Date Published: November 14, 2013
Publisher: Public Library of Science
Author(s): Denise L. Bonilla, Lance A. Durden, Marina E. Eremeeva, Gregory A. Dasch, Joseph Heitman.
Sucking lice (Phthiraptera: Anoplura) are obligate blood-feeding ectoparasites of placental mammals including humans. Worldwide, more than 550 species have been described and many are specific to a particular host species of mammal . Three taxa uniquely parasitize humans: the head louse, body louse, and crab (pubic) louse. The body louse, in particular, has epidemiological importance because it is a vector of the causative agents of three important human diseases: epidemic typhus, trench fever, and louse-borne relapsing fever. Since the advent of antibiotics and more effective body louse control measures in the 1940s, these diseases have markedly diminished in incidence. However, due to 1) increasing pediculicide resistance in human lice, 2) reemergence of body louse populations in some geographic areas and demographic groups, 3) persistent head louse infestations, and 4) recent detection of body louse-borne pathogens in head lice, lice and louse-borne diseases are an emerging problem worldwide. This mini-review is focused on human body and head lice including their biological relationship to each other and its epidemiological relevance, the status and treatment of human louse-borne diseases, and current approaches to prevention and control of human louse infestations.
For over a century, scientists have argued about the exact taxonomic and biological relationships between human head lice and body lice and, in particular, whether they represent a single species with two ecotypes or two distinct species , . The two-species argument considers the body louse to be Pediculus humanus and the head louse to be Pediculus capitis (Table 1) (Figure 1A, B, C, D). The single-species argument treats the body louse as Pediculus humanus humanus and the head louse as Pediculus humanus capitis. Further, although the name Pediculus humanus corporis has been used frequently in the medical literature for the body louse, it is an invalid name according to the rules of the International Commission on Zoological Nomenclature. Whether head and body lice represent distinct species, different subspecies (or strains, phylotypes, or ecotypes) inhabiting different habitats, or a single species is more than a taxonomic issue. This is because all well-investigated outbreaks of louse-transmitted diseases in humans, including many that have shaped our history, have involved pathogen transmission by the body louse, not by the head louse . The recent sequencing and annotation of the small 108 Mb genome of P. humanus humanus, the chromosome and plasmid of its symbiotic bacterium, “Candidatus Riesia pediculicola” , , and the mitochondria of all three human louse taxa  have allowed reevaluation of this argument with potentially important epidemiological ramifications. The sensitivity of lice to sulfamethoxazole-trimethoprim is thought to reflect its lethality for Riesia, which lice depend upon for B vitamin synthesis , . Differences between head and body lice in the complex developmental interactions that maintain Riesia between generations have been described , but whether these differences occur in all louse populations is unknown.
The incidence of louse-borne diseases has decreased in humans since the widespread availability of effective antibiotics and pediculicides. Louse-borne relapsing/recurrent fever (RF), caused by infection with Borrelia recurrentis, has persisted especially in parts of Africa, and it has the potential to infect travelers returning to Europe and North America from endemic regions . Borrelia recurrentis has been detected recently in 23% of head lice in Ethiopia, but whether head lice serve as a vector is unknown . Although the close genetic relationship between Borrelia duttonii and B. recurrentis has made their laboratory differentiation by qPCR difficult , the speculation that acquisition of B. duttonii by body lice could quickly give rise to new strains of B. recurrentis is uncertain considering the massive loss of protein coding capacity, plasmids, and plasmid rearrangements of the latter , .
At present, there are no commercial vaccines against louse-borne diseases of humans. Therefore, louse-borne disease suppression has typically involved elimination and control of lice and, secondarily, treatment of infected patients with doxycycline , , . Single-dose oral administration of doxycycline is most effective in controlling epidemic typhus when permethrin dusting of clothing for louse control is not possible. Body louse infestation is typically associated with poor body and clothing hygiene and crowding, which enables close person-to-person contact that facilitates the spread of lice . However, head louse infestations, especially in developed countries, generally have little to do with hygiene, the socio-economic status, or race of the individual, and most frequently affect children between three and 11 years old , –.
In the 21st century, the prevalence of human louse infestation is still very high worldwide. New molecular tools have been developed and applied to head and body louse ecotypes and to the bacterial agents they transmit. Surprising and novel insights into the evolution of lice, their bacterial disease agents, and the epidemiology of louse-borne diseases have stimulated a renewal of interest in these arthropods. These discoveries may in turn provide new tools for improved understanding and control of these ancient and highly personal scourges of humans.