Widow Spider Envenomation

Background

Widow spiders belong to the genus Latrodectus and include the black widow spider (Latrodectus mactans mactans) in the United States. The term widow spider is used because not all species in the genus Latrodectus are black. Other widow spiders in North America include the brown widow (Latrodectus geometricus), the red-legged widow (Latrodectus bishopi), Latrodectus variolus, and Latrodectus hesperus. The redback spider (Latrodectus hasselti) is endemic to Australia. Latrodectus mactans tredecimguttatus and Latrodectus pallidus are found in Europe and South America, and the button spider (Latrodectus indistinctus) is found in South Africa.

The adult female black widow spider is approximately 2 cm in length and shiny black with a red-orange hourglass or spot on the ventral abdomen. The male is much smaller, brown, and incapable of envenomating humans. Juvenile females are also brown but have the general body morphology of the adult. Males and juveniles have a pale hourglass shape, similar to adult females. The female sometimes eats the male during or after copulation. Webs are irregular, low-lying, and commonly seen in garages, barns, outhouses, and foliage. Other widow spiders are generally black but may have red spots, such as Latrodectus mactans tredecimguttatus, or a dorsal red stripe, such as the redback spider. Latrodectus geometricus is brown with red and yellow markings.

See the images below.

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Female black widow spider, Latrodectus mactans, in the process of spinning her web on a tree branch. Note the characteristic red hourglass located on ....

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Female black widow spider guarding an egg case; species Latrodectus mactans. By Chuck Evans (mcevan). [CC BY 2.5 (https://creativecommons.org/license....

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Black widow spider (Latrodectus mactans) and offspring. Photo by Sean Bush, MD.

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A redback spider (Latrodectus hasseltii) female, found in suburban Sydney, New South Wales, Australia. By Toby Hudson [CC BY-SA 3.0 (https://creative....

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Young female Latrodectus tredecimguttatus; photo taken in Croatia by K. Korlevic. No machine-readable author provided. Kork~commonswiki assumed (based....

Envenomation is an uncommon occurrence with an extremely variable presentation. Treatment of envenomation often is based on speculation and anecdote, and much of the literature is contradictory. This article attempts to keep recommendations in agreement with the most current standards of care. This article serves as a guideline, and the clinician should use judgment for individual patient encounters.

See Arthropod Envenomation: From Benign Bites to Serious Stings and Venomous Spider Bites: Keys to Diagnosis and Treatment, Critical Images slideshows, for help identifying and treating various envenomations.

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Pathophysiology

Alpha-latrotoxin causes the toxic effects observed in humans by opening cation channels (including calcium channels) presynaptically, causing increased release of multiple neurotransmitters. This results in excess stimulation of motor endplates with resultant clinical manifestations. Clinically, the predominant effects are neurological and autonomic, in contrast to the dermonecrotic local effects associated with spiders causing necrotic arachnidism (eg, brown spiders [Loxosceles species]).

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Epidemiology

According to the 2016 Annual Report of the American Association of Poison Control Centers’ National Poison Data System (NPDS), approximately 1300 widow spider bites were reported for 2016,^[1] although this figure is probably conservative because of underreporting. No deaths caused by widow spider envenomation have been reported to the AAPCC since its first annual report in 1983.^[1] Deaths after black widow spider bites were reported in 2001,^[2] 2003,^[3] and 2006 in Spain, Greece, and Albania (2 deaths), respectively.^[4]

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Prognosis

The vast majority of patients with widow spider envenomations recover fully.

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Patient Education

For patient education resources, see the Bites and Stings Center, as well as Black Widow Spider Bite and Brown Recluse Spider Bite.

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History

Initial pain at the bite site is generally trivial and may go unnoticed. It commonly is described as a pinch or pinprick; however, infants may present with unexplained crying.^{[5, 6]}  Within about 1 hour, systemic symptoms begin and may last for a few days.

Muscle cramping may occur locally, around the area bitten. It may extend into large muscle groups, such as the abdomen, back, chest, and thighs. Case reports suggest involvement of smooth muscles, such as bronchial or endometrial.

Other symptoms may include nausea and vomiting, headache, and anxiety.

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Physical Examination

Abnormal vital signs may include hypertension and tachycardia.

Diaphoresis, locally, around the area bitten, or remote from the site of envenomation, may occur.

Tiny fang marks may be visible.

Local effects are usually limited to a small circle of redness and/or induration around the immediate bite site. A central reddened fang puncture site surrounded by an area of blanching and an outer halo of redness is described as a having a target appearance.

Abdominal rigidity may mimic an acute abdomen.

Neurologic effects, including mild weakness, fasciculations, and ptosis, have been described.

Latrodectus facies, characterized by spasm of facial muscles, edematous eyelids, and lacrimation may occur. This can be mistaken for an allergic reaction.

Bronchorrhea and pulmonary edema have been described in Europe and South Africa.

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Complications

Complications may include the following:

• Respiratory difficulty, reactive airway exacerbation
• Spontaneous abortion or preterm labor
• Hypertensive emergency with or without associated seizures (isolated normotensive seizures have not been described), acute myocardial infarction
• Rhabdomyolysis
• Priapism (rare)
• Compartment syndrome (rare)^[7]
• Toxic myocarditis (rare)

Also see Treatment/Complications.

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Laboratory Studies

If the diagnosis is uncertain, laboratory studies to rule out alternative diagnoses (eg, acute abdomen, myocardial infarction) (eg, CBC, pregnancy test, cardiac markers).^[8]

The serum creatine kinase (CK) and peripheral white blood cell count may be elevated.

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Imaging Studies

Consider performing an acute abdominal series or an abdominal CT scan if the diagnosis is unclear in a patient presenting with abdominal pain.

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Other Tests

Skin testing variably may predict immediate hypersensitivity to antivenom and may influence the decision regarding its administration. Antivenom-induced anaphylaxis may be more life threatening than the envenomation itself.

Obtain an ECG, if indicated (eg, comorbidity, chest pain, severe hypertension).

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Prehospital Care

Support the airway, breathing, and circulation per ACLS protocols with oxygen, monitors, and intravenous line.

Negative pressure venom extraction devices (eg, The Extractor - Sawyer Products) have not been evaluated for treatment of widow spider envenomation.

Electric shock and various folk and herbal remedies lack therapeutic value and are potentially harmful.

Do not give antivenom in the field because of the risk of severe allergic complications.^[9]

Attempts to secure the spider may be helpful in confirming widow spider envenomation.

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Emergency Department Care

Antivenom should be given for imminent risk of severe complication of envenomation (see Complications). The risk of allergy to antivenom must be weighed against the benefit of relieving prolonged discomfort, avoiding hospitalization, and preventing complications.^[9]

Grade 1 - Mild envenomation is as follows:

• Local pain at envenomation site
• Normal vital signs

Grade 2 - Moderate envenomation is as follows:

• Muscular pain in the envenomated extremity
• Extension of muscular pain to the abdomen if bitten on a lower extremity or to the chest if envenomated on an upper extremity
• Local diaphoresis of envenomation site or involved extremity
• Normal vital signs

Grade 3 - Severe envenomation is as follows:

• Generalized muscular pain in the back, abdomen, and chest
• Diaphoresis remote from envenomation site
• Abnormal vital signs (blood pressure >140/90 mm Hg, pulse >100)
• Nausea and vomiting
• Headache

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Medical Care

Admission to the hospital is generally indicated for the following patients (subject to clinical judgment):

• Severely symptomatic children^[6]
• Pregnant women
• Patients with a history of hypertension or heart disease
• Patients with intractable pain and contraindications to antivenom
• Patients with unusual complications of envenomation
• Patients who develop anaphylaxis to antivenom

Discharge patients who experience relief with opioid analgesics, sedative-hypnotics, and/or antivenom (after a period of observation). Antivenom administration may reduce the need for hospitalization.

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Consultations

Local poison control centers may assist management of difficult envenomations.

The Antivenom Index, published by the American Zoo and Aquarium Association and the American Association of Poison Control Centers, lists the locations, amounts, and various types of antivenom stores.

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Complications

Antivenom-associated complications

Anaphylaxis, a type I (immediate) hypersensitivity reaction that may be life threatening, is characterized by urticaria, wheezing, and shock. It may occur to some degree in as many as 25% of patients given antivenom. Risk factors may include previous exposure to horse serum or antivenom or a history of reactive airways. It is treated with epinephrine, antihistamines, steroids, and ventilatory/circulatory support. Two deaths have been reported from anaphylaxis to widow spider antivenom in the United States. One case involved the death of an asthmatic patient after undiluted antivenom was given as an intravenous bolus. The patient developed bronchospasm refractory to medical therapy.^[10] The second case also involved an asthmatic patient, but the anaphylaxis occurred during an appropriately diluted antivenom infusion. A subcutaneous test dose had been given prior to the infusion, and no reaction was found. Several minutes into the infusion, the patient developed anaphylaxis and cardiac arrest, required a prolonged resuscitation, and eventually died.^[11]

Serum sickness, a type III (delayed) hypersensitivity reaction characterized by fever, urticaria, lymphadenopathy, and arthritis, may occur 5 days to 3 weeks after antivenom administration. It usually is benign, self-limited, and treated with antihistamines and steroids. Serum sickness is dose related and uncommon following administration of widow spider antivenom because of the small amounts generally needed (1 or 2 vials).

Also see Presentation/Complications.

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Prevention

Pesticides may prevent exposures to widow spiders at home.

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Long-Term Monitoring

Instruct patients to return if any of the following symptoms occur:

• Hematuria
• Rash
• Joint pain
• Swollen lymph nodes
• Difficulty breathing
• Signs of infection

Advise patients that if treated symptomatically with pain medications and benzodiazepines, pain may come and go for up to days to weeks after envenomation.

If patients have been treated with antivenom, discuss signs of serum sickness (as noted above) and warn them of its possible occurrence in 3-21 days.

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Medication Summary

Most widow spider envenomations may be managed with opioid analgesics and sedative-hypnotics. Antivenom may be indicated for patients who have severe envenomations with pain refractory to these measures. Antivenom should be considered when envenomation seriously threatens pregnancy or precipitates potentially limb- or life-threatening effects (eg, severe hypertension, unstable angina, priapism, compartment syndrome^[7] ). On average, antivenom administration results in resolution of most symptoms a half an hour after administration, and it has been shown to decrease the need for hospitalization.

However, the Redback Antivenom Evaluation II (RAVE-II) study from 2014 reported that adding antivenom to standard analgesia did not significantly improve systemic symptoms or pain in patients with redback spider bite and severe pain, with or without systemic symptoms.^[12] After 2 hours, 38 (34%) of 112 patients from the antivenom group had clinically significant improved pain, versus 26 (23%) of 112 in the placebo group (95% confidence interval, -1.1% to 22.6%; P = .10). For systemic symptoms, they resolved after 2 hours in 9 (26%) of 35 patients in the antivenom group, versus 9 (22%) of 41 in the placebo group (95% confidence interval, -15% to 23%; P = .79). Thus, antivenom treatment in latrodectism is not without some controversy.

A new antivenom (Analatro, manufactured by Instituto Bioclon) has undergone phase 3 clinical trials in the United States, but results have not yet been posted and it has not yet been approved for general use. It may be associated with less risk of allergic reaction than the existing antivenom, so its indications for use may differ from the current indications.^{[13, 14]} See Black Widow Spider Antivenin for Patients With Systemic Latrodectism (BWS P3) for more information.

Calcium gluconate is no longer recommended for widow spider envenomation. Studies suggest benzodiazepines are more efficacious than other muscle relaxant agents for the treatment of muscle pain related to widow spider envenomation. Antibiotics are not indicated.

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Morphine sulfate (Duramorph, Infumorph, Astramorph injections)

• Dosing, Interactions, etc.

Clinical Context:  Morphine sulfate is the drug of choice for narcotic analgesia because of its reliable and predictable effects, safety profile, and ease of reversibility with naloxone. Morphine sulfate administered IV may be dosed in a number of ways and commonly is titrated to the desired effect.

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Class Summary

Pain control is essential to quality patient care. It ensures patient comfort and promotes pulmonary toilet. Most analgesics have sedating properties that are beneficial for patients who have sustained trauma.

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Lorazepam (Ativan)

• Dosing, Interactions, etc.

Clinical Context:  Lorazepam is a sedative hypnotic in the benzodiazepine class that has a short onset of effect and relatively long half-life. By increasing the action of GABA, a major inhibitory neurotransmitter in the brain, it may depress all levels of the CNS, including the limbic and reticular formation.

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Diazepam (Valium)

• Dosing, Interactions, etc.

Clinical Context:  Diazepam depresses all levels of the CNS (eg, limbic and reticular formation), possibly by increasing the activity of GABA. It is a third-line agent for agitation or seizures because of a shorter duration of anticonvulsive effects and accumulation of active metabolites that may prolong sedation.

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Midazolam (Versed)

• Dosing, Interactions, etc.

Clinical Context:  Midazolam is used as an alternative in the termination of refractory status epilepticus. Because it is water soluble, it takes approximately 3 times longer than diazepam to peak EEG effects. Thus, the clinician must wait 2-3 minutes to fully evaluate the sedative effects before initiating a procedure or repeating a dose. Midazolam has twice the affinity for benzodiazepine receptors than diazepam. It may be administered intramuscularly if vascular access cannot be obtained. Intranasal midazolam is an option for pediatric widow spider envenomation.

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Class Summary

By binding to specific receptor sites, these agents appear to potentiate the effects of gamma-aminobutyrate (GABA) and to facilitate inhibitory GABA neurotransmission and other inhibitory transmitters.

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Antivenin, black widow spider

• Dosing, Interactions, etc.

Clinical Context:  Black widow spider antivenin is derived from horse serum and produced by Merck & Co., Inc. Consider it for patients with grade 2 or grade 3 envenomations who are refractory to opiates and sedative-hypnotics and do not have risk factors for immediate hypersensitivity reactions. Some authorities advocate antivenom administration for certain patient groups, such as children and elderly persons. The package insert recommends skin testing for possible allergic reaction to the antivenom.

To mix the antivenom, dissolve 1 vial in 2.5 mL of sterile diluent with gentle agitation, then dilute this into a total volume of at least 20-50 mL normal saline. The package insert recommends intravenous injection over 15 minutes. However, adverse reactions may be averted by further diluting the antivenom (eg, to a total volume of 250 mL) and administering the infusion slowly (eg, over 1 h). Symptoms have been shown to improve within 1 hour of antivenom administration and for as long as 48 hours after envenomation.

In Australia, antivenom for Latrodectus envenomation is available from Commonwealth Serum Laboratories and, in South Africa, from the South African Institute of Medical Research. Indications for antivenom use and routes of administration vary around the world.

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Class Summary

These are used to neutralize the toxin of a widow spider bite. Obtain informed consent before antivenom administration, if possible.

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Diphtheria-tetanus toxoid (dT)

• Dosing, Interactions, etc.

Clinical Context:  Diphtheria-tetanus toxoid is used for the passive immunization of any person with a wound that might be contaminated with tetanus spores.

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Class Summary

Tetanus immunization should be instituted following a black widow spider bite. Tetanus results from elaboration of an exotoxin from Clostridium tetani. A booster injection in previously immunized individuals is recommended to prevent this potentially lethal syndrome. Patients who may not have been immunized against C tetani products (eg, immigrants, elderly persons) should receive tetanus immune globulin (Hyper-Tet).

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Diphenhydramine (Benadryl)

• Dosing, Interactions, etc.

Clinical Context:  Diphenhydramine is used for the symptomatic relief of allergic symptoms caused by histamine released in response to allergens.

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Class Summary

Antihistamines prevent the histamine response in sensory nerve endings and blood vessels. They are more effective in preventing the histamine response than in reversing it.

Antihistamines act by competitive inhibition of histamine at the H1 receptor, which mediates the wheal and flare reactions, bronchial constriction, mucous secretion, smooth muscle contraction, edema, hypotension, CNS depression, and cardiac arrhythmias.

In the treatment of black widow spider envenomations, antihistamines are used before antivenom administration to reduce acute adverse reactions to the antivenom.

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