A diagnosis of head lice begins with the discovery of a nit, the technical term for a head louse egg. Nits are notoriously difficult to identify because they are small and easily confused with common scalp debris. Understanding the precise, up-close characteristics of a nit is the definitive way to confirm an infestation and distinguish it from other particles found in the hair.
Detailed Physical Characteristics of the Nit
Head louse eggs are remarkably small, measuring approximately 0.8 millimeters in length, comparable to the size of a grain of salt. The nit’s shape is distinctly oval or teardrop-like, often described as slightly football-shaped with one end rounded and the other tapering to a point. This consistent, uniform shape is a reliable visual identifier during close inspection.
The color changes depending on the stage of development. A viable, live egg containing a developing louse appears translucent, yellowish-white, or tan to coffee-colored. Once the louse nymph has hatched, the remaining shell, which is still firmly attached to the hair shaft, becomes a paler white, pearly, or translucent casing. The shell is hard and impenetrable, composed of proteins similar to hair keratin.
The Critical Factor of Attachment and Placement
The most telling feature of a nit is its method and location of attachment to the hair shaft. A female louse secretes a powerful, waterproof, cement-like substance to secure the egg. This specialized “nit sheath” rapidly hardens, creating a bond so secure that the egg cannot be flicked off, blown away, or easily removed by brushing or washing.
This protein-based cement completely encircles the hair shaft, anchoring the egg to one side. Nits must be laid close to the scalp for successful incubation, typically within 3 to 5 millimeters (about \(1/8\) to \(1/4\) inch) of the skin surface.
Nits found further down the hair shaft, more than a quarter-inch from the scalp, are generally not viable. These older nits have either already hatched, leaving behind the empty casing, or died. Since hair grows roughly a quarter-inch per month, the distance of a nit from the scalp indicates how long the infestation has been present.
Distinguishing Nits from Common Scalp Debris
The secure attachment of the nit is the primary characteristic separating it from all other particles found in the hair. To remove a suspected nit, pulling with fingernails or a specialized fine-toothed comb is required because it remains firmly stuck. Any particle that slides easily along the hair shaft or falls off when touched is not a louse egg.
Dandruff is the most common misidentification, but it consists of irregular, flat flakes of dead skin. Unlike nits, dandruff is found directly on the scalp or loosely scattered throughout the hair, making it easy to brush or shake away. Its color is usually dull white or yellow, and its shape is never the uniform, rigid oval of a nit.
Another common look-alike is the hair cast, a white, tubular sleeve of keratin that encases the entire hair shaft. While hair casts can resemble nits from a distance, their tubular shape and tendency to slide freely up and down the hair shaft make them easily distinguishable. Similarly, particles like Desquamated Epithelial Cells (DEC plugs) are perfectly spherical, creamy balls that are not cemented to the hair and move without resistance.