Black Widow Spider Antivenin
Antivenin (Latrodectus mactans)
Also known as: Black Widow Spider Antivenin · Latrodectus mactans antivenin · Black widow antivenom
Reviewed by the VetSheet Veterinary TeamUpdated Apr 5, 2026Evidence-based veterinary reference
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Dosing by species
🌎 NA — North America🌍 EU — Europe
Dogs
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Black Widow Spider Envenomation | After reconstituting the antivenin, add to 100 mL of normal saline and administer via slow IV over 30 minutes. Pretreatment with 2-4 mg/kg of diphenhydramine SC may help calm the patient and may possibly protect against allergic reactions from the antivenin. | IV | Single dose | 30 minutes | 🌎 NA |
| Black Widow Spider Envenomation | Dissolve contents of one vial and add to 100-200 mL of warm 0.9% NaCl and infuse over 2-6 hours. Administer diphenhydramine at 0.5-1 mg/kg prior to infusion. | IV | Single dose | 2-6 hours | 🌎 NA |
- Black Widow Spider Envenomation: Monitor inner pinna during infusion for signs of anaphylaxis (hyperemia). If hyperemia occurs, discontinue infusion and give a second dose of diphenhydramine. Benzodiazepines may alleviate muscle cramping.
Cats
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Black Widow Spider Envenomation | After reconstituting the antivenin, add to 100 mL of normal saline and administer via slow IV over 30 minutes. Pretreatment with 2-4 mg/kg of diphenhydramine SC may help calm the patient and may possibly protect against allergic reactions from the antivenin. | IV | Single dose | 30 minutes | 🌎 NA |
| Black Widow Spider Envenomation | Dissolve contents of one vial and add to 100-200 mL of warm 0.9% NaCl and infuse over 2-6 hours. Administer diphenhydramine at 0.5-1 mg/kg prior to infusion. | IV | Single dose | 2-6 hours | 🌎 NA |
- Black Widow Spider Envenomation: Monitor inner pinna during infusion for signs of anaphylaxis (hyperemia). If hyperemia occurs, discontinue infusion and give a second dose of diphenhydramine. Benzodiazepines may alleviate muscle cramping.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
Black widow spider antivenin is a life-saving biologic used to treat envenomation caused by the black widow spider (Latrodectus mactans).
Clinical Pearls:
- Species Sensitivity: Cats, camels, and horses are considered to be exquisitely sensitive to black widow venom.
- Toxicity Profile: The primary toxic signs are driven by potent neurotoxins (specifically alpha-latrotoxin) in the venom, which cause massive neurotransmitter release leading to severe muscle cramping, rigidity, and autonomic instability.
- Availability: Antivenins for rare species can be difficult for veterinarians to obtain. Availability is often in flux; contacting a poison control center (e.g., ASPCA Animal Poison Control or local human poison centers) is highly recommended for up-to-date procurement and treatment options.
Mechanism of action
Antivenins act by directly neutralizing the complex protein venoms in the patient's systemic circulation.
- Mechanism: The product provides passive immunization via concentrated serum globulins obtained from horses previously immunized with black widow spider venom.
- Pharmacodynamic Pathway: The antibodies bind to alpha-latrotoxin → prevents the toxin from forming pores in presynaptic neuronal membranes → halts the massive, uncoordinated influx of calcium and subsequent depletion of neurotransmitter vesicles (like acetylcholine and norepinephrine).
- In humans, clinical symptoms typically begin to subside within 1-2 hours after administration.
Safety & warnings
Contraindications
- Known severe hypersensitivity to equine serum (relative contraindication in life-threatening envenomation)
Adverse effects
- Anaphylaxis (incidence <2%)
- Anaphylactoid reactions (nausea, pruritus, hyperemia of the inner pinna)
- Serum sickness (delayed hypersensitivity)
Precautions
Anaphylaxis Risk: Because this product is derived from horse serum, there is a significant risk of anaphylaxis or anaphylactoid reactions.
- Sensitivity Testing: A 1:10 dilution given intracutaneously (0.02-0.03 mL) is suggested to test for hypersensitivity (wheal/erythema within 30 mins indicates a positive reaction). However, a negative response does not guarantee safety.
- Administration Technique: Slow intravenous administration is usually sufficient to identify reacting animals.
- Pre-treatment: Administering diphenhydramine prior to the antivenin is strongly recommended to sedate the patient and reduce allergic reactions.
- Reaction Management: If an anaphylactoid reaction occurs (e.g., hyperemia of the inner pinna), stop the infusion, administer additional diphenhydramine, and restart 5 minutes later at a slower rate.
- Fluid Therapy Caution: Use care with IV fluids, as black widow envenomation can cause significant hypertension.
Drug interactions
May mask the early signs associated with anaphylaxis
Monitoring
- Signs associated with an allergic response to the antivenin (anaphylaxis, anaphylactoid-reactions, serum sickness)
- Respiratory/cardiac rate
- Blood pressure
- Serum chemistry (blood glucose mandatory)
- CBC
- Urine output; urinalysis
Overdose
Specific overdosage information is not provided. However, excessive administration of serum products increases the risk of volume overload and severe hypersensitivity reactions, including anaphylaxis and serum sickness.
Available products
Formulations
- Powder for Injection
Human-labeled
- Antivenin (Latrodectus mactans) Powder for Injection: greater than or equal to 6000 antivenin Units/vial in single-use vials with 1 vial diluent (2.5 mL vial of sterile water for injection) and l mL vial of normal horse serum (1:10 dilution) for sensitivity testing
Regulatory status
No regulatory data for: 🇺🇸 US · 🇪🇺 EU · 🇬🇧 UK · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Product should be stored in the refrigerator (2-8°C). It is reconstituted by adding 2.5 mL of the diluent provided; shake the vial to completely dissolve the contents. Do not freeze the reconstituted solution. For IV use, further dilute the solution in 10-100 mL of normal saline injection.
Client information
Black widow spider envenomation is a life-threatening emergency, particularly for cats, which are highly sensitive to the venom.
- Treatment Risks: The antivenin is highly effective but is made from horse serum. You must be aware of the potential for anaphylaxis (a severe allergic reaction) during administration.
- Intensive Care: Your pet will require close monitoring, intravenous fluids, and potentially other medications (like antihistamines and muscle relaxants) to manage the symptoms and prevent reactions.
- Cost: Due to the rarity and manufacturing process of the antivenin, along with the required hospitalization and intensive monitoring, the expenses associated with this treatment can be significant.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.