While it goes without saying that practical rulings need a more authoritative answer, It seems that lice are a problem and must be removed but once you have done due diligence that is sufficient.
שלחן ערוך יורה דעה קצח:מז
מין כנים שדבוקים בבשר ונושכים בעור במקום שיער ונדבוקים בחוזק בבשר צריך
להסירן ע"י חמין ולגורדן בצפורן ואם אינו יכול להסירן אינו חוצץ
...Similarly, if at times a louse or flea is found on her skin after immersion, after the fact, [the immersion] counts for her. However, those lice which are called filz lice which are very adherent to the skin must be removed by hot water and scraping them. If one does not remove them, the constitute an intervening substance even after the fact. However, if it is impossible to remove them, they do not intervene, since it is a small part [of the body] and one is not disturbed [by it]. (Chochmas Adam 119:4, translation from "the Laws of Tevilah" by Dr. Yehoshua Cohen)
And a more contemporary guide:
Once she completes the normal process of removing lice and nits, any remaining nits that cannot be removed are not a chatzitzah. (A Woman's Guide to the Laws of Niddah, Rabbi Forst, page 183)