Cutaneous and systemic effects of varying doses of brown recluse spider venom in a rabbit model.

Objective: To ascertain whether a dose response exists between the dose of brown recluse spider venom (BRSV) and the cutaneous and coagulation effects in a rabbit model. Cutaneous necrosis is a serious complication of brown recluse spider envenomation (spider bite with venom). Disseminated intravascular coagulation (DIC) is a dreaded complication of brown recluse envenomation in humans. New Zealand white (NZW) rabbits have proved to be a model for the study of therapeutic regimens to prevent skin necrosis after spider bites. We studied the venom's effects on the skin and the coagulation mechanism in this rabbit model to determine if a clear dose-response relationship could be established. Establishment of a dose-response relationship is an important first step in determining if the NZW rabbit is a suitable model to study both cutaneous and systemic effects of the venom.

Design: Thirty-six NZW rabbits were divided into three groups. One group received a saline injection, and the other two groups received a 4.0 microg or a 10.0 microg dose of purified BRSV intradermally into the skin on the dorsum of the back.

Methods: Blood was collected at baseline, 24, 48, and 72 hours. Tissue specimens were obtained after seven days during the animal necropsy and gross and microscopic pathology examination was conducted to assess tissue damage. Measurements included complete blood count (CBC); platelets; PT; activated partial thromboplastin time (APTT); fibrinogen (clottable, immunological); coagulation factors II, V, VII, VIII, IX, X, XI, XII; anti-thrombin (AT); alpha-2 antiplasmin (AP); Protein C (PC); mixing studies; lupus anticoagulant screening; plasminogen; thrombin-antithrombin; fibrin degradation products (FDP); d-dimer; and thrombin time.

Results: Gross pathology results were consistent with previous studies that used higher doses of BRSV. The WBC and platelet counts decreased at 24 hours in the two groups receiving the BRSV (p < 0.05). BRSV produced a dose related prolongation in the APTT (p < 0.05). Levels of fibrinogen as well as factors V, VII, VIII, IX, X, AT, and AP (p < 0.05) were increased in response to the BRSV. Protein C decreased at 24 hours (p < 0.05) and remained low in other time points. Mixing studies corrected the prolonged APTTs to normal ranges. Factor IIXI and XII showed no significant alteration in response to the BRSV.

Conclusions: In the model, both the size and depth of the eschar were dose-related. We also observed a dose related elevation in the APTT that corrected with mixing studies. The dose-response relationship suggests direct interference by a component of the venom, rather than an idiosyncratic response. We did not detect a deficiency of commonly measured coagulation factors or evidence of a lupus anticoagulant. Protein C demonstrated a decrease. Although DIC did not occur in this rabbit model, a dose-related elevation in APTT was noted. The finding that the elevation corrected with mixing studies suggests that a plasma factor is essential in the coagulopathy associated with brown recluse envenomation. Further studies to identify this factor could shed light on human coagulopathy following envenomation.