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Latrodectism: Diagnosis and Treatment


Of over 20,000 species of spiders in the United States, only about 50 species have fangs that can penetrate human skin and only 2 species are commonly implicated in human illness: the black widow (genus Latrodectus) and the brown recluse (genus Loxosceles). Widow spiders will be discussed in this issue of CALL US. Bites by the black widow spider are relatively common and can result in an extremely painful clinical syndrome known as Latrodectism.

Case presentation

A 33-year-old-male feels sharp pain to the right arm just after putting on a shirt that has been stored in a dark closet. As he reaches for the area of pain, he notices that it feels wet. Fifteen minutes later, he notices a progressively increasing, cramping pain at the site. He removes the shirt and a shiny black spider falls onto the floor. He notices a red area with central clearing at the pain site. Over the next 2 hours, crampy pain progresses to his trunk with difficulty taking deep breaths and severe discomfort. At this point, he seeks medical care. On arrival at an Emergency Department, he is hypertensive (168/102), tachycardic (124), and tachypneic (26), with a normal temperature (98.8°F) and oxygenation (98% on room air). He is noted by the Emergency Physician to be diffusely diaphoretic and have a rigid abdomen on examination.


  1. What is the pathophysiology of Latrodectism?
  2. What are the common presenting symptoms and signs?
  3. What treatments are available and what are their effectiveness and adverse effects?


black widow spider

The black widow spider belongs to the genus, Latrodectus, and is found world-wide in temperate and tropical zones. Although the most common species in the U. S. are jet black with a red-orange hourglass spot on the ventral abdomen, the genus is usually described as widow spiders because not all species are black. It is found throughout the United States except for Alaska. The widow spider is non-aggressive except when disturbed and tends to live in dark, protected areas such as corners of garages, woodpiles, and outhouses. Adult females are much larger than the males with 8-10 mm vs. 4-5 mm bodies. The female is solely responsible for envenomation due to the larger size of her fangs being able to penetrate the skin of the victim. Due to the small size of the male, his fangs are unable to penetrate the skin and he is unable to envenomate humans.

In 2003, 2720 black widow bites were reported to the American Association of Poison Control Centers with 635 in children and adolescents and the remainder in adults. 860 were treated in health care facilities where 380 were reported as “moderate outcomes” and 13 as “major outcomes.” No deaths were reported.


All Black Widow species have similar toxins. Alpha-latrotoxin is specific for mammals and is responsible for effects in humans. The toxin acts as a cation pore in the presynaptic neuron allowing influx of calcium which causes depolarization and release of neurotransmitters (primarily acetylcholine). This results in stimulation of skeletal and cardiac muscle fibers, pain, and autonomic hyperactivity.

Clinical presentation

Typically, the bite is felt as a pinprick sensation and the victim will usually recognize the injury. Onset of symptoms is usually within one hour (range: immediate to 12 hours) and can persist for a prolonged period (average: 22 hours +/- 25 hours, with symptoms persisting longer than 72 hours not unusual), often with a waxing and waning pattern.

skin symptoms

Symptoms usually progress from local pain and diaphoresis to pain in regional muscle groups and finally to the chest and abdomen. Severe muscle cramping is most commonly reported in the abdomen and may be severe enough to mimic appendicitis, colic, or peritonitis. However, patients commonly move to find positions of comfort rather than lie still as is common with peritonitis. Priapism has been reported in children. Signs include a “target” lesion at the bite site, hypersalivation, lacrimation, conjunctivitis, diaphoresis, tremors, tachycardia, and hypertension. Latrodectus facies has been described as spasm of facial muscles, edematous eyelids, and lacrimation. There is one report of death due to myocarditis secondary to widow spider envenomation in Greece.

Table 1. Grading of Envenomation

Grade 1
Mild Envenomation

Grade 2
Moderate Envenomation

Grade 3
Severe Envenomation

  • Localized pain at bite
  • Normal vital signs
  • Muscular pain in envenomated extremity
  • Extension of muscular pain to chest or abdomen
  • Local diaphoresis at bite
  • Normal vital signs
  • Generalized muscular pain in back, abdomen and chest
  • Diffuse diaphoresis
  • Latrodectus facies
  • Abnormal vital signs
  • Nausea and vomiting
  • Headache


Diagnosis is primarily clinical based upon symptoms and signs with a suggestive history. Diagnosis may be difficult in young children where an accurate history may not be available. There are currently no specific laboratory studies to assist in diagnosis. However, leukocytosis, elevated creatinine phosphokinase and lactate dehydrogenase levels are common. In adults with cardiac risk factors, consider obtaining EKGs and cardiac enzymes to assess for ischemic injury when indicated.


In the United States, treatment is primarily supportive with the goal of symptomatic relief, although an antivenom is commercially available.

Wound care

These bites rarely become infected, but the bite area should be cleaned and tetanus immunization updated if indicated.


In the past, intravenous calcium gluconate infusions have been recommended, but were found to be ineffective; in one large study relief was only reported in 4% of those treated. Therefore, calcium is no longer a recommended therapy.

Opioid analgesics in conjunction with benzodiazepines are most commonly recommended for all grades of envenomation. Approximately 70% of envenomated patients will obtain relief with these medications. Occasionally, very large and repeated doses may be required and may necessitate hospitalization for monitoring of respiratory status.


Antivenom may be recommended for grade 2 and 3 envenomations. Because of interspecies similarities of the a-latrotoxin, antivenoms against one Latrodectus species have repeatedly been found to be effective against other Latrodectus species.


A Latrodectus antivenom manufactured by Merck is FDA-approved and available in the United States. It is a whole IgG preparation derived from horse serum and carries a small risk of anaphylactic reactions. The volume of protein delivered is very small, however. There is a single report of death from Latrodectus antivenom therapy in the world’s literature, a case where the antivenom was administered as a bolus to a patient with asthma and possible horse serum allergy who developed anaphylaxis. A single vial of antivenom is completely effective in resolving symptoms with no recurrence reported after treatment. It is usually dosed as 1 vial in 250 mL of normal saline infused at 1 mL/min for 15 minutes and the remainder over 1 hour. A skin test for horse serum hypersensitivity can be done, but is neither sensitive nor specific. A F(ab)2 antivenom that has been used effectively and safely in Mexico since 1999 is undergoing clinical trials currently in the U.S.

Table 2. Pharmacologic Therapy




Maximum Dose (may be exceeded in some cases)



0.1 – 0.2 mg/kg


5 – 10 mg

2 – 4 hours


1 – 3 mcg/kg


100 mcg

30 – 60 min


0.1 – 0.2 mg/kg


5 – 10 mg (child)
30 mg (adult)

2 – 4 hours


0.05 – 0.1 mg/kg


5 mg

4 – 8 hours

Merck Antivenom

1 vial in 250 mL normal saline over 30-60 minutes


1 vial

Once, may be repeated

Patients with good pain control, normalized vitals signs and no adverse effects of the treatments may be discharged home.

Discussion of case questions

  1. The active component of the venom known as alpha-latrotoxin acts as a cation pore inducing release of neurotransmitters from presynaptic neurons.
  2. Initial symptoms and signs include a pin-prick pain at the bite site followed by muscle cramping and pain that progresses centrally to the chest and abdomen along with hypertension, tachycardia and diaphoresis.
  3. Standard treatment includes symptomatic care using opioid analgesics and benzodiazepines. The adverse effects of which include sedation and respiratory depression. More severe envenomations may require antivenom, but the antivenom has been reported to cause anaphylaxis in rare cases.