Direct correlation between diffusion of Loxosceles reclusa venom and extent of dermal inflammation

Objectives: Envenomation by Loxosceles species (brown recluse) spiders resu lts in large dermal inflammatory lesions. Venom-induced dermal inflammation occurs indirectly via soluble mediators of inflammation. This study aimed to explore whether the anatomic extent of dermonecrotic arachnidism is due to the cascade of soluble proinflammatory mediators elicited by venom depos ited at the bite site, or due to diffusion of the venom per se. Methods: Th ree New Zealand white rabbits received intradermal L. reclusa venom (3-mug) injections in the flank. At the time of maximum dermal inflammation (24 hr ), paired 4-mm dermal biopsies were obtained in 2-cm intervals extending 0 to 12 cm from the inoculation site. Normal dermal tissue was obtained from the opposite flank to serve as a negative control. One biopsy sample from e ach interval was homogenized and assayed for myeloperoxidase (MPO) activity and for the presence of venom via an enzyme immunoassay (EIA). The other p aired dermal biopsy was sectioned, and examined for the presence of polymor phonuclear neutrophils (PMNs) by microscopy. Lesional areas were measured u sing digital images imported into imaging software. Results: Mean +/- SD le sional diameter 24 hours post inoculation measured 9.18 +/- 0.64 cm. Venom was detected in biopsies 0 to 10 cm from the injection site. As expected, t he highest venom concentrations were measured at the inoculation site (4.28 +/- 3.9 ng/4 mm). In addition, PMNs and MPO were detected up to 8 and 10 c m from the inoculation site, respectively. Neither PMNs nor MPO was detecte d in tissue absent of venom (kappa = 0.88, p < 0.001). Conclusions: Loxosce les venom diffuses from the envenomation site. The extent of dermal inflamm ation mirrors the extent of Loxosceles venom diffusion. This observation im plies that the venom itself defines the extent and magnitude of tissue inju ry following Loxosceles envenomation.