Arachnolingo 2, part II: Medically significant, dangerous, deadly

This is part 2 of an article that was so long it became a trilogy. You can click here for part one, which explains what "venomous" really means, and here for part three, about why the distinction matters.

 "-Is this spider venomous?
-It's not medically significant."

Unlike "venomous" or "venom", which zoologists try to define based on the  biology and evolution of these substances in the natural world, terms describing the relationship of animals with us, humans, such as "dangerous" or "deadly", are colloquial, so their definitions are not as strict and unified.

One could argue, for instance, that any species which has ever killed at least one human being should be deemed "deadly", or that anything that represents any risk, even purely theoretical, to human life or health, could be classified as "dangerous".

However, calling "deadly" animals whose bite is fatal in only about 1 in 500 to 3000 cases still sounds like a bit of an exaggeration, doesn't it?
It feels kinda silly to call a horse or a dog "deadly", yet these animals kill more people annually than any species of spider (and more than all spider species combined), including those the media readily call "deadly", such as black widows.
"Deadly" is one of these words that will make a venomous species sound much more dangerous than it really is.

Although it has killed people before, the bite of the "deadly" Mediterranean black widow (Latrodectus tredecimguttatus) has a survival rate of about 99.8%. While it is undeniably medically significant, calling it "deadly" is kind of a stretch...*

If "deadly horses" sounds ridiculous to you but "deadly spider" doesn't, keep in mind that horses kill more people in a year, and are therefore technically deadlier, than all spider species combined

All in all, a needlessly scary term which is best avoided, or at least reserved for the few animal species that do cause a fairly high, or at least significant, number of fatalities every year (and therefore not applicable to any spider).
In addition to that, death is the most extreme negative outcome of an envenomation, but just because a venomous species is not typically (or never) deadly, doesn't mean its bite/sting can not be a harrowing experience.

Although "dangerous" is somewhat more reasonable, there are still a lot of situations where it would heavily overstate how high the chances of a serious envenomation actually are.

It takes more than just potent venom to reasonably call a species "dangerous". Generally, zoologists only use this term if all these criteria are met:
- The species in question is armed with venom (or any type of other weapon, chemical or physical) potent enough, in amounts large enough to potentially cause significant harm or even death in a human
- This weapon is dangerous enough to cause clinically serious or life-threatening symptoms in a significant or high proportion of bites or sting (or other types of poisoning) cases
- In case of confrontation, the species is likely to use that weapon instead of fleeing or using more benign types of defences
- The species tends to be often found in places where humans live or work, resulting in a high risk of encounters and confrontation.

Well-armed with their horns and considerable mass, Cape buffaloes (Syncerus caffer) are notoriously unpredictable and tend to react aggressively towards percieved threats; they are dangerous, and should always be treated with the greatest caution

Many animals potentially capable of seriously harming humans actually fail to meet several of these criteria.
For instance, African six-eyed sand spiders (genus Hexophthalma) are claimed, based on experiments conducted on rabbits, to be "among the most dangerously venomous spiders on Earth". Yet, the exact extent of the effects their bite can have on a human is still to be determined, as there are so few records of verified bites that none are described in detail in medical literature. They live in sparsely populated, semi-arid to arid areas of Southern Africa. In addition to that, six-eyed sand spiders are very uncommon, and spend most of their time buried in loose sandy soil; finding one generally requires scratching around in the sand with a small rake. Finally, these spiders (like their South American cousins in the genus Sicarius) have an extremely passive disposition towards potential threats, and do not tend to bite, even when handled (which is not a reason to treat them recklessly!). All in all, while their bite is probably quite dangerous, these spiders themselves can hardly be regarded as such.

While often claimed to be "among the most dangerously venomous spiders on the planet", African six-eyed sand spiders (genus Hexophthalma) are rare, live in sparsely populated areas, and are not inclined to bite; they can hardly be called "dangerous" (photo: Jarrod Michael Todd)

While a black mamba, a puff adder or a crocodile can reasonably be called "dangerous", that word exaggerates the threat represented by even the most "dangerous" spider species (except maybe for males of the Australian family Atracidae, which may arguably be called "dangerous" for their defensive disposition, dangerous venom, and tendency to wander around and sometimes cross a human's path).

Few people would consider eating oysters a dangerous activity, yet the risk of falling seriously ill or dying from eating an oyster is much, MUCH higher than the risks associated with having recluse spiders or black widows in or around your home: in the United States alone, it is estimated that about 80 000 people a year get severely ill, and about 100 of them die, from vibriosis after eating a bad oyster. And this is from vibriosis alone, not considering other types of food poisoning and seafood allergies.

Meanwhile, black widows bite about 2600 Americans a year, of which about 1% (something like 20-30 people) experience life-threatening symptoms, causing fewer than ONE fatality per year!
The scarcity of black widow bites in the US is not caused by a rarity of encounters; unlike some other Latrodectus species such as the Mediterranean black widow (L. tredecimguttatus) or the African "black button spiders" (such as L. indistinctus, L. renivulvatus or L. cinctus) who typically live away from human settlements, North American black widows (particularly L. mactans) can quite often be found around houses.
The main reason why there are so few envenomations is that most encounters with black widows do not result in a bite, because these spiders (just like most spiders) are very shy and passive animals; tests have shown that in the face of a threat, a black widow will only be likely to bite if pressure is applied on its body. Therefore, not only is the risk of serious reaction to a bite relatively low, but the risk of getting bitten in case of encounter is also very low.
In other words, the danger posed by widow spiders is actually very low, although these numbers of bite cases and severe reactions still makes them the most medically important spider genus on Earth.

Black widows are reluctant to bite humans and will only do so in case of direct and prolonged contact, even when they're guarding egg sacs (however, females with eggs will be more likely to stand their ground instead of fleeing or dropping, which increases the risk of direct contact)*

Of course, the point of this comparison is not to make you afraid of eating oysters. It is to show that if the thought of having a black widow or a recluse anywhere near your home makes you want to burn the house down, but on the other hand you happily eat oysters, drive a car or live with a large dog without thinking twice about it, your perception of spiders may be severely skewed. Eating oysters is largely safe, so are cars and dogs. However, all of these casual, ordinary things still come with a much greater risk to health and life than spiders.

Calling them "deadly" or "dangerous" is, thus, a needlessly scary overstatement. Calling them "harmless", on the other hand, would also be exaggerated and irresponsible. Widow spiders are definitely NOT harmless; bites may be rare, and severe cases even more, but they still happen, and a severe envenomation by a black widow is still an excruciating, and temporarily incapacitating experience. Although fatalities are extremely rare, adult people have died from Latrodectus bites.
Bites are unlikely, but care is still advised when working with or around widow spiders, or in places where they are likely to be found, to avoid accidentally squishing one against your skin (which is how bites happen).
Same applies to the few other spider species whose bites can have significant consequences.

Recluse spider (Loxosceles spp.) envenomations are almost never life-threatening and rarely severe, but the sores they can cause are painful, prone to secondary infection, and notoriously slow to heal

There is, therefore, a need for a term which expresses the possibility of clinically significant consequences of the bite/sting of a species, but doesn't overstate the threat posed by the animal itself.
That term is "medically significant". 

Unlike colloquial terms such as "dangerous", whose meaning is vague and can vary depending on who is talking, "medically significant" strives to have a clear, much more precise definition.
Widely used among arachnologists, it describes a species whose bite (or sting) can cause, by the direct effects of the venom, symptoms severe enough to require and justify medical assistance to manage them. Every word in that sentence matters. 

It doesn't necessarily mean that the animal's bite or sting can kill people, or that every envenomation will be a medical emergency; it doesn't even have to be most cases. A species will be considered medically significant even if serious symptoms only occur in a small proportion of bite cases, and even if only very young children or vulnerable people are at risk of developing these symptoms. 

It is, however, important to keep in mind that only symptoms that are direct effects of the venom are considered in the definition, so complications such as allergic reactions or secondary infection are not. While some substances are more commonly allergenic than others (for instance, allergic reactions to wasp and bee venoms are very common, while allergies to spider and scorpion venoms are almost unheard of), allergy and other reactions caused by the immune response of the body (such as necroptosis) can be triggered by virtually any venom (or even non-venomous secretion), and vary randomly from person to person; thus, including allergic reactions in the definition would mean every animal species, from bees and mosquitoes to recluse spiders, would be called medically significant.
Same goes for secondary infections, which can develop from spider bites as well as from any wound inflicted with a non-sterile object, being a mosquito's rostrum, a cat's tooth, or a fingernail scratching* an itchy bite, as they are caused by bacteria common on many living and non-living surfaces.
Think about it that way: just because people can suffer life-threatening food poisoning or seafood allergy from eating prawns, doesn't justify regarding them as significantly toxic to humans, because these complications are not direct results of prawn flesh toxicity. Same goes for venoms.
Of course, if the animal in question carries a pathogen which is specifically transmitted by its bite, that's a different story; there are unfortunately many specific mosquito-borne diseases, for instance, that justify calling these insects "medically significant" (and also dangerous and deadly; ironically, they're both nonvenomous, and the deadliest animals on the planet).

Finally, the main part of the definition is about medically significant symptoms requiring and justifying medical management. Medical practitioners frequently prescribe antibiotics and anti-histamine as a preventive measure for bites and stings (by the way, antibiotics are no use against venoms, the reason why doctors prescribe them is to prevent or fight infection of the bite until the body is done healing). Seeking medical attention for a bite or sting and receiving treatment is a wise move, particularly if some unusual symptoms are developing, but just because medical attention was sought doesn't automatically make the bite medically significant.
Symptoms are called medically significant (at least in the context arachnologists mean it) when their intensity and/or duration go beyond what can be safely and easily handled at home.
For instance, a painful bite won't be considered medically significant as long as the pain is mild enough to be ignored, handled successfully with over-the-counter pain medication, or more intense but short-lived enough to make it through without assistance. However, it will be if the pain is intense and long-lived enough to be unbearable without the help of prescription-only painkillers, or even hospital care.
Same goes for other symptoms: mild systemic effects (short-lasting and tolerable nausea and dizziness, for instance) may not necessarily be regarded as medically significant, but they will if they persist for days, or become severe and debilitating. A small, superficial skin lesion at the bite site is not medically significant, but one that spreads to several centimetres in diameter and/or takes a long time to heal, leaving it vulnerable to secondary infection, or even threatens deeper tissue, will definitely be regarded as such.

Bites from the brown widow/brown button spider (Latrodectus geometricus) rarely cause very serious symptoms (and there's no record of a life-threatening envenomation), but sometimes cause severe pain and painful muscle cramps, which justify regarding this species as medically significant

Of course, everyone's tolerance to pain and disease is different, so in that regard, the notion of "medically significant symptoms" includes a part of subjectivity. That's why it's important, rather than drawing conclusions solely from anecdotal cases, to conduct cohort studies that, ideally, include dozens of verified bite cases to assess (be it to confirm or rule out) a species' medical significance (although that is not always possible, as many species very rarely bite humans).

All in all, it's quite a simple notion (as it's meant to be): if its venom can cause any type of symptoms to a human that are too much to safely manage at home, even if it's only to very young children, even if it's only in 0.1% of all bite cases, the species will be considered medically significant.
If it's not (which is the case of 99% of all spider species), you simply don't need to worry about it, or wonder if it would still be a problem to your baby or elderly relatives. Just treat it like you should treat any insect bite, scratch or cut: disinfect it thoroughly, keep it clean and dry, don't tamper with it, and you'll be fine.

There is, therefore, quite a substantial difference between "medically significant" and "dangerous"; these words aren't really synonyms. To be considered dangerous, a species not only has be medically significant, but also to present a high risk to human life or health in case of bite (or sting), AND a propensity to use its venom (or other weapon) in case of confrontation with a human.
Not every venomous species is medically significant, and not every medically significant species is dangerous.⏹

Except when the source is explicitly cited, the images illustrating this blog are mine and are not free to use without permission.

References are integrated into the text of the article; the words in blue are clickable and will redirect you to the sources of the information.

* I am a trained professional, experienced in working with medically significant arachnids and other venomous wild animals in the field. Please do not attempt to replicate what you see on this image; free handling wild animals, particularly those potentially capable of harming humans, is never advised, and is not something I do or condone outside of specific circumstances.