Crab Louse (Pubic Louse)

Pthirus pubis — the crab louse, also called the pubic louse — is the third louse species that parasitizes humans (alongside Pediculus humanus capitis the head louse and P. h. humanus the body louse) and the most distantly-related to the other two human lice. While head and body lice diverged from each other only ~170,000 years ago (and both belong to genus Pediculus), the crab louse belongs to a separate genus (Pthirus) that diverged from the Pediculus lineage tens of millions of years ago. The crab louse's specific origin in humans is one of the most-cited examples of parasite host-jumping in molecular anthropology: the human crab louse and the gorilla louse (Pthirus gorillae) share a most recent common ancestor approximately 3-4 million years ago — long after humans and gorillas split (~7 million years ago). The most parsimonious explanation is that the crab louse host-jumped from gorillas to early hominins 3-4 million years ago, by direct contact between the two species. The exact mechanism is debated (sleeping in gorilla nests, predation events, etc.) but the timing aligns with several other independent measures of human-gorilla contact in early hominin evolution. The species is anatomically distinctive: wide flattened body shape (the source of the 'crab' common name), strong claws on the second and third leg pairs that clamp onto coarse hair shafts, and a smaller body size than head and body lice. Crab lice live almost exclusively on coarse human body hair: pubic hair primarily, but also eyebrows, eyelashes, beards, chest hair, and armpit hair (the specific hair-shaft diameter and spacing of the head scalp does NOT support crab louse attachment, which is why crab lice are not found on the scalp). The species is sexually transmitted in adult humans (close body-to-body contact during sexual activity is the primary transmission route) and is occasionally transmitted to the eyebrows of children sharing close contact with infested adults. Modern infestation rates have declined sharply since the 1970s, possibly correlated with increased pubic hair removal practices, but the species remains a continuing low-level concern in sexual health.