As you're unsure whether it's mites or fungal (and I'm inclined to suspect both may be present to some degree), I would definitely recommend seeing a guinea-savvy vet.
How old is your guinea?
Mites do need treating with ivermectin, dosage depends on the weight of the pig and may be given topically (on the skin, "spot-on" ), orally or parenterally (by injection).
Fungal infectons will require bathing in an anti-fungal shampoo; Nizoral is a good one, or the vet may prescribe Malaseb, or you can follow the Gorgeous Guineas treatment recommendations. Whichever shampoo you use, you will need to wash and rinse, then lather the pig up and allow to soak for five minutes before rinsing and drying.
For localised patches of fungal infection, then a miconazole-based cream will help, as Laura (aalur) said above. Daktarin Cream is one. Others have used Canesten 1% (clotrimazole) with success.
You can treat for both at the same time, dosing schedule as follows:
Day 1 - Anti-fungal bath
Day 3 - Ivermectin
Day 8 - Anti-fungal bath
Day 13 - Ivermectin
Day 15 - Anti-fungal bath
Day 23 - Ivermectin
This covers the usual guidelines of:
- No baths inside 48 hours of an Ivermectin dose.
- One week between anti-fungal baths (for non-severe infections).
- 10 days between Ivermectin doses.
