Pediculus humanus. Phthirus pubis. Cimex lectularius. Pulex irritans

At a glance

1. The human lice are wingless insects that live only on people and die within a day or two off them. They grow by incomplete metamorphosis: egg, three nymphal stages and adult; nymphs and adults feed on blood.
2. Pediculus humanus has two forms: the head louse, common in schoolchildren and passed on by head-to-head contact, and the body louse, which lives in clothes and spreads where people cannot wash or change them.
3. The body louse carries epidemic typhus, trench fever and louse-borne relapsing fever. The bacteria enter not with its bite but when its faeces are scratched into the skin, or a louse is crushed on it.
4. Pthirus pubis, the crab louse, is broad and short, with huge claws on its second and third legs; it lives in pubic hair and is passed on by sex.
5. The bed bug, Cimex lectularius, hides near beds and bites at night; the human flea, Pulex irritans, jumps between hosts. The bed bug is not known to pass on any infection; the human flea is a poor vector, though a swallowed flea can pass on a tapeworm, and the rat flea carries plague.

01What they have in common

All four are blood-sucking insects without wings, ectoparasites that live on the outside of their host. The human lice spend their whole lives on a person or in the clothes they wear; the bed bug and the flea visit only to feed. Their bites itch from an allergic reaction to their saliva, and scratching can infect the skin with bacteria.

• Lice and the bed bug grow by incomplete metamorphosis: the egg hatches into a nymph that looks like a small adult, which moults through its nymphal stages into an adult. Nymphs and adults alike feed on blood.
• The flea grows by complete metamorphosis: a legless larva that lives in dust and debris, feeding on organic matter, a pupa in a cocoon, and the adult.

02Pediculus humanus, the head and body louse

Pediculus humanus lives only on people. Its two forms look the same and may be ecotypes of one species: the head louse, P. h. capitis, and the body louse, P. h. humanus.

• The adult is about the size of a sesame seed: 2 to 4 mm long, the female the larger, greyish white to tan, long and narrow. Its six short legs each end in a curved claw that closes against a thumb-like spur, gripping a hair.
• It feeds on blood several times a day and dies within one to two days off its host.
• The life cycle has three stages. The female lays up to 8 eggs a day, the nits, 0.8 by 0.3 mm: the head louse cements hers to the base of a hair close to the scalp, the body louse lays hers in the seams of clothes. They hatch in about a week into nymphs, which moult three times and are adult about a week later. An adult lives up to 30 days on its host.

The head louse

Head lice are common worldwide, most of all in children of 3 to 11, and more often girls. They pass mostly by head-to-head contact, less often by shared hats, combs or bedding. They are not a sign of poor hygiene, and they are not known to pass on any disease.

• Symptoms: many children have none. Others have itching from an allergic reaction to the louse's saliva, a tickling feeling of movement in the hair, and scratches that can become infected.
• Diagnosis: a live louse, best found by combing wet hair with a fine-toothed comb. Many nits within 6 mm of the scalp suggest an active infestation; nits only further out, an old one.
• Treatment: everyone in the household found to have lice is treated on the same day. Lice in many places now resist the insecticides permethrin and malathion, so a lotion that kills them physically is much used, such as dimeticone, which coats them and to which no resistance is expected. Combing wet hair with a fine-toothed comb removes lice and nits; on its own it is less sure, and is kept for babies and in pregnancy. Oral or topical ivermectin is kept for lice that persist. A lotion that does not kill the eggs is repeated after about a week, once they have hatched.

The body louse

The body louse lives and lays its eggs in the seams of clothes and bedding, and comes to the skin only to feed. It is found where people cannot wash or change their clothes: in poverty, homelessness, refugee camps, prisons and war.

• Symptoms: intense itching and a rash; after long infestation the skin of the waist, groin and thighs thickens and darkens.
• Diagnosis: lice and nits in the seams of the clothes.
• Treatment: washing, and clean clothes. Clothes and bedding are washed hot, at least 54 °C, and machine-dried, or sealed in a bag for two weeks. A pediculicide is seldom needed.

Diseases carried by the body louse

The body louse, not the head louse, is the vector of three bacterial infections. None of them is injected with the bite.

• Epidemic typhus, Rickettsia prowazekii: the rickettsiae multiply in the louse's gut and come out in its faeces; the patient scratches the itching bite and rubs the faeces into it. They stay infective in dried faeces for days. It has followed armies and refugees through history.
• Trench fever, Bartonella quintana: louse faeces in a bite or a scratch, the same way. Today it is seen in homeless people, where B. quintana can also cause a culture-negative endocarditis.
• Louse-borne relapsing fever, Borrelia recurrentis: it gets in when a louse is crushed on the skin and its body fluid reaches a bite or a scratch. It persists in Ethiopia, Eritrea and Somalia, and in 2015 cases reached Europe with refugees from there.

03Pthirus pubis, the crab louse

Pthirus pubis (also spelt Phthirus) is a separate genus. It is broad and short, 1.5 to 2 mm long and nearly as wide, so it looks like a tiny crab, and its second and third legs end in huge claws that close against a thumb-like spur, fitted to the thick, widely spaced hairs it lives on.

• Where: mostly the pubic and perianal hair; also the hair of the armpits, chest, beard and moustache, and the eyebrows and eyelashes.
• Spread: mostly by sexual contact, rarely by bedding or clothes.
• Life cycle: like the head louse's. The female lays about 30 nits in her life of 3 to 4 weeks; they hatch in about a week, and the nymphs moult three times.
• Symptoms: itching, which may be intense; small blue-grey spots where the lice have fed (maculae caeruleae); long infestation can leave the skin discoloured.
• Treatment: a lotion of 1% permethrin or pyrethrins, applied to all the hairy areas. Lice on the eyelashes are removed by hand, or treated with an eye ointment of petroleum jelly, never with a lice lotion near the eyes. The treatment is repeated after 7 to 10 days if live lice are still found. Sexual partners of the past three months are traced and treated, and the patient is tested for other sexually transmitted infections.

In a child, crab lice on the eyelashes or eyebrows are most often caught from a parent by close contact, but may be a sign of sexual abuse.

04Cimex lectularius, the bed bug

Cimex lectularius, of the order Hemiptera, the true bugs, is found worldwide; the tropical bed bug, C. hemipterus, joins it in the tropics and subtropics. It is about 5 mm long, oval, flat and reddish brown, with piercing and sucking mouthparts folded under its head. It cannot fly: its forewings are reduced to two small leathery pads.

• Where it lives: in the seams of mattresses, cracks in bed frames and furniture, behind skirting boards, never far from where people sleep. It travels in luggage, clothes and furniture, and is found in the cleanest hotels: an infestation says nothing about hygiene.
• Feeding: it comes out at night and bites sleeping people, taking a full meal in 5 to 10 minutes; its saliva holds an anaesthetic and an anticoagulant, so the bite is not felt. It can live for months without feeding.
• Life cycle: the female lays about five eggs a day in a hiding place; they hatch in 4 to 12 days into nymphs, smaller and paler versions of the adults, which pass through five nymphal stages, each needing a blood meal.
• Bites: itchy red swellings, often in a row, appearing one to several days after the bite, or up to two weeks later in some people. Rusty spots of blood and droppings on the sheets and mattress give it away. Rarely, a severe allergic reaction.
• Disease: bed bugs have been found carrying blood-borne pathogens, but they are not known to pass on any infection to people, so no antibiotics or tests for infection are needed.
• Treatment and control: an antiseptic or a soothing cream, and an antihistamine for itching; professional pest control for the dwelling.

05Pulex irritans and the fleas

Fleas, order Siphonaptera, are small, wingless and flattened from side to side, so they slip between hairs, with long hind legs for jumping. Many species will bite people.

Pulex irritans, the human flea, is 1 to 4 mm long and has no combs of spines on its head or thorax. Its bite itches, but it is a poor vector: it can carry the plague bacterium, though far less well than the rat flea. Fleas are the usual intermediate host of the dog tapeworm Dipylidium caninum, and an optional one of Hymenolepis nana: a child who swallows an infected flea is infected.

• Life cycle: complete metamorphosis. Eggs dropped in the surroundings hatch in 3 to 4 days into larvae that feed on organic debris; the larvae spin cocoons and pupate; larva and pupa take 3 to 4 weeks. The adult comes out and looks for a warm-blooded host.

Other fleas carry serious diseases:

Murine typhus, like epidemic typhus, enters with the flea's faeces scratched into the bite; plague is passed on with the bite itself, as the flea feeds.

Check yourselfAnswer, then say how sure you were: a lucky guess still counts as a weak point.

1A girl of seven has an itchy scalp. Nits are found on her hair, all more than 2 cm from the scalp, and combing her wet hair finds no lice. What does this mean?

An active infestation, to be treated at onceMost likely an old infestation, since live eggs lie within about 6 mm of the scalpBody liceCrab lice

2How does Rickettsia prowazekii, the cause of epidemic typhus, get into a person?

Injected with the body louse's saliva as it bitesThrough the bite wound or broken skin, when the patient scratches in the louse's infected faecesBy swallowing a louseThrough the bite of a bed bug

3Which is the main way Pthirus pubis is passed on?

Sharing combs and hatsSexual contactFlea bitesContaminated water

4A hotel guest wakes with a row of itchy red bites on her arm. A bed bug is found in the mattress seam. Which infection must she now be treated for?

Epidemic typhusNone, since bed bugs are not known to pass on infections to peoplePlagueLeishmaniasis

5Which flea is the main vector of plague?

Pulex irritans, the human fleaXenopsylla cheopis, the Oriental rat fleaCtenocephalides felis, the cat fleaTunga penetrans, the chigoe flea