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Abstract
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The name "spitting cobra" leaves little to one's imagination. Indeed, when threatened, snakes belonging this group can project their venom up to 3 m through the air to defend themselves.[1] The ultimate goal is to hit the opponent's eyes, which causes pain, blindness, and thus confusion, so that the snake can flee.[1],[2],[3] This maneuver requires highly developed skills, since it is only effective if the cornea is hit.[4] The snakes do not actually spit but eject venom through a hole in their fangs while making a spitting motion.[3],[4],[5] Inoculation of the venom is known as venom ophthalmia, a term used for a variety of symptoms. Spitting cobras are a group of elapids that live naturally in Africa and in Asia, including India. Most snakes are of the Naja (true cobra) genus. The African Hemachatus haemachatus (ring-necked spitting cobra) is not a true cobra, but a close relative.[6]
Corneal damage is theorized to develop in two phases; at first, there is direct injury to the epithelium and stroma caused by the venom itself, after which liberated inflammatory mediators (like prostaglandins and leukotrienes) continue to damage the underlying tissue. Snake venom in general consists of a multitude of proteins, varying among species, with different fractions responsible for different symptoms, for example, a hematotoxic, neurotoxic, or cardiotoxic fraction. The cardiotoxic fraction is found to be responsible for ocular symptoms, but only the compound venom of the spitting cobras can cause corneal damage due to its overall different composition.[7]
Venom dosage correlates to the severity of its effects, as tested in live rabbit models.[8] Low dosages of Naja nigricollis venom (0.1 mg) give mild conjunctival and corneal edema, while higher dosages (1.0-2.5 mg) result in corneal erosion, opacification, and scarring. Even higher doses (5.0 mg) result in extensive corneal melting with perforation. One dose of spitting cobra "spit" is estimated to contain 1.2-6.8 mg of venom. While corneal sensitivity correlates with melanin content of the eye, its effects appear to differ per type of spitting cobra.[9] Inoculation of N. nigricollis venom in albino rabbits eventually results in corneal neovascularization, whereas pigmented rabbits' corneas get clear over time. Conversely, H. haemachatus toxin results in corneal neovascularization in pigmented rabbits, while albino rabbits' corneas get clear in 3 weeks...
Language: en |