Lyme disease and erythema migrans

Lyme disease · erythema migrans · EM rash · tick bite rash · bullseye rash · borreliosis

An expanding rash after a tick bite is Lyme disease - NICE says diagnose and treat it without waiting for a blood test.

GPBand C

Red flags

Facial nerve palsy, especially bilateral, or other cranial nerve involvement - neuroborreliosis, which needs a different regimen and duration
Radicular pain, meningitis or focal neurological signs
Heart block, palpitations or syncope - Lyme carditis
A swollen joint, usually the knee, weeks to months after exposure
Pregnancy - discuss antibiotic choice. This is a BNF point, not an NG95 one: NG95 does not name a drug to avoid in pregnancy, it tells you to treat using antibiotics appropriate for the stage of pregnancy and to consult the BNF

Recognise

On light skin

An expanding red or pink patch, sometimes with central clearing giving a target appearance, not usually itchy, hot or painful.

On brown and black skin

Erythema migrans is defined entirely by redness and is one of the most consistently missed diagnoses in brown and black skin, where it may appear as a darker, dusky, violaceous or hyperpigmented patch, or resemble a bruise, and the central clearing that gives the bullseye its name is often absent. The tone-independent features are the history and the behaviour of the lesion: an expanding patch appearing days to weeks after possible tick exposure, enlarging over time, and not itchy, hot or painful. Measure it and mark it rather than judging by colour, and if there is any doubt in a patient with plausible exposure, treat.

In any skin tone

NICE describes erythema migrans as a rash that increases in size, may sometimes have central clearing, is not usually itchy, hot or painful, becomes visible from 1 to 4 weeks after a tick bite (but can appear from 3 days to 3 months), and lasts several weeks
A reaction to a tick bite is different: it develops and recedes within about 48 hours of the bite, and is more likely to be hot, itchy or painful
Often no remembered tick bite - absence of one does not exclude Lyme disease
Ask about exposure: woodland, moorland, long grass, gardens, and occupational or recreational risk
Systemic features may accompany it: fever, headache, fatigue, joint and muscle pain

Distribution

At the site of a tick bite, often the legs, groin, axillae or hairline. Expands outwards over days to weeks and can become large. NICE describes it as becoming visible from 1 to 4 weeks after a tick bite, though it can appear from 3 days to 3 months, and lasting several weeks.

Photographs

Erythema migrans. A large expanding red ring with central clearing and a more intense red centre at the bite site, giving the target pattern — Erythema migrans. A large expanding red ring with **central clearing** and a more intense red centre at the bite site, giving the target pattern. It is flat, not scaly, and not especially itchy or painful - which is why it is so often ignored. Note that most erythema migrans is in fact uniformly red without a bull's-eye at all. On brown and black skin the ring may read as a faint dusky or violaceous patch and is easily mistaken for a bruise or for cellulitis. The only openly licensed photograph found for this entry shows light skin. **That is a limitation of this page, not of the disease** - a darker-skin photograph is still being sought.
Fitzpatrick II (Fitzpatrick type estimated from the photograph, not stated by the source) · CDC Public Health Image Library #9875 · Wikimedia Commons - Erythema migrans - erythematous rash in Lyme disease - PHIL 9875.jpg · Public domain

Provenance for every photograph in this library: image licences. How much of the corpus still has no darker-skin photograph: skin tone coverage.

Mimics

Cellulitis — Cellulitis is hot, painful and tender; erythema migrans is typically none of these, and antibiotic choice differs
Tinea corporis, cruris and pedis — Tinea has a scaly raised edge and itches; erythema migrans has neither
Erythema multiforme major — Both can look targetoid, but EM lesions are multiple, acral and fixed in size

What to do in the ED

Diagnose and treat Lyme disease without laboratory testing in anyone with erythema migrans - this is an explicit NICE NG95 recommendation, and serology is frequently negative early
Treat per NG95: for erythema migrans without focal symptoms in adults and young people aged 12 and over, oral doxycycline 100 mg twice daily or 200 mg once daily for 21 days; alternatives are amoxicillin 1 g three times daily for 21 days, or azithromycin 500 mg daily for 17 days - but NG95 says do not use azithromycin in anyone with cardiac abnormalities associated with Lyme disease, because of its effect on the QT interval. Children have separate age and weight-based regimens in NG95 table 2; do not extrapolate the adult doses
Discuss anyone under 18 with a specialist - NG95 recommendation 1.3.2 makes this a duty, exempting only a child with a single erythema migrans lesion and no other symptoms, so fever, headache or arthralgia takes them out of the exemption. Ask about pregnancy before prescribing, including in young women under 18
Distinguish it from a simple tick-bite reaction, which appears and settles within about 48 hours of the bite and is hot, itchy or painful
Measure and record the size, and photograph with consent where policy allows - expansion over time is the diagnostic behaviour
Examine the cranial nerves and ask about palpitations and syncope, so that focal disease is not treated with the simple regimen
Remove any attached tick promptly and properly, grasping it close to the skin, and advise on prevention
Advise that a negative blood test in someone with erythema migrans does not exclude the diagnosis

Disposition

Treat and discharge with GP follow-up for uncomplicated erythema migrans. Refer or admit for neurological, cardiac or joint involvement, which need different regimens and specialist input.

Safety-netting

Finish the full antibiotic course. Return if you develop weakness of the face, severe headache, neck stiffness, palpitations, fainting, chest pain, or a swollen painful joint - these can appear weeks later and need different treatment. A negative blood test does not mean you did not have Lyme disease. Check yourself for ticks after being outdoors and remove any promptly.

Sources

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