Brown recluse spider bite or MRSA: an important differential diagnosis?

February 3, 2011, 6:04 pm


Systemic Loxoscelism in the Age of Community-Acquired Methicillin-Resistant Staphylococcus aureus. Rogers KM et al. Ann Emerg Med Feb 2011;57:38-140.


Cutaneous abscesses caused by community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) are often misdiagnosed as a bite by a brown recluse spider (Loxosceles reclusa).  This commonly occurs in areas of the country where L. reclusa is not endemic:

This  report, from the Tennessee Poison Center and Vanderbilt University Medical Center in Nashville, describes case in which the opposite occurred:  Loxoscelism was misdiagnosed as a systemic staphylococcal infection.

An 11-year-old boy presented with a low-grade fever and a large, violaceous, tender area of induration in the right axilla with erythema extending across much of the chest and abdomen, rapidly progressing down his body to his toes.  Associated findings included leukocytosis, hemolysis, jaundice, and hematuria. The patient never developed hemodynamic compromise. He underwent surgical exploration of the affect area for detection and drainage of possible abscess; serosanguinous fluid but no pus or abscess was detected.  Ultimately the diagnosis was changed to presumed systemic loxoscelism.

The authors note that the rapidly progressive rash without shock is not consistent with stretococcal or staphylococcal skin infection.  The following are the characteristics they cite by which the two conditions can be distinguished:

  • Systemic staphylococcal infection: clinically toxic; hemodynamically unstable; thrombocytopenia; dissemininated intravascular coagulation (DIC)
  • Systemic Loxoscelism: uncomfortable but nontoxic; hemodynamically stable except for possible tachycardia secondary to hemolysis; increased bilirubin; increased lactate deyhdrogenase

Note that this case occurred in Tennessee, an area where the brown recluse spider is endemic. It would have made much more valuable reading had it contained photographs illustrating the features of a Loxosceles bite compared with those of a CA-MRSA lesion.

The authors suggest that the attempt of surgical drainage in this case was unnecessary, as was administration of antibiotics.  However, I don’t know any clinician who would not have treated this case as a bacterial infection initially.  Nor am I convinced that a soft tissue ultrasound  not demonstrating an abscess would have been reliable enough to alter the initial management.


  1. Anders Nielsen Says:


    Anyone bitten by a spider should collect the spider for further identification. From both California and Florida there are many reports about bites by Brown Recluse Spiders although no spiders are present in either state.

  2. Leon Says:


    Thanks for the comment. The problem is that very often the actual “bite” — or whatever — goes unnoticed. Later, when a lesion appears, it is attributed to a spider bite when it very well might be caused by MRSA or something else.