Rickettsia Infection vs. MRSA infection: A Misclassification
Recently, a patient of mine came to the office for follow-up care after being seen over the prior weekend at an urgent care center for a cellulitis of the right lower extremity. According to the patient, he was doing yardwork the week before with his wife when he experienced what he initially thought was a spider bite just above the right ankle. Despite cleaning the wound with antimicrobial soap and water, within 24 hours the surrounding area became erythematous and tender. In addition, the center of the wound began to show necrosis of the dermal layer.
Based on this patient’s presentations, as well as the appearance of his wound, conventional wisdom lead the clinicians at the urgent care center to treat this infection as an MRSA infection, due to the bite of a Brown Recluse spider. The patient received intravenous antibiotics and was placed on oral Bactrim. Unfortunately, after three days of oral antibiotic therapy, the wound and cellulitis were worsening.
This patient unfortunately has several immunocompromised states. First he has underlying rheumatoid arthritis and is receiving Remicade for it. In addition, he has recently been diagnosed with subclass IgG deficiency and is on immunoglobulin therapy.
My initial concern, when first evaluating this wound, was that MRSA may not have been the culprit. For the next several days, the patient received intramuscular Ceftriaxone and Bactrim and was continued until we had a finalized report based on DNA sequencing. Progression of the cellulitis was halted with intramuscular ceftriaxone and oral Bactrim.
DNA sequencing revealed that the patient had a complex wound involving a Rickettsia species, plus E. coli and three different types of fungi. The fungi included Curvularia, Sporisorium and Malasessia. The Bactrim was replaced with Doxycycline, and oral Ketoconazole was added. With the change in medications, the cellulitis began to resolve and granulation tissue, where the necrotic area was, began to form.
In retrospect, after discussing these findings with the patient and his wife, it is believed that it was not a brown recluse spider that bit him, but possibly a horsefly. This makes sense considering what was found in the DNA sequencing. Rickettsia infections are usually associated with insect bites, mostly tick and lice, but other insects may also carry Rickettsia. If the bite was truly from a horsefly, it also explains the fungi associated with the wound. Horseflies are usually found around horses and horse manure, which is heavily colonized with fungus. One could easily see how a horsefly may have been colonized with the mold spores, and that when it inoculated the patient, it introduced the mold spores into the subcutaneous area. As far as the E. coli is concerned, that could easily have come from either the horse or the patient himself during showering – contaminating the wound due to gastrointestinal fall-out.
As already stated, conventional wisdom pointed in the direction of a MRSA infection. However, when the patient did not respond to conventional therapy, the ability of using DNA sequencing was an important adjunct, not only in treating the wound, but also giving a better understanding of how this wound came about. As DNA sequencing becomes more readily available, it will benefit many patients who have complex presentations and who are not responding to first-line therapy.