Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Seek more urgent medical advice for spreading redness, increasing pain, pus, fever, or other signs of skin infection.
Fever with severe headache, marked weakness, muscle pain, or a new widespread rash also needs prompt assessment because of the possibility of a louse-borne or unrelated infection.
Routine follow-up should confirm that the person has clean clothing and bedding and that live lice are no longer being found.
Clothing seams should be checked again after laundering, especially if itching continues.
Ongoing access to bathing and at least weekly changes of properly laundered clothes helps prevent recurrence.
Arrange a routine medical appointment if the diagnosis is uncertain, itching is not improving, skin changes are extensive, or lice keep returning despite complete clothing and bedding measures.
A clinician can check for another cause of itching and decide whether a lice treatment is appropriate.
In group settings, follow-up may involve shelter staff, outreach teams, infection-control staff, or local public health services.
Lasting control often depends on practical support with showers, clean replacement clothing, laundry, and safe storage rather than repeated medicine alone.
The article also says
Persistent itching does not always mean treatment failed.
Skin irritation can last after the insects are gone.
However, finding live lice suggests that infested clothing or bedding remains in use, laundering was not effective, or exposure is continuing.
Every sign above is quoted from the sofidex article on Body lice. If you are not sure, err upward — it is always reasonable to be checked.