Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
† phthīrĭăsis, is, f., = φθειρίασις, the lousy disease, phthiriasis, Plin. 26, 13, 86, § 138; 20, 6, 23, § 53; 25, 5, 25, § 61 al.