The chewing louse is considered ancestral to the sucking louse. Three louse species infesting humans are known, the head louse (Pediculus humanus capitis ) infesting human heads (Fig. 1A), the body louse (P. humanus humanus ) infesting undergarments or clothing in contact with a human body (Fig. 1B), and the pubic louse (Pthirus pubis ) mainly infesting the pubic hair (Fig. 1C).
The head louse and the body louse are almost morphologically indistinguishable from each other, though the former sucks blood more frequently, has slender body, and is slightly smaller in size. The evolution of lice infesting humans is closely related to the history of clothes of the human ancestor. Some considers that the body louse is derived from the head louse during the evolution. However, some others consider them as ecological variants adapted to different location of the human body because their DNA sequences are so similar.
Adult head louse measures 2-3 mm in length (3 mm for female and 2-3 mm for male) and is ash-colored; the intestine is seen black on account of the sucked blood. It spends its life from the nymph to the adult holding onto the hair with hook-like claws found at the end of each of their 6 legs. Each female lays about 100 eggs in a month laying 3-4 eggs per day. The egg hatches and releases the nymph in a week, which sucks the blood at an interval of several hours. It sheds its skin three times and becomes adult in 2 weeks. The infestation starts with a small number of lice, but attains large number through repeated ovulation. The saliva excreted during sucking sensitizes the host, and causes unbearable itching about in a month after the start of the infestation. The head louse is found throughout the year, though there are two epidemic peaks in a year, June-July and November (IASR 20: 133-134, 1999 and see p. 350 & 351 of this issue).
The body louse is a size larger than the head louse. The female louse lays eggs along the seam of clothes and underclothes, stays there with its nymphs, and moves to the human body to suck the blood.
The pubic louse is 1-2 mm in length. It is morphologically different from the above two louse species, and often called “crab louse” on account of its morphology. It infests mainly pubic hairs but occasionally eyelashes of children (phthiriasis palpebrarum). In such cases, infestation of the louse in the family should be suspected, and the carriers should be appropriately treated.
The bed bug belongs to Hemiptera and is unrelated to any of the louse species above.
Modes of transmission of lice and how to interrupt their transmission and infestation: The head louse is transmitted in a family or among children directly through contact of heads or indirectly through shared use of bedclothes, pillowcases, towels, caps, combs, brushes, lockers, etc. (IASR 20: 133-134, 1999 and see p. 350 & 351 of this issue). To avoid the transmission, such practices should be avoided. In addition, clothes, sheets, pillowcases and other materials used by children found infested by lice should be soaked in hot water (above 55°C) for 10 min, which will eliminate lice at all stages of their development. When head lice are found from children in a nursery school, a kindergarten or a primary school, the school should inform families of the louse infestation so that they can take appropriate measures immediately. It is important to let the public know the existence of lice that are already resistant to the available pesticides (see p. 352 of this issue). Head lice infestation, pediculosis capitis, can compromise the school hygiene. Though, it is classified as Class 3 school infectious disease, and the school master is allowed, when necessary, to stop attendance of the infested children until the school doctor judges that there is no risk of further transmission (School Health and Safety Act, Enforcement Regulation, Article 18), the patients usually do not have to avoid attending school.
Infestation of body lice is often found among old persons living alone in a small room or persons without housing (a homeless wearing clothes infested by more than 1,000 body lice has been found among those who wore the same clothes for more than a month). Very often homeless persons acquire the body lice through clothes when they pick them up early autumn. Though such a situation makes countermeasures difficult, the local government should advance the anti-louse campaign through persistent education and diffusion of necessary information.
The pubic louse is transmitted through sexual contact. So, it is considered as a sexually transmitted disease. Chronically infested persons generally do not experience itching. Only newly infested persons experience intense itching several weeks after the acquisition, and come to know that the louse infested them. Currently no data are available for estimating the prevalence of the pubic louse infestation in Japan. No pesticide-resistant pubic lice have been reported yet. However, as patients generally treat themselves with the over-the-counter drugs and rarely visit clinics, it is difficult to exclude possible emergence of the pesticide-resistant lice.
Diseases transmitted by lice: Body lice are vectors of Rickettsia prowazekii , Borrelia recurrentis and Bartonella quintana , the etiological agents of epidemic typhus, relapsing fever and trench fever, respectively (Table 1). The former two infections claimed many lives of humans in the history.
1. Epidemic typhus: There were big outbreaks during the First World War and the Second World War. In 1946, one year after the end of the Second World War, more than 32,000 persons were infected and more than 3,000 died. As for Japan no cases have been reported since 1957 when one patient was reported. The recent big outbreak in the world was the one in the Republic of Burundi from 1995 to 1997. In 1995, epidemic typhus in association with infestation of body lice was confirmed in a jail. In 1996, there were 3,500 patients, and from January to May of 1997 24,000 infections were reported. R. prowazekii was detected from the blood of 87% of the patients and 25% of the body lice sampled. Since then, only two incidents have been reported, one was a small outbreak in a psychiatric hospital in Russia and the other was an incident involving few cases in associated with flying squirrels, natural reservoir of R. prowazekii , in USA .
2. Relapsing fever: In 1991, there was a big epidemic of relapsing fever in Southwest of Ethiopia. Body lice infested two-thirds of the local population, and 15% of the local families were affected by relapsing fever. Since then, relapsing fever epidemic occurred in Ethiopia intermittently involving estimated 10,000 patients every year. From 1998 to 1999, the epidemic in Sudan claimed several hundreds of lives.
One imported case of tick-borne relapsing fever from Uzbekistan was reported in Japan in 2010 (see p. 358 of this issue).
3. Trench fever: B. quintana is a Gram-negative brevibacillus (Fig. 2) that grows in the body louse’s intestine. Infection to humans occurs by rubbing infected louse feces into scuffed skin (see p. 357 of this issue). Fever, bone pain and arthralgia are main symptoms, but asymptomatic bacteremia may also occur. Endocarditis and angioma may occur in immunocompromized hosts such as alcoholics and HIV patients. Trench fever severely affected soldiers during the two World Wars. Since then no significant epidemics were reported until 1998 when isolation of B. quintana from the blood specimens and increased antibody titer to the pathogen indicated that 10 among 71 Marseillaise homeless persons were infected with the bacteria; all the infected cases carried body lice. The B. quintana genome has been detected from the blood of homeless people in Tokyo, too (see p. 354 of this issue). B. quintana was recently detected from head lice that had not been considered disease vectors in Nepal, USA and Philippines (see p. 355 of this issue).