Sphingomyelinases D induce direct association of C1q to the erythrocyte membrane causing complement mediated autologous haemolysis

Impact of antivenom administration on the evolution of cutaneous lesions in loxoscelism: A prospective observational study

BACKGROUND

Spiders of the genus Loxosceles are distributed throughout tropical and temperate regions worldwide. Loxosceles spp. bites may evolve to necrosis, with or without intravascular hemolysis. There is no consensus regarding the best treatment to prevent necrosis. The objective of this study was to evaluate the factors associated with the development of necrosis and the impact that antivenom administration has on the evolution of cutaneous loxoscelism.

METHODOLOGY/PRINCIPAL FINDINGS

This was a prospective observational study carried out at a referral center for envenoming. Over a 6-year period, we included 146 patients with a presumptive or definitive diagnosis of loxoscelism. Depending on the symptom severity, a polyvalent anti-arachnid antivenom was administered or not-in 74 cases (50.7%) and 72 cases (49.3%), respectively. Cutaneous and systemic manifestations were assessed at admission and weekly thereafter. Adverse reactions to the antivenom were also evaluated. Cutaneous loxoscelism was observed in 141 cases (96.6%), and the spider was identified in 29 (19.9%). The mean time from bite to antivenom administration was 41.6 ± 27.4 h. After discharge, 130 patients (90.9%) were treated with corticosteroids, antihistamines and analgesics being prescribed as needed. The probability of developing necrosis was significantly lower among the patients who were admitted earlier, as well as among those who received antivenom (p = 0.0245). Among the 74 patients receiving antivenom, early and delayed adverse reactions occurred in seven (9.5%) and four (5.4%), respectively. Local infection was observed only in three (2.3%) of the 128 patients for whom that information was available.

CONCLUSIONS/SIGNIFICANCE

Necrosis after a Loxosceles sp. bite appears to more common when hospital admission is delayed or when antivenom is not administered. In addition, the administration of a polyvalent anti-arachnid antivenom appears to be safe, with a relatively low rate of adverse reactions.

Laboratory Predictors of Hemolytic Anemia in Patients With Systemic Loxoscelism

OBJECTIVES

To develop a sensitive and specific protocol for detecting preclinical hemolysis in patients with brown recluse spider (BRS) bites by comparing a large cohort of individuals with brown recluse spider (BRS) bites with and without hemolytic anemia.

METHODS

A cross-sectional, retrospective analysis of clinical features and laboratory values, including urinalysis (UA) and peripheral blood results, and timing of positive laboratory values prior to a significant drop in hematocrit was performed to evaluate effective predictors of clinically significant hemolysis.

RESULTS

In total, 275 patients with BRS bites were identified (64 with hemolytic anemia). Sensitivity and specificity of UA positive for blood (with and without microscopic hematuria) for detecting hemolysis were 72% and 75%, respectively. The combination of elevated serum total bilirubin (TB) and lactate dehydrogenase (LDH) had greater sensitivity (94%) and specificity (91%) for detecting patients developing hemolysis. When TB and LDH were evaluated prior to a significant decrease in hematocrit, they were positive in 82% of cases, while UA was positive for blood prior to a hematocrit decrease in 38% of cases.

CONCLUSIONS

Serum TB and LDH levels are more effective at detecting preclinical hemolysis than UA and should be serially analyzed to triage patients with BRS bites before life-threatening hemolysis occurs.

A new insight into the cellular mechanisms of envenomation: Elucidating the role of extracellular vesicles in Loxoscelism

Envenomation by the Loxosceles genus spiders is a recurring health issue worldwide and specially in the Americas. The physiopathology of the envenomation is tightly associated to the venom's rich toxin composition, able to produce a local dermonecrotic lesion that can evolve systemically and if worsened, might result in multiple organ failure and lethality. The cellular and molecular mechanisms involved with the physiopathology of Loxoscelism are not completely understood, however, the venom's Phospholipases D (PLDs) are known to trigger membrane injury in various cell types. Here, we report for the first time the Loxosceles venom's ability to stimulate the production of extracellular vesicles (EVs) in various human cell lineages. Components of the Loxosceles venom were also detectable in the cargo of these vesicles, suggesting that they may be implicated in the process of extracellular venom release. EVs from venom treated cells exhibited phospholipase D activity and were able to induce in vitro hemolysis in human red blood cells and alter the HEK cell membranes' permeability. Nonetheless, the PLD activity was inhibited when an anti-venom PLDs monoclonal antibody was co-administered with the whole venom. In summary, our findings shed new light on the mechanisms underlying cellular events in the context of loxoscelism and suggest a crucial role of EVs in the process of envenomation.

Sphingomyelinase D Activity in Sicarius tropicus Venom: Toxic Potential and Clues to the Evolution of SMases D in the Sicariidae Family

The spider family Sicariidae includes three genera, Hexophthalma, Sicarius and Loxosceles. The three genera share a common characteristic in their venoms: the presence of Sphingomyelinases D (SMase D). SMases D are considered the toxins that cause the main pathological effects of the Loxosceles venom, that is, those responsible for the development of loxoscelism. Some studies have shown that Sicarius spiders have less or undetectable SMase D activity in their venoms, when compared to Hexophthalma. In contrast, our group has shown that Sicarius ornatus, a Brazilian species, has active SMase D and toxic potential to envenomation. However, few species of Sicarius have been characterized for their toxic potential. In order to contribute to a better understanding about the toxicity of Sicarius venoms, the aim of this study was to characterize the toxic properties of male and female venoms from Sicarius tropicus and compare them with that from Loxosceles laeta, one of the most toxic Loxosceles venoms. We show here that S. tropicus venom presents active SMases D. However, regarding hemolysis development, it seems that these toxins in this species present different molecular mechanisms of action than that described for Loxosceles venoms, whereas it is similar to those present in bacteria containing SMase D. Besides, our results also suggest that, in addition to the interspecific differences, intraspecific variations in the venoms' composition may play a role in the toxic potential of venoms from Sicarius species.

Cutaneous-hemolytic loxoscelism following brown recluse spider envenomation: new understandings

Background: Brown recluse spider (BRS) (Loxosceles reclusa) envenomation can cause local dermonecrotic lesions, constitutional symptoms, and potentially fatal hemolysis (i.e., cutaneous-hemolytic loxoscelism). As the incidence of hemolysis is low and the spider habitat is limited, little is known regarding the clinical course of cutaneous-hemolytic loxoscelism.Methods: We performed a retrospective observational study of patients following BRS envenomation over an eight-year period. Demographics, clinical course, laboratories, and interventions were assessed. Wilcoxon rank-sum tests and Pearson chi-square tests were used in the univariate analyses. Logistic regression assessed the independent contribution of symptoms in a multivariate analysis.Results: Of the 97 patients, 40.2% (n = 39) developed hemolysis; the majority (66.7%) were 18 years old or younger. Univariate analysis revealed that constitutional symptoms were associated with hemolysis, but multivariate analysis showed only myalgia (aOR: 7.1; 95% CI: 2.2-22.7; p < .001) and malaise (aOR: 12.76; 95% CI: 1.4-119.9; p = .026) were independently associated with hemolysis. The median time to hemolysis onset was 1.0 days (IQR: 1.0-2.5) and all occurred within a week of envenomation. Hemolysis durations were longer in patients DAT positive for IGG antibodies (7.5 vs. 4.0 days; p = .042). Most (76.9%) of hemolyzing patients received blood. In patients with cutaneous-hemolytic loxoscelism, hematuria occurred in 32.4%, rhabdomyolysis occurred in 60.9%, and elevated transaminases with normal hepatic synthetic function occurred in 29.4% but all of these patients developed rhabdomyolysis. Hemolysis was both intravascular and extravascular. Complications (hyperkalemia, INR ≥2.0, metabolic acidosis requiring bicarbonate, hypotension requiring vasopressors, and hypoxia requiring intubation) occurred only in patients with profound hemolytic anemia (hemoglobin <4 g/dL); one patient died.Conclusions: Constitutional symptoms occur in both cutaneous and cutaneous-hemolytic loxoscelism, although they occur more frequently in patients who develop hemolysis. Children may be at a higher risk of hemolysis after envenomation. Renal involvement (as evidenced by hematuria) and rhabdomyolysis may occur more frequently than has been previously reported. Hemolysis was both intravascular and extravascular.

CD28 deficiency leads to accumulation of germinal-center independent IgM+ experienced B cells and to production of protective IgM during experimental malaria

Protective immunity to blood-stage malaria is attributed to Plasmodium-specific IgG and effector-memory T helper 1 (Th1) cells. However, mice lacking the costimulatory receptor CD28 (CD28KO) maintain chronic parasitemia at low levels and do not succumb to infection, suggesting that other immune responses contribute to parasite control. We report here that CD28KO mice develop long-lasting non-sterile immunity and survive lethal parasite challenge. This protection correlated with a progressive increase of anti-parasite IgM serum levels during chronic infection. Serum IgM from chronically infected CD28KO mice recognize erythrocytes infected with mature parasites, and effectively control Plasmodium infection by promoting parasite lysis and uptake. These antibodies also recognize autoantigens and antigens from other pathogens. Chronically infected CD28KO mice have high numbers of IgM⁺ plasmocytes and experienced B cells, exhibiting a germinal-center independent Fas⁺GL7^-CD38⁺CD73^- phenotype. These cells are also present in chronically infected C57BL/6 mice although in lower numbers. Finally, IgM⁺ experienced B cells from cured C57BL/6 and CD28KO mice proliferate and produce anti-parasite IgM in response to infected erythrocytes. This study demonstrates that CD28 deficiency results in the generation of germinal-center independent IgM⁺ experienced B cells and the production of protective IgM during experimental malaria, providing evidence for an additional mechanism by which the immune system controls Plasmodium infection.

Loxosceles venom Sphingomyelinase D activates human blood leukocytes: Role of the complement system

Envenomation by Loxosceles spiders can result in severe systemic and local reactions, which are mainly triggered by Sphingomyelinase D (SMase D), a toxic component of Loxosceles venom. SMase D induces a systemic inflammatory condition similar to the reaction observed during an endotoxic shock. Considering the potent pro-inflammatory potential of Loxosceles venom and the SMase D, in this study we have used the whole human blood model to study the endotoxic-like shock triggered by SMase D. Recombinant purified SMase D from L. intermedia venom, similarly to LPS, induced activation of blood leukocytes, as observed by the increase in the expression of CD11b and TLR4, production of reactive oxygen and nitrogen species (superoxide anion and peroxynitrite) and release of TNF-α. Complement consumption in the plasma was also detected, and complement inhibition by compstatin decreased the SMase D and LPS-induced leukocyte activation, as demonstrated by a reduction in the expression of CD11b and TLR4 and superoxide anion production. Similar results were found for the L. intermedia venom, except for the production of TNF-α. These findings indicate that SMase D present in Loxosceles venom is able to activate leukocytes in a partially complement-dependent manner, which can contribute to the systemic inflammation that follows envenomation by this spider. Thus, future therapeutic management of systemic Loxosceles envenomation could include the use of complement inhibitors as adjunct therapy.

Defining the complex phenotype of severe systemic loxoscelism using a large electronic health record cohort

Objective

Systemic loxoscelism is a rare illness resulting from the bite of the recluse spider and, in its most severe form, can lead to widespread hemolysis, coagulopathy, and death. We aim to describe the clinical features and outcomes of the largest known cohort of individuals with moderate to severe loxoscelism.

Methods

We performed a retrospective, cross sectional study from January 1, 1995, to December 31, 2015, at a tertiary-care academic medical center, to determine individuals with clinical records consistent with moderate to severe loxoscelism. Age-, sex-, and race-matched controls were compared. Demographics, clinical characteristics, laboratory measures, and outcomes of individuals with loxoscelism are described. Case and control groups were compared with descriptive statistics and phenome-wide association study (PheWAS).

Results

During the time period, 57 individuals were identified as having moderate to severe loxoscelism. Of these, only 33% had an antecedent spider bite documented. Median age of individuals diagnosed with moderate to severe loxoscelism was 14 years old (IQR 9.0–24.0 years). PheWAS confirmed associations of systemic loxoscelism with 29 other phenotypes, e.g., rash, hemolytic anemia, and sepsis. Hemoglobin level dropped an average of 3.1 g/dL over an average of 2.0 days (IQR 2.0–6.0). Lactate dehydrogenase and total bilirubin levels were on average over two times their upper limit of normal values. Eighteen individuals of 32 tested had a positive direct antiglobulin (Coombs’) test. Mortality was 3.5% (2/57 individuals).

Conclusion

Systemic loxoscelism is a rare but devastating process with only a minority of patients recalling the toxic exposure; hemolysis reaches a peak at 2 days after admission, with some cases taking more than a week before recovery. In endemic areas, suspicion for systemic loxoscelism should be high in individuals, especially children and younger adults, presenting with a cutaneous ulcer and hemolysis or coagulopathy, even in the absence of a bite exposure history.

Bacterial Sphingomyelinases and Phospholipases as Virulence Factors

Bacterial sphingomyelinases and phospholipases are a heterogeneous group of esterases which are usually surface associated or secreted by a wide variety of Gram-positive and Gram-negative bacteria. These enzymes hydrolyze sphingomyelin and glycerophospholipids, respectively, generating products identical to the ones produced by eukaryotic enzymes which play crucial roles in distinct physiological processes, including membrane dynamics, cellular signaling, migration, growth, and death. Several bacterial sphingomyelinases and phospholipases are essential for virulence of extracellular, facultative, or obligate intracellular pathogens, as these enzymes contribute to phagosomal escape or phagosomal maturation avoidance, favoring tissue colonization, infection establishment and progression, or immune response evasion. This work presents a classification proposal for bacterial sphingomyelinases and phospholipases that considers not only their enzymatic activities but also their structural aspects. An overview of the main physiopathological activities is provided for each enzyme type, as are examples in which inactivation of a sphingomyelinase- or a phospholipase-encoding gene impairs the virulence of a pathogen. The identification of sphingomyelinases and phospholipases important for bacterial pathogenesis and the development of inhibitors for these enzymes could generate candidate vaccines and therapeutic agents, which will diminish the impacts of the associated human and animal diseases.

Animal venoms/toxins and the complement system

Nature is a wealthy source of agents that have been shown to be beneficial to human health, but nature is also a rich source of potential dangerous health damaging compounds. This review will summarise and discuss the agents from the animal kingdom that have been shown to interact with the human complement (C) system. Most of these agents are toxins found in animal venoms and animal secretions. In addition to the mechanism of action of these toxins, their contribution to the field of complement, their role in human pathology and the potential benefit to the venomous animal itself will be discussed. Potential therapeutic applications will also be discussed.

Identification of new sphingomyelinases D in pathogenic fungi and other pathogenic organisms

Sphingomyelinases D (SMases D) or dermonecrotic toxins are well characterized in Loxosceles spider venoms and have been described in some strains of pathogenic microorganisms, such as Corynebacterium sp. After spider bites, the SMase D molecules cause skin necrosis and occasional severe systemic manifestations, such as acute renal failure. In this paper, we identified new SMase D amino acid sequences from various organisms belonging to 24 distinct genera, of which, 19 are new. These SMases D share a conserved active site and a C-terminal motif. We suggest that the C-terminal tail is responsible for stabilizing the entire internal structure of the SMase D Tim barrel and that it can be considered an SMase D hallmark in combination with the amino acid residues from the active site. Most of these enzyme sequences were discovered from fungi and the SMase D activity was experimentally confirmed in the fungus Aspergillus flavus. Because most of these novel SMases D are from organisms that are endowed with pathogenic properties similar to those evoked by these enzymes alone, they might be associated with their pathogenic mechanisms.

Sphingomyelinase D in sicariid spider venom is a potent insecticidal toxin

Spider venoms have evolved over hundreds of millions of years with a primary role of immobilizing prey. Sphingomyelinase D (SMase D) and homologs in the SicTox gene family are the most abundantly expressed toxic protein in venoms of Loxosceles and Sicarius spiders (Sicariidae). While SMase D is well known to cause dermonecrotic lesions in mammals, little work has investigated the bioactivity of this enzyme in its presumed natural role of immobilizing insect prey. We expressed and purified recombinant SMase D from Loxosceles arizonica (Laz-SMase D) and compared its enzymatic and insecticidal activity to that of crude venom. SMase D enzymatic activities of purified protein and crude venom from the same species were indistinguishable. In addition, SMase D and crude venom have comparable and high potency in immobilization assays on crickets. These data indicate that SMase D is a potent insecticidal toxin, the role for which it presumably evolved.

Sphingomyelinase D activity in model membranes: structural effects of in situ generation of ceramide-1-phosphate

The toxicity of Loxosceles spider venom has been attributed to a rare enzyme, sphingomyelinase D, which transforms sphingomyelin to ceramide-1-phosphate. The bases of its inflammatory and dermonecrotic activity, however, remain unclear. In this work the effects of ceramide-1-phosphate on model membranes were studied both by in situ generation of this lipid using a recombinant sphingomyelinase D from the spider Loxosceles laeta and by pre-mixing it with sphingomyelin and cholesterol. The systems of choice were large unilamellar vesicles for bulk studies (enzyme kinetics, fluorescence spectroscopy and dynamic light scattering) and giant unilamellar vesicles for fluorescence microscopy examination using a variety of fluorescent probes. The influence of membrane lateral structure on the kinetics of enzyme activity and the consequences of enzyme activity on the structure of target membranes containing sphingomyelin were examined. The findings indicate that: 1) ceramide-1-phosphate (particularly lauroyl ceramide-1-phosphate) can be incorporated into sphingomyelin bilayers in a concentration-dependent manner and generates coexistence of liquid disordered/solid ordered domains, 2) the activity of sphingomyelinase D is clearly influenced by the supramolecular organization of its substrate in membranes and, 3) in situ ceramide-1-phosphate generation by enzymatic activity profoundly alters the lateral structure and morphology of the target membranes.

Clinical picture and laboratorial evaluation in human loxoscelism

Loxosceles spiders are found globally, especially in South and North America. In Brazil, approximately 10,000 cases of Loxosceles spp. spider bites are reported annually. Herein we analyzed 81 patients diagnosed as either cutaneous or cutaneous-hemolytic loxoscelism, in a geographical area where most accidents are caused by Loxosceles gaucho, and we report their clinical and laboratory data obtained during week 1 and 2 after the bite. Massive hemolysis was noticed in only 2 cases, but high serum bilirubin and LDH levels, suggestive of hemolysis, were noticed in 25 cases on admission. Anemia was not frequent (14.7%), and reticulocytosis was particularly noticed during week 2 (in 56% of patients). High D-dimer levels were suggestive of endothelial cell activation and intravascular thrombin generation, but thrombocytopenia was noticed in only 17.6% of patients in week 1. Acute kidney injury (AKI) only occurred in patients with massive hemolysis. The definitive diagnosis of overt disseminated intravascular coagulation (DIC) could not be established on admission. Fever was associated with the presence of hemolysis (p = 0.03). Altogether, these findings provide evidence that mild hemolysis is frequent in loxoscelism and suggest that AKI is uncommon, exclusively occurring in patients with massive hemolysis.

Heminecrolysin, a potential immunogen for monospecific antivenom production against Hemiscorpius lepturus scorpion

Serotherapy against Hemiscorpius (H.) lepturus scorpion sting is based on the administration of equine polyvalent antivenom prepared against a mixture of six venoms. In a previous study, we reported the identification of Heminecrolysin, a 33 kDa H. lepturus venom protein endowed with a sphingomyelinase D, hemolytic and dermonecrotic activities. We aimed herein to investigate the capacity of Heminecrolysin to generate antibodies able to neutralize the major physiopathological properties of H. lepturus envenomation, e.g. hemolysis and dermonecrosis. The efficiency of anti-Heminecrolysin antibodies was compared to that of anti-whole venom. Our results demonstrated that Heminecrolysin elicits high levels of specific IgGs. Anti-Heminecrolysin, similarly to anti-whole venom antibodies, totally inhibited H. lepturus hemolytic effect when up to 5 times the half maximal effective concentration of venom were used. Phosphatidylserine exposure on the external lipid monolayer of human red blood cells treated with whole venom was also fully blocked by both anti-sera. Experimental envenomation of rabbits showed that anti-Heminecrolysin antibodies were as potent as anti-H. lepturus antibodies to neutralize dermonecrotic effects when up to 4 times the minimal necrotic dose of venom were injected. However, inflammatory reaction was better controlled with anti-whole venom sera. In conclusion, Heminecrolysin elicits protective antibodies of comparable potency to those elicited by immunization with whole venom.

Factor H as a regulator of the classical pathway activation

C1q, the first subcomponent of the classical pathway, is a charge pattern recognition molecule that binds a diverse range of self, non-self and altered self ligands, leading to pro-inflammatory complement activation. Although complement is required for tissue homeostasis as well as defence against pathogens, exaggerated complement activation can be damaging to the tissue. Therefore, a fine balance between complement activation and inhibition is necessary. We have recently found that factor H, a polyanion recognition molecule and soluble regulator of alternative pathway activation in blood and on cell surfaces, can directly compete with C1q in binding to anionic phospholipids (cardiolipin), lipid A and Escherichia coli (three known activators of the classical pathway) and acts as a direct down regulator of the complement classical pathway. This ability of factor H to dampen classical pathway activation is distinct from its role as an alternative pathway down-regulator. Thus, by directly competing for specific C1q ligands (exogenous as well as endogenous), factor H is likely to be involved in fine-tuning and balancing the C1q-driven inflammatory processes in autoimmunity and infection. However, in the case of apoptotic cells, C1q-mediated enhancement of uptake/adhesion of the apoptotic cells by monocytes was reduced by factor H. Thus, factor H may be important in controlling the inflammation, which might arise from C1q deposition on apoptotic cells.

Loxosceles gaucho spider venom and its sphingomyelinase fraction trigger the main functions of human and rabbit platelets

Loxosceles venoms can promote severe local and systemic damages. We have previously reported that Loxosceles gaucho spider venom causes a severe early thrombocytopenia in rabbits. Herein, we investigated the in vitro effects of this venom and its sphingomyelinase fraction on the main functions of platelets. Whole venom and its fraction induced aggregation of both human and rabbit platelets. Aggregation was dependent of plasma component(s) but independent of venom-induced lysophosphatidic acid generation. There was no increase in the levels of lactate dehydrogenase during platelet aggregation, ruling out the possibility of platelet lysis. The increased expression of ligand-induced binding site 1 (LIBS1) induced by L. gaucho venom and its sphingomyelinase fraction, as well as of P-selectin by the whole venom, evidenced the activation state of both human and rabbit platelets. Adhesion assays showed an irregular response when platelets were exposed to the whole venom, whereas the sphingomyelinase fraction induced a dose-dependent increase in the platelet adhesion to collagen. These findings evidence that L. gaucho venom and its sphingomyelinase fraction trigger adhesion, activation, and aggregation of both human and rabbit platelets. Thus, this work justifies the use of rabbits to investigate Loxosceles venom-induced platelet disturbances, and it also supports research on the role of platelets in the pathogenesis of loxoscelism.

Complement activation by phospholipids: the interplay of factor H and C1q

Complement proteins in blood recognize charged particles. The anionic phospholipid (aPL) cardiolipin binds both complement proteins C1q and factor H. C1q is an activator of the complement classical pathway, while factor H is an inhibitor of the alternative pathway. To examine opposing effects of C1q and factor H on complement activation by aPL, we surveyed C1q and factor H binding, and complement activation by aPL, either coated on microtitre plates or in liposomes. Both C1q and factor H bound to all aPL tested, and competed directly with each other for binding. All the aPL activated the complement classical pathway, but negligibly the alternative pathway, consistent with accepted roles of C1q and factor H. However, in this system, factor H, by competing directly with C1q for binding to aPL, acts as a direct regulator of the complement classical pathway. This regulatory mechanism is distinct from its action on the alternative pathway. Regulation of classical pathway activation by factor H was confirmed by measuring C4 activation by aPL in human sera in which the C1q:factor H molar ratio was adjusted over a wide range. Thus factor H, which is regarded as a down-regulator only of the alternative pathway, has a distinct role in downregulating activation of the classical complement pathway by aPL. A factor H homologue, β2-glycoprotein-1, also strongly inhibits C1q binding to cardiolipin. Recombinant globular domains of C1q A, B and C chains bound aPL similarly to native C1q, confirming that C1q binds aPL via its globular heads.

Molecular diversity of spider venom

Spider venom, a factor that has played a decisive role in the evolution of one of the most successful groups of living organisms, is reviewed. Unique molecular diversity of venom components including substances of variable structure (from simple low molecular weight compounds to large multidomain proteins) with different functions is considered. Special attention is given to the structure, properties, and biosynthesis of toxins of polypeptide nature.

Brown recluse spider (Loxosceles reclusa) envenomation in small animals

OBJECTIVE

To provide a comprehensive review of relevant literature regarding the brown recluse spider (BRS) and to define those criteria that must be satisfied before making a diagnosis of brown recluse envenomation.

ETIOLOGY

The complex venom of the BRS contains sphingomyelinase D, which is capable of producing all the clinical signs in the human and some animal models.

DIAGNOSIS

There is no current commercially available test. In humans there are many proposed guidelines to achieve a definitive diagnosis; however, there are no established guidelines for veterinary patients.

THERAPY

Currently, no consensus exists for treatment of BRS envenomation other than supportive care, which includes rest, thorough cleaning of the site, ice, compression, and elevation.

PROGNOSIS

Prognosis varies based on severity of clinical signs and response to supportive care.

SMase II, a new sphingomyelinase D from Loxosceles laeta venom gland: molecular cloning, expression, function and structural analysis

Sphingomyelinase D (SMase D) present in the venoms of Loxosceles spiders is the principal component responsible for local and systemic effects observed in the loxoscelism. By using "expressed sequencing tag", it was possible to identify, in a L. laeta venom gland library, clones containing inserts coding for proteins with similarity to SMase D. One of these clones was expressed and the recombinant protein compared with the previously characterized SMase I from L. laeta, in terms of their biological, biochemical and structural properties. The new recombinant protein, SMase II, possesses all the biological properties ascribed to the whole venom and SMase I. SMase II shares 40% and 77% sequence similarity with SMase I and Lb3, respectively; the latter, a SMase D isoform from L. boneti, catalytically inactive. Molecular modeling and molecular dynamics simulations were employed to understand the structural basis, especially the presence of an additional disulfide bridge, in an attempt to account for the observed differences in SMases D activity.