Ivermectin paste 1.87% (internal per weight OR external/topical application - note though that absorption rate when using paste topically is unknown - but it has helped me), or pour-on ivermectin (NOT for internal use). Both available at "farm stores" like Tractor Supply or online at several vet suppliers for animal meds that don't require prescriptions.
CZ users ICU and Sharkman have a lot more detailed accurate dosage info