Order Acari. Medical importance of ticks and mites

At a glance

1. Ticks and mites form the order Acari, in the class Arachnida. They have no antennae and no wings, and their body is in two parts: the mouthparts in front and one fused body behind. The larva has six legs; the nymph and the adult have eight.
2. Hard ticks (Ixodidae) have a shield on the back and mouthparts that show from above, and feed for days. Soft ticks (Argasidae) have no shield, their mouthparts are hidden underneath, and they feed in about an hour.
3. Ixodes ricinus, the castor bean tick, is Europe's main tick. Larva, nymph and adult each feed once, on a new host, and drop off between meals. It passes on Lyme disease and tick-borne encephalitis; Hyalomma carries Crimean-Congo haemorrhagic fever, and Rhipicephalus Mediterranean spotted fever.
4. Sarcoptes scabiei, the itch mite, burrows in the outer layer of the skin and causes scabies: night-time itch and burrows between the fingers, on the wrists and round the waist. The whole household is treated together.
5. Other mites matter too: Demodex lives in hair follicles, trombiculid larvae (chiggers) bite and carry scrub typhus in Asia, and house dust mites cause asthma and rhinitis.

01The order Acari

Ticks and mites are arachnids, like spiders and scorpions, and differ from insects in the same ways: no antennae, no wings, and eight legs in the adult. Their body plan is simpler still than a spider's.

• The body has two parts. In front is the gnathosoma, called the capitulum in ticks, which carries the mouthparts: a palp either side and, in ticks, the barbed hypostome that anchors the tick in the skin. Behind it, the rest of the body is fused into one sac, the idiosoma, with no waist between a cephalothorax and an abdomen as in a spider.
• They develop from egg to a six-legged larva, then to eight-legged nymphs and the adult, each stage much like the next.
• Ticks have no eyes in many species, Ixodes included. They find a host with Haller's organ, a pit on each front leg that senses warmth, carbon dioxide and smells.

Ticks are all blood-sucking parasites, and large: an unfed adult is a few millimetres long, a fed female a centimetre or more. Mites are mostly under a millimetre, and most live freely; only a few are parasites of people.

02Hard ticks and soft ticks

A hard tick's long meal is what makes it such a good vector: it stays attached for days, its saliva numbs the skin and stops the blood clotting, and the longer it feeds, the more likely it is to pass on what it carries.

03Ixodes ricinus, the castor bean tick

Ixodes ricinus is the commonest tick in Europe, in deciduous woods and mixed forest, heath and long grass. The unfed female is 3 to 3.6 mm long and swells to 10 to 11 mm when fed; the male is smaller, and the nymph is about 1.5 mm, the size of a poppy seed.

It is a three-host tick. The larva feeds for 3 to 5 days, usually on a shrew or a mouse, and drops off; on the ground it moults into a nymph, which feeds on a bird or a small or medium-sized mammal, drops off and moults into an adult. The adults feed on large mammals, such as deer, sheep, cattle and dogs, and usually mate there. The fed female drops off, lays up to 2000 eggs and dies. The whole cycle usually takes 2 to 3 years.

Between meals the tick waits on the tips of grass and low plants, holding out its front legs to catch onto a passing host: questing. It cannot fly or jump.

What a tick takes up with blood in one stage, it keeps through its moult and can pass on in the next: transstadial transmission. Some pathogens, such as the tick-borne encephalitis virus and some rickettsiae, also pass through the female to her eggs: transovarial transmission. Borrelia seldom does, so larvae rarely carry it; ticks pick it up from mice, other rodents and birds, the reservoir hosts. Deer feed most of the adults and keep ticks numerous, but do not keep Borrelia going.

Most cases of Lyme disease follow the bite of a nymph: adults bite people too, but a nymph is so small that it often feeds unnoticed for the day or more that Borrelia usually needs to cross into the skin.

04Diseases carried by ticks

Ticks also do harm without an infection:

• Tick paralysis: a toxin in the saliva of a feeding female, usually of Dermacentor in North America or Ixodes holocyclus in Australia, causes a paralysis that climbs from the legs, like Guillain-Barré syndrome. Finding and removing the whole tick is the treatment, so a child with a new weakness is searched for one, in the hair too. Most recover within hours; after the Australian I. holocyclus the weakness can still worsen, so the child is watched, with breathing support if needed.
• Alpha-gal syndrome: a tick bite can sensitise a person to a sugar, galactose-α-1,3-galactose, found in the meat of mammals. Hours after eating red meat they get hives or anaphylaxis, and the same sugar on the drug cetuximab and in gelatine-based plasma expanders can cause reactions at once.

Lyme borreliosis in brief

The first sign, in most patients, is erythema migrans: a red patch that spreads slowly out from the bite, usually 7 to 14 days later, often clearing in the middle. The diagnosis of typical erythema migrans is clinical: no blood test is needed. Untreated, the infection can spread to the nerves (a facial palsy, a painful radiculitis, meningitis), the heart (heart block) or the joints (arthritis), and in Europe to the skin, as a bluish-red swelling of the earlobe or nipple (borrelial lymphocytoma) or, years later, acrodermatitis chronica atrophicans. It is treated with an antibiotic, such as doxycycline or amoxicillin.

Removing a tick

• Remove it as soon as possible: the shorter it feeds, the less Borrelia it passes on. The tick-borne encephalitis virus passes within minutes of the bite, so only the vaccine protects against it.
• Grip it as close to the skin as possible with fine-tipped tweezers, never by its swollen body, and pull it straight out with steady, even pressure, without twisting or jerking.
• If the mouthparts stay in the skin, they can be lifted out, or left to come out as the skin heals.
• Do not use petroleum jelly, heat or nail polish, which may make the tick push infected fluid into the skin.
• Clean the bite and your hands afterwards, and look for other ticks.
• Watch the bite: a spreading rash or a fever in the weeks that follow is a reason to see a doctor.

05Sarcoptes scabiei, the itch mite

Sarcoptes scabiei var. hominis causes scabies. The female is 0.3 to 0.45 mm long, a round, eyeless sac of a mite, just visible as a speck. Her two front pairs of legs end in stalked suckers, her hind legs in long bristles, and her back carries rows of spines that brace her in her burrow.

• Life cycle: the fertilised female burrows into the outer layer of the skin, never below the stratum corneum, and lays 2 to 3 eggs a day for the rest of her life of 1 to 2 months. The eggs hatch in 3 to 4 days into six-legged larvae, which make short moulting pouches in the skin and become adults in 1 to 2 weeks.
• Spread: by close skin-to-skin contact, as in a family, and by sex. Off the skin the mites die in 2 to 3 days, so bedding and clothes rarely pass on ordinary scabies.
• Symptoms: the itch is an allergic reaction to the mites, their eggs and their faeces, so it begins only 4 to 6 weeks after a first infestation, and within days of a second. It is intense, and worse at night. Burrows show as thin, wavy, greyish lines, up to a centimetre or more long, between the fingers, on the wrists and elbows, round the waist and buttocks, and on the penis and breasts. Infants may have it on the palms, soles and scalp.
• Complications: scratched skin is infected by Streptococcus pyogenes and Staphylococcus aureus, causing impetigo. In the tropics this leads on to post-streptococcal glomerulonephritis and possibly rheumatic heart disease.
• Diagnosis: usually clinical. A mite, its eggs or its faeces can be seen in a scraping from a burrow under the microscope, or the mite at the end of its burrow with a dermatoscope.

Crusted scabies (Norwegian scabies) develops in people who are immunosuppressed, old or frail: their skin carries thousands or millions of mites in thick, grey, scaly crusts, often on the hands and feet, which may hardly itch. It is very contagious, also through bedding and clothes, and causes outbreaks in care homes and hospitals.

Treatment:

• Permethrin 5% cream on the whole body from the neck down, and over the scalp, face and ears too in infants, the elderly, the immunosuppressed and crusted scabies; washed off after 8 to 12 hours, and repeated after about a week, since it does not kill all the eggs.
• Or oral ivermectin, 200 µg/kg, two doses one to two weeks apart; it is not given in pregnancy or to children under 15 kg. Benzyl benzoate 25% lotion is the third first-line choice in the European guideline.
• Everyone in the household and close contacts are treated at the same time, even without symptoms. Sexual partners of the past two months are treated too, and offered screening for other sexually transmitted infections. Bedding and clothes of the past few days are washed hot, or sealed in a bag for a week.
• Crusted scabies needs both: oral ivermectin in several doses with a topical scabicide, and a cream to soften the crusts.
• The itch can get worse for a week or two after treatment and last several weeks, without meaning that the treatment has failed.

06Other mites

• Demodex: two species live in our skin, D. folliculorum in hair follicles, mostly of the face and eyelashes, and D. brevis in the sebaceous glands. They are long and slender, about 0.3 mm, D. brevis a little shorter, and nearly every adult carries them harmlessly. In large numbers they are linked to demodicosis, a rosacea-like rash, and to blepharitis.
• Trombiculid mites, chiggers or harvest mites: only the six-legged larvae are parasites. They feed on skin, not blood, by dissolving it with their saliva, leaving intensely itchy red papules on the legs and round the waist; in Europe the harvest mite, Neotrombicula autumnalis, bites in late summer and autumn. In Asia and the western Pacific the larvae of Leptotrombidium carry scrub typhus, Orientia tsutsugamushi: fever, a black eschar at the bite and a rash, treated with doxycycline.
• House dust mites, Dermatophagoides pteronyssinus in Europe: free-living, not parasites. They live in beds, carpets and soft furnishings and feed on flakes of human skin. Proteins in their faeces, such as Der p 1, are among the commonest allergens, a major cause of allergic asthma, rhinitis and eczema.

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

1A tick is found with a hard shield on its back and mouthparts that stick out in front, visible from above. Which family is it, and how long does it feed?

Argasidae, the soft ticks, for about an hourIxodidae, the hard ticks, for several daysIxodidae, the hard ticks, for about an hourArgasidae, the soft ticks, for several days

2Why are most cases of Lyme disease caused by the bite of a nymph rather than an adult Ixodes tick?

Only nymphs carry BorreliaNymphs are so small that they often feed unnoticed for long enough to pass it onAdults do not bite peopleLarvae pass Borrelia to nymphs through the egg

3A walker finds a tick attached to her leg. What should she do?

Cover it with petroleum jelly so that it lets goGrip it close to the skin with fine-tipped tweezers and pull it straight out with steady pressureBurn it with a matchLeave it until it drops off, then see a doctor

4A mother and her two children all have an itchy rash, worse at night, with thin wavy lines between the fingers and on the wrists. What is the treatment?

Permethrin 5% cream for the patient onlyPermethrin 5% cream for the whole household at the same time, repeated after about a weekAn oral antibioticA steroid cream until the itch settles

5An elderly man in a care home has thick, crusted, scaly skin on his hands that barely itches, and several staff now have itchy rashes. What is the likely diagnosis?

PsoriasisCrusted scabies, carrying huge numbers of mitesDemodicosisContact dermatitis