Body Louse

Pediculus humanus humanus — the body louse — is one of three lice that parasitize humans (alongside the head louse P. h. capitis and the pubic/crab louse Pthirus pubis) and by far the most medically consequential. Despite the close taxonomic relationship to the head louse (the two are typically classified as different subspecies of P. humanus), the body louse has a fundamentally different ecology: she lives in the SEAMS OF HUMAN CLOTHING, not on the body itself. She crawls onto the skin only to feed (typically multiple times per day), then returns to clothing seams to digest, lay eggs, and rest. The body louse / head louse divergence is one of the most-cited findings in molecular anthropology: genetic data (Kittler, Kayser, Stoneking, 2003) date the divergence to approximately 170,000 years ago, when humans began wearing clothes regularly enough to provide a stable habitat for body lice. The body louse is the vector of three serious bacterial diseases: epidemic typhus (Rickettsia prowazekii — fever, headache, rash, septic shock; mortality 10-40% untreated), trench fever (Bartonella quintana — fever, leg pain, fatigue; less acutely lethal but chronic), and louse-borne relapsing fever (Borrelia recurrentis — recurring fever, jaundice, hemorrhage; mortality up to 40% in epidemics). These diseases have killed an estimated 30+ million humans across documented history. Major epidemics include the 1812 Napoleonic typhus catastrophe (more French soldiers died of typhus than from Russian forces or freezing combined on the retreat from Moscow), the 1918-1922 Russian Civil War typhus epidemic (3 million deaths), and the World War I trench fever pandemic (1 million Allied soldier cases). The body louse is now relatively rare in developed countries because of widely-available laundering and bathing — but louse-borne typhus and trench fever remain epidemic risks in refugee camps, prisons, war zones, and homeless populations worldwide.