Snake venom is complex mixtures of compounds that cause lethal toxicity. The scientific study of venoms is known as toxinology.
When a snake feels threatened, it can eject streams or sprays of venom from its modified fangs. The venom of the spitting cobra Naja, an elapid, is primarily neurotoxic with some myotoxic properties.
Snake venom contains toxins that bind to multiple receptors in different parts of the body. Some toxins act on the nervous system, preventing neurons from transmitting signals and causing paralysis. Others bind to the circulatory system, causing blood cells to burst and lowering blood pressure. Still other toxins, such as those of Russell’s vipers (Daboia russelii) and some Australian elapids, kill muscles. The dead muscle cells cause edema, which can lead to hypovolemia and shock.
Symptoms of snake bite poisoning vary depending on the type of snake that bit the victim and the amount of venom injected. The symptoms of a serious snakebite include severe pain at the bite, swelling that may extend up the bitten limb or throughout the entire body, numbness and loss of sensation at the bite site, nausea, vomiting, breathing difficulties, anxiety, confusion and bleeding at the bite site and other body openings.
A victim should call 911 and describe the type of snake to help first responders figure out the best treatment. While waiting for help to arrive, the victim should lie down in the recovery position with the bitten limb below heart level and avoid moving. Remove any tight clothing such as rings and watches, which could cut into the skin if it swells. If possible, splint the bitten limb to keep it still.
Venom from many snakes contains a mixture of lethal components. The proteolytic venom destroys cell membranes and disrupts procoagulant and anticoagulant activities in blood; the hemotoxic component of the venom interferes with normal heart function; the neurotoxic venom affects nerves; and the cytotoxic venom causes local tissue death.
The first step in treating a snakebite is to keep the victim calm and still, if possible. Remove jewelry and any shoes from the bitten limb. Use a heavy crepe or elasticized roller bandage to firmly immobilize the limb, starting at the fingers or toes and working upwards. Keep the wound below the heart and avoid tight arterial tourniquets. If the bite is on the neck or muzzle, a tracheal tube may be needed to help with breathing. Paracetamol may be given to reduce the pain.
Snakebite victims who receive antivenom in time benefit from fewer and less severe symptoms, shorter hospital stays and a quicker return to their homes and communities. However, many people are unable to access antivenom for a number of reasons including distance from health facilities with antivenom, cultural barriers influencing health-seeking behaviour and poor understanding of snakebite envenoming.
Snakebite victims should be treated as seriously as those with any other life-threatening injury. The patient should be monitored closely by an interprofessional team for signs of systemic venom toxicity. Those whose symptoms include shock, severe bleeding from the puncture site, a collapsed or unresponsive patient, and/or evidence of gastrointestinal or hepatic dysfunction should be managed with intravenous fluids and vasopressor agents.
The most important preventative measure is to never harass or kill snakes. Snakes rarely bite unless they feel threatened, such as when humans invade their territory or if children play too rough with them. Bites are most common in Africa and Asia, where venomous snakes occur in large numbers and people live close to their habitat.
Venom contains a complex mix of compounds, some of which cause pain, while others cause paralysis and death. Neurotoxins cause the muscles to seize up and suffocate, while haemotoxins interfere with blood circulation, breaking down clotting agents and leading to bleeding.
Some snakes are able to release only a small amount of venom with each bite (dry bites), while other snakes may release significant amounts when biting humans. Some snakes also eject venom directly into their victims’ eyes, which causes pain and may damage the cornea.
If bitten, seek medical attention immediately. Antivenom is available, which works by stimulating the body’s own antibodies to neutralize the venom. There are antivenoms that treat venom from only one type of snake (monovalent), and there are antivenoms that can be used for many types of snake found in a geographic area (polyvalent). Victims should also make sure they have a current tetanus booster vaccine. Also, if a person is in an area that commonly holds snakes such as grasslands or irrigated gardens, they should wear high-top leather boots and avoid sitting down.
Medicinal drugs based on snake venom have been used for centuries. For example, tirofiban — an anticoagulant and vasodilator derived from the venom of the saw-scaled viper (Echis carinatus) — is widely used in surgery. Many more potential therapies are under investigation.
The evolutionary origins of venom are an important subject of research, particularly when a trait is thought to have had a major impact on the survival of the organism that developed it. The idea that venom originated as a single early recruitment event from physiological protein families is commonly accepted in the literature7,8. However, the inclusion of a large body of Toxicoferan physiological protein sequences in toxin gene family phylogenetic analyses has led to a radical reassessment of these ideas9,10.
Venom toxins are complex proteins, often consisting of multiple sub-units. These proteins are recruited into venom from protein families that fulfill ordinary physiological functions9. It was not until an efficient delivery system evolved to deliver these molecules deeply into prey or attacker tissues that genuine functional novelty in the protein allowed it to be selected for as a venom toxin.
This suggests that, whereas it is common for convergent evolution to give rise to analogous structures such as wings in bats and birds, it may not be so simple to produce functional novelty in the context of venoms. Furthermore, the evidence that non-toxin branches of gene families nested within toxin clades show evidence of positive selection complicates remaining interpretations of toxin gene tree phylogenies.