Snakebite Update: How Well Do You Know Your Pit Viper Anti-Venom?

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According to the CDC, between 7,000 to 8,000 people are bit by venomous snakes each year, and that number is going up. Stay up to date on the latest anti-venom updates.

John, a 25-year-old avid hiker, is trekking  through the woods enjoying the sunshine and fresh air of the countryside. As he looks off into the distance, dreaming about what he is going to have for lunch, he hears a rustling in the grass and the sound of a lawn sprinkler next to the trail. He  looks down, suddenly feeling  the pain of 10 red hot pokers to his right ankle, just above his hiking boot. As he examines  the area of intense pain, he notices the familiar triangular head of a rattlesnake as it slithers off into the distance. Panicked, he finds his phone and dials 911 for help. As the EMS crew arrives, his skin has a mottled appearance and two fang marks appear just above the lateral malleolus.

He arrives at your emergency department where he is delivered to the critical care area. As a swarm of nurses and technicians initiate care with IVs, monitors, you step into the room barking out orders to start two large bore IV and immediately give 2 L of normal saline and fentanyl for pain. John has beads of sweat and the look of fear on his face when he starts to vomit. You yell out, “Give him ondansetron!.” As you inspect the wound, the swelling has now progressed to just below the knee, and there is a large area of ecchymosis around the lateral malleolus with the skin becoming tense with swelling. Next you say, “Continue to measure the leading edge of the swelling.” You feel his inguinal canal, and he yelps in discomfort. Vital signs are normal except for sinus tachycardia at 120. You call Jane, the local pit viper treatment expert, who recommends Fab pit viper antivenom over one hour. John’s labs return showing the arrows of abnormality. His fibrinogen was 60, platelets 36, and prothrombin time greater than 120 seconds. He is whisked away to the intensive care unit as the antivenom bag swings rhythmically from the IV pole. You speak to the toxicologist later who sighs, “He received over 20 vials of antivenom.”

Pit viper envenomation are caused by rattlesnakes, cottonmouth and copperheads in the United States. Venom from these snakes may cause local and systemic problems. Neurologic symptoms may be seen from a few species of pit viper in the United States, namely the Mohave and Southern Pacific rattlesnakes. Symptomatic treatment depends on the severity of the snakebite. It is estimated that 20-25% of pit viper bites are “dry” or no venom is released into the bite. The Snakebite Severity Score may be useful to guide anti-venom administration. A score of 0-3 is considered mild, a score of 4-7 is moderate, and 8-20 would be considered severe envenomation (link to Snakebite Severity Score on epmonthly.com).

CroFab [Crotalidae polyvalent immune Fab (ovine)] (FabAV) is currently the only FDA approved anti-venom for the treatment of pit viper bites. The recommended dose for symptomatic pit viper bites range from 4-12 vials initially in both children and adults depending on the presenting signs and symptoms. Bolus doses of anti-venom are used to “control” envenomation effects of swelling, pain, hematologic effects, unstable vital signs and any neurotoxic findings. More than one bolus dose may be required in some cases. Once effects stabilize then maintenance doses to prevent recurrent venom effects may be recommended, which is often two vials of CroFab every six hours for 3 doses. Maintenance dosing is dependent on the clinical situation.

Patients should be instructed to monitor for serum sickness after antivenom treatment. This reaction is an IgG mediated antibody response to animal proteins. Have the patient look for these symptoms 5 to 14 days after given antivenom. Symptomatic treatment with antihistamines and steroids may be needed and rarely require hospitalization.

Prior to the paper published in June 2017 Annals of Emergency Medicine by Gerardo, et al, mild to moderate copperhead (Agkistrodon contortrix) bites were managed symptomatically with pain medication with some patients receiving FabAV. The most common sign and symptom from these bites were pain and swelling. Rarely would copperhead bites cause fatalities and hematologic issues. However, some copperhead bites cause permanent tissue loss and disability. This multicenter, randomized, double-blind, placebo controlled study involved the treatment of 74 patients with 45 receiving FabAV and 29 placebo. They used the Patient Specific Functional Scale (PSFS) at 14 days after copperhead envenomation to evaluate limb function. PSFS is used to quantify activity limitation and functional outcomes in a variety of musculoskeletal problems. Treatment with FabAV reduced limb disability at 14 days (8.6 for FabAV and 7.4 for placebo). Other retrospective studies have also found FabAV may improve limb function. Essentially this means patients may recover quicker. Another significant finding was opioid analgesia use was less in those treated with FabAV.

Successful treatment of pit viper envenomation whether severe or minor may be managed with the help of your local expert or regional poison control center. As with any intervention the risk and benefits as well as cost must be considered and discussed with the patient. Victims should be reminded that tissue damaged by venom is irreversible and disability from envenomation may impair limb function, cause disfigurement, persistent symptoms such as numbness, pain and paresthesia despite receiving antivenom.

Signs & Symptoms

Signs of Envenomation

  • Swelling – local and diffuse
  • Tenderness
  • Erythema
  • Ecchymosis
  • Blebs
  • Elevated PT/INR
  • Decreased fibrinogen
  • Decreased platelets
  • Systemic signs such as hypotension, hemorrhage, respiratory failure, angioedema, neurotoxicity

Signs of serum sickness, which may occur with antivenom treatment

  • Fever
  • Joint pain
  • Rash or itching

ABOUT THE AUTHOR

Dr. Quan is an emergency physician and medical toxicologist at Maricopa Integrated Health System in Phoenix, AZ.

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