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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. The three species of sucking lice that parasitize human being belong to two genera: Pediculus and Phthirus. head lice (Pediculus humanus capitis), body lice (Pediculus humanus corporis), and pubic lice (Phthirus pubis). Human head and body lice (genus Pediculus) share a common ancestor with chimpanzee lice, while pubic lice (genus Phthirus) share a common ancestor with gorilla lice. Using phylogenetic and cophylogenetic analysis, Reed et al. hypothesized that the lice genera Pediculus and Phthirus are sister taxa and monophyletic. In other words, both genera descended from the same common ancestor. The age of divergence between Pediculus and its common ancestor is estimated to be 6-7 million years ago and matches the age predicted by chimpanzee-hominid divergence. Because parasites rely on their hosts, host-parasite cospeciation events are likely to occur.
For example, genetic evidence suggests that our human ancestors acquired pubic lice from gorillas approximately 3-4 million years ago. Unlike the genus Pediculus, the divergence in Phthirus does not match the age of host divergence that likely occurred 7 million years ago. Reed et al. propose a Phthirus species host-switch around 3-4 million years ago. While it is difficult to determine if a parasite-host switch occurred in evolutionary history, this explanation is the most parsimonious (contains the fewest number of evolutionary changes).
Additionally, the DNA differences between head lice and body lice provide corroborating evidence that humans using clothing 72,000 +/- 42,000 years ago. Human head louse and body louse occupy distinct ecological zones. Head lice live and feed on the scalp, while body lice live on clothing and feed on the body. Because clothing lice require clothing to survive, it is thought that this date roughly estimates the introduction of clothing in human evolutionary history.
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.
Analysis of mitochondrial DNA in human body and hair lice reveals that greater genetic diversity existed in African than in non-African lice. Human lice can also shed light on human migratory patterns in pre-history. The dominating theory of anthropologists regarding human migration is known as the Out of Africa Hypothesis. Genetic diversity accumulates over time and mutations occur at a relatively constant rate. Because there is more genetic diversity in African lice, it can be assumed that lice (and their human) existed in Africa before anywhere else.
- Ricinus bombycillae (Denny, 1842).JPG
Ricinus bombycillae, an Amblyceran louse from the bohemian waxwing
- Trinoton anserinum (Fabricius, 1805).JPG
Trinoton anserinum, an Amblyceran louse from a mute swan.
- Lice image01.jpg
Damalinia limbata is an Ischnoceran louse from goats. The male is smaller than the female.
- Louse diagram, Micrographia, Robert Hooke, 1667.jpg
Diagram of a louse, by Robert Hooke, 1667.
- Pest (organism)
- Use of DNA in forensic entomology
- Whale louse, unrelated crustaceans that live parasitically on marine mammals
- H. V. Hoell, J. T. Doyen & A. H. Purcell (1998). Introduction to Insect Biology and Diversity (2nd ed.). Oxford University Press. pp. 407–409. ISBN 0-19-510033-6.
- Rózsa L 1997. Patterns in the abundance of avian lice (Phthiraptera: Amblycera, Ischnocera). Journal of Avian Biology 28, 249–254.
- Rékási J et al. 1997. Patterns in the distribution of avian lice (Phthiraptera: Amblycera, Ischnocera). Journal of Avian Biology 28, 150–156.
- Felső B et al. 2006. Reduced taxonomic richness of lice (Insecta: Phthiraptera) in diving birds. Journal of Parasitology 92, 867–869.
- Felső B et al. 2007. Diving behaviour reduces genera richness of lice (Insecta: Phthiraptera) of mammals. Acta Parasitologica 52, 82–85.
- Møller AP et al. 2005. Parasite biodiversity and host defenses: Chewing lice and immune response of their avian hosts. Oecologia 142, 169–176.
- Møller AP et al. 2010. Ectoparasites, uropygial glands and hatching success in birds. Oecologia 163, 303–311.
- Rózsa L 1993. Speciation patterns of ectoparasites and "straggling" lice. International Journal for Parasitology 23, 859–864.
- Paterson AM et al. 1999. How Frequently Do Avian Lice Miss the Boat? Implications for Coevolutionary Studies Systematic Biology 48, 214–223
- MacLeod C et al. 2010. Parasites lost – do invaders miss the boat or drown on arrival? Ecology Letters
- Rózsa L et al. 1996. Relationship of host coloniality to the population ecology of avian lice (Insecta: Phthiraptera). Journal of Animal Ecology 65, 242–248.
- 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.
- Reed D.L., J.E. Light, J.M. Allen, and J.J. Kirchman. 2007. Pair of lice lost or parasites regained: the evolutionary history of anthropoid primate lice. BMC Biology 5(1):7-18.
- Reed D.L., J.E. Light, J.M. Allen, and J.J. Kirchman (2007). "Pair of lice lost or parasites regained: the evolutionary history of anthropoid primate lice". BMC Biology 5: 7. PMC 1828715. PMID 17343749. doi:10.1186/1741-7007-5-7.
- John Travis (August 23, 2003). "The naked truth? Lice hint at a recent origin of clothing" 164 (8). Science News. p. 118.[dead link]
- Kittler R., M. Kayser, and M. Stoneking. 2003. Molecular evolution of Pediculus humanus and the origin of clothing. Current Biology 13:1414-1417.
- Shao R, X.Q. Zhu, S.C. Barker, and K. Herd. 2012. Evolution of extensively fragmented mitochondrial genomes in the lice of humans. Genome Biol. Evol. 4(11):1088-1101
- Light J.E., J.M. Allen, L.M. Long, T.E. Carter, L. Barrow , G. Suren, D. Raoult, and D.L Reed. 2008. Geographic distribution and origins of human head lice (Pediculus humanus capitis) based on mitochondrial data. Journal of Parasitology 94(6)1275-1281.
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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