DESCRIPTION

Black widow spider antivenom is an antidote to black widow spider venom.

FORMS AND USES

The formulation is ANTIVENIN (Latrodectus mactans), USP (Merck).

• Each package contains 6,000 units of powdered antivenom.
• The powder is reconstituted with a 2.5-ml vial of water for injection (included in package).
• Also included is a 1-ml vial of normal horse serum (1:10 dilution) for sensitivity testing.
• The inactive ingredient is thimerosal 1:10,000.

MECHANISM OF ACTION

• This antivenom contains immunoglobulin G from the serum of horses hyperimmunized to the venom of Latrodectus mactans (black widow spider); it confers passive immunity by binding constituents of black widow spider venom.
• Due to cross-reactivity with the venom of other subspecies, the antivenom is considered effective treatment for all Latrodectus subspecies.

PREGNANCY AND LACTATION

• US FDA Pregnancy Category C. The drug exerts animal teratogenic or embryocidal effects, but there are no controlled studies in women, or no studies are available in either animals or women.
• Black widow antivenom should be used in pregnant women as it is in other patients.

Section Outline:

• DESCRIPTION
• FORMS AND USES
• MECHANISM OF ACTION
• PREGNANCY AND LACTATION

• Severe hypertension
• Persistent pain not controlled by opioids and muscle relaxants
• Respiratory failure
• Suspected myocardial ischemia
• Premature labor

CONTRAINDICATIONS

Known hypersensitivity to horse serum or black widow spider antivenom precludes its use.

ADVERSE EFFECTS

Acute Allergic Reactions (Rash, Bronchospasm)

• Frequency of acute reactions is unknown, but occurs in 20% to 25% of patients receiving other equine antivenoms.
• Antivenom infusion should be discontinued.
• Patient should receive antihistamine such as diphenhydramine; adult, 25 to 50 mg intravenously or orally every 6 to 8 hours; pediatric, 1 mg/kg intravenously or orally up to 50 mg every 6 to 8 hours.
• Bronchospasm should be treated with a bronchodilator, such as albuterol 0.15 mg/kg (maximum of 10 mg), in saline with humidified oxygen via nebulizer every 20 to 30 minutes.

Anaphylaxis

• Frequency is unknown; one death has been reported from anaphylaxis induced by the horse serum skin test.
• Patient should receive 100% oxygen, and a secure airway and intravenous access should be maintained.
• H₁ and H₂ antihistamines should be administered.
  • Diphenhydramine. Adult, 25 to 50 mg intravenously every 6 to 8 hours; pediatric, 1 mg/kg intravenously up to 50 mg every 6 to 8 hours.
  • Cimetidine. Adult, 300 mg intravenously every 6 hours; pediatric, 40 mg/kg per day intravenously divided every 6 hours up to 300 mg/dose.

Bronchospasm

Bronchospasm is treated with a bronchodilator, such as albuterol 0.15 mg/kg up to 10 mg, in saline with humidified oxygen via nebulizer every 20 to 30 minutes; use of epinephrine 1:1,000 should be considered in refractory cases: adult, 0.3 to 0.5 ml subcutaneously; pediatric, 0.01 ml/kg subcutaneously up 0.5 ml.

Hypotension

If hypotension develops, crystalloid should be administered in an initial bolus of 0.9% NaCl, 10 to 20 ml/kg; epinephrine can be added if needed.

• Initial bolus of epinephrine 1:10,000 diluted 10:1 is administered.
  • Adult. Intravenous push 3 to 5 ml over 5 to 10 minutes.
  • Pediatric. Intravenous push 0.1 ml/kg up to 5 ml over 5 to 10 minutes.
• Bolus should be followed with infusion if necessary: 1 mg of 1:1,000 epinephrine in 250 ml D5W; 1 µg/min initially should be titrated to desired blood pressure.

Methylprednisolone

Methylprednisolone 60 to 125 mg (1-1.5 mg/kg) should be given intravenously (pediatric, 1-2 mg/kg) every 6 to 8 hours.

Type IV Hypersensitivity (Serum Sickness)

• Frequency after use of black widow antivenom is unknown.
• Serum sickness typically begins 3 to 14 days after antivenom infusion as malaise and diffuse arthralgia and progresses to diffuse rash, pruritus, and (rarely) pericarditis and glomerulonephritis.
• Antihistamine such as diphenhydramine should be administered.
  • Adult, 25 to 50 mg orally every 6 to 8 hours
  • Pediatric, 1 mg/kg orally up to 50 mg every 6 to 8 hours
• Prednisone should be administered to most symptomatic patients.
  • Adult, 40 to 60 mg/day orally for 7 to 10 days
  • Pediatric, 1 mg/kg per day orally for 7 to 10 days
  • Tapering regimen acceptable, but not needed, in patients not treated chronically with steroids
• Antipyretics and pain control
  • Acetaminophen. Adult, 1 g orally every 4 to 6 hours, up to 4 g/day; pediatric, 10 mg/kg every 4 to 6 hours up to 4 g/day.
  • Ibuprofen. Adult, 600 to 800 mg orally every 8 hours; pediatric, 5 to 10 mg/kg every 6 to 8 hours.

Section Outline:

• CONTRAINDICATIONS
• ADVERSE EFFECTS
  • Acute Allergic Reactions (Rash, Bronchospasm)
  • Anaphylaxis
  • Type IV Hypersensitivity (Serum Sickness)

• Skin test should be performed with 0.02 ml of the horse serum provided.
  • Serum should be injected intradermally.
  • Patient should be observed for 10 to 20 minutes for wheal and flare reaction; the test should be read using the same parameters as for the tuberculin skin test (wheal and erythema larger than 10 mm in diameter).
• If the skin test is negative, one vial of antivenom should be reconstituted with 2.5 ml sterile water for injection (included), and then diluted in 50 to 100 ml intravenous fluid (D5W or 0.9% NaCl).
• The infusion should begin slowly (10-25 ml/h) using an infusion controller; in the absence of reaction, the rate can slowly be increased every few minutes while observation for evidence of acute allergic reaction continues.
  • Infusion should be completed over 20 to 30 minutes as tolerated.
  • Patient should be monitored continuously during administration; airway management and anaphylaxis treatment may be required.
  • If reaction occurs, infusion should be terminated immediately.
• Clinical improvement is usually evident within 30 to 60 minutes.
• If patient shows no response or partial response, one additional vial can be administered.
• If there is no clinical response after the administration of two vials of antivenom, the diagnosis should be reconsidered.

• Rapid intravenous bolus administration may produce an anaphylactoid reaction.
• Antivenom use is generally not indicated in minor envenomation syndromes because the risk of anaphylaxis outweighs the potential benefits.
• Inadequate monitoring may allow anaphylaxis to develop without treatment.

Toxic effect of venom.

See Also: SECTION IV, Black Widow Spider chapter.

RECOMMENDED READING

Clark RF, Wethern-Kestner S, Vance MV, et al. Clinical presentation and treatment of black widow spider envenomation: a review of 163 cases. Ann Emerg Med 1992;21:782-787.

Moss HS, Binder LS. A retrospective review of black widow spider envenomation. Ann Emerg Med 1987;16:188-191.

Author: Katherine M. Hurlbut

Reviewer: Richard C. Dart