Haemotoxicity of snakes: a review of pathogenesis, clinical manifestations, novel diagnostics and challenges in management

Abstract

Haemotoxicity is the most common complication of systemic envenoming following snakebite, leading to diverse clinical syndromes ranging from haemorrhagic to prothrombotic manifestations. Key haematological abnormalities include platelet dysfunction, venom-induced consumption coagulopathy, anticoagulant coagulopathy and organ-threatening thrombotic microangiopathy. Diagnostic methods include the bedside whole blood clotting test, laboratory coagulation screening and other advanced methods such as thromboelastogram and clot strength analysis. The primary management strategies are venom neutralisation with antivenom and correction of coagulopathy with blood component transfusions, while options such as plasma exchange are utilised in certain cases. Recent advancements in understanding the pathogenesis of haemotoxicity have facilitated the development of new diagnostic and treatment modalities. This review summarises current knowledge on the pathogenesis, diagnosis, clinical and laboratory manifestations and treatment of the haematological effects of snake envenoming. Furthermore, it highlights important challenges concerning diagnosis and management. Addressing these challenges is crucial for achieving the WHO's goal of reducing deaths and disabilities caused by snakebites by 2030.

Introduction

Snakebite envenoming remains a significant public health issue, particularly in tropical and subtropical regions, where it disproportionately affects rural and impoverished communities. Despite being classified by the WHO as a neglected tropical disease, snakebite envenoming continues to exert a considerable global burden, causing substantial morbidity and mortality. It is estimated that, annually, snakebites result in approximately 1.8–2.7 million cases of envenoming, leading to 81 000–138 000 deaths and leaving hundreds of thousands with permanent disabilities and disfigurement. The true burden is likely under-reported because of inadequate health infrastructure and limited access to medical care in many affected regions.¹ In 2019, South Asia bore the highest burden of snakebites, with 54 600 deaths, representing 86% of global snakebite fatalities. Sub-Saharan Africa had the second highest toll, with 6790 deaths. Within South Asia, India reported the greatest number of snakebite-envenoming deaths, totalling 51 100, followed by Pakistan.²

The pathogenesis of haemotoxicity induced by snake venoms is complex and multifaceted. Snake venoms are a potent mixture of enzymes, peptides and proteins that can disrupt haemostasis through various mechanisms. The resultant haemotoxic effects can manifest clinically as spontaneous systemic bleeding, local tissue necrosis and disseminated intravascular coagulation (DIC), which can be life-threatening if not promptly and effectively managed.³

Clinical manifestations of haemotoxic snakebite envenoming vary widely depending on the species of snake, the amount of venom injected and the site of the bite. Coagulopathy is a common feature and may progress to hypovolaemic shock secondary to significant blood loss and organ dysfunction like acute kidney injury (AKI), which could be due to hypoperfusion or direct toxic effects of snake venom. Timely diagnosis and intervention are critical to prevent these severe outcomes.⁴ Despite advances in understanding the pathophysiology of snakebite envenoming, the development of novel diagnostics and effective management strategies remains challenging. Laboratory diagnostic methods, such as clotting assays, often fall short in resource-limited settings where snakebites are most prevalent, calling for bedside tests.⁵ Diagnostic accuracy studies of these tests have yielded varying results. There is a pressing need for rapid, reliable and accessible diagnostic tools that can facilitate early and accurate identification of envenoming and its severity. Recent research efforts have focused on the development of point-of-care diagnostics, which hold promise for improving clinical outcomes.⁶

The management of haemotoxic snakebite envenoming primarily involves the administration of antivenom, which remains the cornerstone of treatment. However, antivenom availability, issues related to specificity, adverse reactions and high costs are common challenges to its use.⁷ Moreover, poor infrastructure in high snakebite-prevalent areas for immediate hospitalisation increases the risk of coagulopathy and severe systemic envenomings.⁸

Addressing these challenges is crucial to achieving the goals set forth by the WHO, which aims to reduce the number of deaths and disabilities caused by snakebites by 2030. It is in this context that this review is positioned and is intended to provide a comprehensive overview of the current state of knowledge regarding pathogenesis and clinical manifestations, while focusing on the diagnosis and management associated with haemotoxicity of snake venom.

Methods

Eligibility criteria

We formulated three search frameworks (see Supplementary Material Section 1 for frameworks and eligibility criteria for each framework): (i) pathogenic mechanisms, clinical and laboratory manifestations of haematological effects of snake envenoming; (ii) diagnostic tests to detect the haematological toxicity of snake envenoming; and (iii) treatment of the haematological manifestations of snake envenoming. Eligibility criteria for each framework are detailed in Supplementary Material Section 1. In brief, we searched for: (i) systematic reviews, meta-analyses and observational studies describing pathogenic mechanisms, clinical and laboratory manifestations among humans with haemotoxicity due to snake envenoming; (ii) diagnostic accuracy studies and comparative studies of diagnostic tests among humans with haemotoxicity due to snake envenoming, comparing two or more studies to diagnose haemotoxicity; and (iii) meta-analyses, systematic reviews, randomised controlled trials (RCTs), quasi-experimental studies or other comparative studies reporting efficacy (with or without safety) outcomes in treating humans with haemotoxicity due to snake envenoming.

Information sources and search strategy

The search process is summarised in Figure 1.⁹ Potentially eligible studies were identified by an electronic database search of PubMed and CENTRAL databases (see Supplementary Material Section 2 for the detailed search strategy). We searched for records published from 1 January 2000 to 31 January 2024 to identify recent developments in the field. The search was limited to publications in the English language. We used the following keywords and their MeSH terms: ‘snake bite’, ‘snake envenoming’, ‘haematological toxicity’, ‘coagulopathy’, ‘diagnosis’, ‘treatment’, ‘antivenom’, ‘blood component transfusion’, ‘plasma exchange’ and ‘management’.

Selection of studies

Titles and abstracts of selected records were screened independently by two authors (BYA and HAD), using the Rayyan web-based literature screening tool (RAYYAN, Cambridge, USA).¹⁰ Full texts of selected records were screened by the same two authors. Conflicts were resolved by discussion and, when needed, through discussion with a third reviewer (PNW or PACDP). Additional publications were identified through forward and backward citation tracking.

Data extraction and synthesis of findings

Data collection for studies meeting the inclusion criteria after the full-text review was based on the following topics: pathogenesis and clinical manifestations, diagnosis, treatments and management and novel diagnostic/treatment methodologies. HAD, BYA, PNW and PACDP performed the data extraction. Statistical analysis was not employed as this review did not include a meta-analysis. A descriptive synthesis of the findings is presented.

First, an overview of haemotoxic snake species is provided, leading to a section on the pathogenic attributes of snake venom on the haematological system. The translation of these pathogenic mechanisms is then linked to clinical manifestations, diagnosis and management modalities.

Results

Haemotoxic snake species

Vipers, Australasian elapids and colubrids are the common snake species known to cause haemotoxicity.^11,12 These haemotoxic snake species and their respective venom actions are summarised in Table 1. They are widely distributed across the continents of Asia, Australia and Africa. Additionally, Cerastes species, in the Middle East, and Vipera species and Crotalus species in North and South America, are known to cause coagulopathy. The disease mechanisms and severity vary among and depend on the dose of venom inoculated. Even within a snake species, the effects of venom can change based on habitat prey type and its escape capacity.¹³

Among these species, coagulopathy and haemorrhage are significantly observed in the Viperidae family because of the effect of disintegrins and haemorrhagins. Some species may cause rare and life-threatening complications such as DIC and thrombotic microangiopathy (TMA).¹⁴ The variability in venom effects underscores the importance of understanding the specific characteristics of the venom of each snake species.

Pathogenesis

The pathogenesis of snakebite coagulopathy involves several mechanisms, depending on the nature of the toxins present in the snake venom. These can include procoagulant enzymes, anticoagulant proteins and fibrinolytic enzymes, as well as direct cytotoxic effects on blood cells and the endothelium. The major pathways include procoagulant effects, anticoagulant effects, fibrinolytic effects, platelet effects and vascular effects (Figure 2).

Snake venom contains two types of coagulant toxins: procoagulant toxins that activate the clotting cascade and anticoagulant toxins that inhibit it. Venom-induced consumptive coagulopathy (VICC) arises from the consumption of clotting factors due to these procoagulant toxins. Prothrombin activators, which are snake venom serine proteases (SVSPs), convert prothrombin to thrombin (SVSPs and factor Va for optimal activity in Australian elapid snakes), while snake venom metalloproteinases (SVMPs) of Viperidae (such as Ecarin in E. carinatus) and carinactivase-1 (SVMPs that require Ca²⁺ to function) convert activated prothrombin to meizothrombin. Excessive activation of Factors V and VIII leads to their depletion, and the subsequent formation of fibrin depletes fibrinogen, ultimately resulting in bleeding.^15,16 Procoagulant toxins, such as Factor V and Factor X activators from Daboia russelli, the Echis genus and Australian elapids, are associated with a high degree of coagulopathy and haemorrhage, contributing to multiple systemic effects alongside other venom toxins.¹⁷ Thrombin-like enzymes (TLEs) in the snake species Borthrops, Hypnale and Crotalus consume fibrinogen directly without activating the clotting pathway. TLE is a zinc metalloproteinase that cleaves either alpha or beta chains of fibrinogen, forming unstable fibrin clots¹⁸ (Figure 3).

Venom proteomic assays have revealed that disintegrins can bind to glycoprotein IIb/IIIa, which is the platelet fibrinogen receptor, and prevent fibrinogen–platelet binding, thereby inhibiting platelet aggregation. In addition, phospholipase A2, C-type lectins and other platelet aggregation inhibitors can result in a risk of bleeding, leading to thrombocytopenia.¹⁴ By contrast, certain C-type lectins (convulxin in Bothrops species and alboaggregin in Crotalus species), metalloproteinases such as bothrojaracin in Bothrops jararaca and TLEs in Hypnale species, act as platelet aggregation activators in snake venom, leading to the formation of clots, which results in critical clinical manifestations such as deep vein thrombosis, pulmonary emboli, cerebral emboli and myocardial infarction.^19,20

Anticoagulant proteins and serpin inactivators cause inhibition of coagulation cascade. Serpins (serine protease inhibitors) in human blood act as potent inhibitors of serine protease toxins. By inhibiting these proteases, serpins can counteract the toxic effects of the venom on various physiological processes, including blood coagulation, inflammation and tissue damage. However, serpin inactivators in snakes inhibit the function of serpins, thereby affecting their regulatory roles in proteolysis.²¹

However, the concept of ‘serpin inactivators’ specifically in snake venom is less commonly discussed than the roles of serpins and other protease inhibitors in the venom. If serpin inactivators are identified, they would likely function by binding to serpins and preventing their interaction with target proteases, or by degrading/modifying serpins to reduce their inhibitory function. The direct identification and characterisation of specific serpin inactivators in snake venom are limited, highlighting an area for further research.

Figure 3 provides an overview of the toxic effects of venom that contribute to the pathogenesis of coagulopathy associated with snakebite envenoming. In-depth reviews of pathogenic mechanisms have been published.^11,14

Clinical and laboratory manifestations

Overall, 30–70% of snakebite victims develop some form of haemotoxicity.^22,23 The most common overt clinical manifestation is bleeding and this develops in 20–35%.^24,25 This can take the form of prolonged bleeding from the bite site, spontaneous mucocutaneous bleeding (ecchymoses, nasal or gum bleeding) or more severe visceral bleeding (e.g. bleeding in the gastrointestinal tract, urinary tract or visceral haematoma).²⁶ About 1–20% of patients may develop major bleeding (life-threatening or requiring blood transfusion).^22,24 The key pathogenic mechanisms involved are thrombocytopenia, which affects 10–60% of snake-envenoming cases,²⁷ and VICC, which affects 30–90%.²⁸ DIC and TMA are rare, affecting 5–10% of patients. But these lead to widespread clotting, organ dysfunction resulting from microcirculatory failure and subsequent bleeding caused by the consumption of clotting factors.¹¹ Table 1 provides a summary of clinical manifestations by snake species. Key clinical syndromes of snake venom-induced haemotoxicity and their laboratory manifestations are summarised in Table 2.

VICC

VICC results from the consumption of clotting factors caused by the activation of the clotting cascade by procoagulant toxins in the venom (Table 1). This leads to widespread formation of clots and subsequent depletion of clotting factors (Factor V and VIII), resulting in severe bleeding.^12,16 VICC is characterised by prolonged bleeding times, prothrombin time (PT) and activated partial thromboplastin time (aPTT).²⁹ VICC manifests rapidly within hours of a snakebite and resolves within 24–48 h without any fibrin deposition.¹⁷ However, in certain patients, delayed onset of VICC, longer duration and recurrence after initial resolution with antivenom, have been described.³⁰ Slow release from subcutaneous inoculation, sustained release of toxins distributed to deep tissues and redistribution of toxins from tissues to plasma space, longer half-lives of toxins outlasting antivenom and underdosing of antivenom are possible explanations.³¹

Anticoagulant coagulopathy, which results from toxins with direct inhibitory effects on clotting cascade, often causes a deranged clotting profile without clinically significant bleeding. This is seen with Australian black snake (Pseudechis spp.), Mulga snake (Pseudechis australis) and some cobras.³²

Often, these phenomena are clinically silent, but significant delayed bleeding has also been reported in rare instances.³³ Re-administration of antivenom was employed in several patients and resulted in rapid correction of clotting functions; its clinical significance remains unclear.³⁴ In some studies, long half-life F(ab’)2 antivenom has been used successfully to prevent recurrent coagulopathy.³⁵

DIC

By contrast, DIC is observed in patients with severe coagulopathy, with the features of fibrin deposition, evidence of systemic microthrombi, endothelial injury and end organ injury with a higher mortality rate. It is often characterised by prolonged PT and aPTT, thrombocytopenia, high D-dimer and fibrin degradation product levels and very low fibrinogen levels, resulting in incoagulable blood.^36–38

Thrombin generation in DIC is mediated by tissue factor/factor VIIa pathway. This cannot be corrected by the normal anticoagulant system of the body; depression of the fibrinolytic system leads to impaired fibrin removal. Hence, in such patients, excessive blood clotting and vascular damage resulting in internal haemorrhage has a protracted resolution. Similar events are not observed in VICC, which provides further evidence that DIC is a different entity to VICC. As a result of severe coagulopathy, fatal conditions such as pulmonary haemorrhage, intracranial haemorrhage and cardiac sequalae are reported following hump-nosed viper (HNV) envenoming.^12,39–41 Hence, VICC and DIC are not similar.

TMA

Snake venom-induced TMA is now a well-recognised entity.³⁶ The exact mechanism is uncertain,^16,32 but probably involves microvascular thrombosis, as demonstrated in postmortem studies of renal tissue.⁴² TMA is now defined as a triad of thrombocytopenia, microangiopathic haemolytic anaemia (MAHA) with >1% schistocytes in peripheral blood smear, with or without AKI. TMA occurs in a subset of patients with VICC. It is known that VICC occurs within hours of the bite and resolves before TMA develops. However, we believe that more advanced methods need to be employed to understand the exact pathophysiology of TMA linking to VICC and AKI.

Classic thrombotic thrombocytopenic purpura (TTP) is a prototypic TMA with predominant neurological involvement and marked ADAMTS13 deficiency. However, there is no evidence of decreased levels of ADAMTS13 or any other complements (C3, C4) related to snakebite-associated TMA. Hence, TMA associated with snakebite is a secondary condition, and distinct from TTP.²⁸

TMA occurs in envenoming by several haemotoxic snake species, but only in a small subset of patients. Australian elapidae (Pseudonaja spp., Oxyuranus scutellatus, Notechis scutatus and Tropidechis carinatus) and some snakes in the Viperidae family (Daboia spp., Cerastes cerastes, Proatheris superciliaris, Hypnale hypnale, Echis carinatus, Echis coloratus, Bitis arietans) are responsible for causing TMA³⁶ (Table 1). Therefore, it is unlikely that TMA is mediated by a separate toxin. Whether different doses of toxin have TMA-inducing properties is uncertain. Why TMA in snakebite is almost entirely confined to renal tissue also remains an unanswered question.

Vascular endothelial basement membrane damage is a key feature of snakebite-associated TMA. Metalloproteinases or haemorrhagins in snake venom make small vessels vulnerable to rupture and haemorrhage. Those act by degrading collagen, gelatin, elastin, laminin, fibronectin and thrombospondin in the vessel wall, and may contribute to significant bleeding, even with minor clotting factor deficiency.⁴³ Moreover, mechanical red cell fragmentation is universally present in all snakebite-associated TMA patients.⁴⁴ Haemolysis appears to be an early event in TMA, as a recent study demonstrated high circulating microvesicles (MV) derived from injured red cells early in the disease to predicted progression to overt TMA.⁴⁵ In its severest form, TMA causes extensive red cell fragmentation, severe thrombocytopenia and acute renal impairment. It differs from DIC as the former does not lead to multiorgan dysfunction, a major manifestation of DIC. Although TMA itself does not cause derangement of coagulation profile, it may overlap with VICC, thus mimicking DIC (microangiopathic haemolysis with deranged coagulation).³⁶

End organ injury due to haemotoxicity

The above-mentioned haematological effects can result in a spectrum of clinical effects ranging from asymptomatic coagulopathies to severe organ injury. The most common cause of organ injury secondary to haemotoxicity is haemorrhage causing hypovolaemic shock. Endothelial injury as a direct effect of toxins or secondary to inflammatory response of the host may lead to a systemic capillary leakage syndrome resulting in hypovolaemic shock.²⁴ Other mechanisms of hypotension and shock after snakebite are myocardial infarction, cerebrovascular ischaemia, AKI,⁴⁶ hypocortisolism due to pituitary or adrenal ischaemia or haemorrhage⁴⁷ and autonomic dysfunction.⁴⁸

Diagnostic evaluation

The 20-min whole blood clotting test (WBCT), PT, aPTT and full blood count are considered essential initial laboratory tests for determining snakebite coagulopathy. Although thrombin time and fibrinogen assays are not routinely performed, they are valuable in assessing the severity of coagulopathy, such as in cases of DIC. A blood smear can provide crucial information for diagnosing TMA. Additionally, thromboelastometry studies, which are more sensitive, can offer a clearer picture of haemotoxic envenoming following a snakebite (Figure 4).^5,49–51 The sections below provide more information on these test modalities and VICC results from the consumption of clotting factors due to the activation of the clotting cascade by procoagulant toxins in the venom (Table 1). This leads to widespread formation of clots and subsequent depletion of clotting factors (Factor V and VIII), resulting in severe bleeding.^12,16

Full blood count and cell indices

Complete blood count is useful for the detection and monitoring of haemoglobin and platelet counts as well as for leucocyte response. Neutrophil leucocytosis is an indicator of systemic envenoming and predictor of worse outcomes, including secondary infections.^52,53 Furthermore, the delta neutrophil index (marker of immature neutrophil percentage) was shown to predict coagulopathy.⁵⁴ Distinguishing between leucocytosis due to injury/inflammation and that caused by the effects of snake venom toxins requires a comprehensive evaluation of the clinical context, differential white blood cell counts, cytokine profiles and overall patient presentation.

The neutrophil-to-lymphocyte ratio and platelet-to-lymphocyte ratio may be useful as inflammatory markers of systemic inflammation followed by snakebite. Moreover, mean platelet volume (MPV), which is related to platelet activation, may also indicate inflammation. During severe inflammation, MPV decreases due to increased consumption of platelets at inflammatory sites.⁵⁵

Köse et al. investigated the effectiveness of platelet distribution width (PDW) and PDW-to-lymphocyte ratio (PDWLR) in predicting the severity of envenoming and clinical outcomes in snakebite patients. Envenoming graded as 0 or 1 was categorised as the none/minimal group, while those graded as 2 or 3 were placed in the moderate/severe group. PDW and PDWLR were significantly higher in the moderate and severe envenoming group (p<0.001) and correlated with more severe envenoming. Blood product replacement, thrombocytopenia, haematologic abnormalities, advanced local findings, compartment syndrome/fasciotomy, antivenom dosing and moderate/severe envenoming were found to be associated with a PDWLR>6.15 (p<0.05), while multivariate analysis identified PDWLR as an independent predictor of severe envenoming. Therefore, PDWLR may be used to predict severe envenoming and adverse outcomes.⁵⁶

Thrombocytopenia is a common manifestation seen in VICC patients. It is particularly common in snakebites in North and South America. In fact, it is the most common haematological manifestation after crotalid bites in the Americas, affecting 60% of victims.²⁷ Thrombocytopenia, along with hypofibrinogenaemia, was the strongest factor predicting severe haemotoxicity and systemic envenoming.^57,58 The exact mechanism of thrombocytopenia is not certain, but the action of phospholipase-A2 resulting in membrane injury, consumption due to intravascular thrombosis following endothelial dysfunction, induction of apoptosis through reactive oxygen species and matrix metalloprotease-mediated mitochondrial apoptotic mechanisms, are possible mechanisms implicated in the lowering of platelet number.^59,60

Bedside clotting time tests

Currently, the 20-min WBCT is the most widely used method to detect haemotoxicity. Five millilitres of the patient's blood are placed in a clean glass tube and allowed to stand undisturbed for 20 min. The test is considered positive if a clot does not form after 20 min.⁶¹ The performance of this test has shown marked variation across studies. A meta-analysis of 12 comparative studies concluded that the 20-min WBCT is a useful bedside test, with a sensitivity and specificity for detecting International Normalised Ratio (INR)>1.4 of 84% (95% CI 61 to 94%) and 91% (95% CI 76 to 97%), and for detecting fibrinogen concentrations of <100 mg/dL of 72% (95% CI 58 to 83%) and 94% (95% CI 88 to 98%), respectively.^62,63 However, >20% of patients eligible for antivenom could be missed.⁶⁴ It is recommended that if a 20-min WBCT is positive without clinical bleeding, PT/INR is needed prior to antivenom administration due to high false positive rates.⁶⁵

While a 20-min WBCT is sensitive enough to detect moderate and severe coagulopathies from haemotoxic snakebites, it often misses mild coagulopathy, such as that caused by Hypnale spp. Additionally, the WBCT underperforms in diagnosis of VICC in patients with Russell's viper envenoming, failing to identify up to two-thirds of coagulopathic patients.^39,62,66

Several variants of the WBCT have been reported. The 15-min WBCT showed superior sensitivity (47%) compared with 20- and 25-min WBCT tests (30–35%) for detecting VICC (defined as INR>1.5) after snakebite.⁶⁶ Additionally, there may be discrepancies between the 20- and 30-min WBCT in detecting the resolution or recurrence of coagulopathy. Performing an INR test can help confirm coagulopathy and prevent unnecessary treatment.⁶⁷

The Lee–White clotting time test is another bedside screening tool for coagulopathy. Here, 1 mL of venous blood is drawn into a glass syringe and transferred to a glass tube with a diameter of 8 mm. This tube is inverted every 30 s. The first timepoint at which the blood does not flow is considered the end of the test (i.e. clot formed).⁶⁸ A modified Lee–White clotting test has been used in South America: 1 mL of venous blood, drawn by a plastic syringe, is immediately transferred to a glass tube, then left to stand for 5 min at room temperature; it is gently tilted every minute to examine for clot formation. The test is deemed normal if a clot develops within 9 min.⁶⁹ Its sensitivity and specificity to diagnose coagulopathy (reference test fibrinogen concentration <200 mg/dL) was 78% and 40.7%, respectively.⁶⁹ A study from India reported using another modified version of the Lee–White clotting test. A venous blood sample was transferred to three glass tubes, which were assessed every minute by tilting to 60 degrees for a visual determination of clot formation, starting with one tube then sequentially examining the other tubes once the first tube has clotted. Once all three tubes are clotted (the time to this point is regarded as the clotting time), then the last tube is examined every minute for 10 min to detect clot lysis. Delayed clot formation (no clot within 20 min) or clot lysis within 20 min is considered a positive test. This test was more sensitive in detecting VICC compared with the 20-min WBCT among snakebite patients (85% [95% CI 61.1 to 96.0%] vs 55.0% [95% CI 32.0 to 76.2%]) using PT/INR>1.4 as the reference test.⁷⁰

These differences in bedside clotting time tests probably reflect variations in methods, in particular the types and nature of tubes used and subjective interpretation of clot formation. However, in a small-scale study comparing the use of syringe and bottle, it was concluded that both methods had comparable sensitivities (88.9% and 83.3%) and specificities (82.4% and 90%).⁶¹,⁶⁷ To complicate issues further, studies evaluating these tests are frequently limited by small sample size, uncertainties over case definitions, the use of arbitrary observation times and inconsistencies in the reference test.⁵¹

PT/INR

PT-INR is the most widely used laboratory test for the diagnosis of coagulopathy, and it is the most commonly reported reference test for evaluating bedside diagnostic tests to detect coagulopathy following snakebite. However, with the advent of more sophisticated methods like rotational thromboelastometry, the limitations of PT/INR, particularly in detecting mild coagulopathy in patients with snake envenoming, have been exposed, leaving a gap for a more sensitive test. PT/INR, as well as the other traditional coagulation tests (e.g. aPTT, thrombin time), is unable to fully explain the mechanisms involved in VICC or may be too slow to become deranged, thus limiting its clinical utility.^5,71

Modern point-of-care devices for INR testing are known to give rise to false positive results. These devices measure clotting based on an electrochemical reaction that depends on active thrombin formation rather than fibrinogen formation, which is the endpoint in laboratory PT tests. It is thought that prothrombin activators in snake venom can activate thrombin, thus giving rise to a normal PT/INR test result. Therefore, in VICC patients, a point-of-care INR can appear normal, whereas laboratory tests would show an unrecordably prolonged PT/INR.⁷² Moreover, it has been reported that reagents used for routine aPTT testing are relatively insensitive to the anticoagulant effects of snake envenoming, hence extra consideration should be taken when interpreting the results.⁷³

Serial analysis of laboratory parameters, including INR and aPTT, at admission, 1, 6 and 12 h postbite, aids in the early detection of VICC and systemic envenoming. This timely monitoring facilitates the early administration of an adequate dose of antivenom.⁷⁴

D-dimer, fibrinogen assays and clotting factor levels

An increase in D-dimer and decrease in the fibrinogen level are the earliest detectable haematological changes in patients with consumption coagulopathy due to snake envenoming, followed by prolongation of prothrombin time, aPTT and thrombin times.⁷⁵ Although D-dimer testing is not frequently performed in under-resourced clinical settings, it could aid in the early diagnosis of haemotoxic snake envenoming and facilitate the prompt administration of an adequate dose of antivenom.³⁸ It is useful to distinguish consumption coagulopathy from other types of coagulopathies.⁵¹ Additionally, deficiencies in factors II, V, X, VIII and fibrinogen can help determine the coagulopathy associated with haemotoxic snakebites.^16,37,76 However, data on their diagnostic accuracy and use in routine clinical practice are limited.

Thromboelastography and rotational thromboelastometry

Two novel methods, thromboelastography (TEG) and rotational thromboelastometry (ROTEM), provide qualitative and quantitative assessments of all three pathways of coagulation cascade through clot waveform analysis.^5,77 Two studies were recently conducted on Sri Lankan vipers using ROTEM. One study on Sri Lankan Russell's viper showed high sensitivity (93%), specificity (86%) and a positive predictive value (98%) in predicting patients who required antivenom treatment.⁷⁸ The second study on HNV bites showed that ROTEM parameters were more likely to pick up subtle changes in coagulation compared with conventional coagulation tests.⁷⁹ ROTEM findings were sensitive for the detection of hypocoagulation in viper bite patients in Vietnam, showing moderate correlation with standard coagulation parameters and platelet counts. ROTEM also identified patterns of generalised clotting factor deficiency or isolated fibrinogen deficiency, which were not detected by standard clotting tests.⁸⁰ Moreover, TEG and ROTEM can serve as alternatives to standard care. If TEG or ROTEM results are within normal limits, patients can be monitored clinically without the need for repeat coagulation studies.⁸¹

Both TEG and ROTEM have demonstrated effectiveness in detecting hyperfibrinolysis and patterns indicative of generalised clotting factor deficiencies or isolated fibrinogen deficiencies, conditions that standard clotting tests do not provide. These advanced assays provide dynamic, real-time information about haemostatic disorders induced by snake envenoming quickly, often within the first 24 h postbite.^77,80 While conventional coagulation tests primarily evaluate the initiation of clotting, they do not fully reflect the haemostatic capacity of an individual in vivo. By contrast, TEG and ROTEM offer comprehensive insights into the dynamics of clot development, stabilisation and dissolution, closely mirroring the actual haemostasis process within the body.⁸⁰ Clinically, TEG and ROTEM can identify haemostatic disorders early and guide treatment with antivenom.⁸² Abnormal ROTEM results can guide clinicians on which blood components (including fresh frozen plasma [FFP]) to transfuse based on identified deficiencies in clotting factors or fibrinogen, ensuring targeted and effective treatment. Although the cost of TEG limits its adoption in resource-poor settings, the detailed information provided by both TEG and ROTEM can significantly enhance the understanding of snakebite-related coagulopathy.⁸³

Complement testing

A systemic inflammatory response is mounted immediately after exposure to snake venom. Endothelial injury and innate immune response results in the release of many cytokines. Toxins activate the complement system via the classical pathway.⁸⁴ Often, this causes systemic symptoms and a transient pro-inflammatory state with minimal consequences. IL-6, IL-10, TNFα, C3a, C5a, C4a, mast cell tryptase and histamine levels in blood were found to be increased following snakebite envenoming.⁸⁵ However, those levels correlated with non-specific systemic symptoms but not with coagulopathy or neurotoxicity.⁸⁵ At least in severe cases, this extensive cytokine surge may contribute to organ injury.

Flowcytometry

MV are small membrane-bound particles shed from cells, often as a result of cell activation or damage.⁸⁶ In flow cytometry analysis of circulating MV in snakebite patients, those with MAHA had significantly higher red cell MV compared with those without MAHA, while platelet MV levels showed no significant difference. Additionally, endothelial MV were reduced in all snakebite patients compared with controls. Measuring red cell MV at presentation could serve as a predictive marker for MAHA in snakebite patients.⁴⁵ However, these analyses are currently limited to research settings.

Management

Management of snake venom haemotoxicity is mostly based on conventional approaches with a limited evidence base. Antivenom therapy and replacement of deficient clotting factors constitute the mainstay of therapy. A summary of management strategies for haemotoxic effects of snakebite envenoming is presented in Table 3.

Antivenom

Antivenom sera comprise antibodies that neutralise the toxins in snake venom. Most antivenom products are derived from sensitised equine, porcine or sheep sera. Novel products manufactured using recombinant technology are under investigation.¹¹⁴

A few RCTs have evaluated the effectiveness of polyclonal antibody antivenoms against West African carpet viper-induced coagulopathy and the local effects of green pit vipers, as well as antivenom dosages for neurotoxic snakebite envenoming. However, trials to demonstrate the benefits of antivenom in the treatment of coagulopathy following snakebite envenoming, especially by viper species in the Asian region, are limited.^115–118 This is an important gap in the evidence as these species account for widespread morbidity and mortality. In a recent review of observational cohort studies, the authors concluded that the benefit of antivenom is uncertain in neutralising systemic effects, including common coagulopathy.^12,119

The observed variability in the efficacy of antivenom could be due to several factors. First, a uniform dosing regimen may not be effective against larger amounts of venom. Some studies have shown improved outcomes with higher loading doses and sustained infusions of antivenom,^120,121 while others found no significant differences across various doses or timings of administration.¹¹⁹ Second, the antibodies in antivenom may not neutralise all the toxins in the venom. This limitation is particularly evident when antivenom for one snake species is used to treat envenoming by a different species based on presumed cross-reactivity from in vitro studies. Even among snakes with similar toxins, antivenom efficacy can vary due to co-factors affecting toxin activity.¹²² Third, the optimal timing of antivenom administration remains poorly understood.¹²³ Fourth, systemic envenoming can alter vascular permeability and organ function, affecting the pharmacokinetics of antivenom distribution and elimination. These physiological changes can result in unpredictable clinical responses among different individuals.¹²⁴ These complexities highlight the challenges in ensuring the effectiveness of antivenom treatment in early correction of haemotoxicity.

Furthermore, there appears to be significant diversity in response to antivenom across geographies. VICC due to African viper bites responds rapidly to antivenom while it is less dramatic in Australian snakebite VICC.^125,126

Hence, even although polyvalent antivenoms are used globally, both interspecific and intraspecific variations in venom compositions in different geographical sites can affect the neutralisation capacity of antivenoms. Therefore, antivenom efficacy tests, molecular studies and clinical trials are needed to evaluate the actual efficacy of available antivenoms on different snake species in different countries.^127,128

Another important determinant of the efficacy of antivenom treatment is the route of administration. Antivenoms are large molecules, such as F(ab')2 fragments or whole IgG, which are absorbed slowly via the lymphatics after intramuscular injection. This route has poor bioavailability, particularly after intragluteal injection, and blood levels of antivenom never reach those achieved through intravenous (direct bloodstream) administration.¹²⁹ In addition, a large-scale animal model study examining the effect of antivenom on the lymphatic absorption and pharmacokinetics of coral snake venom found that, although antivenom administered intravenously immediately neutralises venom in the bloodstream, this neutralisation is often slow and incomplete for venom absorbed by the lymph. Consequently, subjects may continue to experience systemic envenoming. Clinical data on human subjects are insufficient to recommend a specific route of administration for antivenom.¹³⁰

A major challenge with antivenom administration is the high risk of hypersensitivity reactions. Nearly 20–25% of patients develop acute hypersensitivity reactions, a major proportion of which are anaphylaxis.^131,132 Delayed hypersensitivity reactions, including sick sinus syndrome, affected 29% of patients in one series.¹³¹ Although various regimens of premedication have been attempted to mitigate the risk, results have been disappointing, except for the use of adrenaline.¹³² While there is no specific evidence on the effectiveness of antivenom on TMA, the use of antivenom remains the gold standard of care at the earliest sign of envenoming.¹³³

Replacement of clotting factors

The primary mechanism of coagulopathy in snakebite envenoming is consumptive coagulopathy. Therefore, it is possible to postulate that replacement clotting factors would mitigate the pathology. However, it also raises the theoretical risk of aggravating thrombosis, by fuelling the process. In a recent review, the authors concluded that replacement of clotting factors may accelerate recovery.¹² While one study showed that the benefit of FFP is minimal with early administration (within 4 h of the bite),¹⁰⁸ another showed that the earlier the administration then the better the outcome.¹¹⁰ A RCT on Daboia russelii envenoming indicated that FFP after antivenom administration did not hasten recovery of coagulopathy, whereas low-dose antivenom/FFP did not worsen VICC either, suggesting that low-dose antivenom is sufficient.¹⁰⁷ Low molecular weight heparin in addition to antivenom was tried in two randomised comparative studies assuming that suppression of activation of clotting cascade would improve outcome.¹³⁴ However, no added benefit was observed in either study.

In cases of Hypnale envenoming in South Asia, clinicians encounter significant challenges in patient management because of the lack of an available antivenom. Consequently, FFP has become the commonly employed treatment to prevent VICC and AKI in Hypnale bites when administered early. The authors used Hypnale species as a case study to illustrate the critical need for alternative treatments in regions where specific antivenoms are not accessible.¹³⁵ However, the administration of FFP in VICC associated with HNV envenoming found that, of the 20 patients who received FFP, only 55% experienced early correction of VICC, while the rest developed coagulopathy. Hence, the development of coagulopathy or death due to snakebite envenoming is unpredictable even if FFP is administered early.¹⁰⁶ In some patients, the coagulopathy was aggravated with the administration of FFP and a further elevation of D-dimer was detected, which suggested consumption of transfused clotting factors. Hence, the benefits of FFP in treating VICC are doubtful in envenoming by snakes with procoagulant venoms, such as HNVs.¹³⁶

The current evidence does not support routine administration of FFP or clotting factors in patients with VICC or the use of anticoagulants to prevent its progression. However, in the presence of major bleeding, clotting factor replacement should be of use in correcting coagulopathy, preferably following antivenom. In the absence of antivenom, the theoretical risk of worsening VICC with clotting factor replacement cannot be disregarded and further studies are required. Nevertheless, in the case of an unidentified snakebite and coagulopathy causing significant haemorrhage, clotting factor replacement remains a reasonable option.

Therapeutic plasma exchange

Therapeutic plasma exchange (TPE) has been used to treat TMA with varying degrees of success.^137-140 TPE in TMA helps to remove toxins that are bound to proteins, which cannot be cleared by haemoperfusion or dialysis, while reducing the toxin concentrations inside and outside the venous system.^111,141

The main drawback of TPE is the removal of antivenom during plasma exchange. Therefore, it is potentially useful for victims who have not received antivenom. Although some retrospective small-scale studies have suggested faster recovery,¹³⁸ no unequivocal evidence exists to support or refute the use of plasma exchange in TMA.¹² Unfortunately, although administration of TPE for TMA is still practised, currently there is no evidence to suggest that it reduces the risk of progression to chronic kidney disease (CKD), duration of hospitalisation, dialysis dependency, or improves the survival rate.¹¹² RCTs would be needed to determine the best treatment option for TMA, and to investigate the efficacy and safety of TPE on patients with TMA. This would enhance management of snakebites in those areas of the world most affected by snakebites with resource limitations.¹³³

Novel therapeutics

Photobiomodulation therapy.

Low-level laser therapy is a novel photobiomodulation therapy that is used to treat Bothrops atrox envenoming in reducing local manifestation. Bothrops venom was identified as a rapidly spreading venom around the bite site, causing tissue damage. A clinical trial has shown that the technique was feasible, safe and effective in reducing the local inflammatory responses. As secondary outcomes, photobiomodulation therapy has improved the Lee–White clotting time compared with patients who only receive antivenom treatment (43% vs 23%).¹⁴² In mice, photobiomodulation showed an effectiveness in reducing local effects, oedema and haemorrhage induced by Bothrops moojeni.⁹⁷

Oral small molecule inhibitors: varespladib and marimastat.

Next-generation snakebite therapies are being widely investigated, and recently an oral drug has been introduced to treat snakebite envenoming. The main aim of the use of antivenom as an oral preparation is to reduce the cost and to improve efficacy, as it can be distributed among communities with high exposure for snakebites, and thereby victims can ingest the drug as soon as they have been bitten by a snake. A few oral drugs, namely, marimastat, prinomastat hydrochloride, tanomastat, batimastat, doxycycline, dimercaprol, DMPS (2,3-dimercapto-1-propane-sulphonic acid sodium salt monohydrate), varespladib and nafamostat mesylate, are still under development and in the validation process for targeting snake venom SVMPs, which are responsible for haemorrhage and coagulopathy.¹⁴³

Varespladib is an innovative oral phospholipase A2 (PLA2) inhibitor. Snake venom-associated PLA2 has haemotoxic potential and can cause bleeding and effects on platelet aggregation. Varespladib works by blocking this enzyme's activity, thereby mitigating the local and systemic effects of the venom. Preclinical studies have shown that varespladib can effectively reduce venom-induced tissue damage and improve survival rates in animal models. It is now in phase 2 human clinical trials in the USA and India.^89,90

Marimastat is a matrix metalloproteinase inhibitor that degrades extracellular matrix components contributing to the tissue destruction and haemorrhage seen in snake envenomation. By inhibiting these enzymes, marimastat can help to preserve tissue integrity and reduce bleeding. This drug has shown potential in animal studies to lessen the severity of snakebite symptoms.^90,91

DNA-based studies against snakebite envenoming.

DNA-based approaches represent a cutting-edge frontier in the treatment of snakebite envenoming. These studies involve the use of DNA vaccines or gene therapy to induce the production of antivenom antibodies within the patient's own body. DNA vaccines work by introducing genetic material encoding venom antigens into the host cells, prompting the immune system to generate a protective response. This method can potentially offer a rapid and scalable way to produce antivenom without the need for complex manufacturing processes associated with traditional antivenoms. Additionally, gene therapy techniques aim to deliver genes encoding specific antivenom antibodies directly into patients, providing a continuous supply of therapeutic antibodies.^94,144

Radioactive ruthenium-based treatments.

The use of radioactive ruthenium compounds is a novel approach that leverages the unique properties of ruthenium to neutralise venom toxins. These compounds can be designed to bind selectively to venom components, and their radioactive properties enhance their ability to disrupt the function of these toxins. This treatment method offers a new mechanism to counteract envenomation that is different from traditional antibody-based antivenoms. While still in the experimental stages, radioactive ruthenium-based treatments could provide an additional tool in the arsenal against snakebite envenoming, especially in cases where other treatments are ineffective.⁹⁵

Monoclonal antivenoms.

Monoclonal antivenoms are produced using cloned immune cells that generate a single type of antibody, which can be highly specific to certain venom components. This specificity allows for more precise targeting of venom toxins, increasing the effectiveness of the treatment and reducing the likelihood of side effects associated with non-specific immune responses. Monoclonal antibodies can be engineered to bind strongly to specific venom proteins, neutralising their toxic effects. Theoretically these could then be used to target proteins with specific effects on the haemostasis pathway. Advances in biotechnology have made it possible to produce these antibodies in large quantities, and they are being explored as a complement to or replacement for traditional polyclonal antivenoms. Monoclonal antivenoms offer the potential for more consistent and reliable treatment outcomes for snakebite victims.^99,100

Recommendations and conclusions

1. Understanding the mechanisms of TMA: the exact mechanisms underlying snake venom-induced TMA are not fully understood. Advanced haematological and molecular techniques are needed to elucidate the pathophysiology and the link between VICC, AKI and TMA.

2. Development of predictive biomarkers: there is a need to develop and validate biomarkers that can predict early haemotoxicity. These biomarkers should be integrated into clinical decision-making tools to improve early diagnosis and management.

3. Regional variability in venom composition: the variability in venom composition across different regions affects clinical outcomes and treatment efficacy. Multicentre studies should investigate these regional differences and their clinical implications, leading to the establishment of region-specific treatment protocols.

4. Enhanced diagnostic technologies: advanced diagnostic technologies should be implemented in routine clinical practice to enhance the detection and monitoring of venom effects. These include both laboratory-based and point-of-care diagnostics.

5. Development of oral antivenoms: the development and clinical testing of oral antivenom formulations should be accelerated. These formulations should be approved through RCTs to ensure they are effective, affordable and accessible in high-risk communities.

6. Efficacy of TPE: limited evidence supports the use of TPE in treating snakebite-induced TMA. RCTs are required to evaluate the efficacy and safety of TPE, including its impact on long-term outcomes such as CKD and survival rates.

7. Biotechnological advances in therapeutics: modern biotechnological methods are needed to develop new therapeutic agents. Research should focus on monoclonal antibodies, small molecule inhibitors and synthetic biology to create targeted and efficient antivenom therapies.

8. International collaboration and funding: international partnerships and collaborations should be fostered to share knowledge, resources and technology, enabling better research and patient care. Increased funding and infrastructure development should be advocated in low- and middle-income countries to support snakebite research and improve clinical management.

By addressing these gaps, the understanding and management of snakebite-induced haemotoxicity can be significantly improved, aligning with the WHO 2030 road map for reducing the global burden of snakebites.

Authors’ contributions

HAD and BYA conducted the literature search. HAD, PACDP and PNW reviewed the selected literature. HAD, PACDP, PNW and BYA drafted the manuscript. CAG and LVG critically reviewed the manuscript structure and content. HAD and PNW are guarantors of the paper.

Acknowledgements

Not applicable.

Funding

None.

Competing interests

None.

Ethical approval

Not applicable.

Data availability

No new data were generated or analysed as this is a review article.

References