Sicariidae

Distribution is the primary diagnostic

Loxosceles reclusa has a well-bounded native range covering roughly the south-central United States, from southern Iowa and Illinois south through Texas and east to western Georgia. Surveys conducted outside that range consistently fail to recover established populations, even in states where recluse bites are reported in large numbers.

For identification purposes, geography carries as much weight as morphology. A six-eyed spider recovered inside the native range is very likely Loxosceles. A specimen reported from coastal California or the Pacific Northwest is far more likely to be a different family entirely, with the notable exception of the Mediterranean recluse in isolated heated structures.

Venom composition

The active dermonecrotic component of Loxosceles venom is sphingomyelinase D, an enzyme that hydrolyzes sphingomyelin in cell membranes and initiates a complement-mediated inflammatory cascade. This enzyme is essentially absent from other North American spider venoms, which is why dermonecrosis is not a plausible outcome of most spider bites.

Lesion severity varies widely. The majority of confirmed bites resolve without necrosis. Where necrosis develops, the lesion typically becomes apparent between 24 and 72 hours after envenomation and may take weeks to months to heal.

Misdiagnosis burden

Published case series have repeatedly shown that lesions clinically diagnosed as recluse bites are more often caused by other conditions, most commonly community-acquired methicillin-resistant Staphylococcus aureus infection, but also by pyoderma gangrenosum, diabetic ulcers, herpes zoster, and Lyme disease. Diagnosis in the absence of a captured and identified specimen is unreliable.