My guinea pigs had lice and I took them to the vet, the vet gave them needles to get rid of the lice but it didn't work we had to keep on going every two weeks they had 4 injections altogether every two weeks but these lice didn't wanna go so they tried the spot on instead (Xeno) after 4 weeks of treatment the lice had gone!
But I suppose it depends on the guinea pig!
It also depends upon the parasite!
Lice, which CAN be seen with the naked eye; are surface parasites that feed on skin not blood, so will not be killed by ivermectin injections but can be killed by ivermectin topical (ie"spot-on treatements)
Sarcoptic mange mites burrow into the skin, can't be seen with the naked eye, are the most serious (and deadly) of infections and are best killed by ivermectin injections
Hay mites can;t be seen with the naked eye (or look a bit like walking dandruff) and require either injections or topical treatment
Fungal needs a completely different treatment altogether.
Beaphar is NOT a good mite treatment as it is weak and ineffective compared to vet meds. Also piggies need any medication, whatever the symptoms, to be dosed by weight according to specific guidelines in the National Pet formulary, and for small pigs, again you need to be very careful what you use.
There is NO point in trying to give a treatment prior to an imminent vets visit as all it will do is interfere with what the vet can give them in terms of the correct treatment for a properly diagnosed condition
HTH
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