protein and phosphates and this reduces the load on his
kidneys.

But what's the Viscum for and how does it work? What other sources are
there for this - the Plantavet supplied packs are quite pricey?

Any clues would be gratefully appreciated.

S.