Gizzy - I didn't say that ivermectin was not toxic to guinea pigs, I said
"...are not toxic to guinea pigs unless you overdose them."
If a guinea pig requires treatment for something, that treatment should not be withheld. There's a difference between unnecessary treatment, perhaps as a preventative and legitamate treatment when something is wrong.
Not many pharmaceuticals are licensed for guinea pigs: Metacam is not licensed for guinea pigs, there are no arthritic medications licensed for guinea pigs, there are no heart medications licensed for guinea pigs and, according to my vet, the only antibiotic licensed for guinea pigs is Baytril, which is hardly helpful if said guinea pig has something resistant to that antibiotic.
Keeping this post in context: the Original Poster has a guinea pig who is losing fur. The original poster believes that he or she has found lice in her pigs fur and has decided that lice are in the fur. Would you have her not treat the animal because breeders in another location are giving their animals preventative doses. Treating an animal who has parasites is hardly "blitzing their systems with allsorts" it's treating a problem with a known method of prevention.
Different pharmaceuticals are used on these animals in controlled measures because they have been shown to make the guinea pig more comfortable. They are used on guinea pigs because the guinea pig is unwell and needs to be made comfortable. You are currently discussing medical issues with a group of guinea pig pet owners not a bunch of breeders trying to prevent things from happening but owners who are willing to treat aliments in their pets.
I'd very much like to read whatever research paper you got your statistics on the biggest reported killer of guinea pigs. Keep in mind that not all kidney failure is caused by medicine, but can also be due to a long-term illness where the pig hasn't been drinking enough.
So far I've found illnesses such as malocclusion or annorexia and heart/lung issues to have been the biggest cause of death in guinea pigs in the guinea pig forums I've frequented.