The Deadly 40

Spider bites kill people. No really, they do. But then so do car crashes, heart attacks and narcotics. It's all a matter of stats, to really get a grip on what to be worried about then mathematics can be pretty useful*.

The main trouble with statistics is that most people, myself included, have a very limited grasp on how they work. This is something that Professor David Nutt, head of the UK's drug advisory body, discovered in 2009 when he quite correctly pointed out that you are as likely to be killed horse riding as you are taking ecstasy. Tabloid outrage ensued.

People die taking ecstasy, about 30 every year in the UK. They also die, riding horses, driving cars and yes, by being bitten by spiders. But how many people actually die from spider bites?

There are around 200 species of spider that are considered dangerous, and of those, a few (about 40) that are known to have a potentially** deadly bite. This is not many out of the 50,000 currently estimated species of spider in the world (Cesaretli & Ozkan 2010).

Of "The Deadly 40" (been watching the Deadly 60, at all? it's great!), nearly all are web-building Araneomorphs (often called 'true spiders') rather than the more primitive Mesothelae and Mygalomorphs. I mention this only because tarantulas, in the commonly thought of sense (big hairy spiders from South America) are all Mygalomorphs, and no tarantula could kill by biting a human (unless you happened to be very allergic to tarantula venom). So, for the record, tarantulas do not kill people. End of. Not that Mygalomorphs are all nice and friendly - Sydney funnel-web spiders, Atrax robustus, are Mygalomorphs and are pretty dangerous critters.

Grammostola rosea - a Mygalomorph spider, and not in any way deadly. Unless you're a cricket.

Anyway, of those deadly ones one of the most significant groups is the genus Latrodectus, or to most people, the widow spiders.

Black widows are the archetypal dangerous spider, but given the level of news coverage every time a 'deadly' black widow turns up in a truck in county Durham or in an imported car in Chatham docks, exactly how deadly are we talking? And what precisely is a black widow spider?

American widow spiders Latrodectus macrons, L. various and L. hesperus are the three species commonly referred to as black widows. The Australian red back (Latrodectus hasseltii) is a closely related but distinct species from the main three American black widows. All of these spiders have a potentially lethal bite, but the operative word is, of course, potentially.

Ricky Langley published a paper in 2005 in Wilderness and Environmental Medicine about animal-related fatalities based on data from the Centers for Disease Control and Prevention (CDC). The most deadly animal in America is… "Other specified" animals. A slightly disappointing grouping consisting on large animals such as bears, sharks, cows and horses. Of this, cows and horses are likely to have caused around 90% of the 846 deaths recorded in America between 1991 and 2001. Next up, hornets, bees and wasps with 533 deaths. Then dogs with 208 deaths. Spiders lie disappointingly in 5th place with 66 deaths recorded between 1991 and 2001. That's an average of just over 6 deaths cause by spider bites in the US per year.

Most of these deaths would have been caused or at least attributed to black widows. It was estimated by Jelinek (1997) that since the introduction of antivenin in the US around 1% of cases of black widow bites end in fatalities. Before, the figure was around 6%. Given 6.6 cases of fatality per year, and assuming that all spider bite fatalities are caused by black widows, this suggests that there are around 660 cases of black widow envenomation in the US annually. In a country containing over 300 million people you don't need to be a statistician to realise that the threat posed to you from a black widow spider is infinitesimally small.

In Australia, Jelinek estimates around 2000 L. hasseltii bites are recorded every year (there are likely to be more that are never reported) and the number of deaths annually from L. hasseltii is…. None. Nil. Nada. Zilch. Since the introduction of antivenin in 1956, nobody has died from the bite of a red back spider. Before then, there are only 13 recorded deaths ever.

Interestingly, the Latrodectus bite data shows a few unusual quirks. Firstly, in most countries you are more likely to be bitten or killed if you are male. Around 60% of cases of death in the US and bites in Australia were on males. You are also much more likely to die from a black widow bite in the US if you are white and aged between 20 and 64, going against the commonly healed view that most victims of spider bites are children.

That is not to say that the bites of any of these creatures is not to be taken seriously. Antivenin reduces the mortality rate from Laterodectus bites from around 6% to 1% or lower, and the symptoms of a black widow bite are not pleasant. Localised pain, swelling, sweating progressing to muscle cramps, spasms and paralysis with associated nausea, vomiting, fever and hypertension. But these spiders are not aggressive, the vast majority of bites are reported as occurring because of close contact/crushing of the spider against the skin. Bites appear to only occur in self defence. If you're careful, and know what you're doing, there's no reason why you couldn't carefully handle these spiders.

I doubt that the commonly held view on widow spiders is likely to change any time soon. Every news story about exotic spiders discovered in somewhere unusual is likely to contain the adjective "deadly." They can be deadly, although statistically much less deadly than a cow.

Or a horse. Or ecstasy, for that matter.

*DISCLAIMER - Stats are useful, but are a pain in the ass to calculate. That's for someone else to do.

** "potentially" - statistics again… It might kill you, but it also might not. Like a cigarette.