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Why can humans only be treated with antivenom once?

Published in Antivenom Hypersensitivity 3 mins read

Humans are generally advised against receiving antivenom multiple times due to the significant risk of developing severe, potentially life-threatening allergic reactions upon subsequent exposures.

The Immune System's Response to Antivenom

Antivenom is a life-saving medication derived from animal antibodies (typically horses or sheep) that neutralize venom from snakes, spiders, or scorpions. While highly effective, these foreign proteins can trigger a strong immune response in humans.

During a patient's first antivenom treatment, they may experience immediate hypersensitivity reactions. These initial reactions are often not IgE-mediated, meaning they are less severe and more manageable, sometimes with pre-treatment or by adjusting the infusion rate.

Risk of IgE-Mediated Immediate Hypersensitivity

The primary reason subsequent treatments are avoided is the body's development of specific antibodies. After the initial exposure, the human immune system can recognize the animal proteins in the antivenom as foreign invaders. Upon second or subsequent administration, the body may mount a much more aggressive response, leading to a severe form of allergic reaction known as IgE-mediated immediate hypersensitivity.

This type of reaction is characterized by the rapid release of histamine and other inflammatory mediators, which can cause:

  • Anaphylaxis: A severe, systemic allergic reaction that can be life-threatening. Symptoms include difficulty breathing, a sudden drop in blood pressure, swelling of the throat, rash, and dizziness.
  • Serum Sickness: A delayed reaction that can occur days or weeks after treatment, causing fever, rash, joint pain, and kidney problems. While not immediate, it's also a significant concern.

Managing Subsequent Allergic Reactions

If an IgE-mediated immediate hypersensitivity reaction occurs during antivenom administration, the treatment must be stopped promptly. Immediate anti-allergy treatment, such as epinephrine, antihistamines, and corticosteroids, is then administered to stabilize the patient and manage the severe allergic response.

The high risk of these severe reactions means that while a second bite might necessitate antivenom again, the decision to re-administer it comes with extreme caution and in settings where immediate life support is available, making it not a routinely viable option.

Summary of Antivenom Treatment Risks

To illustrate the difference in reaction types:

Aspect First Antivenom Dose Subsequent Antivenom Dose(s)
Primary Risk Non-IgE-mediated immediate hypersensitivity (e.g., chills, nausea) IgE-mediated immediate hypersensitivity (anaphylaxis)
Immune Response Initial exposure to foreign proteins Sensitized immune system, rapid IgE antibody response
Severity of Reaction Generally milder, more manageable with precautions Potentially severe and life-threatening
Management Often managed by slowing infusion or pre-medication Immediate cessation of antivenom, aggressive anti-allergy treatment (e.g., epinephrine)

Due to the elevated and potentially fatal risks associated with IgE-mediated reactions, healthcare providers typically aim to limit antivenom administration to a single course whenever possible, or only proceed with subsequent doses under extreme medical necessity and rigorous monitoring.