The word ᴱQ. itis “a fly bite” appeared in the Qenya Lexicon of the 1910s derived from the early root ᴱ√ITI “peck, bite (of flies), annoy” (QL/43). Tolkien first wrote this form as itis(s-) implying a stem from of itiss-, but he erased the second s and wrote “(-s)” separately, which seems to imply a stem form of itis-.

Neo-Quenya: I would adapt this word as ᴺQ. itis [þ] based on the Neo-Root ᴺ√ITITH, chosen to preserve Late Quenya s in all forms, and extend its meaning to include both “fly bite” and “*itch” based on the adjective itisin “itching”.