A 30-year-old male presents to the emergency department with a swollen, tender red nodule on his right side for one week. ”

How did it get there?,” you ask.

“Oh– I was bitten by a spider,” he assuredly pronounces.

Our 8-legged, somewhat creepy little friends are often blamed for soft tissue infections presenting to the ED and Urgent Care settings.  If I had a nickel for every furuncle/abscess/cellulitis that a patient blamed on the bite of a spider, I could probably retire from my job in Emergency Medicine!

The reality is, spiders get a bad rap.  Spider bites are likely not culprit for the majority of soft tissue lesions, as most practitioners come to realize with experience.  And, even if an everyday, garden variety spider did give you a bite at the site of an infection, it’s likely inoculation with your own skin bacteria during the biting event lead to the problem.  Thus, the spider bite may as well be a minor abrasion, splinter, or your own ingrown hair follicle.

But what about when a spider bite really is to blame for a soft tissue lesion?

This said, as medical professionals in the acute care setting we can’t be totally oblivious to the potential pathology a spider can cause.  On the contrary, we should aim to be the most knowledgeable people in the room at all times!

What are the spiders we really need to worry about?  Where do these ominous creatures live, and how do we identify a skin lesion actually caused by one of these spiders?  And, why does it matter? How would your initial management differ from the treatment of any other infectious skin lesion?

My interview with Dr. Alonso Miguel, a Chilean Emergency Physician at Universidad Católica who has plenty of experience to share regarding bites from the Chilean recluse spider.

I tackled these questions during my visiting professorship at the Universidad Católica in Santiago, Chile, where physicians regularly deal with the more venomous and more deadly cousin of the brown recluse spider, the araña de rincon (Loxosceles laeta).  This post complements a segment released January 2018 on EMRAP.  There’s a lot to cover so I will tackle this in two-parts.

Spiders 101

A few fun (and practical) facts about spiders:

black widow spider.jpg
The well-known black widow spider is easily recognized by its red hourglass shaped marking on its abdomen.  Image from: terro.com
  • All spiders are venomous.  However, few spiders possess fangs long enough to puncture human skin, and thus potentially cause a threat to us.
  • Medically important spider syndromes worldwide include Lactrodectism (caused by the black widow spider bite) and Loxoscelism (caused by the brown recluse spider).
  • There are other even more dangerous spiders out there.  For example, the Australian funnel-web spider actually wins the prize for the most dangerous spider known to exist. A significant bite causes rapid onset of life-threatening effects including neuro-excitation,  autonomic dysfunction, pulmonary edema, and even death–but fortunately it lives in such a geographically limited area that it doesn’t pose a threat to most of us.


Spiders of the Genus Loxosceles have the distinction of being the only spider where enough evidence exists (via human case series and bench research on its venom) to support a causal relationship between the bite and pathophysiologic effects. For this reason, the rest of this post will focus on this bite syndrome.

Two forms exist: cutaneous and viscero-cutaneous loxoscelism. Since this is a website about wound care, the cutaneous form is described in detail below.  Suffice it to say that in viscero-cutaneous loxocelism, a severe, sometimes life-threatening illness develops including hemolysis, coagulopathy, shock, renal failure, and multiple organ dysfunction.


Scientific Classification: 

  • Family: Sicardiidae
  • Genus: Loxosceles
  • species: multiple exist. reclusa and deserta (North America) and laeta (Chile, Brasil) are most often implicated in causing deleterious effects on humans.

What does the recluse spider look like?

In cases where a person has concern about a spider bite and actually brings the dead spider in for examination, there is some value to examining it closely.

  • The eye pattern is its most distinctive feature.  While most spiders have 8 eyes arranged in 2 rows of 4, the loxosceles spider has 6 eyes arranged in dyads, with one pair anterior and two lateral.  However, without magnification and lots of experience looking at spiders up close, this can be hard for a layperson or medical practitioner to recognize.
  • Its leg-to-thorax ratio is high (long legged, not chubby).  But this is true of many spiders.
  • Loxosceles typically has a violin shaped pattern on its back, but this is not diagnostic.  This can be missing in young recluse spiders, fades in older spiders, and can be seen in other spiders as well.
Screen Shot 2018-01-03 at 11.37.30 PM
From the 2016 Chilean guidelines on management of bites from Loxosceles laeta.

Where does the recluse spider live?

Roughly 100 species exist worldwide.  Most live in South America, where they present a major health issue. The first case of Loxoscelism was actually described in the 1930s in Chile.

Cases of loxoscelism in North America and Mexico are attributable to Loxosceles reclusa and Loxosceles deserta. In Chile, the primary offender is Loxosceles laeta, aka the Chilean recluse spider, aka the arana de rincon. In Brazil the same spider is often referred to as the aranha marrom (brown spider).

What does the loxosceles bite look like?

The bite of the brown recluse spider in North America is not very painful (though reportedly bites from the Chilean araña de rincon certainly can be!) and will likely be ignored by most people if they don’t notice the spider. Patients thus present for medical attention (1) when they see the spider and are nervous about potential adverse effects, or (2) when they actually develop a cutaneous syndrome. Given this, the true extent of minor cases is likely under-reported.

The cutaneous manifestations of the bite vary depending upon the time of presentation.

  • 0-12 hours: initially, mild erythema and pain are all that characterize the bite, often mistaken for simple cellulitis.
In the early stages, the bite of a brown recluse spider can be mistaken for a simple cellulitis.
  • 12-24 hours: painful edema, irregular erythema, sometimes blisters.
A small blister forms at the site of a bite from Loxesceles laeta.
  • 24-48 hours: not much change occurs in the wound itself, but non-specific, constitutional symptoms may occur during this time frame.
  • ~72 hours: skin necrosis occurs.
A purplish hue is noted as skin necrosis from the bite of a brown recluse spider sets in.
  • Days 5-7: the cutaneous lesion delimits itself and forms a dry, necrotic eschar.
Several days after the bite of a brown recluse spider, the margins of the wound become more well demarcated and a dry, necrotic central ulceration is present.
  • 2-3 weeks: the eschar detaches, and from there the lesion can take months to heal.

How does one get bitten?

Confirming a brown recluse bite is tough. Though an assay exists to confirm the presence of the venom in the bite in humans, it’s not commercially available.

Thus, we are left with trying to make a clinical diagnosis based on the story the patient provides, the appearance of the lesion, and sometimes, the spider itself.

The first consideration should always be geography.  If you don’t live in one of these areas in the United States, your chances of a bite are essentially nil.

recluse spiders in the US.png
From the website, spiders.ucr.edu

And, even if you do, don’t panic.  The recluse is just that–it’s a fraidy-cat and will run away from you, not attack you.  A 1970 survey performed of the most infested homes in Chile (average 163 spiders in the house) had no people with bite-related syndromes. A man in Kansas collected 2055 confirmed brown recluse spiders over a 6 month period and no one in his household was bitten during this time period (2002 study).  What this says is that, even in areas where the spiders have high densities, bites are unlikely; so in areas where they are not present in high densities, bites causing issues should be rare to non-existent.

So how do bites occur?  The recluse tends to nest in clothing and bedsheets. Bites occur when the spider is frightened.  Thus, highest risk occurs when putting on the clothing in which they were hiding or rustling the sheets when heading to bed.

I am concerned my patient has a bite!

Read through all of this, and still not sure if the patient in front of you with a cutaneous lesion was in fact bitten by a brown recluse? Fear not! In Part II, I’ll present a handy mnemonic device to help you rule out a recluse spider bite, and discuss various treatment options for your patient who actually is bitten.


  • Swanson DL, Vetter RS.  Bites of Brown Recluse Spiders and Suspected Necrotic Arachnidism.  N Engl J Med 2005;352:700-7.
  • Isbister GK, Fan HW.  Spider bite.  Lancet 2011; 378: 2039–47.
  • Isbister GK, White J.  Clinical consequences of spider bites: recent advances in our understanding.  Toxicon 43 (2004) 477–492.