Tag Archives: scary animals

Dangerous animals—spiders

In this installment of the Dangerous Animals series we look at a group that is very misunderstood, and often erroneously indicted for being dangerous—spiders. In the summary chart of dangerous animals, summarized from various sources, spiders are accused of causing 6 deaths a year, on average, in North America. This is more deaths than caused by bears, mountain lions, and wolves combined, and I am highly suspicious of the figure.

In his review of the Centers for Disease Control and Prevention (CDC) data, Langley (2005) summarizes death by all sorts of wild animals, and spider bites have their own classification code, suggesting that the medical community has decided it is worth watching for. For example, the data suggest that between 1991 and 2001 there were 5 fatalities by alligators, and a whopping 66 deaths by spider. People seem to be dropping dead left and right from spider bites. What gives?

In North America, there are two types of spiders known to cause medically significant envenomations in humans: the widows and the recluse. Let’s look at each.

Latrodectus, the Black Widow

Latrodectus, the black widow

Latrodectus, the black widow, showing a characteristic pose, upside down in the web.

There are currently 30 species of spiders within the genus Latrodectus, commonly called widows in North America. The species are distributed world-wide and are on every continent except Antarctica. The venom of the widow contains neurotoxins that inhibit neurotransmission. The spiders like dark and quiet places, with bites occurring when people unintentionally grab or sit on the spider, perhaps under a porch, on lawn furniture, in the tool shed, or in gloves or other item clothing. In the past bites sometimes occurred in outdoor toilets. Symptoms of bites tend to be local and radiating pain, and sometimes back, abdominal, and chest pain, sometimes accompanied by fever, agitation, hypertension, and interestingly, priapism (Vetter and Isbister 2008). People have described it to me like a case of the flu. Untreated, symptoms can last from hours to days. Despite their infamy, death is very uncommon.

Loxosceles reclusa, the Brown Recluse

Loxosceles reclusa, the Brown Recluse

Loxosceles reclusa, a Brown Recluse female guarding her egg sac on a cardboard box in Kansas.

Few spiders generate as much passion and aversion as the brown recluse (Loxosceles reclusa). I currently live in an area where black widows are extremely common, and local people are very casual about them, but are terrified of the brown recluse. I have done many educational programs where I have displayed live spiders, including black widows, and unvaryingly I am treated to several stories by visitors about how they (or someone they know) were bitten by a brown recluse, usually with very bad consequences. (I literally had one person tell me that his aunt had her entire arm removed because of a bite). The thing is brown recluse spiders do not live here! Nothing generates fear like the unknown.

Prior to living where I do now, I lived in an area with gobs of brown recluses, and the people there were generally nonchalant about their presence, as there were almost no cases of bites resulting in horrible wounds.

Distribution map of species within the genus Loxosceles, including Loxosceles reclusa, or the Brown Recluse

Distribution map of species within the genus Loxosceles, including Loxosceles reclusa, or the Brown Recluse (from Vetter 2008).

To be clear, Loxosceles is confirmed to have bitten people and caused wounds that in rare cases take a long time to heal and can leave disfiguring scars, or even death. They are a spider of medical concern. But, having said this, the threat is far over blown.

They are named “recluse” because they like very quite areas, and can frequent homes and storage sheds in quite places. They like corners of basements, and particularly cardboard boxes. Sometimes they crawl into clothing and shoes left on the floor or in the closet. Like with the widows, people are most often bitten when they catch the spider between their body and where the spider is—the bite is defensive.

In a majority of cases, the bite results in local discomfort and nothing more. In some cases a larger wound forms that is tender, but most of these heal with minimal medical intervention, usually within days. Sometimes the wound heals slower, and in rare instances does grow large and can leave a scar. And in very rare cases (<1%) there are more significant systemic issues that can affect major organs and cause death. (Vetter and Isbister 2008).

As mentioned, I lived in an area with known recluse populations. In fact, in one case, 2,055 individual recluse spiders were captured in 6 months from one home in Kansas where the family lived for many years without a single incident attributed to the spiders (Vetter 2008). However, popular perception about these spiders is very different. Why is this?

The most likely explanation is that when the recluse was implicated in bites the most extreme cases got widely reported, heightening awareness in the public and medical community. Diagnoses of recluse bites have become common place, often in areas where the spiders have not been found in the wild, and usually without clear evidence that the symptoms presented were actually caused by a spider, or any other bite for that matter. For example, in Florida, an area without a known population of recluses, during a six year period, 844 brown recluse bites were reported: 124 by medical personnel, 198 by people seeking information about bites, and 522 from people reporting bites treated at a non-healthcare facility (Vetter and Furbee 2006). Physicians are thus occasionally guilty of “practicing Arachnology” by identifying bites, and even spider species, from clinical symptoms alone. The truth is, there are numerous conditions that have been, or could be, misdiagnosed as a recluse bite (Vetter 2008) (see below).

Given the obvious over-diagnosis and misdiagnosis of spider bites, and of recluse bites in particular, I find the assertion that 6 deaths a year in North America are caused by spiders to be highly doubtful. At the very least, this is an undeserved slam against our eight-legged friends, and at worst is misleading the public and medical community, causing potential misdiagnoses and poor treatment choices.

Conditions that have, or could be, misdiagnosed as a bite from a brown recluse (Loxosceles reclusa), from Vetter 2008.


Atypical mycobacteria


– Streptococcus

– Staphylococcus (especially MRSA)

– Lyme borreliosis

– Cutaneous anthrax

– Syphilis

– Gonococcemia

– Ricketsial disease

– Tularemia

Deep Fungal

– Sporotrichosis

– Aspergillosis

– Cryptococcosis

Ecthyma gangrenosum (Pseudomonas aeruginosa)

Parasitic (Leishmaniasis)

Viral (herpes simplex, herpes zoster (shingles))

Vascular occlusive or venous disease

Antiphospholipid-antibody syndrome

Livedoid vasculopathy

Small-vessel occlusive arterial disease

Venous statis ulcer

Necrotising vasculitis

Leukocytoclastic vaculitis

Polyarteritis nodosa

Takayasu’s arteritis

Wegeners granulomatosis

Neoplastic disease

Leukemia cutis

Lymphoma (e.g., mycosis fungoides)

Primary skin neoplasms (basal cell carcinoma, malignant melanoma, squamous cell carcinoma)

Lymphomatoid papulosis

Topical and Exogenous Causes

Burns (chemical, thermal)

Toxic plant dermatitis (poison ivy, poison oak)

Factitious injury (i.e., self-induced)

Pressure ulcers (i.e., bed sores)

Other arthropod bites


Other Conditions

Calcific uremic arteriolopathy


Diabetic ulcer

Langerhans’-cell histiocytosis

Pemphigus vegetans

Pyoderma gangrenosum

Septic embolism

Related posts:
See the rest of the Dangerous Animals series
Pesky house bugs–bed bugs


Langley, R. L. 2005. Animal-related fatalities in the United States–an update. Wilderness & Environmental Medicine 16:67-74.

Vetter, R. S. 2008. Spiders of the genus Loxosceles (Araneae, Sicariidae): a review of biological, medical and psychological aspects regarding envenomations. The Journal of Arachnology 36:150-163.

Vetter, R. S., and R. B. Furbee. 2006. Caveats in interpreting poison control centre data in spider bite epidemiology studies. Public Health 120:179-181.

Vetter, R. S., and G. K. Isbister. 2008. Medical aspects of spider bites. Annual Review of Entomology 53:409-429.

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Dangerous animals–Sharks

We can all hear the ominous music, building slowly, frightfully, until the climax when Jaws attacks! Movies like Jaws have burnt this fearsome group of animals into our psyche, and I think the thought lurks somewhere in our minds whenever we visit the ocean that there are really big fish out there. There are somewhere around 440 species of shark worldwide. They are an ancient group of fish, whose overall lineage dates to before the Age of Dinosaurs. Of these hundreds of species, only 4 are known to have been involved in a significant number of fatal attacks on humans. Worldwide there is an average of 4.3 fatalities per year. 

The four most dangerous species have different habits, and therefore the patterns of attacks and their danger to people are different. They are the great white shark (Carcharodon carcharias), oceanic whitetip shark (Carcharhinus longimanus), bull shark (Carcharhinus leucas), and the tiger shark (Galeocerdo cuvier).

Great White Shark

Great White Shark

The great white, of Jaws fame, is a fearsome predator. Adult sharks are 12-17 feet in length. They are found along both coasts from Mexico into high latitude waters of Alaska, and along the east coast up to Hudson Bay. However, it is also known from deeper ocean waters, and new findings suggest that at least the west coast populations congregate in the open ocean between California and Hawaii for parts of the year. They are predators of marine mammals, turtles, large fish, and even whales. Humans are incidental targets.

During the hot summer of 1916, the Jersey Shore was the scene of a series of attacks that shuck the public, and was the inspiration for the book Jaws. Between July 1 and July 12, four people were killed. The great white is often blamed, but it could have also been a bull shark. Why there were so many attacks at that time is hard to say, and has not been repeated since.
Oceanic Whitetip Shark Oceanic Whitetip Shark


The oceanic whitetip shark is actually responsible for more fatal attacks on humans than all other species combined, and it does not even frequent the coasts. It is found worldwide, and prefers warm waters and deep ocean areas. It is the unfortunate events of the twentieth century that allowed this shark to become superlative in human fatalities—it is the shark reasonable for attacks on survivors of shipwrecks and downed aircraft. During the war in the Pacific, when ships and planes were regularly shot down, many hundreds of stranded sailors were attacked and killed. Horrific stories from survivors testify to this gruesome bloodshed and led to much research by the Navy in shark repellents. Hopefully, this scale of human carnage will never occur again. And while this species is the most deadly shark, its habits make it of little concern to the average beach-goer.

Tiger Shark Tiger shark


The tiger shark is found worldwide in mostly equatorial waters. It prefers tropical and sub-tropical warm water, and does not get into the high latitudes like the great white. It tends to stay in deep waters that line reefs, but it does occasionally move into shallow water and channels where it might encounter people. This shark is large, commonly attaining lengths of 10-14 feet, but because of its habits encounters with humans are relatively rare.

Bull shark Bull shark


In contrast, the bull shark is likely the most dangerous species to humans overall. It too is common in warm waters along coasts, but it also tolerates fresh water, and can migrate into rivers. They have been found far up the Amazon River in South America, and as far north in the Mississippi River as Illinois (yikes!) (Thomerson et al. 1977). These sharks are unpredictable and are often aggressive, and because of their habit of being in shallow waters are probably responsible for the majority of near-shore attacks.

However, having said all of that, the chances of being attacked are very remote. You can see the per year compared with other animals on the chart. Was that deep, ominous music I just heard?

Average number of deaths per year caused by various animals

Average number of deaths per year caused by various animals

Thomerson, J. E., T. B. Thorson, and R. L. Hempel. 1977. The bull shark, Carcharhinus leucas, from the upper Mississippi River, near Alton, Illinois. Copeia 1977(1):166-168.

Several of the images come from sources recommended in the Nature Wallpaper post.

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Dangerous Animals

In his song “Sail away,” Randy Newman sings about how good we have it in America. The land is bounteous and its wild inhabitants are peaceful. “Ain’t no lion or tiger, ain’t no mamba snake, just the sweet watermelon and the buckwheat cake.” Of course, Newman is known for lyrics that intentionally are outrageous contradictions, but in this case, he is mostly right.

North America is blessed with a general paucity of dangerous beasts, and it is very safe to venture into the wilderness and not worry about being eaten or molested by nature, aside from flies and mosquitoes. However, for many, dangerous animals are exciting, and there is much misinformation about the wilds. So, this is the first of a series on the dangerous animals of North America, or at least the ones that often get a bad rap as being dangerous.

I undertook a survey of animals that are known to have caused human death, and worked to separate truth from fiction. Death caused by animals does happen. However, it is a relatively rare event in North America, even if it generates a lot of media interest when it does happen. And the identity of the most dangerous animals might just surprise you. So explore all the dangerous animals.

Posts in the Dangerous Animals Series:

Venom, poison, and toxicity
Number of venomous snakebites a year
Venomous snakes of North America
Mountain Lions

Average number of deaths per year caused by various animals

Average number of deaths per year caused by various animals

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