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Please have your girls checked by a vet for a proper diagnosis and appropriate treatment. Egg cases on the bum end are usually those of fur or hay mites, otherwise also known as static lice; they often look like somebody turned a pepper mill over the back of the piggies. They are the least harmful of the guinea pig parasites and usually come with the hay.
These do look like static lice to me. Ivermectin won't work on them, but a antiparasitic shampoo weekly for 3 weeks will clear them up. As Wiebke says get a guinea savvy vet to confirm the diagnosis. I use Lyclear shampoo, or Lice n Easy from Gorgeous Guineas is also popular.