Envenomations: Initial Management of Common U.S. Snakebites
Nothing says “emergency” like a bite from a venomous reptile. If you work in an area populated by snakes, which covers most of the United States and the world, then chances are good that you will see a patient with a snake bite in the Emergency Department (ED). The severity of the symptoms and the treatment vary greatly with different snakes. In this post, we will outline the ED approach to and management of common U.S. snake envenomation.
A 3 year-old presents to an ED in the Southeastern U.S. with a diffusely swollen arm. She was in their yard playing when her mother heard her cry. Her mother noticed a few drops of blood trickling down her hand and put a bandaid on it. During the next few hours the child’s hand became diffusely swollen, with ecchymosis of her hand and digit, with possible puncture wounds on her right hand at the base of the metacarpal joints of the third and fourth digits. Her mother immediately brought the child to the ED with concerns for a copperhead bite, since copperheads are endemic to the area. By the time she arrived, the swelling and pain had spread to her elbow.
Figure 1: Swelling and ecchymosis from a copperhead bite
Summer is snake bite season! Almost all bites (save those from exotic pet reptiles) occur between April and October. So as the weather warms up, U.S. EDs will begin to see more of the nearly 8,000 venomous snakebites that occur annually.1 Fortunately, very few bites are fatal.2 99% of all venomous snakebites are from the Crotalinae family (formerly known as Crotalidae), which are pit vipers and include rattlesnakes, copperheads, and cottonmouths.2 Most fatalities from snake bites in the U.S. are due to rattlesnakes because of their wide prevalence and more potent venom. More rarely snake bites can be from the Elapidae family which includes coral snakes. As with the example case, almost all bites occur on the extremities, particularly the hands and arms.
Crotalinae (Pit Vipers)
Pit vipers in the US include rattlesnakes, copperheads, and cottonmouths. Their venom has 3 effects:
- Local tissue inflammation
- Hematologic effects leading to coagulopathy
- Systemic effects such as hypotension and angioedema
The venom contains enzymes and proteins that cause: local necrosis, endothelial damage and third-spacing of fluids, hemolysis, and in severe cases, venom-induced consumptive coagulopathy,3 acute renal failure, distributive shock, and death. Shock and hypotension are fortunately rare, occurring in only about 7% of envenomed bites.2
Bites from copperheads, in contrast to cottonmouth or rattlesnake bites, tend to be less severe and are rarely fatal. Certain rattlesnake species, such as the Southern Pacific Rattlesnake and Mojave rattlesnakes, can produce severe neurotoxicity in addition to hemotoxic effects. There may be significant genetic variance in venom composition even among snakes of the same species such as Timber rattlesnakes and Eastern Diamondbacks, and the presence of muscle fasciculations or “myokymia” is strongly suggestive of neurotoxin presence in a rattlesnake bite.2,4
CroFab (Crotalindae Polyvalent Immune Fab) can counteract all 3 of the envenomation effects. While CroFab is much less likely to cause anaphylaxis compared with the horse antivenom used in the past, there is still about a 5-6% chance of developing an allergic reaction, which is treated with usual care and halting the infusion if the reaction is severe.
Figure 2: Copperhead snake – Bites are rarely fatal or severe but can lead to limb or digit loss. Patients can have anaphylaxis if they have had a previous bite. Bites rarely require antivenom.
Figure 3: Rattlesnake – Bites can be life threatening, with the highest risk of hematologic venom effects of the Crotalinae.
A Step-wise ED Approach to Crotalinae Snake Bites5
Step 1: Assess the patient
Check their vitals and assess if they require concurrent resuscitation at the same time as their evaluation and diagnosis. Mark the leading edge of the swelling and pain, immobilize and elevate the extremity, treat their pain, draw initial labs including:
- Complete cell count
- Chemistry panel
- Liver function tests
- PT/INR, PTT
- Creatine kinase
Update the patient’s tetanus vaccination if needed. Avoid treatments such as a tourniquet, ice, incision, or suction of the wound as these are not beneficial.
Step 2: Evaluate for envenomation
Not all bites involve the transmission of venom into the patient, even when the bite is from a venomous snake. About 25% are non-envenomed, or “dry” bites. You may observe ecchymosis and puncture wounds. Indications of envenomation include the classic signs of inflammation: tumor, dolor, calor, rubor. Patients may also have systemic signs, such as bleeding at other sites due to coagulopathy, vomiting, diarrhea, hypotension, paresthesias, or angioedema.
Laboratory indications that the bite was envenomed include:
- Elevated PT
- Decreased fibrinogen
These are signs of intravascular clot formation. If there are no immediate signs of envenomation, then the patient should be monitored for at least 8-12 hours, and labs should be repeated prior to discharge to ensure no change.2,4,5
Step 3: Determine the severity
If the bite does appear to be envenomed, then the next step is to determine the severity and whether antivenom is needed. Indications for use of antivenom include any of the following:
- Moderate to severe swelling that is progressing
- Any systemic symptoms such as hypotension or angioedema
- Abnormalities of the PT, fibrinogen, or platelet levels
The majority of envenomed bites from copperheads do not initially appear severe. However, it is important to remember that envenomation is a dynamic process that progresses over many hours to days. A major pitfall is to discharge the patient too soon before the full effect of the envenomation is evident, or to fail to give antivenom as this can lead to long-term limb or digit dysfunction.
If there is evidence of envenomation but it is minor, then the patient should be monitored and observed for 12-24 hours, and the labs repeated.
If there is moderate or severe envenomation, then CroFab is indicated. At this point, it is a good idea to contact the poison control center (at 1-800-222-1222 in the U.S.), which is an invaluable resource for snake bite management. Their consultants can provide treatment algorithms, which may be particularly helpful in areas where snake bites are less common, and physicians may not be as familiar with indications for the effective but highly expensive CroFab.
Important caveat: In management of bites directly to a digit, be aware that there is likely to be more localized damage in this smaller space. Digit loss, even with copperhead bites, can occur in bites initially graded as “minor” or “mild” due to the time it takes for the swelling and pain to progress more proximally. In addition bites on the face or head that are near the larynx are considered more severe, as edema can lead to airway compromise.
Table 1. Severity of Envenomation4
|Local effects||Systemic effects||Coagulopathy|
|Minor||Swelling, pain, and bruising around the site||None||Normal, and no bleeding|
|Moderate||Swelling, pain and bruising extending no more than 50 cm from the bite or less than the entire extremity||Minor symptoms such as nausea, vomiting, paresthesias, diarrhea, tachycardia, mild hypotension (SBP>90 mmHg)||Abnormal coagulation studies, but only minor bleeding such as gum bleeding, epistaxis, or hematuria|
|Severe||Swelling, pain, or bruising involving an entire extremity, extending more than 50 cm, threatening the airway (eg for head/neck bites), or signs of compartment syndrome||Severe or life threatening hypotension, confusion, seizures, respiratory failure, or other severe vital sign abnormalities||Very abnormal coagulation studies, and serious bleeding|
Step 4. Administer antivenom if needed
CroFab should be given as soon as the patient meets criteria for it. It is given as 4-6 vials mixed in 250 cc of normal saline and infused intravenously over 1 hour. For patients in shock or with signs of bleeding from coagulopathy, a higher dose of 8-12 vials may be used. You can expect a response within about 1 hour of the infusion. If symptoms are not controlled or are progressing after the first dose, then an additional 4-6 vials can be given until the symptoms and systemic signs are normalizing. After this, a maintenance dose of 2 vials every 6 hours for up to 18 hours can be used.
The dose for pediatrics is the same as adults, although studies have been in children 11 years and older in clinical trials. The antivenom is an antibody fragment that acts by binding directly to the venom. So the dose is based on the quantity of venom, not the weight of the victim. The downsides of CroFab are the risks of an allergic reaction and the cost (about $2,300 per vial).
Step 5. Monitor and reassess
The patient should be monitored closely. If they are severely ill, with hypotension or angioedema that requires intubation, they will require an intensive care unit admission. If the envenomation was moderate or not associated with severe systemic symptoms, then they should be observed for 18-24 hours, and given additional antivenom as above. After discharge the patients should have follow up in 2-3 days to recheck their PT, fibrinogen, hemoglobin, and platelets.
Step 6. Recheck before discharge
To be safe for discharge, the patient should show no signs of progression, be clinically stable, have mild symptoms, and have no worsening of their PT, fibrinogen, or platelets. There is only a 3% rate of secondary infections, so prophylactic antibiotics are not routinely recommended.6
|Dry bite, no envenomation||8-12 hours||Initial laboratory studies of CBC, chemistry, LFTs, PT/INR, PTT, CK, fibrinogen, UA. Repeat studies of CBC, PT/INR, fibrinogen||No antivenom|
|Minor envenomation||12-24 hours||Initial laboratory studies; repeat laboratory studies every 4-6 hours||Consider antivenmon only if high-risk areas affected (e.g. face and swelling close to airway)|
|Moderate envenomation||Admit||Initial laboratory studies; repeat every 1 hour after antivenom until initial control||Antivenom administration, supportive care|
|Severe envenomation||Admit to ICU||Initial laboratory studies; repeat every 1 hour after antivenom until initial control||Antivenom administration (consider higher initial dose and repeat doses), supportive care, airway management|
Table 2. Summary of emergency medicine care of Crotalinae envenomations.4,7
Elapidae (Coral snakes)
Coral snakes are found in the southern U.S. but cause fewer than 100 bites per year on average. Bites are typically due to mishandling or misidentification. They can be differentiated from similar appearing non-venomous snakes in the U.S. by the phrase “red on yellow, kill a fellow, red on black’s a friendly jack.” The important distinction from the pit vipers is that their bites deliver a neurotoxin, which can cause a descending paralysis, akin to rapid onset myasthenia gravis.
Unlike pit viper bites, these patients will need antivenom even if they are initially asymptomatic, as the neurotoxicity may not appear immediately following the bite. In addition to watching for local tissue necrosis and coagulopathy, these patients need to be monitored and assessed for muscular weakness and respiratory failure. Patients will experience a descending paralysis with bulbar muscles affected first. If neurotoxic symptoms become evident, the next step is to assess the airway and respiratory function with negative inspiratory force (NIF) and forced vital capacity (FVC) monitoring as they may require intubation. Coral snake antivenom is not readily available at most hospitals, so a patient with severe envenomation will likely require mechanical ventilation before the antivenom can be administered. If there will be a delay to care, the affected extremity can be treated with pressure immobilization, which may delay systemic effects, but is a technique not indicated in pit viper bites.8
Figure 4: Coral Snake – These small snakes carry a potent neurotoxin, and patients may initially be asymptomatic after a bite.
Exotic snakes, such as mambas and cobras [news story], can be easily purchased online, and their ownership seems to be gaining in popularity. So the odds are good that someone near your ED owns one. Bites from these snakes are an order of magnitude more severe than endemic poisonous snakes, and without treatment are often non-survivable. King Cobra bites in particular cause severe neurotoxicity and require aggressive airway control and fluid resuscitation. To procure antivenom, U.S. providers can call the poison control center, where their staff can help arrange consultation with an expert and shipment of antivenom to your hospital.
Figure 5: The King Cobra – Bites can cause rapid shock and patients will require early intubation and fluid resuscitation until antivenom can be acquired.
The 3 year-old girl who presented to the ED showed signs of moderate Crotalinae envenomation, with swelling spreading up to the elbow, and she was from an area with a large, endemic copperhead population. While most copperhead envenomations do not require CroFab due to their milder symptoms and extremely low fatality, bites in children can be more severe. The patient was treated with CroFab and admitted for monitoring, repeat laboratory checks, pain control, and observation for resolution of symptoms for 24 hours.
Take Home Points
- Most venomous snake bites in the U.S. are from Crotalinae, or pit vipers.
- The first step is to assess the patient, resuscitate if critically ill, and determine the severity of the envenomation.
- Moderate and severe Crotalinae envenomation should be treated with CroFab and the patients monitored for resolution of symptoms.
- Crotalinae bites that do not show signs of envenomation should still be monitored for at least 8 hours, as swelling and pain may develop some time after the bite. Most patients will present immediately after a bite, and if they are discharged too soon, the full extent of the envenomation may not be apparent.
- The pediatric CroFab dosing is the same as for adults as the antivenom binds to the venom, so the dose is based on the amount of venom present, and not the weight of the patient.
- Rattlesnake bites can be life-threatening and require significant amounts of anti-venom to control coagulopathy and hemodynamic instability. Almost every state in the U.S. falls within the coverage area of rattlesnakes, so when in doubt, monitor the patient closely, as envenomation may become severe.
- Patients with coral snake bites should be treated with antivenom early even if they are asymptomatic, as the venom contains a neurotoxin that is initially asymptomatic but will later present with descending weakness that can require intubation.
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