Why does local edema persist? Immunological mismatch between phospholipase A2 potency and antivenom affinity in Protobothrops mucrosquamatus envenomation

Background Protobothrops mucrosquamatus is the primary species responsible for snakebite morbidity in Taiwan, typically resulting in severe localized tissue edema. Although freeze-dried hemorrhagic antivenom (FHAV) is the standard treatment, persistent swelling remains a frequent clinical challenge. This study aimed to identify primary edematogenic toxins and evaluate the neutralizing efficacy of FHAV and small-molecule inhibitors (SMIs) to address this therapeutic gap. Methodology/principal findings Multiple linear regression of a retrospective cohort of 50 patients was used to identify predictors of edema remission. Experimentally, a murine paw edema model was established to evaluate the potency of crude venom and isolated fractions (PLA 2 , SVSP, and SVMP/CTL), whereas ELISA was used to determine FHAV immunoreactivity. Clinically, total FHAV dosage was the strongest independent predictor of the remission timeline ( β = 0.658, T = 5.167, P < 0.001), accounting for 69.8% of the variance. Experimentally, the PLA 2 -enriched fraction was identified as the primary edematogenic driver, eliciting swelling at 100 μg/mL, which was statistically indistinguishable from that of crude venom, whereas the other fractions required fivefold higher concentrations to achieve similar effects. However, ELISA revealed a significant immunological mismatch: the SVMP/CTL fraction sequestered 81.49% of the FHAV, indicating that highly potent PLA 2 was poorly targeted. Furthermore, SMIs (varespladib, marimastat, and batimastat) at 10 μM exhibited limited efficacy against crude venom-induced edema, suggesting that single-target interventions against the complex venom proteome are insufficient. Conclusions/significance Persistent localized edema arises from an immunological mismatch in which FHAV recognizes primarily high-molecular-weight immunogens instead of the primary edematogenic driver, PLA 2 . Current clinical management necessitates increasing FHAV dosages to compensate for low specific affinity through increased total antibody volume. These findings highlight that robust prospective multicenter trials are urgently needed to transition from empirical dosing to standardized, potency-aligned FHAV protocols, potentially incorporating multimodal therapeutic cocktails to achieve comprehensive neutralization.