ReviewThe origin and distribution of human lice in the world
Introduction
Lice are obligate ectoparasites, and each host species carries its own type of louse. Parasite speciation frequently occurs at approximately the same time as the “cospeciation process” in the host species (Veracx and Raoult, 2012). In 2010, an investigation of sucking louse relationships using 18S, EF-1α and COI genes (Light et al., 2010) estimated the diversification of sucking lice (Anoplura) to be approximately 77 million years old, a time period coinciding with the emergence of the placental mammal superorders. There are conflicts between host and parasite phylogenies: these conflicts can be the result of events that intervene in the distribution of parasites on hosts, the speciation of the parasite independent of the host and colonization failure (Page RDM, 2003). Lice have been recorded in most parts of the world, including in mummies in the New World and most likely achieved worldwide distribution because of their human host, which indicates that the species share a long record of co-evolution (Ascunce et al., 2013). Because of this long association with humans, lice have become a model for the study of cophylogenetic relationships between hosts and parasites (Demastes et al., 2012), although the origins of parasitism in lice remain inadequately understood.
Section snippets
Taxonomic status of human head and body lice, morphology, biology and ecology
Lice are small, wingless insects that cannot live independently from their host (Weiss, 2009). The order of Phthiraptera is divided into two major groups: Anoplura (the hematophagous sucking lice of placental mammals) and Mallophaga (the chewing or biting lice of birds, marsupials and placental mammals) (Lance, 2008). Although the Mallophaga group is likely paraphyletic, it is widely agreed that sucking and chewing lice originated from a common nonparasitic ancestral group closely related to
Human louse as a vector of human pathogens responsible for epidemics
Pediculosis is the cause of parasitism by the body louse (Chosidow, 2000). A louse-infested person can be infested by thousands of lice, each of which bites five times per day (Roux and Raoult, 1999). The louse injects the skin with biologically active proteins that include an anticoagulant and an anesthetic (Roux and Raoult, 1999). These antigens provoke an allergic reaction within 3–4 weeks after the bite, which can lead to pruritus (Raoult and Roux, 1999). Severe infestations of body lice,
Trench fever
Trench fever was first described during World War I, so named because the disease affected Allied and German troops crowded into trenches during World War I (Badiaga and Brouqui, 2012). The disease is caused by B. quintana, a Gram negative bacteria. The incidence of B. quintana dramatically fell after World War II. In the early 1990s, trench fever was recognized as a major reemerging disease in the poor living conditions of urban homeless populations in developed countries (Brouqui and Raoult,
Epidemic typhus
Epidemic typhus has caused more deaths than all of the wars in history (Zinsser, 1935); its transmission by the body louse was demonstrated by Charles Nicolle (Gross, 1996, Nicolle, 1910). It is caused by R. prowazekii, an obligate intracellular bacterium that also kills the lice within one week of infection (Andersson and Andersson, 2000).The origin of typhus is controversial; some consider it to be an old European disease that caused the Athens plague (Zinsser, 1935), while others believe
Relapsing fever
Epidemic relapsing fever is caused by the spirochete B. recurrentis, and humans are the sole reservoir (Cutler, 2006). Although the disease has disappeared in extensive regions of the world, it remains an important endemic disease in northeastern Africa (Mitiku and Mengistu, 2002). It was initially described in Ireland and was one of the first infectious lice disease identified by microscopy (Mackie, 1907). Relapsing fever spreads in humans through feces (Houhamdi and Raoult, 2005) and, as with
Other louse-associated diseases
The role of body lice in the transmission of pathogenic bacteria to humans is being investigated. Acinetobacter baumannii was found in 21% of the 622 body lice collected worldwide (La Scola and Raoult, 2004), although no A. baumannii infections are known to be transmitted by body lice. Yersinia pestis, an etiological agent of plague, was recovered from a body louse collected from a septicemic patient in Morocco in the 1940s (Drancourt et al., 2006, Houhamdi and Raoult, 2008) and was observed in
Pathogenic bacteria in head lice
Body lice are much more potent vectors of pathogens than head lice, likely because of the reduced phagocytic activity of their immune system (Kim et al., 2011). Yet, although it is not clear if head lice can act as vectors of human pathogens, they can carry pathogens. DNA from B. quintana was collected in head lice from Nepalese children in 2006 (Sasaki et al., 2006), in head lice from homeless individuals in the USA in 2009 (Bonilla et al., 2009), in head louse nits collected from a Marseille
Genetic studies and different types of lice
The major phenotypic differences between the head and body louse relate to the ecology and color (Veracx and Raoult, 2012). No congruence among those characteristics could be assessed (Veracx et al., 2012a, Veracx et al., 2012b), and phenotypic studies were subsequently challenged by genetic studies (Veracx and Raoult, 2012). Three clades (A, B and C) of head lice were described by analyzing the mitochondrial DNA (cytochrome b and cytochrome oxidase subunit 1), and of them, only one (clade A)
The sympatric life of lice and recombination between mitochondrial genes
P. humanus females have lost their spermatheca, the sperm storage organ, and must mate before laying eggs; frequent mating is essential, and this process encourages outbreeding (Maunder, 1983, Mukerji and SEN, 1951). The high mobility of lice is rarely recognized, and lice do not exist as isolated groups but as actively intermingling specimens (Maunder, 1983). This intermingling allows them to mate more frequently, which increases gene exchange and recombination (Veracx and Raoult, 2012).
Two
Evolutionary history of human lice
The intimate association of lice with their hosts can explain why lice show more cospeciation with hosts than other groups of insects (Grimaldi and Engel, 2006). The oldest human head louse nit was found on a hair from an archeological site in northeastern Brazil and was dated to 8000 B.C. (Araujo et al., 2000). The oldest such finding in the Old World was 9000 years old, obtained from a hair sample from an individual who lived in the Nahal Hemar cave in Israel (Mumcuoglu, 2008). Head lice have
Distribution of lice before globalization and association with the different human migrations
An exciting aspect of studying the diversity of lice is its application to the history of human evolution. Lice have been associated with humans for millions of years and dispersed throughout the world by early human migrants (Ascunce et al., 2013). P. humanus shows genetic evidence of population expansion from Africa approximately 100,000 years ago, which is consistent with the host evolutionary history because humans originated in Africa (Reed et al., 2004, Reed et al., 2007) and dispersed to
Conclusion
The combination of phenotypic characters and genetic data is crucial to understand lice epidemiology, and efforts to obtain more data on that parasite are essential to prevent disease reemergence because body lice can carry and spread severe diseases in human populations and head lice can serve as a reservoir. From a genetic standpoint, the most important paradigm is that there are three major clades of head lice with one that also comprises a body louse, which represents a different ecotype of
Conflict of interest
None.
Funding source
None.
Author contributions
D. Raoult conceived and designed the review.
A. Boutellis conducted the literature search and wrote the drafts of the review.
L. Abi Rached critically revised all of the draft.
Acknowledgments
We thank Dr Bernard Davoust, Jean Michel Berenger, Samir Benkouiten and Dr Mario Rivera for their help in figures.
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