Head lice (Pediculus humanus capitis) are small, wingless insects that live on the human scalp and feed on blood. Infestations, known as pediculosis, are common, especially among school-age children. Although lice do not transmit disease, the intense itching often prompts immediate treatment. The term “super lice” describes a specific strain of head louse that is resistant to standard household remedies. These resistant lice are physically indistinguishable from their non-resistant counterparts.
Defining Resistance in Super Lice
The designation “super” refers to the louse’s genetic ability to survive common chemical treatments, not its size or speed. Super lice possess the knockdown resistance, or kdr, mutation. This modification affects the louse’s nervous system by desensitizing the voltage-sensitive sodium channels within the nerve cells. While normal lice are paralyzed and killed when these channels are overstimulated by specific chemicals, the kdr mutation prevents this toxic effect.
This resistance evolved due to the widespread use of a specific class of insecticides over decades. The affected chemical group is pyrethrins and their synthetic derivatives, pyrethroids, such as permethrin, which are the active ingredients in many over-the-counter products. When lice are treated with permethrin, those carrying the kdr gene survive and reproduce, passing resistance to the next generation. This natural selection process has created strains of head lice nearly immune to conventional treatments.
The Life Cycle and Transmission
Understanding the life cycle of the head louse is necessary for successful eradication, as treatment must interrupt reproduction. The cycle begins with the egg stage; eggs, called nits, are firmly attached to the hair shaft close to the scalp with a glue-like substance. Nits typically take six to nine days to hatch into the nymph stage. Viable eggs are usually found within six millimeters of the scalp, requiring the head’s warmth to develop.
The nymph stage involves three molts over seven to twelve days before the louse matures into an adult. Adult head lice are roughly the size of a sesame seed and are capable of reproduction. Adult female lice can lay up to eight eggs per day and live for up to 30 days on a human head, requiring multiple blood meals daily.
Head lice are spread almost exclusively through direct head-to-head contact, common in social settings like schools and playdates. Lice move by crawling rapidly and cannot jump or fly. Transmission through indirect contact, such as sharing hats or combs, is far less common. The louse quickly dies when separated from its human host, so efforts should focus primarily on checking and treating the hair of close contacts.
Effective Non-Resistant Treatments
When a permethrin-based treatment fails or resistance is suspected, alternative methods that do not rely on the pyrethroid mechanism must be used. One effective, non-chemical approach is mechanical removal, often called wet-combing. This method involves saturating the hair with conditioner and meticulously combing through small sections with a fine-toothed nit comb every few days for several weeks.
Several non-pyrethroid chemical and physical treatments are also available, working by different mechanisms. Suffocation agents, such as products containing dimethicone, work by coating the louse, blocking its respiratory system, and physically immobilizing it. Dimethicone is also ovicidal, meaning it can kill the eggs, which reduces the need for repeated treatments. Prescription options are available for confirmed resistant lice, often targeting the nervous system in a way that bypasses the kdr mutation.
These prescription medications include topical agents like spinosad suspension and ivermectin lotion. Spinosad is effective against live lice and often against nits, requiring only a single application followed by a check seven days later. Successful eradication depends on a strict retreatment schedule to ensure that newly hatched nymphs are killed before they can mature and lay new eggs.