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Medicine for the Outdoors
Medicine for the Outdoors

Dr. Paul Auerbach is the world's leading outdoor health expert. His blog offers tips on outdoor safety and advice on how to handle wilderness emergencies.

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"A Diagnostic Dilemma" Update


On September 17, I wrote a post entitled "A Diagnostic Dilemma," in which I described a friend who had developed an illness that we attributed to an insect bite, species undetermined. It was a possible spider bite, but the differential diagnosis included tick bite, and even the possibility of Lyme disease.

As it turns out, his blood tests for Lyme disease have come back positive, so the rash portrayed here and in the original blog is most likely a true case of erythema migrans (also sometimes called erythema chronicum migrans), which is the hallmark presentation for an early localized infection with Borrelia burgdorferi, the causative infectious organism of Lyme disease.

The ability of patients to remember a tick bite varies, frequently by species of tick, as some bites are more painful than others. Early localized disease typically begins as a localized erythema migrans rash or lesion, which occurs 7 to 10 days (range, 3 to 32 days) after a tick bite. It has been stated that 75 to 90% of patients with Lyme disease will develop an erythema migrans lesion.

Erythema migrans may appear anywhere on the body, but usually occurs at or near the site of the tick bite. In cases with a single erythema migrans lesion, the most common sites (in order of descending frequency, which likely reflects the propensity of a tick to land and bite) include the head and neck region, arms and legs, back (as was the case with this particular victim), abdomen, armpits, groin, and chest.

The erythema migrans rash is variable in size, ranging from two centimeters to over 60 centimeters in diameter, and is usually in a circular pattern. To meet the Centers for Disease Control (CDC) case definition of Lyme disease, the lesion must be at least 5 cm. It usually begins as a red flat spot or bump, with an area of central clearing that becomes more apparent as the lesion expands in size. The central portion of the rash may become inflamed and lumpy. The borders, which are usually bright red, may expand as much as one centimeter a day. These borders are usually flat, although rarely they may be raised or inflamed. Occasionally, there are multiple, alternating concentric rings of redness and central clearing, a rash pattern referred to as “bulls-eye.” The rash is often warm to the touch.

The lesions sometimes are difficult to differentiate from local immune reactions to tick salivary proteins, and are sometimes confused with secondary bacterial infections. In contrast, local allergic reactions usually occur within hours of the tick bite and are very itchy. Secondary infections typically occur within a few days of the tick bite and lack the central clearing and rapid expansion.

Patients often describe the lesion as burning, but may also report itching or pain. Children may develop fevers to 104ºF (40ºC), although low grade fevers are more common in adults. Constitutional symptoms, such as fatigue and muscle aching, may also be present.

Erythema migrans fades after an average of 3 to 4 weeks (range, 1 to 14 weeks) without treatment; with antibiotics, the lesion resolves after several days and seldom comes back. Although erythema migrans lesions resolve without treatment, untreated patients are at risk for developing more severe Lyme disease.

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photo by Paul Auerbach
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About the Author

Dr. Paul S. Auerbach is the world’s leading authority on wilderness medicine.

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