Antivenin (Latrodectus mactans) (equine) is a preparation of equine immunoglobulins capable of neutralizing venom toxins of black widow spiders ( Latrodectus mactans ).1, 4
Antivenin (Latrodectus mactans) (equine) is used in the treatment of symptoms resulting from bites inflicted by the black widow spider ( Latrodectus mactans ).1, 4 When indicated, early use of the antivenin is emphasized for prompt relief.1 However, delayed administration (up to 90 hours after envenomation) reportedly has accelerated recovery and alleviated manifestations in a patient experiencing a prolonged or refractory course of illness.2 Sensitivity testing (skin test or conjunctival test) should be conducted in all individuals prior to administration of antivenin (Latrodectus mactans) (equine). 1 (See Dosage and Administration: Sensitivity Testing and Desensitization.)
Management of Latrodectus mactans Envenomation
The initial signs and symptoms of black widow spider bite are sharp pain at the time of bite (similar to that caused by needle puncture) and local muscular cramps that begin 15 minutes to several hours after the bite.1, 7 The sequence of manifestations depends on the location of the bite.1, 7 Black widow spider venom acts on myoneural junctions or on nerve endings, and can cause an ascending motor paralysis or destruction of peripheral nerve endings.1 Muscles most frequently affected at first are those of the thigh, shoulder, and back; pain can become severe, spreading to the abdomen, and weakness and tremor usually develop.1 Abdominal muscles may assume a boardlike rigidity with light tenderness.1 There may be cardiac manifestations (tachycardia, hypertension, shock or cardiovascular collapse), respiratory symptoms (chest pain/tightness, shortness of breath, respiratory distress), and other symptoms (nausea, vomiting, headache, numbness, weakness, paresthesia, restlessness, agitation, irritability, generalized diaphoresis).7 Convulsions may occur, especially in small children.1 Manifestations usually increase in severity for several hours (perhaps a day) and then slowly become less severe, usually subsiding over the next 2-3 days.1 There may be residual symptoms (e.g., generalized weakness, tingling, nervousness, transient muscle spasm) for weeks or months after recovery from the acute stage.1 Black widow spider bite envenomation can be severe in young children, geriatric adults, and individuals with serious underlying medical conditions.7 Fatalities associated with black widow spider envenomation occur only rarely.11
Management of black widow spider bite depends on the severity of envenomation and associated manifestations.5, 7, 10, 11 Hospitalization may be required in some cases.1, 11 The bite wound should be cleaned,7 but use of a tourniquet, incision, and suction is no longer recommended in the treatment of spider bite.1 Oral analgesics with general supportive care may be adequate for mild envenomation (local pain at bite site, no systemic manifestations, normal vital signs);7, 10, 11 however, additional measures to control pain and muscle contractions are indicated for moderate to severe cases and use of antivenin (Latrodectus mactans) (equine) should be considered based on the condition of the patient.1, 5, 7, 9, 10, 11 A regimen of parenteral benzodiazepines and opiates usually is recommended for symptomatic relief.1, 4, 5, 11 Morphine also has been used to control pain and barbiturates have been used for extreme restlessness; however, since black widow spider venom is neurotoxic and may cause respiratory paralysis, CNS depressants should be used cautiously.1 Corticosteroids have been used with varying degrees of success.1 Although IV calcium gluconate has been used in the past in the management of black widow spider bite,1, 4, 7, 11 calcium gluconate generally is ineffective for symptomatic relief compared with a regimen of benzodiazepines and opiates and is no longer recommended.4, 7, 10, 11
Antivenin (Latrodectus mactans) (equine) can be highly effective for the management of manifestations of black widow spider envenomation and may shorten the duration of symptoms and reduce the need for hospitalization.2, 4, 5, 7, 9, 11 There may be symptomatic relief within 30 minutes of administration of antivenin (Latrodectus mactans) (equine),4, 11 and manifestations usually subside within 1-3 hours after the dose.1 In one study, the mean duration of manifestations was 9 hours in patients whose treatment included antivenin (Latrodectus mactans) (equine) compared with 22 hours in those who did not receive the antivenin and 12 and 52%, respectively, required hospitalization.4 The manufacturer suggests that, in otherwise healthy individuals 16-60 years of age, use of the antivenin may be deferred and treatment with a skeletal muscle relaxant considered.1 Some clinicians recommend use of the antivenin in moderate to severe cases,5, 9, 11 including when there are serious systemic or local manifestations of envenomation, or when manifestations are refractory to other strategies (e.g., regimen of benzodiazepines and opiates).7, 11 Consultation with experts experienced in treating black widow spider envenomation is recommended to guide treatment decisions regarding individual patients.1, 4
Reconstitution and Administration
Antivenin (Latrodectus mactans) (equine) is administered by IM injection or slow IV infusion.1 IV infusion is the preferred route of administration for the management of severe black widow spider envenomation or when the patient is younger than 12 years of age or is in shock.1
The possibility of severe immediate sensitivity reactions (e.g., anaphylaxis) exists since antivenin (Latrodectus mactans) (equine) is prepared using horse serum, and sensitivity testing should be performed in all patients prior to administration of the antivenin. 1(See Dosage and Administration: Sensitivity Testing and Desensitization.)
Each vial of antivenin (Latrodectus mactans) (equine) labeled as containing at least 6000 units of antivenin should be reconstituted by adding 2.5 mL of sterile diluent provided by the manufacturer or 2.5 mL of sterile water for injection; the vial should be shaken (with the diluent needle still in the rubber stopper) until the contents are completely dissolved.1 The reconstituted antivenin should be inspected visually for particulate matter prior to administration.1
IM injections of reconstituted antivenin (Latrodectus mactans) (equine) should be made preferably into the anterolateral thigh so that a tourniquet can be applied if an adverse systemic reaction occurs.1
For IV administration, the manufacturer recommends that each vial of reconstituted antivenin (Latrodectus mactans) (equine) be diluted in 10-50 mL of 0.9% sodium chloride injection and infused over a 15-minute period.1 Alternatively, the antivenin has been diluted in 50-100 mL of 0.9% sodium chloride injection2, 7 or has been infused over 20-60 minutes.2, 5, 7, 11
The antivenin should not be given by rapid IV injection or infusion and should not be given IV undiluted.5, 7
Dosage of antivenin (Latrodectus mactans) (equine) usually is expressed in terms of the number of vials of the antivenin.1
The recommended dosage of antivenin (Latrodectus mactans) (equine) in adults and children is the entire contents of a single reconstituted vial (2.5 mL).1 Symptoms usually subside in 1-3 hours.1 A single 1-vial dose of antivenin is adequate for most patients;1, 2, 7, 11, 12 a second 1-vial dose may be necessary in some patients.1
Sensitivity Testing and Desensitization
Because of the risk of severe immediate sensitivity reactions (e.g., anaphylaxis), a skin or conjunctival sensitivity test should be performed prior to administration of antivenin (Latrodectus mactans) (equine).1 If sensitivity tests are negative, the antivenin may be administered;1 however, negative reactions do not preclude the possibility of an immediate hypersensitivity reaction.7
The skin sensitivity test consists of intradermal injection of not more than 0.02 mL of the 1:10 dilution of normal equine serum provided by the manufacturer; the test is read after 10 minutes.1 To facilitate interpretation of the results, an intradermal control should be administered on the contralateral extremity using an equal volume of 0.9% sodium chloride injection1 A positive skin test reaction consists of an urticarial wheal surrounded by a zone of erythema.1
The conjunctival sensitivity test consists of placing 1 drop of the normal equine serum (1:10 dilution in adults or 1:100 dilution in children) into the conjunctival sac and noting the results for 10 minutes.1 A positive conjunctival test reaction usually occurs within 10 minutes and consists of itching of the eye and reddening of the conjunctiva.1
Although desensitization can be attempted in individuals with a history of allergy or with mildly or questionably positive sensitivity tests, desensitization should be attempted only when administration of the antivenin is considered necessary to save the patient's life and epinephrine should be readily available. 1 For the desensitization procedure, 1:10 and 1:100 dilutions of antivenin (Latrodectus mactans) (equine) should be prepared in separate vials or syringes using 0.9% sodium chloride injection.1 The desensitization procedure involves subcutaneous injection of 0.1, 0.2, and 0.5 mL of the 1:100 dilution of antivenin (Latrodectus mactans) (equine) at 15- or, preferably, 30-minute intervals; this is repeated using the 1:10 dilution of the antivenin, and then using undiluted antivenin.1 If an immediate sensitivity reaction occurs at any time during the desensitization procedure, a tourniquet should be applied proximal to the injection sites and epinephrine administered proximal to the tourniquet or into another extremity.1 After at least 30 minutes, the desensitization procedure may be continued using the last dose that did not produce a reaction.1 If no reaction occurs after 0.5 mL of undiluted antivenin has been administered, additional 0.5-mL doses of undiluted antivenin (Latrodectus mactans) (equine) can be administered at 15-minute intervals until the entire 1-vial dose has been injected.1
Immediate Sensitivity or Anaphylaxis
Immediate sensitivity reactions (e.g., shock, anaphylaxis, anaphylactoid reactions, urticaria) have been reported rarely in patients receiving antivenin (Latrodectus mactans) (equine).1, 4, 5, 12, 14 A few fatalities related to severe hypersensitivity reactions (severe bronchospasm, severe anaphylaxis resulting in cardiac arrest) have been reported;4, 5, 11, 14 at least 1 case occurred after rapid IV administration of undiluted antivenin (see IV Infusion under Dosage and Administration: Reconstitution and Administration).5, 11 Anaphylactic reactions and death have been reported in patients with a history of asthma.1
The risk of immediate sensitivity reactions to antivenin (Latrodectus mactans) (equine) may be increased in individuals with atopic sensitivity to horses.1 Signs and symptoms may develop within minutes after beginning, or during, administration of the antivenin and may include apprehension; flushing; pruritus; urticaria; edema of the face, tongue, and throat; cough; dyspnea; bronchospasm; cyanosis; vomiting; hypotension; and cardiovascular collapse.
If a severe immediate sensitivity reaction occurs during administration of antivenin (Latrodectus mactans) (equine), administration should be immediately discontinued, at least temporarily, and the patient given appropriate therapy (e.g., epinephrine, antihistamines, IV fluids, IV vasopressor agents, maintenance of an adequate airway, oxygen) as indicated.5 Immediate sensitivity reactions may usually be managed by temporarily interrupting administration of the antivenin or decreasing the rate of administration, and administering an antihistamine and/or epinephrine.5 If administration of the antivenin is temporarily interrupted and then reinitiated after control of the reaction, administration should be at a slower rate.5
Delayed Hypersensitivity or Serum Reactions
Serum sickness has been reported rarely following use of antivenin (Latrodectus mactans) (equine),1, 7 and usually is evident within 2-14 days.7, 6 The manufacturer recommends that patients be observed for serum sickness for an average of 8-12 days after receiving the antivenin.1 6 The usual manifestations of serum sickness are malaise, fever, urticaria, lymphadenopathy, edema, arthralgia, nausea, and vomiting. Pain and muscle weakness are frequently present, and permanent atrophy may develop. Although their efficacy is not clearly established in the treatment of envenomation or venom shock, corticosteroids and antihistamines are the treatment of choice for the treatment of serious serum sickness reactions.6
Antivenin (Latrodectus mactans) (equine) contains thimerosal, a mercury-containing preservative.1, 22 The 1:10 dilution of normal equine serum provided by the manufacturer for sensitivity testing also contains thimerosal.1 (See Cautions: Precautions and Contraindications.)
Hypersensitivity reactions to thimerosal have been reported rarely in individuals receiving vaccines containing thimerosal.21, 31, 33 These reactions usually manifest as local, delayed-type hypersensitivity reactions (e.g., erythema, swelling),21, 23, 25 but a generalized reaction manifested as pruritus and an erythematous, maculopapular rash on all 4 extremities has been reported rarely.33 Even when patch or intradermal tests for thimerosal sensitivity are positive, most individuals receiving thimerosal-containing vaccines do not develop such hypersensitivity reactions.21, 23
Precautions and Contraindications
Prior to administration of antivenin (Latrodectus mactans) (equine), careful inquiry should be made concerning a history of prior exposure to preparations containing equine serum or any allergies.1 Sensitivity testing (skin test or conjunctival test) should be performed prior to administration of the antivenin, regardless of the patient's clinical history.1 (See Dosage and Administration: Sensitivity Testing and Desensitization.) However, an anaphylactic reaction to the antivenin may still occur in patients with negative skin or conjunctival sensitivity tests.1
Patients receiving antivenin (Latrodectus mactans) (equine) should be monitored continuously and appropriate equipment for maintenance of an adequate airway and other supportive measures and epinephrine or other agents should be readily available for the treatment of anaphylaxis or other severe systemic reactions.4, 7 In addition, patients should be monitored for signs and symptoms of serum sickness for an average of 8-12 days after administration of the antivenin.1
Antivenin (Latrodectus mactans) (equine) and the normal equine serum provided by the manufacturer for sensitivity testing contain mercury in the form of ethyl mercury from thimerosal, a mercury-containing preservative.1, 22 Each vial of lyophilized antivenin (Latrodectus mactans) (equine) and each vial of 1:10 normal equine serum contain 0.01% thimerosal (1:10,000).1 Only limited toxicology data are available regarding ethyl mercury, but high dose and acute exposures to methyl mercury have been associated with neurologic and renal toxicities.20 Developing fetuses and young children, especially neonates and infants younger than 6 months of age, are at greatest risk of mercury toxicity.18, 19, 20 Although it has been suggested that thimerosal added as a preservative or used during the manufacturing process of vaccines or plasma-derived products theoretically could have adverse effects in recipients, there is no conclusive evidence that the low concentrations of thimerosal contained in vaccines cause harm in vaccine recipients.27, 28, 29, 32, 34, 35, 36, 37, 38, 39 Efforts to eliminate or reduce the thimerosal content in such products are recommended as a prudent measure to reduce mercury exposure in infants and children and part of an overall strategy to reduce mercury exposures from all sources, including food and drugs.18, 19, 20, 23, 27 For additional information on risks associated with thimerosal and mercury, see Thimerosal Precautions under Cautions: Precautions and Contraindications, in Influenza Virus Vaccine Inactivated 80:12.
Although safety and efficacy studies have not been conducted in pediatric patients,1 antivenin (Latrodectus mactans) (equine) has been used safely in children without unusual adverse effects.1, 2, 11
Clinical experience with antivenin (Latrodectus mactans) (equine) has not identified differences in responses between geriatric adults and younger patients.1 Because of the increased risk of complications from envenomation in geriatric adults, patients older than 60 years of age should receive the antivenin when indicated.1
Mutagenicity and Carcinogenicity
No long-term animal studies have been performed to evaluate the mutagenic or carcinogenic potential of antivenin (Latrodectus mactans) (equine).1
Pregnancy, Fertility, and Lactation
Animal reproduction studies have not been performed with antivenin (Latrodectus mactans) (equine).1 The manufacturer states that it is not known whether antivenin (Latrodectus mactans) (equine) can cause fetal harm when administered to a pregnant woman or can affect fertility, and the antivenin should be used during pregnancy only when clearly needed.1 Antivenin (Latrodectus mactans) (equine) has been used in pregnant women without unusual adverse effects.9, 15, 16
It is not known whether antivenin (Latrodectus mactans) (equine) is distributed into milk.1 Because many drugs are excreted in human milk, the antivenin should be used with caution in nursing women.1
Antivenin (Latrodectus mactans) (equine) contains equine venom-neutralizing immunoglobulins capable of neutralizing toxic effects of black widow spiders ( Latrodectus mactans ) venom.1 The exact pharmacologic mechanism of action is unknown.1
Data are not available regarding the pharmacokinetics of antivenin (Latrodectus mactans) (equine) following IM or IV administration.1
Antivenin (Latrodectus mactans) (equine) is a sterile, nonpyrogenic preparation of specific venom-neutralizing immunoglobulins capable of neutralizing venom toxins of black widow spiders ( Latrodectus mactans ).1, 4 The antivenin is prepared from the serum of healthy horses immunized against the venom of black widow spiders ( Latrodectus mactans ).1 Commercially available antivenin (Latrodectus mactans) (equine) meets standards established by the Office of Biologics of the US Food and Drug Administration.3 Antivenin (Latrodectus mactans) (equine) is standardized by its ability to neutralize the action of L. mactans venom given IV to mice (mouse LD50 neutralizing units), and each vial of the antivenin contains not less than 6000 mouse LD50.1, 3
Antivenin (Latrodectus mactans) (equine) is a lyophilized preparation that occurs as a white to gray crystalline powder.1 Following reconstitution as directed by the manufacturer, the antivenin is opalescent, ranging in color from light straw to very dark; the color has no effect on potency.1 The reconstituted antivenin contains not more than 20% solids.1, 3 Antivenin (Latrodectus mactans) (equine) and the 1:10 dilution of normal equine serum provided by the manufacturer for sensitivity testing contain thimerosal 0.01% (1:10,000) as a preservative (50 mcg of mercury per mL).1
Antivenin (Latrodectus mactans) (equine) should be stored at 2-8°C, and should not be frozen or exposed to excessive heat.1, 3
Additional Information
The American Society of Health-System Pharmacists, Inc. represents that the information provided in the accompanying monograph was formulated with a reasonable standard of care, and in conformity with professional standards in the field. Readers are advised that decisions regarding use of drugs are complex medical decisions requiring the independent, informed decision of an appropriate health care professional, and that the information contained in the monograph is provided for informational purposes only. The manufacturer's labeling should be consulted for more detailed information. The American Society of Health-System Pharmacists, Inc. does not endorse or recommend the use of any drug. The information contained in the monograph is not a substitute for medical care.
Excipients in commercially available drug preparations may have clinically important effects in some individuals; consult specific product labeling for details.
Please refer to the ASHP Drug Shortages Resource Center for information on shortages of one or more of these preparations.
Routes | Dosage Forms | Strengths | Brand Names | Manufacturer |
|---|---|---|---|---|
Parenteral | For injection | Containing at least 6000 antivenin units | Antivenin (Latrodectus mactans) Black Widow Spider Antivenin Equine |
AHFS® Drug Information. © Copyright, 1959-2025, Selected Revisions June 10, 2024. American Society of Health-System Pharmacists, Inc., 4500 East-West Highway, Suite 900, Bethesda, MD 20814.
1. Merck & Co, Inc. Antivenin (Latrodectus mactans) (Black Widow Spider Antivenin) Equine Origin prescribing information. Whitehouse Station, NJ; 2014 Feb.
2. O'Malley GF, Dart RC, Kuffner EF. Successful treatment of latrodectism with antivenin after 90 hours. N Engl J Med . 1999; 340:657. [PubMed 10049091]
3. The United States pharmacopeia, 25th rev, and The national formulary, 20th ed. Rockville, MD: The United States Pharmacopeial Convention, Inc; 2002:158.
4. 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;21:782-7 [PubMed 1351707]
5. Clark RF. The safety and efficacy of antivenin Latrodectus mactans. Clin Toxicol . 2001;39:125-7. [PubMed 11407497]
6. Erffmeyer JE. Serum sickness. Ann Allergy . 1986;56:105-9. [PubMed 3511779]
7. Utah Poison Control Center for Health Professionals. Black widow spider envenomation. 2002 Dec. From website. [Web]
9. Monte AA, Bucher-Bartelson B, Heard KJ. A US perspective of symptomatic Latrodectus spp. envenomation and treatment: a National Poison Data System review. Ann Pharmacother . 2011; 45:1491-8. [PubMed 22116992]
10. Isbister GK, Fan HW. Spider bite. Lancet . 2011; 378:2039-47. [PubMed 21762981]
11. Offerman SR, Daubert GP, Clark RF. The treatment of black widow spider envenomation with antivenin latrodectus mactans: a case series. Perm J . 2011; 15:76-81. [PubMed 22058673][PubMedCentral]
12. Nordt SP, Clark RF, Lee A et al. Examination of adverse events following black widow antivenom use in California. Clin Toxicol (Phila) . 2012; 50:70-3. [PubMed 22175789]
14. Murphy CM, Hong JJ, Beuhler MC. Anaphylaxis with Latrodectus antivenin resulting in cardiac arrest. J Med Toxicol . 2011; 7:317-21. [PubMed 22052335][PubMedCentral]
15. Handel CC, Izquierdo LA, Curet LB. Black widow spider (Latrodectus mactans) bite during pregnancy. West J Med . 1994; 160:261-2. [PubMed 8191768][PubMedCentral]
16. Sherman RP, Groll JM, Gonzalez DI et al. Black widow spider (Latrodectus mactans) envenomation in a term pregnancy. Curr Surg . 2000; 57:346-348. [PubMed 11024247]
18. Centers for Disease Control and Prevention. Recommendations regarding the use of vaccines that contain thimerosal as a preservative. MMWR Morb Mortal Wkly Rep . 1999; 48:996-8. [PubMed 10577494]
19. American Academy of Pediatrics Committee on Infectious Diseases and Committee on Environmental Health. Thimerosal in vaccines: an interim report to clinicians (RE9935). Pediatrics . 1999; 104:570-4. [PubMed 10469789]
20. Centers for Disease Control and Prevention. Thimerosal in vaccines: a joint statement of the American Academy of Pediatrics and the Public Health Service. MMWR . 1999; 48:563-5. [PubMed 10418806]
21. Aberer W. Vaccination despite thimerosal sensitivity. Contact Dermatitis . 1991; 24:6-10. [PubMed 2044374]
22. Food and Drug Administration (FDA). Mercury in plasma-derived products. From FDA website. Accessed 2012 Mar 12. [Web]
23. National Center for Immunization and Respiratory Diseases. General recommendations on immunization --- recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR Recomm Rep . 2011; 60:1-64.
25. Food and Drug Administration. Thimerosal in vaccines. From FDA website. Accessed 2008 Oct 27. [Web]
27. Food and Drug Administration. Thimerosal in vaccines. Frequently asked questions (FAQ). From FDA website. Accessed 2008 Oct 27. [Web]
28. Institute of Medicine. Immunization safety review: thimerosal-containing vaccines and neurodevelopmental disorder. Washington DC; National Academy Press; 2001. From IOM website. Accessed 2003 Jul 24. [Web]
29. Thompson WW, Price C, Goodson B et al. Early thimerosal exposure and neuropsychological outcomes at 7 to 10 years. N Engl J Med . 2007; 357:1281-92. [PubMed 17898097]
30. Pichichero ME, Gentile A, Giglio N et al. Mercury levels in newborns and infants after receipt of thimerosal-containing vaccines. Pediatrics . 2008; 121:e208-14. [PubMed 18245396]
31. Zheng W, Dreskin SC. Thimerosal in influenza vaccine: an immediate hypersensitivity reaction. Ann Allergy Asthma Immunol . 2007; 99:574-5. [PubMed 18219843]
32. Madsen KM, Lauritsen MB, Pedersen CB et al. Thimerosal and the occurrence of autism: negative ecological evidence from Danish population-based data. Pediatrics . 2003; 112:604-6. [PubMed 12949291]
33. Lee-Wong M, Resnick D, Chong K. A generalized reaction to thimerosal from an influenza vaccine. Ann Allergy Asthma Immunol . 2005; 94:90-4. [PubMed 15702823]
34. Parker S, Todd J, Schwartz B et al. Thimerosal-containing vaccines and autistic spectrum disorder: a critical review of published original data. Pediatrics . 2005; 115:200. [PubMed 15630018]
35. Schechter R, Grether JK. Continuing increases in autism reported to California's developmental services system: mercury in retrograde. Arch Gen Psychiatry . 2008; 65:19-24. [PubMed 18180424]
36. Andrews N, Miller E, Grant A et al. Thimerosal exposure in infants and developmental disorders: a retrospective cohort study in the United kingdom does not support a causal association. Pediatrics . 2004; 114:584-91. [PubMed 15342825]
37. Verstraeten T, Davis RL, DeStefano F et al. Safety of thimerosal-containing vaccines: a two-phased study of computerized health maintenance organization databases. Pediatrics . 2003; 112:1039-48. [PubMed 14595043]
38. Hviid A, Stellfeld M, Wohlfahrt J et al. Association between thimerosal-containing vaccine and autism. JAMA . 2003; 290:1763-6. [PubMed 14519711]
39. Institute of Medicine. Immunization safety review: vaccines and autism. Washington DC; National Academy Press; 2004. From IOM website. Accessed 2008 Oct 28. [Web]
40. Food and Drug Administration. Information for health care professionals: extension of expiration date for black widow spider antivenin packaged lot H019984 until January 3, 2015 and instructions for diluent use. Accessed 2014 Nov 10. [Web]
41. Perry ES. Dear Health Care Provider letter regarding important product supply information for antivenin (Lactrodectus mactans) (Equine Origin). Extension of expiration dating to January 3, 2015 packaged lot H019984. Merck; 2014 Jul 3. From FDA website. Accessed 2014 Nov 10. [Web]