How many head lice are there? - briefly
Worldwide, there are an estimated 300–400 million head‑lice infestations at any given time, affecting roughly 6–12 % of school‑age children. The prevalence varies by region, socioeconomic factors, and seasonal patterns.
How many head lice are there? - in detail
Head lice (Pediculus humanus capitis) are obligate ectoparasites that live on the human scalp. An adult female lays 5‑8 eggs (nits) per day, and a typical infestation contains 5‑10 adult insects, 20‑30 nymphs, and 50‑100 viable eggs. The total number of live lice on a single host rarely exceeds 150, but heavy cases can approach 200.
Epidemiological surveys show that 2‑5 % of school‑age children in developed nations harbor lice at any given time, while prevalence rises to 10‑15 % in densely populated or low‑income regions. Assuming a global child population of roughly 1.2 billion, the number of infested children ranges from 24 million to 180 million. Adult infestation rates are lower, estimated at 0.5‑1 % of the worldwide adult population (≈5 billion), adding 25‑50 million additional cases.
To estimate the total lice population, multiply the average lice count per infested individual by the estimated number of cases. Using a median value of 100 live lice per host:
- Lower bound: (24 million + 25 million) × 100 ≈ 4.9 billion lice.
- Upper bound: (180 million + 50 million) × 100 ≈ 23 billion lice.
Thus, the global head‑lice population is likely between 5 billion and 25 billion individuals at any moment.
Factors influencing these figures include age (children host more lice), crowding (schools, shelters), and treatment accessibility (availability of effective pediculicides). Seasonal peaks occur during school terms, when close contact increases transmission rates.