Jan Welzl (1868–1948), a Czech traveller, explorer and gold prospector, lived in Northern Siberia for about 30 years. While staying with natives, young women visited his hut and playfully threw lice at him. This, Welzl discovered, was, ‘the customary manner of indicating love, and a notice of serious intentions. A sort of “My louse is thy louse” ceremony.’1
In cultures across the world, humans touch heads to indicate friendship or love. Some researchers believe that this behaviour evolved so that we acquire head lice infestations early in life. The infestation would provoke an immune response against head and body lice. This, in turn, offers some defence against diseases carried by lice, such as typhus, trench fever and plague.2
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Today, thinking about body, head or pubic lice is more likely to evoke repulsion than romance. The very thought can make us itch. Scratching defends against skin irritants, pathogens or parasites including arthropods, the phylum that includes insects, crabs and centipedes.3,4 Arthropods evolved to live in almost every habitat on earth. Many became parasites, such as bed bugs (see IN March 2023).4 This feature focuses on three other arthropod parasites: mites, ticks and lice.4 NICE and the British Association of Sexual Health and HIV (www.bashh.org) cover diagnosis and treatment. Here, we introduce the creatures themselves.
Mites
Ticks and mites belong to a group of arthropods called Acari.4 Mites, which tend to be 0.2 to 0.8 mm long, are remarkably diverse.4 Biologists have named almost 60,000 mite species. But the estimated total of mite species is anywhere between 500,000 and 10.2 million.5
You may harbour mites without being any the wiser. Demodex folliculorum, which is about 0.3 mm, lives in the hair follicle above the sebaceous gland. The smaller (0.2mm) relative Demodex brevis lives in the sebaceous gland4 – a perfect example of how arthropods adapt to almost every habitat. Often Demodex infestations go unnoticed. But Demodex may contribute to some cases of rosacea and juvenile acne.4
Scabies
Sarcoptes scabiei causes scabies in humans and some cases of mange in animals.4 People with Norwegian (‘crusted’) scabies develop large crusted lesions, generalised scales, thickened skin and are infested with thousands or millions of highly contagious mites. Crusted scabies is most commonly found in immunocompromised, malnourished and disabled people. If not diagnosed rapidly, crusted scabies can cause complications including secondary impetigo, cellulitis and potentially fatal sepsis.6
Male S. scabiei scurry around on the surface of human skin seeking a mate. Mature females burrow into the host’s skin, especially in moist areas such as the genitalia, back of the knees and elbows, and between the fingers and toes. Female mites excavate between 0.5–5 mm a day.4 After 1–2 months, the accumulation of eggs, mites and faeces in the tunnels triggers an allergic reaction characterised by a rash and intense itching that is worse at night.4,7
Several factors increase the risk of contracting scabies, such as overcrowding, poor hygiene and diet, homelessness, dementia, sexual contact and living in a tropical region.7 S. scabiei seems to cause epidemics of scabies every 20–30 years, although no-one is quite sure why.4
Ticks
Ticks tend to be larger than mites: up to 30 mm long.4 There are approximately 800 species of ticks, all of which are parasitic, divided into two families: Ixodidae (hard ticks) and Argasidae (soft ticks).4,8 Unfed Ixodidae are about 3 mm long, but can grow to 11 mm after a blood meal. The engorged ticks detach and moult to the next stage in their lifecycle.8
Tick saliva contains chemicals that help them feed. For example, the chemicals stop the host’s blood from coagulating; relax blood vessels; and block inflammation. The host produces bradykinin as part of the immune response, which causes pain at the site of the bite. Chemicals in tick saliva breakdown bradykinin. So, the tick continues to feed painlessly.4
Only a few ticks infest humans, but can cause serious health problems. Paralysis can follow a tick bite near the base of the skull. The tick can produce toxic secretions. Heavy tick infestations can lead to anaemia. Ticks can also transmit certain viruses, bacteria and other pathogens.4
Lyme disease
In the mid-1970s, for example, researchers investigated clusters of recurrent attacks of asymmetric swelling and pain in large joints, such as knees, in children and adults linked to Old Lyme, Connecticut, USA. Most attacks occurred during the summer and early autumn.9 Further investigation revealed that Ixodes ricinus, a tick infecting deer and sheep, can transmit the bacterium Borrelia burgdorferi, which causes Lyme disease.8 Ticks collected from seabirds, hedgehogs, voles, mice, dogs are among the other species that can harbour B. burgdorferi.10
While Lyme disease was first identified in the mid-1970s, researchers found B. burgdorferi in a tick collected in 1897.9,10 Genetic studies of the Tyrolean Iceman, a 5300-year-old corpse discovered by hikers in the Ötztal Alps in 1991, found about 60% of B. burgdorferi’s genome.11
There are about 1500 laboratory-confirmed cases of Lyme disease in England and Wales annually. Doctors, however, treat many cases without laboratory tests. So, there are an estimated 3000 to 4000 new cases in England and Wales and perhaps more than 8000 across the UK year.12,13 Scotland has the largest number of cases in the UK and about 15% of patients contract Lyme disease abroad.12,13
Lyme disease often presents non–specific symptoms (eg headaches, neck stiffness, flu–like symptoms, facial palsy, arthralgia, heart palpitations or dizziness).8 About 60% of patients develop erythema migrans, a bull’s eye rash, between 2 and 30 days after being bitten. Fever, headache and regional lymphadenopathy may accompany the rash.8,14 Prompt diagnosis is important: patients with suspected Lyme diseases need antibiotics.8 Ticks attached for less than 24 to 36 hours are unlikely to transmit B. burgdorferi. So, advise people to rapidly remove any ticks from themselves or their animals.8
Lice
As we’ve seen, attitudes to lice have changed over the years. During the 18th century, lice had a reputation for protecting children from disease. Even in the 1950s, some English people thought that a large population of head lice indicated robust health.15 Western healers used preparations containing lice to treat epilepsy, malaria, toothache and jaundice: the last with a dozen crushed lice in wine. Healers even introduced lice directly into the urethra to treat urinary tract problems.15
Body lice
Three types of louse call humans home: head (Pediculus humanus capitis), body (Pediculus humanus humanus) and pubic lice (Pthirus pubis).4 Body lice usually live in clothes, especially along seams, where each louse lays about 8–12 eggs a day.4,16 They leave their sartorial home to feed on blood 1–5 times a day.4,16 Infested people present with generalised pruritus and lesions (eg excoriations and eczema–like patches) where clothes touch the skin, such as the neck, shoulders, upper back, flanks and waist. Scratching predisposes to impetigo, ecthyma and cellulitis.17
Body lice die if detached from their host for about 3–5 days.17 When the knights of Henry II murdered Archbishop Thomas Becket, his corpse lay on the floor of Canterbury cathedral overnight. His body cooled and body lice ‘boil[ed] over like water in a simmering cauldron’.15 Today, in developed countries, body lice are relatively rare. We wear fewer clothes to keep warm, and change and launder our clothes more often than our medieval ancestors.15 But up to 30% of homeless people may have body lice.17
Lice are disease vectors. Bartonella quintana, a bacterium transmitted by body lice and, possibly, head lice causes trench fever.16 The homeless and other socioeconomically deprived groups can develop ‘urban trench fever’, typically characterised by headache, dizziness, conjunctival injection (red eye), severe shin pain, lymphadenopathy, a macular rash that lasts for less than 24 hours and relapsing fevers over 4–8 days.17
Head lice
Head lice are much more common than body lice. Researchers in Essex found that 2% of primary school children had head lice at the time of the survey. The annual incidence was 37.4%.18 A Norwegian study reported that 1.7% primary school children had head lice and 44.3% had been infested with at least once.19
Adult head lice are about 2–4 mm long with ‘sickle–shaped’ claws that wrap around a hair.20 Most children with head lice are infested with fewer than 20 mature parasites. After mating, a female louse lays about 4–10 eggs a day, sticking each to a hair shaft close to the scalp. Eggs hatch after 9–10 days leaving the empty casing (a ‘nit’). Larvae (‘nymphs’) mature in 9–15 days and head lice live for 3–4 weeks.20
Head lice eggs remain viable for up to three days away from their host, but they need higher temperatures (eg near the scalp) to hatch.20 Whether pillowcases, hats and hair products and other ‘fomites’, spread infestations is controversial.20 You could advise concerned parents to wash bedding, hats, brushes and combs at more than 60˚C and dry in a hot dryer for at least 15 minutes and store un–washable items for at least two weeks in a plastic bag.20
Patients can become sensitised to lice saliva causing itching, which usually takes four to six weeks to develop. Heavy infestations can lead to secondary bacterial infections and iron–deficiency anaemia.20,21
Pubic lice
Body and head lice evolved from the chimpanzee louse. The gorilla louse evolved into the human pubic louse: Phthirus pubis. Estimating the prevalence of pubic lice is difficult, partly because of patients’ understandable reluctance to seek help, but seems to vary from 0.3% to 4.6%. P. pubis is transmitted by close contact.22 So, pubic lice are more common in crowded areas, during war or natural disaster, and where sanitation is poor. The fashion for removing body hair seems to have reduced the number of people with pubic lice.22
Pubic lice are the smallest human lice (0.8–3 mm length) with pincer-like claws (the colloquial name is crabs) that match the diameter of pubic hair.22 P. pubis may also infest scalp and body hair, eyebrows and eyelashes.22 An adult female lays about 3–10 eggs daily. P. pubis hatch over 6–10 days, mature into adults over 2–3 weeks and have an average lifespan of 3–4 weeks.22
P. pubis can cause pruritus, red papules and rust or brown deposits from feeding or faecal matter.22 Itching secondary to allergic sensitisation from louse saliva often occurs 2–6 weeks after first exposure. Subsequent exposures cause pruritus after 1–2 days.22
P. pubis infestation may indicate that the person has other sexually transmitted infections or, in some cases, been the victim of sexual abuse.22,23
More exotic pathogens carried by ticks also cause numerous diseases among travellers to other countries including tick–borne encephalitis (TSE) and rickettsioses, caused by members of the Rickettsia genus of bacteria. Species of Rickettsia can cause Rocky Mountain spotted fever, Mediterranean spotted fever and African tick bite fever.8 In the UK, TSE and rickettsioses tend to occur in returning travellers. However, British ticks now host pathogenic rickettsiae.8
Other arthropods parasites cause health problems. You may, for example, encounter the human bot fly (Dermatobia hominis) if you visit central and south America. The bot fly sticks fertilised eggs underneath the abdomen of a mosquito or another blood–sucking anthropoid. At the next meal, the body heat triggers the larvae to hatch. These burrow into the skin, where they mature during the next 6–8 weeks. Bot fly larvae are notoriously difficult to remove.4
This feature may have made you itch. But mites, ticks and lice are fascinating. This article only ‘scratches’ the surface. As Patel and colleagues note: ‘Lice are ancient, flexible and resourceful creatures so it is likely that they will beat mankind in the evolutionary race and be here long after humans are gone.’22
Mark Greener is a freelance medical writer
References
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3. Wimalasena NK, Milner G, Silva Ret al. 2021 .Dissecting the precise nature of itch-evoked scratching. Neuron. 109(19):3075-87.e2. https://doi.org.10.1016/j.neuron.2021.07.020.
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5. Zhang YX, Chen X, Wang JPet al. 2019 .Genomic insights into mite phylogeny, fitness, development, and reproduction. BMC Genomics. 20(1):954. https://doi.org.10.1186/s12864-019-6281-1.
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7. Gunning K, Kiraly B and Pippitt K. 2019 .Lice and scabies: treatment update. Am Fam Physician. 99(10):635-42.
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10. Hubbard MJ, Baker AS and Cann KJ. 1998 .Distribution of Borrelia burgdorferi s.l. spirochaete DNA in British ticks (Argasidae and Ixodidae .since the 19th century, assessed by PCR. Med Vet Entomol. 12(1):89-97. https://doi.org.10.1046/j.1365-2915.1998.00088.x.
11. Keller A, Graefen A, Ball Met al. 2012 .New insights into the Tyrolean Iceman’s origin and phenotype as inferred by whole-genome sequencing. Nat Comm. 3(1):698. https://doi.org.10.1038/ncomms1701.
12. UK Government. 2022 .Lyme disease: signs and symptoms. Available at https://www.gov.uk/government/publications/lyme-disease-signs-and-symptoms/lyme-disease-signs-and-symptoms. Accessed September 2023.
13. Cairns V, Wallenhorst C, Rietbrock Set al. 2019 .Incidence of Lyme disease in the UK: a population-based cohort study. BMJ Open. 9(7):e025916. https://doi.org.10.1136/bmjopen-2018-025916.
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15. Drisdelle R. 2010 .Parasites: Tales of Humanity’s Most Unwelcome Guests. Berkley: University of California Press.
16. Bonilla DL, Durden LA, Eremeeva MEet al. 2013 .The biology and taxonomy of head and body lice--implications for louse-borne disease prevention. PLoS Pathog. 9(11):e1003724. https://doi.org.10.1371/journal.ppat.1003724.
17. Coates SJ, Thomas C, Chosidow Oet al. 2020 .Ectoparasites: pediculosis and tungiasis. J Am Acad Dermatol. 82(3):551-69. https://doi.org.10.1016/j.jaad.2019.05.110.
18. Harris J, Crawshaw JG and Millership S. 2003 .Incidence and prevalence of head lice in a district health authority area. Commun Dis Public Health. 6(3):246-9.
19. Birkemoe T, Lindstedt HH, Ottesen Pet al. 2016 .Head lice predictors and infestation dynamics among primary school children in Norway. Fam Pract. 33(1):23-9. https://doi.org.10.1093/fampra/cmv081.
20. Canadian Paediatric Society. 2008 .Head lice infestations: A clinical update. Paediatr Child Health. 13(8):692-6. https://doi.org.10.1093/pch/pxx165.
21. Althomali SA, Alzubaidi LM and Alkhaldi DM. 2015 .Severe iron deficiency anaemia associated with heavy lice infestation in a young woman. BMJ Case Rep.bcr2015212207. https://doi.org.10.1136/bcr-2015-212207.
22. Patel PU, Tan A and Levell NJ. 2021 .A clinical review and history of pubic lice. Clin Exp Dermatol. 46(7):1181-8. https://doi.org.10.1111/ced.14666.
23. British Association of Sexual Health and HIV. 2007 .United Kingdom National Guideline on the Management of Phthirus pubis infestation. Available at https://www.bashhguidelines.org/media/1074/28.pdf (Accessed September 2023).