Pediculus humanus, human louse
- Lice is general name for small, wingless, parasitic insects that feed on blood. Though exact taxonomy is still controversial, they can be grouped in the suborders Anoplura (sucking lice), Amblycera (chewing lice), Ischnocera (biting or chewing lice), and Rhynchophthirina (elephant lice and wart-hog lice).
- Elephant lice suborder has just one family and one genus, Haematomyzus. They parasitize primarily Asian elephants and to a lesser degree, African elephant (H. elephantus ), the warthog (H. hopkinsi), and the bush pig of eastern Africa (H. porci). They are unusual in that their drill-like mouthparts allow them to penetrate the thick skin of their host, so they are firmly attached to the skin in a "tick-like" manner (which is understandable given their hosts do not have much hair on their bodies). One fascinating fact about H. elephantus is that populations of the louse in African and in Asian elephants does not appear to be separable, yet the elephant species are not believed to co-exist in close proximity with each other for thousands of years. Four hypotheses were advanced to explain this phenomenon.
- Pediculus humanus belongs to order Anoplura. Anoplura parasitize eutherian (placental) mammals exclusively. Note: non-placental mammals are monotremes (duck-billed platypus, short-beaked echidna), and marsupials (kangaroos, opossums, bandicoots, and wombats).
- Two kinds of Pediculus humanus are recognized: Pediculus humanus capitis (human head louse) and Pediculus humanus corporis (human body louse, human clothing louse). The question whether they represent two distinct species or are merely subspiecies of Pediculus humanus is still under dispute. Genetic analysis had shown significant differences between the two forms of lice, however even greater differences were found between lice from different countries. In addition, head and body lice are able to interbreed under laboratory conditions, and it seems that sometimes head lice can infest clothing (which is different ecological niche) as successfully as the body lice.
Brief facts
- Pediculosis capitis is medical name for head lice infestation.
- Louse is a vector of "classic" form of epidemic typhus caused by Rickettsia prowazekii. Currently, lice are not associated with serious medical problems.
- Most common manifestation of lice infestation is itching of the scalp caused by louse salivary or fecal antigens. Intense scratching can damage scalp skin and lead to secondary infections.
- Lice infestation causes anxiety and embarassment which are increased by children' exclusion from school under "no-nit" policy. Lice infestations combined carry huge economical burden: lost work for parents, missed education, lost governmental grants for schools due to absenteeism, cost of therapy, etc. It was estimated that pharmacotherapy alone for head lice infestations costs the US economy hundreds of million dollars per year.
- The golden standard for diagnosing head lice infestation before and after treatment is finding a mobile louse form (adult or nymph) in the hair. Presence of eggs (nits) that remained on the hair after the treatment does not signify failure of the treatment especially when they were located more than 1 cm from the scalp.
- Immunization of the organism to the lice may occur during active infestation and protect the host from next infestations: antibodies circulating in the blood cause digestive failure in the feeding lice, lessen their fecundity and egg viability. This is why lice infestations on sick hosts with immune deficit are usually more severe. There was a case when after infested person was cured from primary illness (meningitis), lice "died out naturally" without special anti-lice treatment (personal communications).
Developmental stages (life cycle)
The development from egg to egg-laying adult takes from 15 to 23 days. The optimal laboratory conditions for rearing head lice eggs are 27-31 °C and 45-75% relative humidity.