These Are the World's Most Dangerous Spiders

Some spider bites can be dangerous to humans—and can even cause death. But which arachnids around the world pose the biggest risk to us?

Spiders have a bad reputation—research suggests that between three and 11 percent of the world's population is arachnophobic.

But while almost all spiders are venomous, it is estimated that less than one percent of the roughly 50,000 known spider species have venom that is toxic enough to have adverse effects in humans, Rick Vetter, a retired entomologist and spider expert, who was previously affiliated with the University of California, Riverside, told Newsweek.

An even smaller percentage of spider species have been responsible for human deaths. There are several reasons why spider bite fatalities are extremely rare.

Why Are Human Deaths So Rare?

Firstly, very few spiders are even capable of penetrating human skin with their bites. Of the species that can, only some have venom toxic enough to cause severe illness in humans and produce sufficient quantities of that venom to have any adverse effects on us.

Of these potentially dangerous species, some rarely or almost never come into contact with humans because of the particular habitats they live in. Health clinics and hospitals also often have access to effective anti-venom, which can mitigate the risks of a bite from one of the more dangerous species.

Spiders don't deliberately target humans either and usually only bite in defense or when they feel threatened. Even when they do bite us, venom is not always delivered, or it is not delivered in sufficient quantities to cause serious harm.

Spider bites from the most dangerous species can sometimes cause significant tissue damage, such as skin lesions, or severe illness in humans—and in very rare cases, even death. In general, young children, the elderly and people with underlying health problems are at highest risk from spider bites.

Most Deadly Spiders

According to Vetter, there are four primary groups of spiders that pose the most danger to humans when it comes to the toxicity of their venom: the Australian funnel-web spiders (Atrax and Hadronyche species), widow spiders (Latrodectus species), recluse spiders (Loxosceles species) and the South American banana spiders (Phoneutria species).

Several Australian funnel-web spiders, of which there are at least 40 species, have venom that is "highly toxic" to humans, according to Vetter. This group is named after the funnel-shaped webs they create.

Funnel-web Spiders

The Sydney funnel-web spider (Atrax robustus) is perhaps the most dangerous of this group having been responsible for all recorded deaths (at least 13) among this group of spiders in Australia and many medically serious bites, according to the Australian Museum.

All of these recorded deaths have been the result of bites from male Atrax robustus individuals. The venom of the males contains a substance that can severely affect the nervous systems of humans and monkeys, but not other mammals.

A Sydney funnel-web spider
A Sydney funnel-web spider in strike stance. This species has venom that is highly toxic to humans. iStock

This neurotoxin, known as delta-hexatoxin, can cause muscle spasms, dangerous drops in blood pressure, coma and even organ failure. If left untreated, a bite can result in death, sometimes within a few hours.

However, since the introduction of an effective anti-venom in 1981, there have been no recorded deaths resulting from a Sydney funnel-web bite.

Major city and regional hospitals across Australia stock this anti-venom, which has also been used against other dangerous funnel-web spiders.

The fact that the range of this spider with particularly toxic venom is centered on the Sydney region, which is home to around four million people, means it comes into contact with humans more often than many other funnel-webs.

Aside from Atrax robustus, the funnel-webs that pose the most risk to humans are the Blue Mountains funnel-web spider (Hadronyche versuta) and the southern and northern tree funnel-web spiders (Hadronyche cerberea and Hadronyche formidabilis).

Widows

Another group of potentially dangerous spiders are the true-widows, of which there are more than 30 species. They are found on every continent except Antarctica.

The venom of widow spiders contains a neurotoxin known as alpha-latrotoxin, which affects the nervous system and can cause symptoms such pain, redness and swelling around the bite site, abdominal cramps, nausea and vomiting, tremors, and sweating.

But despite their fearsome reputation, people who are bitten by widow spiders often don't suffer any serious complications, although they may experience varying levels of discomfort and may require medical treatment.

A black widow spider
Stock image: A black widow spider. True-widows are found on every continent except Antarctica. iStock

The bites of female widow spiders tend to pose more of a risk to humans than those of males.

In very rare cases, widow bites can result in death—caused by the severe disruption of nerve signals in the body.

Among the widow species that have been documented as causing human deaths are the Mediterranean black widow (Latrodectus tredecimguttatus) and the redback spider (Latrodectus hasselti), which is native to Australia but has also appeared in other countries, such as New Zealand and Japan (as an invasive species).

There are anti-venoms available for some widow spiders, including the redback. While redbacks have caused deaths in Australia, there have been no confirmed fatalities from these spiders since 1956 when the anti-venom for this species was introduced, despite the thousands of bites, according to the Australian Museum. An individual apparently died from a reported redback bite in 2016, but this was not confirmed as a main cause of death, the Australian Daily Telegraph reported.

Banana Spiders

The Phoneutria group of spiders, which are found in forested areas of South America and some parts of Central America, also have venom that is highly toxic humans.

This venom affects the nervous system, causing symptoms such as pain, sweating, salivation, cardiac issues, visual problems and even priapism—or in other words, prolonged erections of the penis—according to Vetter.

Also known as banana spiders because they are sometimes found on banana leaves, these large arachnids, whose legs measure up to six inches in length, have been responsible for thousands of bites in these regions and a small number of deaths. In cases where deaths have occurred, the cause is usually respiratory arrest.

Phoneutria spiders don't make webs and instead spend their time wandering the forest floor, which can sometimes bring them into contact with humans. In populated areas, for example, these spiders sometimes like to hide in or around human dwellings, which can increase the risks of bites occurring if they are accidentally disturbed.

Some research has indicated that the venom of Phoneutria spiders is even more potent than even that of the Latrodectus and Atrax groups.

But one 2000 study which documented 422 patients who had been bitten by Phoneutria spiders between 1984 and 1996, and found that that the vast majority of cases (around 90 percent) only suffered mild effects, with just 0.5 percent experiencing severe envenomation.

Among the severe cases were two children, one of whom died around nine hours after being bitten.

While there is an anti-venom available for use to treat a Phoneutria spider bite, only around two percent of the bites in the study were serious enough to warrant the use of this treatment.

Perhaps the most dangerous member of this group is the species Phoneutria nigriventer found in Brazil, Uruguay and Argentina, which is responsible for most bite cases because it is often found in highly populated areas.

A Phoneutria nigriventer spider
Stock image: A spider of the species Phoneutria nigriventer. Spiders of the Phoneutria group are found in forested regions of South America and some areas of Central America.

Recluse Spiders

Finally, the recluse spiders, which are found in many parts of the world, including North America, also have venom that is toxic to humans. The bite of these spiders can cause a condition known as loxoscelism, characterized by the development of necrotic skin lesions—where tissue around the site of envenomation dies.

These necrotic lesions only occur in a minority of recluse spider bites, and severe systemic reactions—where the venom spreads throughout the body—that can lead to death are extremely rare, most frequently occurring in high-risk groups such as children. Most recluse bites are mild or unremarkable.

Many experts consider the Chilean recluse spider (Loxosceles laeta) to be the most dangerous of this group, with its potent venom more likely to cause serious systemic effects following a bite than other species.

One study that examined more than 200 cases of Chilean recluse bites between 1955 and 1988 found that patients experienced systemic loxoscelism in around 15 percent of cases, while about four percent of cases resulted in death.

America's Most Dangerous Spiders

In the U.S., there are two spiders of medical significance that are of most concern to humans—the black widow and the brown recluse.

Black widows, which can be identified by red markings on the underside of their abdomen, are found around across North America, primarily in southern and western regions of the United States.

According to Boston Children's Hospital, symptoms of a black widow bite include: pain, burning, swelling, and redness at the site of the bite; cramping pain and muscle rigidity in some parts of the body; headache; dizziness; rashes and itching; restlessness and anxiety; sweating; swelling of the eyelids; nausea or vomiting; salivation; weakness; tremors; and paralysis.

More than 2,500 black widow bites are reported to American poison control centers annually. While some of these bites may be extremely unpleasant, deaths from these spiders are extremely rare.

No deaths from widow bites have been reported to the American Association of Poison Control Centers since it began producing its annual report in 1983, although there are some other reports of fatalities.

A brown recluse spider
Stock image: A brown recluse spider. This species is found in some parts of the U.S. iStock

Brown recluse spiders (Loxosceles reclusa), also known as violin spiders, are found in Midwestern and southern states. This spider likes to hide away in undisturbed areas such as closets, basements or attics.

According to the Children's Hospital, its venom can cause damage to local tissues and may produce a variety of symptoms, such as: burning, pain, itching or redness at the bite site; skin lesions; headache or body aches; fever; and nausea or vomiting.

"Part of the problem with recluses is that you don't know you've been bitten because they'll sneak into your bed at night," Vetter said. "With the spiders in Australia and South America, they're big and so you know you've been nailed by one of them."

"The bites of a recluse are like a pinprick. It will have some effect in a couple of hours, but typically nothing gets serious until around 48 hours. That's when people start seeking a doctor's help."

In the most extreme cases, which usually occur in children, the recluse venom can destroy red blood cells, causing anemia and potentially death, although fatalities are extremely rare.

Recluses might cause the deaths of a couple of children every year on average in the U.S., Vetter said, although the true number of fatalities is unclear.

"I used to say there are no recorded [recluse] deaths. But I think what happened was, that doctors just didn't report them."