Illustrated Encyclopaedia: Everything You Need to Know About Tick Bites

Tick bites – what constitutes a normal skin reaction, and when should Lyme borreliosis be suspected?

Tick bites are one of the most common types of skin injury caused by arthropods in Europe. Although most bites are harmless and do not lead to any infectious disease, any skin changes following a tick bite warrant close attention. In clinical practice, however, one of the greatest challenges is distinguishing between a normal inflammatory reaction and the first skin symptoms of early infection. A significant proportion of patients attribute any skin redness to Lyme disease, whilst others regard even the characteristic erythema migrans as a simple bite reaction.

The tick embeds itself deeply in the skin with its mouthparts, whilst its saliva introduces numerous biologically active molecules into the host’s body. These include proteins with anticoagulant, immunomodulatory, anti-inflammatory and analgesic effects, which facilitate prolonged blood-feeding. As a result, the bite itself is usually painless, and most people only notice the tick by chance. The absence of pain therefore reflects not the absence of infection, but the tick’s evolutionary adaptation.

Within a few hours or one to two days of the tick being removed, a mild local inflammatory reaction usually develops. This usually takes the form of a pale or more vivid red patch or papule, typically a few millimetres in diameter, though rarely 1–2 cm. A small indentation, mild oedema or a central punctum may also be observed at the site of the bite. Patients often report mild itching, and less commonly a burning sensation or minimal tenderness. The lesion does not usually grow progressively, but fades spontaneously within a few days and then resolves without treatment. From an immunological perspective, this reaction is a local hypersensitivity response to tick salivary proteins and, in itself, does not indicate any infectious disease.

The size, intensity and duration of the local bite reaction can vary significantly from person to person. In individuals who have been exposed to ticks on several previous occasions, more pronounced inflammation, more severe itching and larger papules may develop, whilst during first-time exposure the reaction is often barely noticeable. In atopic patients or those with increased sensitivity to insect venom, oedematous, urticarial plaques measuring up to several centimetres may develop; however, these also regress within a few days and do not spread centrifugally.

From a clinical perspective, it is of particular importance that there is a difference between an early bite reaction and erythema migrans, not only in terms of size but also in terms of dynamics. A common insect bite reaction usually occurs within the first 24–48 hours, remains stable in size or shrinks, and typically resolves within 5–7 days. In contrast, erythema migrans appears after an incubation period of at least 3–30 days, grows gradually, usually exceeds 5 cm in diameter, and may persist for weeks without treatment. Recognising the difference between the two conditions is a far more important diagnostic consideration than the shape or colour of the rash alone.

The classic ‘rosette’ appearance commonly recognised by the general public is in fact one form of erythema migrans, but by no means the most common. In a significant proportion of patients, early Lyme borreliosis begins as a homogeneous red patch, without a central pale area. This fact alone explains why the early recognition of the disease poses a challenge for both patients and GPs. Diagnosis is further complicated by the fact that numerous other dermatological conditions – insect bites, cellulitis, erysipelas, tinea corporis, granuloma annulare or contact dermatitis – can mimic the initial skin symptoms of Lyme disease.

When assessing a tick bite, therefore, a decision should not be based on a single snapshot, but rather on an evaluation of the complete clinical picture. The timing of the lesion’s onset, its rate of progression, the patient’s symptoms, the epidemiological background and, where necessary, a follow-up examination are of far greater diagnostic value than the redness alone. The basis for the early detection of Lyme borreliosis remains a thorough clinical examination, which, at present, cannot be replaced by either serological or molecular testing in the early stages of the appearance of erythema migrans.

The clinical presentation of erythema migrans – from the classic ‘cockade’ to atypical forms

Erythema migrans (EM) is the earliest and best-known clinical manifestation of Lyme disease, occurring in only one third of patients in Europe. The appearance of the skin lesion is diagnostic in itself: given an appropriate clinical picture and a history of exposure, serological confirmation is not required to establish the diagnosis, as the antibody response is often not yet detectable in the early stages of infection. Both international and national guidelines emphasise that treatment must be initiated immediately upon the appearance of the characteristic erythema migrans.

According to the classic textbook description, erythema migrans is a gradually enlarging, circular or oval-shaped erythematous plaque, the central red area of which and the outer red ring gradually become lighter in colour over time, thus creating a characteristic ‘cockade’ or ‘bull’s-eye’ pattern. However, this appearance is far less common in everyday practice than was previously assumed. According to modern clinical studies, a homogeneous, uniformly red erythematous plaque is significantly more common, whilst central fading is observed only in a small proportion of cases. Nevertheless, in popular belief – and, unfortunately, even in many health education materials – the cockade-like appearance is presented as the almost exclusive skin symptom of Lyme disease, which may contribute to the underdiagnosis of homogeneous erythema.

Erythema migrans usually appears 3–30 days after a tick bite, most commonly within one or two weeks. Initially, it presents as an erythematous macule measuring a few centimetres or as a mildly infiltrated plaque, then spreads slowly and centrifugally over a period of several days. The diameter usually exceeds 5 cm, but in untreated cases, lesions as large as 20–30 cm or even larger may develop. The rate of growth varies, averaging 1–2 cm per day; therefore, photographs taken by the patient or follow-up examinations often provide significant diagnostic assistance.

Erythema migrans is usually asymptomatic. Some patients report mild itching, a feeling of tightness or minimal tenderness, but significant pain is not typical. In the event of severe pain, marked warmth, tenderness or rapidly developing oedema, other conditions – such as cellulitis or erysipelas – must also be considered. However, it should be emphasised that the presence or absence of itching alone is not sufficient to rule out or confirm Lyme disease.

The clinical presentation is extremely variable. In addition to the homogeneous erythematous form, oval or irregularly shaped plaques are common, as are lesions with a partially annular pattern and those that only begin to fade centrally at a later stage. The peripheral border may be sharply defined or gradually blurred, and the colour may range from pale pink to a livid red hue. Particularly on darker skin types, the erythema may be less conspicuous; in such cases, it is more likely that a change in skin texture or infiltration will attract attention.

The literature describes an increasing number of atypical forms of erythema migrans. It may present with a uniformly livid, haemorrhagic, vesicular or, rarely, bullous appearance. Lesions with purpuric margins, partial desquamation, or a central crust have been described; at first glance, these may resemble a bacterial skin infection or an inflammatory dermatosis. These atypical forms may be particularly common in children, in immunosuppressed patients, or during infections treated with antibiotics. Due to the diversity of the clinical presentation, the diagnosis continues to be based on the progression of the lesion rather than on any single morphological feature.

In Europe – primarily due to the dominance of Borrelia afzelii and Borrelia garinii – certain characteristics of erythema migrans differ from the appearance typically seen in North America. In European patients, homogeneous erythema, less infiltration and a more varied morphological presentation are more frequently observed. This also explains why the classic ‘bull’s-eye’ photographs featured in American textbooks do not always reflect the clinical reality in Europe.

Multiplex erythema migrans is also not uncommon, in which, alongside the primary lesion, several smaller erythematous foci, situated at a distance from one another, appear as a result of haematogenous dissemination. These are usually smaller, less infiltrated and often do not develop at the site of the tick bite. Their appearance is already a sign of early disseminated infection; therefore, recognising them is particularly important, even if the patient’s general condition is good and they are only experiencing mild systemic symptoms – such as a slight fever, fatigue, headache or muscle pain.

The differential diagnosis covers a wide spectrum. An early local tick-bite reaction is usually small, does not enlarge and resolves spontaneously within a few days. Cellulitis, by contrast, is a painful, warm-to-the-touch, rapidly progressing bacterial infection, often accompanied by fever. Tinea corporis causes a scaly, ring-shaped plaque with an active margin, whilst granuloma annulare usually presents as a ring of asymptomatic, skin-coloured or slightly erythematous papules. In the case of contact dermatitis, the medical history, the predominance of itching and exposure to the triggering allergen aid diagnosis. In clinical decision-making, the combined assessment of the temporal course, progression and epidemiological background is always of paramount importance.

Studies in recent years have also highlighted that recognising erythema migrans is not always straightforward, even for experienced clinicians. According to surveys conducted in various countries, the diagnostic accuracy for atypical forms is significantly lower than for the classic presentation. For this very reason, modern medical training is placing increasing emphasis on diverse collections of clinical photographs and atlases that present not only the textbook-perfect examples, but also the forms of erythema migrans.

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