Phthirus pubis is not known to serve as a vector of any infective disease.

The pathological effects of its parasitism on man are on the whole of slight degree. Some persons are more sensitive than others to its presence; on the one hand the louse may produce no reaction by its bite or it may be tolerated upon the person for years, on the other hand it may induce pruritus of a more or less severe character, especially in parts of the body that are most infested and where the skin is most delicate, i.e. about the genitalia, the axillae, and eyelids. All grades of pruritus are observable, scratching of which the individual is unconscious may occur, or in the severest cases the scratching goes on day and night, so that by day it may be evident to spectators and aid in diagnosis. The scratching may only begin from the moment when a person realizes that he is infested.

The pale bluish-grey maculae are a specific sign of the presence of Phthirus upon man, but they are not always present. They are induced by the adult louse, are few in number, painless, deep seated, do not disappear on pressure, and measure 0·2–3 cm. in diameter. The maculae mark the site of the insect's bite; they appear 8–24 hours after the louse commences to feed on the spot affected. The bites of P. humanus do not produce maculae caeruleae. The salivary glands of Phthirus and Pediculus give correspondingly positive and negative results when inoculated subcutaneously. The maculae disappear usually within a week after the removal of the offending parasite, and they disappear more rapidly when the skin is irritated. The nature of the maculae remains to be determined, they may be due to extravasated and altered blood.

Apart from the maculae, Phthirus, like P. humanus, fleas and mosquitoes, may cause a febrile condition owing to skin irritation, although this appears to be rare; with the removal of the lice, the fever promptly subsides. Papular eruptions and eczematous inflammation may supervene upon the presence of the louse, but they appear to be largely due to the added insult of scratching the infested skin. The melanism is of non-specific origin, being due to continued scratching which leads to hyperaemia, pigmentation and desquamation. The lesions induced thereby may lead to secondary skin infection with pyogenic bacteria.