Venom is the general term referring to any variety of toxins used by certain types of animals that inject it into their victims by the means of a bite, sting or other sharp body feature. Unlike poison, which is ingested or inhaled, venom is usually delivered directly into the lymphatic system, where it acts faster.
The potency of different venoms varies; lethal venoms are often characterised by the median lethal dose (LD50, LD50, or LD-50), expressed in terms of mass fraction (e.g., milligrams of toxin per kilogram of body mass), that will kill 50% of victims of a specified type (e.g., laboratory mice).
Venomous invertebrates include spiders, which use fangs - part of their chelicerae - to inject venom (see spider bite); and centipedes, which use forcipules - modified legs - to deliver venom; along with scorpions and stinging insects, which inject venom with a sting. In insects such as bees and wasps the stinger is a modified egg-laying device – the ovipositor. Many caterpillars have defensive venom glands associated with specialized bristles on the body, known as urticating hairs, which can be lethal to humans (e.g., that of the Lonomia moth), although the venom's strength varies depending on the species.
Because they are tasked to defend their hives and food stores, bees synthesize and employ an acidic venom (apitoxin) to cause pain in those that they sting, whereas wasps use a chemically different venom designed to paralyze prey, so it can be stored alive in the food chambers of their young. The use of venom is much more widespread than just these examples. Other insects, such as true bugs and many ants also produce venom. At least one ant species (Polyrhachis dives) has been shown to use venom topically for the sterilisation of pathogens.
There are many other venomous invertebrates, including jellyfish and cone snails. The box jellyfish is the most venomous jellyfish in the world.
Venom can also be found in some fish, such as the cartilaginous fishes – stingrays, sharks, and chimaeras – and the teleost fishes including onejaws, catfishes, stonefishes and waspfishes, scorpionfishes and lionfishes, gurnards, rabbitfishes, surgeonfishes, scats, stargazers, weever, swarmfish, and pufferfish.
There are only a few species of venomous amphibians; certain salamandrid salamanders can extrude sharp venom-tipped ribs.
The reptiles most known to use venom are snakes, some species of which inject venom into their prey via fangs.
Snake venom is produced by glands below the eye (the mandibular gland) and delivered to the victim through tubular or channeled fangs. Snake venoms contain a variety of peptide toxins, including Proteases, which hydrolyze protein peptide bonds, nucleases, which hydrolyze the phosphodiester bonds of DNA, and neurotoxins, which disable signalling in the nervous system. Snakes use their venom principally for hunting, though they do not hesitate to employ it defensively. Venomous snake bites may cause a variety of symptoms, including pain, swelling, tissue necrosis, low blood pressure, convulsions, hemorrhage (varying by species of snake), respiratory paralysis, kidney failure, coma and death.
Aside from snakes, venom is found in a few other reptiles such as the Mexican beaded lizard and gila monster, and may be present in a few species of monitor lizards.
One such reptile that was previously thought of as being nonvenomous is the Komodo dragon, Varanus komodoensis. It was then demonstrated through magnetic resonance imaging that the Komodo dragon possess a mandibular gland with a major posterior compartment and five smaller anterior compartments. The scientists used mass spectrometry to show that the mixture of proteins present in the venom was as complex as the proteins found in snake venom.
Due to these recent studies investigating venom glands in squamates, lizards that were previously thought of as being nonvenomous are now being classified by some scientists as venomous because they possess a venom gland. This hypothetical clade, Toxicofera, includes all venomous squamates: the suborders Serpentes and Iguania and the families Varanidae, Anguidae, and Helodermatidae.
Sinornithosaurus, a genus of feathered dromaeosaurid dinosaur, may have had a venomous bite. This hypothesis is still being disputed. The theropod Dilophosaurus is commonly depicted in popular culture as being venomous, but this portrayal is not considered likely by the scientific community.
Some mammals are also venomous, including solenodons, shrews, the slow loris, and the male platypus.
Euchambersia, a genus of Therocephalia (animals close to the evolution of mammals) is known to have had venom glands attached to its canine teeth, used to help subdue and kill its prey. The potency of its venom is unknown.
Physicians treat victims of a venomous bite with antivenom, which is created by dosing an animal such as a sheep, horse, goat, or rabbit with a small amount of the targeted venom. The immune system of the subject animal responds to the dose, producing antibodies to the venom's active molecules; the antibodies can then be harvested from the animal's blood and injected into bite victims to treat envenomation. This treatment can be used effectively only a limited number of times for a given individual, however, as a bite victim will ultimately develop antibodies to neutralize the foreign animal antigens injected into them as components of the antivenin. This is called sensitization. Even if a bite victim does not suffer a serious allergic reaction to the antivenom, his own, sensitized, immune system may destroy the antivenom before the antivenom can destroy the venom. Though most individuals never require even one treatment of anti-venom in their lifetime, let alone several, those routinely exposed to snakes or other venomous animals may become sensitized to antivenom due to previous exposure.
Aristolochia rugosa and Aristolochia trilobata, or "Dutchman's Pipe", are recorded in a list of plants used worldwide and in the West Indies, South and Central America against snakebites and scorpion stings. Aristolochic acid inhibits inflammation induced by immune complexes, and nonimmunological agents (carrageenan or croton oil).][ Aristolochic acid inhibits the activity of snake venom phospholipase (PLA2) by forming a 1:1 complex with the enzyme. Since phospholipase enzymes play a significant part in the cascade leading to the inflammatory and pain response, their inhibition could lead to relief of problems from scorpion envenomation.
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