Snake venom is a modified saliva consisting of a mixture of proteins, enzymes, and peptides used by snakes to immobilize and begin digesting prey. While the exact chemical composition varies widely, the effects on the victim’s body are broadly categorized into three distinct functional groups. This classification is based on the primary system targeted by the most abundant toxins. These three categories—neurotoxic, hemotoxic, and cytotoxic—describe how these biological agents disrupt essential bodily functions.
Neurotoxic Venom
Neurotoxic venom primarily acts upon the nervous system, disrupting communication between nerve cells and muscles. The toxins rapidly target the neuromuscular junction, where motor neurons transmit signals to skeletal muscle fibers. This targeted attack swiftly leads to muscular paralysis, which is the most life-threatening effect of this type of envenomation.
Some neurotoxins, such as alpha-neurotoxins (3FTxs), function postsynaptically by binding directly to nicotinic acetylcholine receptors on the muscle cell surface. By blocking these receptors, they prevent the neurotransmitter acetylcholine from initiating muscle contraction. Other toxins, often a type of Phospholipase A2 (PLA2), act presynaptically by destroying the nerve endings, preventing the release of acetylcholine altogether.
Neurotoxic envenomation typically begins with signs of cranial nerve dysfunction, such as drooping eyelids (ptosis) and blurred vision. As paralysis descends, it affects the throat and tongue, causing difficulty speaking and swallowing (bulbar paralysis). The most dangerous outcome is respiratory failure, which occurs when the venom paralyzes the diaphragm and other muscles necessary for breathing. Snakes that rely on this quick-acting mechanism include many species of Elapids, such as Cobras, Mambas, Kraits, and Sea Snakes.
Hemotoxic Venom
Hemotoxic venom acts destructively on the circulatory system, targeting blood vessels, blood components, and the body’s clotting cascade. The toxins include Snake Venom Metalloproteases (SVMPs) and Snake Venom Serine Proteases (SVSPs), which work to either prevent blood from clotting or cause widespread, inappropriate clot formation.
Metalloproteases cause internal hemorrhage by degrading the structural integrity of blood vessel walls, leading to leaks and bleeding. Serine proteases, often called thrombin-like enzymes, disrupt the coagulation system by either activating clotting factors prematurely or destroying fibrinogen, the protein necessary for a stable blood clot. This action can result in a consumption coagulopathy, where clotting factors are used up, leading to uncontrolled systemic bleeding.
Symptoms begin with massive local swelling and bruising at the bite site. Systemic effects include internal bleeding, which can lead to cardiovascular collapse and organ damage due to lack of oxygen. Most species of Vipers and Pit Vipers, such as Rattlesnakes, Copperheads, and Russell’s Vipers, possess venoms with dominant hemotoxic properties.
Cytotoxic Venom
Cytotoxic venom focuses on the direct destruction of local tissue at the bite site, causing cellular damage and necrosis. While many venoms contain cytotoxic elements, some snakes use this mechanism primarily to debilitate prey or facilitate digestion. The toxins work by breaking down the fundamental structures of cells and the surrounding extracellular matrix.
Toxins in this category directly disrupt the integrity of cell membranes, causing cells to rupture and die. Enzymes like Phospholipase A2s and metalloproteinases break down the structural proteins that hold tissues together. Metalloproteinases degrade collagen and basement membranes, causing local tissue death (necrosis) and facilitating the rapid spread of other venom components.
Clinical results include immediate, intense pain, significant localized swelling, and severe blistering, often leading to deep tissue damage (myonecrosis and dermonecrosis). This destruction can necessitate surgical debridement or even amputation to prevent secondary infection. Snakes known for a strong cytotoxic component include many Cobras, particularly spitting varieties, and large Vipers like the Puff Adder and Gaboon Viper.