The ocular effects of spitting cobras: I. The ringhals cobra (Hemachatus haemachatus) venom-induced corneal opacification syndrome

Wilderness Medical Society Clinical Practice Guidelines for Treatment of Eye Injuries and Illnesses in the Wilderness: 2024 Update

A panel convened to develop an evidence-based set of guidelines for the recognition and treatment of eye injuries and illnesses that may occur in the wilderness. These guidelines are meant to serve as a tool to help wilderness providers accurately identify and subsequently treat or evacuate for a variety of ophthalmologic complaints. Recommendations are graded based on the quality of their supporting evidence and the balance between risks and benefits according to criteria developed by the American College of Chest Physicians.

The Ex vivo Eye Irritation Test (EVEIT) model as a mean of improving venom ophthalmia understanding

Snakes belonging to the genus Naja (Elapid family), also known as "spitting cobras", can spit venom towards the eyes of the predator as a defensive strategy, causing painful and potentially blinding ocular envenoming. Venom ophthalmia is characterized by pain, hyperemia, blepharitis, blepharospasm and corneal erosions. Elapid venom ophthalmia is not well documented and no specific treatment exists. Furthermore, accidental ejection of venom by non-spitting vipers, as Bothrops, also occurs. The Ex vivo Eye Irritation Test model (EVEIT) has enabled important progress in the knowledge of chemical ocular burns. Considering the lack of experimental animal model, we adapted the EVEIT to study venom ophthalmia mechanisms. Ex vivo rabbit corneas were exposed to venoms from spitting (Naja mossambica, Naja nigricollis) and non-spitting (Naja naja, Bothrops jararaca and Bothrops lanceolatus) snakes, and rinsed or not with water. The corneal thickness and the depth of damage were assessed using high-resolution optical coherence tomography (HR-OCT) imaging and histological analysis. All Naja venoms induced significant corneal edema, collagen structure disorganization and epithelial necrosis. Corneas envenomed by African N. mossambica and N. nigricollis venoms were completely opaque. Opacification was not observed in corneas treated with venoms from non-spitting snakes, such as the Asian cobra, N. naja, and the vipers, B. jararaca and B. lanceolatus. Moreover, Bothrops venoms were able to damage the epithelium and cause collagen structure disorganization, but not edema. Immediate water rinsing improved corneal status, though damage and edema could still be observed. In conclusion, the present study shows that the EVEIT model was successfully adapted to set a new experimental ex vivo animal model of ophthalmia, caused by snake venoms, which will enable to explore new therapies for venom ophthalmia.

Ophthalmic Injuries from a Spitting Chinese Cobra

A man sustained conjunctivitis of the left eye after being spat by the venom of a Chinese cobra (Naja atra). He received fluid irrigation, topical antibiotic and topical steroid treatment. The conjunctivitis resolved without sequalae after 4 days. Various treatment options are discussed.

How the Cobra Got Its Flesh-Eating Venom: Cytotoxicity as a Defensive Innovation and Its Co-Evolution with Hooding, Aposematic Marking, and Spitting

The cytotoxicity of the venom of 25 species of Old World elapid snake was tested and compared with the morphological and behavioural adaptations of hooding and spitting. We determined that, contrary to previous assumptions, the venoms of spitting species are not consistently more cytotoxic than those of closely related non-spitting species. While this correlation between spitting and non-spitting was found among African cobras, it was not present among Asian cobras. On the other hand, a consistent positive correlation was observed between cytotoxicity and utilisation of the defensive hooding display that cobras are famous for. Hooding and spitting are widely regarded as defensive adaptations, but it has hitherto been uncertain whether cytotoxicity serves a defensive purpose or is somehow useful in prey subjugation. The results of this study suggest that cytotoxicity evolved primarily as a defensive innovation and that it has co-evolved twice alongside hooding behavior: once in the Hemachatus + Naja and again independently in the king cobras (Ophiophagus). There was a significant increase of cytotoxicity in the Asian Naja linked to the evolution of bold aposematic hood markings, reinforcing the link between hooding and the evolution of defensive cytotoxic venoms. In parallel, lineages with increased cytotoxicity but lacking bold hood patterns evolved aposematic markers in the form of high contrast body banding. The results also indicate that, secondary to the evolution of venom rich in cytotoxins, spitting has evolved three times independently: once within the African Naja, once within the Asian Naja, and once in the Hemachatus genus. The evolution of cytotoxic venom thus appears to facilitate the evolution of defensive spitting behaviour. In contrast, a secondary loss of cytotoxicity and reduction of the hood occurred in the water cobra Naja annulata, which possesses streamlined neurotoxic venom similar to that of other aquatic elapid snakes (e.g., hydrophiine sea snakes). The results of this study make an important contribution to our growing understanding of the selection pressures shaping the evolution of snake venom and its constituent toxins. The data also aid in elucidating the relationship between these selection pressures and the medical impact of human snakebite in the developing world, as cytotoxic cobras cause considerable morbidity including loss-of-function injuries that result in economic and social burdens in the tropics of Asia and sub-Saharan Africa.

Pharmacokinetics of Naja sumatrana (equatorial spitting cobra) venom and its major toxins in experimentally envenomed rabbits

BACKGROUND

The optimization of snakebite management and the use of antivenom depend greatly on the knowledge of the venom's composition as well as its pharmacokinetics. To date, however, pharmacokinetic reports on cobra venoms and their toxins are still relatively limited. In the present study, we investigated the pharmacokinetics of Naja sumatrana (Equatorial spitting cobra) venom and its major toxins (phospholipase A2, neurotoxin and cardiotoxin), following intravenous and intramuscular administration into rabbits.

PRINCIPAL FINDINGS

The serum antigen concentration-time profile of the N. sumatrana venom and its major toxins injected intravenously fitted a two-compartment model of pharmacokinetics. The systemic clearance (91.3 ml/h), terminal phase half-life (13.6 h) and systemic bioavailability (41.9%) of N. sumatrana venom injected intramuscularly were similar to those of N. sputatrix venom determined in an earlier study. The venom neurotoxin and cardiotoxin reached their peak concentrations within 30 min following intramuscular injection, relatively faster than the phospholipase A2 and whole venom (Tmax=2 h and 1 h, respectively). Rapid absorption of the neurotoxin and cardiotoxin from the injection site into systemic circulation indicates fast onsets of action of these principal toxins that are responsible for the early systemic manifestation of envenoming. The more prominent role of the neurotoxin in N. sumatrana systemic envenoming is further supported by its significantly higher intramuscular bioavailability (Fi.m.=81.5%) compared to that of the phospholipase A2 (Fi.m.=68.6%) or cardiotoxin (Fi.m.=45.6%). The incomplete absorption of the phospholipase A2 and cardiotoxin may infer the toxins' affinities for tissues at the injection site and their pathological roles in local tissue damages through synergistic interactions.

CONCLUSION/SIGNIFICANCE

Our results suggest that the venom neurotoxin is absorbed very rapidly and has the highest bioavailability following intramuscular injection, supporting its role as the principal toxin in systemic envenoming.

Potential targets aimed at by spitting cobras when deterring predators from attacking

When threatened, spitting cobras eject venom towards the face of an aggressor. To uncover the relevant cues used by cobras for face recognition we determined how often artificial targets equipped with or without eyes elicited spitting behavior. In addition, we measured whether and how target shape and size influenced the spitting behavior of cobras. Results show that oval- and round-shaped targets were most effective, while triangles with the same surface area as oval 'face like' targets hardly elicited spitting. The likelihood of spitting depended on neither the presence, the spatial arrangement (horizontal or vertical) nor the surface texture (shiny or matt) of glass eyes. Most likely, cobras do not specifically aim at the eyes of an offender but at the center of the body part closest to them. As this is usually the face of an animal, this strategy will result in at least one eye of the offender being hit most of the time.

Wilderness Medical Society practice guidelines for treatment of eye injuries and illnesses in the wilderness

A panel convened to develop an evidence-based set of guidelines for the recognition and treatment of eye injuries and illnesses that may occur in the wilderness. These guidelines are meant to serve as a tool to help wilderness providers accurately identify and subsequently treat or evacuate for a variety of ophthalmologic complaints. Recommendations are graded based on the quality of their supporting evidence and the balance between risks and benefits according to criteria developed by the American College of Chest Physicians.

Functional bases of the spatial dispersal of venom during cobra "spitting"

Spitting cobras expulse venom toward the face and/or eyes of potential predators as part of their defensive repertoire. Evaluating the accuracy of the cobras is difficult because the spit venom does not land as a point but rather is distributed, in some cases widely, in complex geometric patterns on the surface of the target. The purpose of this study was to explore the functional bases of the venom's spatial distribution. Using a combination of spatial analysis of "caught" venom, morphology, high-speed digital videography, and electromyography (EMG), three hypothesis were evaluated. Two of these hypotheses--that the spatial distribution was due to differential venom pressure produced by the contractile activity of the adductor mandibulae externus superficiali and that the spatial distribution was produced by the morphology of the venom canal within the fang-were both rejected. The third hypothesis--that the spatial distribution was due to rapid rotational movements of the head about the vertebral column--was supported by analyses of EMG activity within the cervical axial muscles and by predictions of venom-distribution patterns based on these cephalic displacements. These results suggest that the ability to "spit" venom is a unique suite of specializations involving both the axial and the cephalic systems.

Effects of Topical Heparin, Antivenom, Tetracycline and Dexamethasone Treatment in Corneal Injury Resulting from the Venom of the Black Spitting Cobra (Naja sumatrana), in a Rabbit Model

BACKGROUND

The Naja sumatrana cobra can spit venom in defense and may result in permanent blindness. The study sought to determine the efficacy of topical heparin, Haffkine antivenom, tetracycline and dexamethasone.

MATERIALS AND METHODS

Male New Zealand White Rabbits were used. Pooled venom was frozen at -30 degrees C. 0.05 mL of 20 times dilute venom was introduced into the conjunctiva, in groups of three rabbits randomly. Heparin at 5000 IU/mL, Haffkine antivenom or saline control was administered repeatedly on each rabbit's eye over 158 minutes, after a specified delay. In other groups, 1% tetracycline, 0.1% dexamethasone or a placebo ointment was applied and repeated at 24 and 48 hours. All the rabbits were assessed after 24, 48, 72 hours, one and two weeks by an ophthalmologist blinded to the treatment arms.

OBSERVATIONS

Following ocular envenomation, there was immediate blepharospasm, lacrimal secretions, redness and chemosis; more intense in the normal saline group. The Roper-Hall grades improved, corneas re-epithelialized and inflammation quietened in the heparin and antivenom-treated rabbit eyes compared to controls. Scarring appeared from the first week, but ameliorated in the heparin and antivenom groups. Heparin treatment remained efficacious up to four minutes delay. The tetracycline, dexamethasone and placebo groups had worsening Roper-Hall trends, greater corneal epithelial loss, inflammation and scarring. Combined heparin-tetracycline therapy was as efficacious with heparin alone.

CONCLUSION

Topical heparin or antivenom therapy significantly improved overall outcomes in rabbit corneas exposed to Naja sumatrana venom, compared to tetracycline, dexamethasone and controls. Heparin treatment remains efficacious up to 4 minutes delay.

The spitting behavior of two species of spitting cobras

Spitting cobras defend themselves by spitting their venom in the face of a harasser. Although it is common belief that spitting cobras direct their venom at the eyes of an aggressor, this has never been investigated. Here, we show that the spitting act of cobras (Naja nigricollis and N. pallida) can readily be triggered by a moving human face or by a moving real size photo of a human face. In contrast, a stationary human face (real or photo) or a moving or stationary human hand does not trigger the spitting act. If threatened, spitting cobras aim their venom, ejected either in two distinct jets (N. pallida) or in a fine spray (N. nigricollis), either between the eyes or at one eye. In both cobra species investigated, the width and height of the area hit by the venom was independent of eye distance (test range 5.5 cm and 11 cm). During the spitting act the cobras performed fast undulating head movements that lead to a larger distribution of their venom. This behavior increases the probability that at least one eye of the aggressor is hit.

The buccal buckle: the functional morphology of venom spitting in cobras

Multiple radiations of Asiatic and African cobras have independently evolved the ability to expel their venom as a pressurized horizontal stream, a behavior commonly referred to as spitting. Though the unique fang morphology of spitting cobras is well known, the functional bases of venom spitting have received little attention. The combined results of gross and microscopic morphology, high-speed digital videography, experimental manipulations of anesthetized cobras and electromyography reveal a two-part mechanism for spitting venom. Contraction of the M. protractor pterygoideus (PP) causes displacement and deformation of the palato-maxillary arch and fang sheath;ultimately this displacement removes soft tissue barriers to venom flow that are normally present within the fang sheath. The M. adductor mandibulae externus superficialis (AMES) is activated simultaneously with the PP; the AMES increases venom pressure within the venom gland, propelling a stream of venom through the venom duct and out the fang. The displacements of the palato-maxillary arch, which form the first part of the spitting mechanism,are very similar to the motions of these bones during prey ingestion (the pterygoid walk), suggesting that venom spitting may have evolved from a specialization of prey ingestion, rather than prey capture.

Taiwan’s venomous snakebite: epidemiological, evolution and geographic differences

Located at the juncture of tropical and subtropical regions, Taiwan has a warm and humid climate with abundant precipitation and food, which coupled with the island's diverse vegetation and landscape, makes it a suitable environment for many snake species. Among these, Naja atra, Bungarus multicinctus, Deinagkistrodon acutus, Trimeresurus mucrosquamatus, Trimeresurus stejnegeri, Daboia russelii siamensis are the 6 principal venomous species, and have caused significant injuries and death over the years. The natural environment of Taiwan has changed tremendously in the last 20-30 years, which is likely to have affected the number and distribution of venomous snakes, thus indirectly affecting incidence of snakebite. A retrospective analysis of 286 snakebite cases at a medical center in central Taiwan analyzed the snakebite-related epidemiological data in the past 30 years. The results showed that the bite rates of various venomous snakes vary geographically, which is related to the overlapping of the human living environment and snakes' habitat. In Taiwan, T. mucrosquamatus and T. stejnegeri bites are most common. Bites by Deinagkistrodon acutus and Daboia russelii siamensis generally occur in the south and east parts of the island and attacks by Naja atra are most common in central Taiwan. Aggressive antivenom treatment can reduce snakebite mortality rate, but for Bungarus multicinctus bites, maintaining the patient's airway and supporting their ventilation is vital to reducing mortality rate in addition to antivenom treatment.

Protein variation in the venom spat by the red spitting cobra, Naja pallida (Reptilia: Serpentes)

The venom spat by red spitting cobras (Naja pallida) was analyzed to document variations in protein composition occurring over short temporal periods (less than 5 min). These cobras exhibited distinct control of venom flow with spits averaging 1.7% of the volume of the venom gland, thus enabling the cobras to rapidly expel over 40 consecutive spits. Variations in the low and high molecular weight proteins were observed when comparing the 1st, 20th and 40th spits produced by the same specimens. The first few spits were characterized by a distinctive 9 kDa protein which was never observed beyond the 7th spit, was present in milked venom and was present when the spitting behavior was preceded by a 5 min period of induced defensive behaviors.

The ocular effects of spitting cobras: II. Evidence that cardiotoxins are responsible for the corneal opacification syndrome

Fractionation of H. haemachatus, N. nigricollis, N. nivea and N. melanoleuca venoms using Amberlite CG-50 and (NH4)HCO3 elution gradient chromatography yielded 11-13 fractions for each venom. One fraction, F X, from H. haemachatus, two fractions, F X and F XI, from N. nigricollis and one fraction, F VIII, from N. melanoleuca venoms possessed the whole of ocular activity of the venoms. The fractions were the only venom fractions that caused cardiac depressant activity; their effect was reversed by raising Ca++ concentration in the physiological solution; they did not influence the twitches of the phrenic nerve hemidiaphragm and guinea-pig ileum preparations. Further purification of the fractions on Sephadex G-50 followed by fractionation on Amberlite CG-50 yielded fractions free from phospholipase A2 activity but possessing the same ocular effects. Similarly, the cardiotoxin from commercial N. nigricollis venom caused the same ocular effects as the crude venom and its purified cardiotoxic fractions. All cardiotoxic fractions as well as N. nigricollis cardiotoxin, caused extensive chemosis, blepharitis and corneal opacification with corneal and subconjunctival neovascularization. On a weight basis, the cardiotoxins were weaker in their oculotoxic activity than the corresponding parent crude venoms possibly because of the potentiating effect of phospholipase A2 in the crude venoms. It is postulated that in spitting cobras the cardiotoxins are responsible for the corneal opacification syndrome. In other cobra venoms the stable binding of cardiotoxins with acidic proteins limits their possible ocular effects. Only in the venoms of the spitting species are the cardiotoxins present in an appropriately free form to cause the ocular opacification syndrome.