I would buy several anti-parasitical essential oils, mix them together with coconut oil, and rub on. I would start with a diluted formulat and slowly add more oil as tolerable. When the mixture has soaked in, I would coat with a thin layer of DMSO cream to take it into deeper tissues. I would put the essential oil formula on as frequently as the itch occurs.
Is the grainy stuff of sand size and consistency? Do you think the grains are eggs or has the tissue been damaged from zapping?