|Light micrograph of Fahrenholzia pinnata|
Louse (plural: lice) is the common name for members of over 3,000 species of wingless insects of the order Phthiraptera; three of which are classified as human disease agents. They are obligate ectoparasites of every avian and mammalian order except for monotremes (the platypus and echidnas), bats, whales, dolphins, porpoises and pangolins.
Most lice are scavengers, feeding on skin and other debris found on the host's body, but some species feed on sebaceous secretions and blood. Most are found on only specific types of animals, and, in some cases, on only a particular part of the body; some animals are known to host up to fifteen different species, although one to three is typical for mammals, and two to six for birds. For example, in humans, different species of louse inhabit the scalp and pubic hair. Lice generally cannot survive for long if removed from their host.
A louse's color varies from pale beige to dark gray; however, if feeding on blood, it may become considerably darker. Female lice are usually more common than the males, and some species are even known to be parthenogenetic. A louse's egg is commonly called a nit. Many lice attach their eggs to their host's hair with specialized saliva; the saliva/hair bond is very difficult to sever without specialized products. Lice inhabiting birds, however, may simply leave their eggs in parts of the body inaccessible to preening, such as the interior of feather shafts. Living lice eggs tend to be pale white. Dead lice eggs are more yellow.
Lice are optimal model organisms to study the ecology of contagious pathogens since their quantities, sex-ratios etc. are easier to quantify than those of other pathogens. The ecology of avian lice has been studied more intensively than that of mammal lice.
A few major trends
- The average number of lice per host tends to be higher in large-bodied bird species than in small ones.
- Louse individuals exhibit an aggregated distribution across bird individuals, i.e. most lice live on a few birds, while most birds are relatively free of lice. This pattern is more pronounced in territorial than in colonial—more social—bird species.
- Host taxa that dive under the water surface to feed on aquatic prey harbor fewer taxa of lice.
- Bird taxa that are capable of exerting stronger antiparasitic defense—such as stronger T cell immune response or larger uropygial glands—harbor more taxa of Amblyceran lice than others.
- Temporal bottlenecks in host population size may cause a long-lasting reduction of louse taxonomic richness. E.g., birds introduced into New Zealand host fewer species of lice there than in Europe.
- Louse sex ratios are more balanced in more social hosts and more female-biased in less social hosts, presumably due to the stronger isolation among louse subpopulations (living on separate birds) in the latter case.
A few effects of lice infestation upon the host
- Lice may reduce host life expectancy.
- Lice may transmit microbial diseases or helminth parasites.
- Ischnoceran lice may reduce the thermoregulation effect of the plumage; thus heavily infested birds lose more heat than other ones.
- Lice infestation is a disadvantage in the context of sexual rivalry.
The order has traditionally been divided into two suborders, the sucking lice (Anoplura) and the chewing lice (Mallophaga); however, recent classifications suggest that the Mallophaga are paraphyletic and four suborders are now recognized:
- Anoplura: sucking lice, occurring on mammals exclusively
- Rhynchophthirina: parasites of elephants and warthogs
- Ischnocera: mostly avian chewing lice, however, one family parasitizes mammals
- Amblycera: a primitive suborder of chewing lice, widespread on birds, however, also live on South-American and Australian mammals
It has been suggested that the order is contained by the Troctomorpha suborder of Psocoptera.
Lice in humans
Human lice and DNA discoveries
Lice have been the subject of significant DNA research in the 2000s that led to discoveries on human evolution. For example, genetic evidence suggests that our human ancestors acquired pubic lice from gorillas approximately 3-4 million years ago. Additionally, the DNA differences between head lice and body lice provide corroborating evidence that humans started losing body hair about 2 million years ago.
The mitochondrial genome of the human species of body lice (Pediculus humanus humanus), the head louse (Pediculus humanus capitis) and the pubic louse (Pthirus pubis) is fragmented into a number of minichromosomes. This fragmentation appears to have been present for at least 7 million years. The body louse evolved from the head louse ~107,000 years ago and since body lice require clothing to survive in the absence of thick body hair, this latter date has been suggested as the approximate time when humans began to wear clothing, perhaps as adaption to some groups of humans having moved to cooler climates than those of Africa.
- Pest (organism)
- Use of DNA in forensic entomology
- Whale louse, unrelated crustaceans that live parasitically on marine mammals
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- Paterson AM et al. 1999. How Frequently Do Avian Lice Miss the Boat? Implications for Coevolutionary Studies Systematic Biology 48, 214–223
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- Brown CR et al. 1995. Ectoparasites reduce long-term surviviorship of their avian host. Proceedings of the Royal Society of London B 262, 313–319.
- Barlett CM 1993. Lice (Amblycera and Ischnocera) as vectors of Eulimdana spp. (Nematoda: Filarioidea) in Charadriiform birds and the necessity of short reproductive periods in adult worms. Journal of Parasitol. 79, 85–91.
- Booth DT et al. 1993. Experimental demonstration of the energetic cost of parasitism in free-ranging hosts. Proceedings of the Royal Society of London B 253, 125–129.
- Clayton DH 1990. Mate choice in experimentally parasitized rock doves: lousy males lose. American Zoologist 30, 251–262.
- Garamszegi LZ et al. 2005. Age-dependent health status and song characteristics. Behavioral Ecology 16, 580–591.
- David L. Reed, Jessica E. Light, Julie M. Allen & Jeremy J. Kirchman (2007). "Pair of lice lost or parasites regained: the evolutionary history of anthropoid primate lice". BMC Biology 5: 7. doi:10.1186/1741-7007-5-7. PMC 1828715. PMID 17343749.
- John Travis (August 23, 2003). "The naked truth? Lice hint at a recent origin of clothing" 164 (8). Science News. p. 118.[dead link]
- Shao R, Zhu XQ, Barker SC, Herd K (2012) Evolution of extensively fragmented mitochondrial genomes in the lice of humans. Genome Biol Evol
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- National Pesticide Information Center – Understanding and Controlling Lice
- Bed-hopping led humans to 3 million-year itch
- Extensive scientific information at Phthiraptera Central
- body and head lice on the University of Florida/Institute of Food and Agricultural Sciences Featured Creatures Web site
- crab louse on the University of Florida/Institute of Food and Agricultural Sciences Featured Creatures Web site
- WebMD Lice Info
- Lice chapter in United States Environmental Protection Agency and University of Florida/Institute of Food and Agricultural Sciences National Public Health Pesticide Applicator Training Manual
- Pediculus humanus capitis head louse facts, myths, life cycle at MetaPathogen