Head lice (Pediculus humanus capitis) infestations are confirmed by identifying their eggs, commonly called nits. A nit is a small, oval capsule laid by an adult female louse and firmly cemented onto a hair shaft. Visual identification is important because nits are often confused with common debris found on the hair and scalp. Knowing the precise characteristics of a viable egg is necessary to determine if an active infestation is present and requires treatment.
The Appearance of Viable Nits
A live, unhatched nit is extremely small, typically measuring about the size of a pinhead. They are distinctively teardrop or oval-shaped and cemented to the hair shaft at a fixed angle, usually to one side of the strand. Their color is often described as translucent, pearly, or a subtle yellowish-white. Just before hatching, they can appear a darker tan or golden-brown.
The most reliable indicator of viability is the nit’s location on the hair. A developing egg requires the warmth of the scalp to incubate, so it must be laid within 1/4 inch (about 6 millimeters) of the skin.
The female louse secretes a powerful, water-insoluble glue to attach the nit to the hair. This glue is so strong that the egg cannot be moved along the hair shaft by brushing, shaking, or flicking. To remove a viable nit, you must scrape it off the hair shaft using a fingernail or a specialized fine-toothed nit comb.
Distinguishing Nits from Common Look-Alikes
The small, static appearance of nits leads to frequent misidentification with various non-parasitic items. The most common look-alike is dandruff or dry skin flakes, which are highly irregular in shape and color, often appearing flat and powdery white. Unlike nits, dandruff is loose and easily brushes or flakes off the hair and scalp with a gentle shake or comb stroke.
Hair spray or other product residues can also form small, sticky particles on the hair shaft, but they generally lack the uniform, teardrop shape of a nit. This residue is typically found irregularly along the hair shaft or only near the surface, not precisely cemented to the base of the hair strand.
Hair casts are another item that can mimic nits, appearing as small, white cylinders that encircle the entire hair shaft. A nit is cemented only to one side of the hair, but a hair cast wraps completely around it. While both nits and hair casts may resist flicking, a hair cast can usually be slid along the hair shaft with modest pressure from the fingers.
The “stickiness test” remains the most effective way to differentiate between a true nit and debris. If the object slides off the hair with ease or can be flicked away, it is not a nit and poses no risk.
What Happens to Nits After Hatching
Once a viable nit has matured, the young louse, called a nymph, emerges from the shell, leaving behind an empty casing. This shell remains glued to the hair shaft and is often referred to as a “nit” in common language. After hatching, the casing loses its initial pearly or golden-brown color and becomes noticeably duller.
These empty shells are opaque white, gray, or simply translucent, having lost the darker color that indicated a developing louse inside. They are no longer viable and cannot hatch, so their presence does not signal an active infestation.
Because the hair continues to grow, these casings are carried further away from the scalp. Finding an empty shell more than 1/4 to 1/2 inch from the scalp generally indicates a past infestation or a successful treatment. Dead nits, which failed to hatch, also remain glued to the hair but may appear shriveled or more opaque than an empty casing.