Venom can turn a seemingly weak and tiny creature into a feared killer. But what, exactly, does it mean for an animal to be "venomous"? And which venomous species are the most deadly to humans? Here's what we know.
Poisonous ≠ Venomous
Though the two terms are often used interchangeably, "venom" is not the same thing as "poison." The difference between the two has to do with the mode of toxin delivery. Let's start with poisonous animals.
Poisons are absorbed through the skin or ingested. That is, a poisonous animal can only deliver its toxic chemicals to you if you eat it or touch it.
The poison dart frog — which actually isn't a single species, but a group of frogs in the family Dendrobatidae — is one of most poisonous animals in the world. One particularly deadly species is called the golden dart frog (Phyllobates terribilis). It has toxins so potent that as little as 2 micrograms of the stuff can supposedly kill an adult human. Interestingly, this group of amphibians isn't innately poisonous. If you take them out of their natural habitat, they lose their toxicity, because they actually generate their poison from the insects they eat (those insects, in turn, get their poisons from plants).
Another notable poisonous animal is the puffer fish (fugu). Considered a delicacy in Japan, this fish has organs laced with neurotoxins (chemicals that attack the nervous system) that can kill you. Of course, many other poisonous animals exist, including some birds, such as New Guinea's Pitohui birds.
Venomous animals, by contrast, have ways of injecting their toxins directly into another animal, such as by biting, stinging or stabbing. A recent review on the evolution of venoms states:
Venom can be broadly defined as "a secretion, produced in a specialised gland in one animal and delivered to a target animal through the infliction of a wound 'regardless of how tiny it could be,' which contains molecules that disrupt normal physiological or biochemical processes so as to facilitate feeding or defense by the producing animal."
The authors stress that venoms serve multiple functions in the animal kingdom, depending on the specific species. Venom is most often a foraging adaptation that helps animals — snakes, scorpions, spiders, centipedes, jellies and more —immobilize, digest or otherwise kill their prey. In other cases, venom is used for defense, as seen in some lizards, fish and insects.
How Does Venom Kill?
So what exactly is venom made of? Back to the review from above:
Most animal venoms are highly complex cocktails of bioactive compounds. Venoms typically comprise a mixture of protein and peptides (commonly referred to as toxins), salts and organic components, such as amino acids and neurotransmitters. The proteinaceous components are usually the most abundant.
Generally, defensive venoms are very "streamlined" and primarily function to immediately bring about intense pain in a localized area (think: bee stings). Predatory venoms are complex and have a wide range of effects on the body.
In any case, venoms are often discussed in terms of the type of toxins they contain. These toxins can generally be placed into a couple of broad categories: neurotoxins and hemotoxins.
As mentioned above, neurotoxins attack cells throughout the nervous system, causing a number of issues, including paralysis, muscle spasms and respiratory arrest. Hemotoxins — a misnomer, really — damage the blood cells and organs. They can destroy red blood cells (which deliver oxygen throughout the body), cause inflammation, disrupt normal blood clotting and even spark organ failure.
Toxins can be broken down further into categories based on their specific targets, or the animals that produce them.
Cobra venom, for example, contains a cardiotoxin (attacks the heart) that messes with heart contractions, leading to cardiac arrest. Some snakes, particularly rattlesnakes, produce myotoxins that cause muscular necrosis, resulting in pain, convulsions and other symptoms. The venom of widow spiders (Latrodectus) contains α-Latrotoxin, which, among other things, interferes with the nerve signals that control muscles — the neurotoxin may cause tremors, nausea and elevated blood pressure.
Importantly, venoms can contain a number of different toxins and each toxin may affect several different systems in the body. For these reasons, experts have long argued that people get away from the habit of labeling venom based on a specific type of toxin. In the U.S., for example, rattlesnakes are sometimes said to have "hemotoxic venom"; however, the venom of the Mojave rattlesnake (Crotalus scutulatus) also contains a potent neurotoxin, Mojave toxin.
What's the Most Toxic Venom?
Determining which venomous species is the most deadly to humans isn't easy.
Physiological ecologist Marshall McCue dissected this question in a recent letter in the Journal of Venomous Animals and Toxins including Tropical Diseases. He notes that if you want to know which snake species is the most deadly to humans, you have to specify if you're asking which snake causes the most deaths or which snake is most likely to have a lethal bite — both questions have their own issues.
Human population density and social habits will naturally affect how many people a certain venomous species will kill, as will an animal's propensity to actually attack someone (some species are more aggressive than others). Additionally, the availability of emergency health care — and antivenom — will partially determine how likely a person will die from the venom.
To measure the toxicity of venom, scientists often rely on the Lethal Dose 50% (LD50) test, which specifies the dose necessary to kill half of a test population. The test is administered to laboratory animals, such as mice, rats and guinea pigs, and the results are sometimes extrapolated to humans. But there may be inherent problems with this methodology.
These laboratory animals, which likely have been inbred for generations, are not necessarily representative of wild animals. What's more, there are physiological differences between the test animals and humans — what's deadly to the animals may not be as deadly to humans (and vice versa). A prime example of this is chocolate, which is toxic to dogs, parrots and other animals (they cannot metabolize the chemical theobromine), but is fine — and delicious — for humans. McCue adds:
The mode of venom administration (i.e., intravenous, subcutaneous, intramuscular, or intraperitoneal) has a tremendous influence on the toxic action of venoms, and minimum lethal dosages may need to be several hundred times greater when delivered subcutaneously than those administered intravenously. Therefore, any extrapolations from contrived laboratory situations must be interpreted with caution.
The Species With the Deadliest Bites
Despite the difficulties in ranking how dangerous venomous animals are to humans, various studies, reports and organizations have identified certain species that may be a step above the rest of their kind.
The inland taipan (Oxyuranus microlepidotus) is often reported to have the most deadly snake venom — made up of neurotoxins, procoagulants (that disrupt blood clotting) and myotoxins — in the world. The LD50 (subcutaneous, mouse) of the Australian native is 0.025 mg/kg. This means that 0.025 milligrams of the venom for every kilogram body weight of a mouse causes the death of 50 percent of the test group (when administered subcutaneously). A single drop of the inland taipan's venom can supposedly kill 250,000 mice or 100 adult men, though human bites are quite rare given the snake's shyness (and, fortunately, an antivenom is available).
There are a few candidates for the title of the world's deadliest venomous spider. A 2009 study claims that the Australian “funnel-web” spiders (Atrax and Hadronyche) are the "spiders most dangerous for humans" — their venom causes local and systemic effects, including hypertension, arrhythmia, coma and death. The venom of male spiders — containing a neurotoxin that only affects primates — can kill a small child within 15 minutes, though there have been no reported deaths since the development of the antivenom in 1981.
The Guinness Book of World Records instead recognizes the Brazilian wandering spider (Phoneutria) as the most venomous spider — just 0.006 mg of the venom can kill a mouse, according to the records. (A 2002 study notes that the LD50 of a female Phoneutria nigriventer with an egg sac is 0.63 mg/kg).
The venoms of scorpions are often harmless to humans, but some can actually kill us. Some experts consider the Indian Red Scorpion to be the most lethal of all scorpions. While not likely to kill healthy adults, the scorpions can prove deadly to children and the elderly — their venom consists of a potent toxin that causes pulmonary edema (fluid accumulation in the lungs). But according to Guinness, the most venomous scorpion is the Tunisian fat-tailed scorpion (Androctonus australis), which is reportedly responsible for 90 percent of deaths from scorpion stings in North Africa.
Surprisingly, octopuses can also be venomous. In fact, a 2009 study suggests that all octopuses are venomous. The tiny blue-ringed octopus (Hapalochlaena maculosa) is widely considered to be the most deadly of all octopuses, as its bite injects venom that can kill an adult human in minutes (and there is no antivenom). The cephalopod's tetrodotoxin, the same neurotoxin in fugu and poison dart frogs, causes paralysis and the cessation of breathing. But don't blame the octopus for its venom — blame the bacteria in its saliva.
Other animals of note: the stonefish (Synanceja horrida) is often regarded as the most dangerous venomous fish in the world, with spines that can cause various problems, including edema, respiratory distress and convulsions, which can ultimately kill a person within six hours. A number of cone snail species — Conus textile and C. geographus, in particular — shoot a venom-infused harpoon that can paralyze and kill people. And the Australian box jellyfish (Chironex fleckeri) is the most venomous marine animal, able to cause paralysis, cardiac arrest and even death within just a few minutes after stinging someone, according to the National Oceanic and Atmospheric Administration.
Be safe out there!