There are about 40,000 different species of spiders worldwide, with only a handful of species being medically relevant causing significant bites and stings. We’ll cover our two most common players: the brown recluse and black widow spiders. Here we go!

Brown Recluse Spider-Loxosceles reclusa

What is the with Brown Recluse Spider envenomations?

In the south and the lower Midwest. Look for the fiddle! It has six eyes! #6EyeGuy #FiddlerSpider
Painless bite, with necrosis. Hyaluronidase (tissue penetration) and sphingomyelinase D (necrosis/thrombosis/ischemia), but no systemic symptoms #VitalSignsStable #HyaluronidaseFirst #SphingoWUTDoesEverything
Erythema, poss scar heals in weeks. Bad ones can cause ulceration, vesiculation, violaceous necrosis, blanching. Eschar in weeks. N/V/F/C, arthralgias, low platelets, rhabdo, DIC, and renal failure can occur (Get CBC/CMP/Coags). Hemolysis can happen at 24-72 hours. Pain control and supportive care .
 #RedWhite&BlueSign #ItsABigDeal #GetLabs #SupportiveCareFTW

When to suspect?
Brown recluse spiders are generally localized within the Midwest and the south. Texas, Oklahoma, Kansas, Missouri, southern Illinois and Indiana, Kentucky, Alabama, Mississippi, Arkansas and Louisiana are home to the brown recluse.

Distribution of the Brown Recluse Spider in the US

Anecdotally, brown recluse spiders have been found in California among other states, although never confirmed. Literature suggests that encounters with spiders beyond endemic regions are rare.
These spiders will measure up to 2 cm long, colored gray, orange, or brown. The brown recluse spider is characterized with a pigmented fiddle-like shape on their thorax. That said, other species of spiders can possess this as well, therefore it is not specific. However, unique to the loxosceles species, are their 6 eyes (one anterior with two lateral pairs), versus the average spider possessing 8. The spider is generally nocturnal and shy—it generally bites when threatened. It lives within dark, and dry areas including basements, wood piles, and closets. It usually bites in the morning, between the spring and fall. It is a very resilient in that it can survive up to 6 months without food or water, while tolerating temperatures between 45-110F. Females, like the black widow, are more dangerous than the males. 

How it hurts:
The brown recluse spider causes a necrotic presentation, because its venom contains hyaluronidase and sphingomyelinase D among other enzymes. Hyaluronidase allows for venom penetration into tissues, while sphingomyelinase causes necrosis of tissue through a chain reaction that produces inflammatory mediators that causes vessel thrombosis, ischemia to tissue, and skin injury.  In comparison to the black widow spider (the other clinically significant spider in North America), it generally does not have any systemic symptoms.

The bite of a brown recluse is generally painless, which creates a diagnostic challenge in terms of the patient knowing when he/she got bitten and identification of the spider.  Patients may present to the ED with skin lesions they attribute to a spider bite, although it often has another etiology. In one study, of 182 patients who complained of spider bite only 3% were confirmed while 84% had a skin and soft tissue infection.

Bites commonly manifests as a mild erythematous lesion/papule with possible scar, but ultimately heals in a week or two. More severe bites or reaction to bite can result in substantial pain, erythema and pruritis with associated swelling. Ulceration can occur in 2-8 hours. This will evolve in 1-3 days where there is ecchymosis. There is usually central hemorrhagic vesiculation, ulceration, then violaceous necrosis surrounded by ischemic blanching of skin and outer erythema and induration. This is often dubbed as the “red, white and, blue sign.” By a week, an eschar can form.

While systemic symptoms of a brown recluse spider bite are rare, a sign of systemic brown recluse envenomation is hemolysis, which generally occurs 24-72 hours after the bite. They may also develop nausea, vomiting, fever, chills, arthralgias, thrombocytopenia, rhabdomyolysis, DIC, hemoglobinuria, and renal failure. Systemic loxoscelism typically affects children and severity cannot be predicted by the extent of the cutaneous lesion.

If your suspicion is high, obtain CBC, CMP, and coagulation studies.

Clean the wound site, given tetanus prophylaxis as indicated. Elevate the bitten extremity and apply cool compresses.

Supportive care (including pain) will be paramount in management of a brown recluse spider bite. If the site appears infected, then antibiotics are warranted. Serial wound evaluations are warranted such that if the wound worsens, surgical debridement may be warranted after adequate tissue demarcation has occurred, typically several weeks after the initial bite. Early excision of the bite site has not been shown to be helpful.

There are other experimental interventions, however these have not been proven to be beneficial: dapsone, colchicine, antivenom, HBO.

Black widow spider—Latrodectus

What is the with Black Widow Spider envenomations?

Look for the hourglass #ButDatHourglassDoe
Where? Everywhere, but west coast (California), the south, and the east coast are common places #ItsEverywhere #NobodyIsSafe

Toxin binds to presynaptic neurexin I-a (Ca2+ dependent), and Ca2+ independent receptor, called latrophilin. Causes a calcium pore to form, with exocytosis of vesicles (NE, dopa, neuropeptides, ACh, glutamate, and GABA. G-protein receptors and PLP) #LeakingCalcium #ItsAllCalciumBaby #NeurotransmitterShotgun
Pinprick bites, spreads up the extremity. Erythema, with a target-like lesion or halo #LymeMimic
Erythema, and neuromuscular cramps/rigidity as early as 15 min. Chest, Abd, face. #latrodectismSUCKS #cramps #rigidityNOTtetanus #MyBellyHurtsEverywhere #ImGonnaDie #PavorMortis
Abx are not routine. Give IV opioids and benzos for pain and spasm control. #BenzosBenzosBenzos #SeeAPatternYet

Antivenom puts at risk for anaphylaxis and serum sickness (HORSE SERUM), and usually reserved for pregnant women.  #GiveWhenWarranted #Horsey

When to suspect?
Latrodectus spiders, or widow spiders are found worldwide. There are 5 species in the US, however the black widow is only three of these (mactans, variolus, hesperus). Many of the latrodectus species, both US and worldwide, have a distinct design on their abdomen, whether it be . The classical hourglass is on the mactans species. Males are small, and they cannot bite. Females however, measure up to 1cm in body width with leg spans up to 4-5cm and their bite can be very toxic. The females lay their eggs in the warmer months, and will defend her eggs aggressively, thus most black widow bites occur between April and October usually on the hands and forearms. Latrodectus spiders are found in similar places as the brown recluse, such as woodpiles, garages, sheds, basements, stone walls, crevices, woodpiles, outhouses, barns, stables, rubbish piles. Geographically, most of the latrodectus live in temperate conditions, on the west coast (California predominantly), the south, or the east coast.

How it hurts:
The venom of the black widow deserves respect. It is more potent than on a volume per volume basis versus a pit viper. There are 6 components to the venom, a-latrotoxin and 5 latroinsectotoxins/latrocurstatoxin. These toxins bind to presynaptic neurexin I-a (Ca2+ dependent) and a Ca2+ independent receptor for a-latrotoxin, known as latrophilin. When it binds, it causes conformational change, which then forms a pore, with a calcium ionophore, with exocytosis of vesicles that contain norepinephrine, dopamine, neuropeptides, Ach, glutamate, and GABA. The mechanism is mediated through G-protein receptors and phospholipase C.

Unlike the brown recluse bite (where it is often unnoticed), most latrodectus bites are felt as a pinprick. Pain starts at the bite site and spreads quickly up the extremity. Erythema arises within an hour of the bite. A small macule may emerge from the bite site, that evolves into a target lesion (blanched center) or a halo presentation.

Latrodectism is a constellation of signs associated with a bite from a widow spider. Like the erythema, severe neuromuscular symptoms can occur within the hour. Time to symptoms is correlated to the severity of the bite. Severe muscle cramps can occur within an hour, as early as 15 minutes. Rigidity can spread to other muscles throughout the body, particularly in the chest, abdomen, and face. It can in fact, mimic a surgical abdomen, but this is secondary to severe muscle spasms. These will resolve in a few hours but can recur over days. In addition, sweating, contorted/grimaced face, with blepharitis, conjunctivitis, rhinitis, and masseter trismus can all occur. Patients will describe a fear of death, known as pavor morits. Symptoms such as nausea, vomiting, sweating, tachycardia, hypertension, restlessness can occur. Severe complications such as malignant hypertension, respiratory distress, and cardiovascular instability, and gangrene can ensue that can cause death. That said, in a span of 20 years, only 2 deaths have been reported, which in part, is due to the development of antivenom.

In all things emergency medicine, ensuring airway, breathing, and circulation is paramount. Evaluate the wound, provide tetanus prophylaxis, and like brown recluse bites, antibiotics are not warranted unless an obvious infection is present. If the envenomation causes muscular pain with migration to the trunk or even worse, with abnormal vital signs with systemic symptoms, give IV opioids and benzodiazepines to manage pain and muscle spasm.

The antivenom of latrodectus is effective, but has been known to cause anaphylaxis and serum sickness given that it is a crude hyperimmune horse serum. Because of this risk, the antivenom is usually reserved for the most severe envenomations, ones that are producing systemic symptoms with vital sign abnormalities, especially pregnant women. The dose is generally 1-2 vials of antivenom diluted in 50-100ml of D5W or NS, infused over an hour. At this time there is no evidence supporting or rejecting use of antihistamines in setting of antivenom administration.


Schneir A, Clark RF. Chapter 311: Bites and Stings-Spiders. In Tintinalli JE, editors. Tintinalli’s Emergency Medicine: A Comprehensive Study Guide, 8th edition.  

Hahn I. Chapter 118: Arthopods. Goldfrank’s Toxicologic Emergencies. 10th edition.

Text written: Alexander Huh, MD
Reviewed by: Anthony Scoccimaro, MD

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