Differentials
Local inflammatory reaction to tick bite
SIGNS / SYMPTOMS
Typically appears sooner than erythema migrans (within hours to days) and recedes within 48 hours. Associated with itching, pain, and warmth. No central clearing of the rash.
INVESTIGATIONS
There are typically no differentiating tests.
Tickbite allergy
SIGNS / SYMPTOMS
There is no central clearing of the rash. Typically itchy.
INVESTIGATIONS
There are typically no differentiating tests.
Cellulitis
SIGNS / SYMPTOMS
There is a homogeneous, warm, tender, indurated area, often accompanied by fever and chills, in most cases of cellulitis.
INVESTIGATIONS
Patients may have leukocytosis or positive blood cultures.
Erythema multiforme
SIGNS / SYMPTOMS
Extensive, disseminated rash often occurs, with blistering and mucosal involvement in erythema multiforme major.
INVESTIGATIONS
Skin biopsy is diagnostic.
Rickettsiosis
SIGNS / SYMPTOMS
Diffuse, generalised maculopapular rash is present in rickettsiosis.
INVESTIGATIONS
Serology for Rickettsia rickettsii is positive.
Ehrlichiosis
SIGNS / SYMPTOMS
There may be no differentiating symptoms.
Co-infection with ehrlichiosis (anaplasmosis) can occur in patients with Lyme disease.
There may be a history of local prevalence.
Absence of rash.
INVESTIGATIONS
Serology is positive for Ehrlichia species.
There may be leukocytopenia.
Babesiosis
SIGNS / SYMPTOMS
There may be no differentiating symptoms.
Co-infection with babesiosis can occur in patients with Lyme disease.
There may be a history of local prevalence.
Absence of rash.
INVESTIGATIONS
Peripheral smear is positive for intraerythrocytic Babesia.
There may be evidence of haemolysis.
Tick-borne encephalitis
SIGNS / SYMPTOMS
There may be a history of local prevalence.
Absence of rash.
Found in Europe.
INVESTIGATIONS
IgM antibody against tick-borne encephalitis virus is present in cerebrospinal fluid.
Southern tick-associated rash illness (STARI)
SIGNS / SYMPTOMS
Bull's-eye rash on skin after a tick bite from lone star tick (Amblyomma americanum).
Usually reported from different geographical distribution than Lyme disease, but there is overlap.
Constitutional symptoms including joint pain may occur similar to Lyme disease, but there are no associated neurological, joint, or cardiac sequelae.
INVESTIGATIONS
Lyme serology is negative in most cases, although there have been a small number of cases where it may be false-positive.
A biopsy of skin, identification of the tick as a lone star tick, and a negative Lyme serology aid differentiation.
Septic arthritis
SIGNS / SYMPTOMS
Predictors for septic arthritis include fever, hip involvement, and pain with short arc motion, whereas predictors of Lyme arthritis include history of a tick bite, isolated knee involvement, and lack of fever. However, there is considerable overlap between Lyme arthritis and septic arthritis in children in the following instances: presence of fever, elevated acute phase reactants, and the inability to bear weight (especially when the hip is involved).
INVESTIGATIONS
Predictors for septic arthritis include a peripheral blood absolute neutrophil count >10,000 and erythrocyte sedimentation rate of >40. When there is any doubt, joint fluid should be obtained for culture for other bacterial causes of septic arthritis.
Chronic fatigue syndrome
SIGNS / SYMPTOMS
Medically unexplained, persistent fatigue, lasting at least 6 months. No objective findings or laboratory anomalies.
INVESTIGATIONS
A standard battery of laboratory testing is typically normal.
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