A30-year-old female presents to your office for the evaluation of a rash on her back. It has been present and growing for about a week. Along with this rash, she has had a fever, headache, myalgias, and fatigue. Her symptoms started about a week after returning from a camping trip to New England. She denies having any bites from ticks or other insects and exposure to poison ivy and has had no wounds to her skin. On examination, her temperature is 99.5°F and her v ital signs are otherwise normal. Her rash is shown in Figure. Her examination is otherwise unremarkable.
What is the most likely cause of her rash?
- contact dermatitis secondary to plant exposure
- infection transmitted by tick bite
- infection transmitted by mosquito bite
- group A Streptococcus suprainfection of small puncture wound
- allergic reaction to ingested (i.e., food) allergen
Answer(s): B
Explanation:
Lyme disease is the most common vector-borne disease in the United States. It is caused by infection with B. burgdorferi, a spirochete that is transmitted to humans through the bite of ticks of the Ixodes family. These ticks are very small, so frequently the victim is unaware of having been bitten. After an incubation of 330 days, a red macule or papule develops at the site of the bite, which expands to form a large annular lesion with partial central clearing or several red rings within an outside ring. The lesion, erythema migrans, is often said to resemble a "bull's-eye" target. Within a few days or weeks of this, the patient often complains of flu-like symptoms fever, chills, myalgias, headache, fatigue caused by the hematogenous spread of the spirochete. Lyme disease has been found in most of the United States, but is most common in the New England states, where over 20% of Ixodes ticks are infected with the spirochete. Left untreated, patients may progress to develop multiple complications, including neurologic, musculoskeletal, or cardiac involvement. Lyme disease is usually diagnosed by recognition of the symptoms and signs, along with serologic testing. However, serologic tests may be negative for several weeks after infection. IgG and IgM should be tested in acute and convalescent samples. Only 2030% of exposures will have positive acute antibody responses, whereas 7080% will have positive convalescent titers. Samples that are positive by ELISA assay should be confirmed by Western blot testing. Empirical antibiotic therapy, preferably with doxycycline, is recommended for patients with a high probability of Lyme disease--such as those with erythema migrans. Doxycycline is the preferred antibiotic for treatment of early stage Lyme disease in adults because of its effectiveness against Lyme disease and other infections, such as human granulocytic ehrlichiosis, which is also transmitted by Ixodes ticks. Waiting to treat until convalescent titers become positive would not be recommended in this patient, who has a high likelihood of having Lyme disease, as it may result in more complications developing and the need for longer and more intensive treatment. For more advanced stages of disease, such as the presence of nervous system involvement or third-degree heart block, parenteral antibiotic treatment is necessary. Ceftriaxone is the treatment of choice in this setting.