Lizard Envenomation

The venom apparatus is much less sophisticated than that of most venomous snakes. A pair of multilobed labial venom glands (modified submandibular glands) lie in the anterior portion of the lower jaw. Venom is conducted from each lobe through a single duct (H horridum) or a series of ducts (H suspectum) and is deposited into a labial mucosal pocket adjacent to the anterior teeth.[9]

The teeth (approximately 20 per jaw) are grooved and loosely attached to the jaws. Venom is conducted via capillary action along these grooves into the victim's tissues as the lizard bites and chews.[2] The more irritated the lizard is when it bites, the more it salivates and the greater the venom yield. Effective envenomation in humans probably occurs in less than 70% of bites. Venom is thought to be defensive rather than digestive or for prey immobilization [2]

The dentition of a Gila monster. The grooved surfaces of the teeth are evident. These grooves allow for venom movement from the venom glands into the victim's tissues. Photo by Michael Cardwell.

The venoms of these lizards are remarkably similar and contain a number of components, including L-amino acid oxidase, hyaluronidase, phospholipase A, serotonin, and highly active kallikreins that release vasoactive kinins. The venom contains no neurotoxins or any enzymes that significantly affect coagulation. In laboratory animals, the venom is as potent as some rattlesnake venoms. Rare hypersensitivity to helodermatid venom has been reported.[10, 11]

Frequency

United States

Bites are very infrequent and usually involve captive specimens. A significant number of bites probably go unreported because private keepers of these protected lizards may be reluctant to seek medical attention.

International

No data regarding current incidence of bites by venomous lizards in Mexico are available.

Timing

At least one study indicates Heloderma encounters are more freqeunt during warmer months [1]

Race-, sex-, and age-related information

No data are available to support any racial predilection to lizard bites.

Males are more frequently bitten[1] , likely due to increased interaction with these animals 

While there are no large studies reporting the age distribution of venomous lizard bites, it is likely that those at greatest risk will be children, adolescents, and young adults. Young children may be at increased risk of bites if they come across a wild Heloderma specimen, as they may not comprehend the risk. Adolescents and young adults may be more prone to intentionally interact with or collect such lizards, despite knowing their venomous nature.

The prognosis is excellent for those with a lizard envenomation, although pain may be an issue for many days. Reports of  ​Heloderma envenomations resulting in death are largely unconfirmed and no deaths have been reported in modern literature.[2] It is believed that a prolonged bite received by a small individual, such as an infant or toddler, could result in death.

Severe pain following a helodermatid bite may last many hours and generalized weakness can persist for several days. Other complications include abdominal pain, vomiting and diarrhea.  More rare and life-threatening complications include shock, airway edema, myocardial infarction, cardiac dysrhythmia, allergic/anaphylactic reaction, and coagulopathy.[1, 11, 12, 7, 8]

The vast majority of individuals who are bitten by Gila monsters or beaded lizards are intentionally interacting with the animals, and the history of the bite usually is clear.

To help estimate the severity of envenomation, it is important to estimate the length of time the lizard remained attached to the victim. While an effective envenomation can occur with any bite, longer attachment time of the animal can be associated with more severe symptoms[2] .

The victim may present with many signs and symptoms, including the following:

• Multiple lacerations that may bleed profusely

• Severe throbbing or burning pain at the bite site that may radiate proximally

• Discoloration at the bite site (eg, cyanosis, ecchymosis)

• Generalized weakness

• Nausea and vomiting

• Shortness of breath

• Sweating

• Numbness

• Dizziness

• Faintness

• Progressive edema

• Painful lymph nodes

• Angioedema

Any of the attendant complications of shock may be encountered.

Myocardial infarction and cardiac dysrhythmias may occur.

Coagulopathy is a rare complication.

Airway edema may occur and may require advanced airway managment

Electrolyte disturbances may occur.

Wound infections may occur, especially in the setting of a retained tooth.

Necrosis is notably rare.

Allergic or anaphylactic reactions are also rare, but have been described.[11]

The most common CBC count finding is an elevation in the WBC count. In rare severe cases, a drop in the platelet count may occur.

Measure serum electrolyte levels, particularly if there is a history of significant underlying disease or evidence of volume loss such as signficnat gastrointestinaly symptoms or diaphoresis.

Helodermatid venoms do not appear to have any anticoagulant fractions. However, very rarely, reports of abnormal coagulation studies with severe bites have been documented. These rare coagulopathies are likely secondary to hemostatic changes occurring as a result of severe endothelial cell damage.

It is reasonable to obtain a prothrombin time, an activated partial thromboplastin time, a fibrinogen level, and a measure of fibrin degradation products, although the clinical significance of abnormalities is not known.

Perform a urinalysis and look for any evidence of blood or casts.

A cardiac panel, including a CK-MB, myoglobin, and troponin should be obtained if the victim has evidence of hemodynamic instability, chest pain, or an abnormal ECG result.

No evidence-based recommendations have been determined for field management of lizard envenomation.

If the lizard remains attached to the victim, it immediately should be removed as soon as possible. Unfortunately mechanical methods for detaching the animal may facilitate injury to the animal including disloged teeth remaining in the wound. Briefly holding flame under the animal's chin/neck or submerging the lizard in cold water has been successful previously. Care should be exercised to prevent additional bites to the victim or rescuer as the lizard may feel threatened.

If possible, the wound should be washed quickly with copious running water. Place a dressing on the wound to control any active bleeding, and apply a splint to limit movement of the extremity. The limb can be kept at approximately heart level during transport to medical care.

No evidence to recommend the use of constrictive bands, compressive dressings, or suction is available. Tourniquets should not be used.

Do not make any incisions into the already damaged tissues.

Although ice has been used to reduce pain, it can exacerbate local vasospasm.

Victims showing evidence of shock should be kept in a supine position with legs elevated. If oxygen and intravenous fluids are available, these should be administered during transport to the hospital.

Closely monitor vital signs.

Immediately begin cardiac and pulse oximetry monitoring.[13]

Establish 2 IV lines with isotonic sodium chloride solution.

If hypotension occurs, begin aggressive fluid resuscitation. If vital signs fail to improve with adequate crystalloid infusion (eg, 20-40 mL/kg), vasopressor agents may be necessary.

Wound care

Clean the wounds with soap and water, and evaluate them for retained foreign bodies.

A local anesthetic, such as lidocaine (without epinephrine), can be injected at the bite site to aid in pain control and to assist in exploration of the wounds for retained teeth or damage to underlying structures.[14] Regional nerve blocks and judicious use of narcotics can be helpful in controlling pain.[15] A non–histamine-releasing narcotic, such as fentanyl, may be preferable.

The extremity should be splinted and kept at (or slightly above) the patient's heart level to reduce edema.

Because of the significant local tissue trauma, prophylactic broad-spectrum antibiotics may be administered for a few days, although this treatment is controversial and untested.

Tetanus immunization status should be updated as necessary.

Inpatient

Admit patients with a significant systemic reaction or with abnormal lab studies and/or ECG findings, to a monitored setting.

The bitten extremity should be placed in a well-padded splint and elevated to the patient's heart level or higher to reduce edema.

Institute standard daily wound care, including cleansing, topical antibiotic application, and dressing.

Physical therapy can help speed the return to full function.

Admission for pain control may be warranted.

Transfer

Transfer of victims with evidence of severe envenoming to a center with experience in dealing with these cases may be prudent. Consultation with an experienced provider is recommended (see Consultations).

Outpatient

All victims of helodermatid bites should be observed in the ED for at least 6 hours for development of systemic or severe symptoms.

Reliable patients who are relatively asymptomatic, with normal vital signs and lab findings, can be discharged with instructions to return for any worsening symptoms. A responsible adult should be available to help the patient return if necessary.

Provide instructions for wound care (eg, wash the wounds twice per day with soap and water; apply a topical antibiotic ointment and dressing).

Signs and symptoms of wound infection should be discussed with the patient.

Arrange a follow-up appointment for a wound check in 24-48 hours.

Give a prescription for antibiotics (eg, cephalexin) to patients who are being prophylactically treated.

A prescription for a narcotic analgesic, such as hydrocodone, may be appropriate.

Consult an expert in reptile envenomations to assist in management, as necessary. In the US, the local Poison Control Center (1-800-222-1222) can help direct or facilitate appropriate care.  An excellent resource for assistance is the University of Arizona Poison and Drug Information Center (telephone: 520-626-6016).

No specific agents are used to treat lizard envenomations. Although 2 experimental antivenoms have been produced for the Gila monster, neither has been made commercially available. Drug therapy is entirely symptomatic (eg, local anesthetics, analgesics, antiemetics). If antibiotics are administered, they should be broad-spectrum and they should provide coverage for gram-negative organisms. For the rare case of anaphylaxis/angioedema, therapy should be instituted with sympathomimetic agents, antihistamines, and/or steroids.

Class Summary

Cardiovascular agents possess alpha- and beta-adrenergic effects that are useful in reversing anaphylactic reactions (eg, angioedema, bronchospasm, hypotension).

Epinephrine (Adrenalin, EpiPen)

Epinephrine is the drug of choice for the treatment of anaphylactoid reactions. Alpha-agonist effects increase peripheral vascular resistance and reverse peripheral vasodilatation and vascular permeability, thus helping to restore vascular tone and blood pressure. The beta-agonist effects increase heart rate and inotropism and cause bronchodilatation.

Class Summary

Corticosteroids onset of action is approximately 4-6 hours, although some synergistic activity may be noted when simultaneously used with sympathomimetic agents. Steroids may be needed in the rare event of an allergic reaction to lizard venom. Corticosteroids have no role in the management of envenomation itself.

Methylprednisolone (Solu-Medrol)

Methylprednisolone decreases inflammation by suppressing the migration of polymorphonuclear leukocytes and reversing increased capillary permeability.

Prednisone (Deltasone, Orasone, Meticorten)

Prednisone decreases inflammation by suppressing the migration of polymorphonuclear leukocytes and reversing increased capillary permeability.

Class Summary

Antihistamines prevent the histamine response in sensory nerve endings and blood vessels. They are more effective in preventing a histamine response than in reversing it.

Diphenhydramine (Benadryl, Benylin, Bydramine)

Diphenhydramine is used for the symptomatic relief of allergic symptoms caused by histamine released in response to allergens.

Cimetidine (Tagamet)

Cimetidine is an H2 antagonist that, when combined with an H1 type, may be useful in treating itching and flushing in anaphylaxis, pruritus, urticaria, and contact dermatitis that do not respond to H1-receptor antagonists alone. Use it in addition to H1 antihistamines.

Class Summary

Local anesthetics can be injected, either locally or regionally, to reduce pain and facilitate exploration of wounds (to rule out damage to underlying structures or retained teeth).

Lidocaine anesthetic

Lidocaine is an amide local anesthetic used in 1-2% concentration. It inhibits depolarization of type C sensory neurons by blocking sodium channels. Epinephrine should be avoided in venomous lizard bites to prevent causing additional local tissue ischemia.

Class Summary

Patients may require narcotic analgesics for pain control after venomous lizard bites. In choosing a narcotic analgesic, to prevent worsening venom effects, it is best to select a narcotic that has less tendency to induce histamine release, such as fentanyl.

Fentanyl citrate (Sublimaze)

Fentanyl is a synthetic opioid that is 75-200 times more potent and has a much shorter half-life than morphine sulfate. It has less hypotensive effects and is safer in patients with hyperactive airway disease than morphine because of minimal-to-no associated histamine release. By itself, it causes little cardiovascular compromise, although the addition of benzodiazepines or other sedatives may result in decreased cardiac output and blood pressure. Fentanyl is highly lipophilic and protein-bound. Prolonged exposure leads to an accumulation in fat and delays the weaning process. Consider using a continuous infusion because of the short half-life of fentanyl. The parenteral form is the drug of choice for conscious sedation analgesia. It is ideal for analgesic action of short duration during anesthesia and the immediate postoperative period. It is an excellent choice for pain management and sedation of short duration (30-60 min), and it is easy to titrate. Its effects are easily and quickly reversed by naloxone.

After initial parenteral dose, subsequent parenteral doses should not be titrated more frequently than every 3 hours or every 6 hours thereafter. Transdermal form is used only for chronic pain conditions in opioid-tolerant patients. When using a transdermal dosage form, the majority of patients are controlled with 72-hour dosing intervals; however, some patients require dosing intervals of 48 hours.