How long do they live on the head? - briefly
Head lice typically survive for about 30 days on a human scalp, completing their life cycle from egg to adult within that period. Without a host, they die within 1–2 days.
How long do they live on the head? - in detail
The lifespan of organisms that inhabit the scalp varies by species, environmental conditions, and host health.
Head lice (Pediculus humanus capitis) progress through three developmental stages: egg (nit), nymph, and adult. An egg requires about 7–10 days to hatch. Nymphs undergo three molts over roughly 9 days before reaching maturity. Adult females live 30–35 days, during which they lay 5–10 eggs per day. Males survive a comparable period but do not reproduce after mating. The entire life cycle, from egg to death, therefore spans approximately 30–45 days under optimal temperature (30–32 °C) and humidity.
Demodex mites (Demodex folliculorum and Demodex brevis) reside in hair follicles and sebaceous glands. Their life cycle lasts 14–16 days: egg, larva, protonymph, deutonymph, and adult. Adults live about 5 days before reproducing, after which they die. Overall, a single generation persists for roughly three weeks.
Scalp fungi, chiefly Trichophyton and Microsporum species, colonize keratinized tissue. Hyphal growth continues as long as nutrients are available, often weeks to months. Treatment can reduce fungal viability within 7–14 days, but untreated infections may persist for several months.
Key factors influencing duration:
- Temperature: higher ambient and skin temperatures accelerate metabolic rates, shortening developmental periods.
- Humidity: moisture levels above 50 % favor egg hatching and mite survival.
- Host hygiene: regular washing removes eggs and reduces adult numbers, effectively truncating life cycles.
- Chemical exposure: insecticides and antifungals disrupt reproductive cycles, leading to rapid population decline.
In summary, scalp-residing parasites and microbes exhibit life spans ranging from a few days (adult mites) to over a month (adult lice), while fungal colonies can endure for months if untreated. Control measures targeting environmental conditions and regular scalp care shorten these periods considerably.