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Snake Envenomation Overview

Author: Brian James Daley, MD, MBA, FACS, Associate Program Director, Professor, Department of Surgery, Division of Trauma and Critical Care, University of Tennessee School of Medicine
Contributor Information and Disclosures

Updated: Aug 20, 2009

Introduction

Background

Most snakebites are innocuous and are delivered by nonpoisonous species. North America is home to 25 species of poisonous snakes. Worldwide, only about 15% of the more than 3000 species of snakes are considered dangerous to humans. The family Viperidae is the largest family of venomous snakes, and members can be found in Africa, Europe, Asia, and the Americas. The family Elapidae is the next largest family of venomous snakes. In North America, the venomous species are members of the families Elapidae and Viperidae, subfamily Crotalidae.

The subfamily Crotalidae (pit vipers) includes rattlesnakes (Crotalus and Sistrurus), cottonmouths (Agkistrodon), and copperheads (Agkistrodon). The Western diamondback, timber, and prairie rattlesnakes are pit vipers (see Media file 1).

Elapidae includes the coral snakes (Micrurus fulvius fulvius and Micrurus fulvius tenere; see Media file 2).

Cobras, mambas, and kraits are also members of the family Elapidae but are not indigenous to the Americas. However, an increasing number of exotic species are kept by both zoos and private collectors making bites by nonindigenous species increasingly common.

Pathophysiology

Venom is produced and stored in paired glands below the eye. It is discharged from hollow fangs located in the upper jaw. Fangs can grow to 20 mm in large rattlesnakes. Venom dosage per bite depends on the elapsed time since the last bite, the degree of threat perceived by the snake, and size of the prey. Nostril pits respond to the heat emission of the prey, which may enable the snake to vary the amount of venom delivered.

Coral snakes have shorter fangs and a smaller mouth. This allows them less opportunity for envenomation than the crotalids, and their bites more closely resemble chewing rather than the strike for which the pit vipers are famous. Both methods inject venom into the victim to immobilize it quickly and begin digestion.

Venom is mostly water. Enzymatic proteins in venom impart its destructive properties. Proteases, collagenase, and arginine ester hydrolase have been identified in pit viper venom. Neurotoxins comprise the majority of coral snake venom. Specific details are known for several enzymes as follows: (1) hyaluronidase allows rapid spread of venom through subcutaneous tissues by disrupting mucopolysaccharides; (2) phospholipase A2 plays a major role in hemolysis secondary to the esterolytic effect on red cell membranes and promotes muscle necrosis; and (3) thrombogenic enzymes promote the formation of a weak fibrin clot, which, in turn, activates plasmin and results in a consumptive coagulopathy and its hemorrhagic consequences.

Enzyme concentrations vary among species, thereby causing dissimilar envenomations. Copperhead bites generally are limited to local tissue destruction. Rattlesnakes can leave impressive wounds and cause systemic toxicity. Coral snakes may leave a small wound that later results in respiratory failure from systemic neuromuscular blockade.

The local effects of venom serve as a reminder of the potential for systemic disruption of organ system function. One effect is local bleeding; coagulopathy is not uncommon with severe envenomation. Another effect, local edema, increases capillary leak and interstitial fluid in the lungs. Pulmonary mechanics may be altered significantly. The final effect, local cell death, increases lactic acid concentration secondary to changes in volume status and requires increased minute ventilation. The effects of neuromuscular blockade result in poor diaphragmatic excursion. Cardiac failure can result from hypotension and acidosis. Myonecrosis raises concerns about myoglobinuria and renal damage.

Frequency

United States

Snakebites frequently go unreported. Approximately 4000-7000 bites are reported to national centers each year. North Carolina has the highest frequency, with 19 bites per 100,000 persons. The national average is approximately 4 bites per 100,000 persons.

International

Generally, only localized reporting of international data are available. Most snakebites and deaths due to snakebites are not reported, especially in the developing world.

Mortality/Morbidity

A 20-year review of data from the National Vital Statistics Systems identified 97 fatalities. The state of Texas had the most fatalities (17), followed by Florida (14), and Georgia (12).

• Deaths secondary to snakebites are rare. With the proper use of antivenin, they are becoming rarer still. The national average has been less than 4 deaths per year for the last several years.
• A review of morbidity associated with snakebites from Kentucky was published. Most bites were from copperheads and resulted in 8 days of pain, 11 days of extremity edema, and 14 days of missed work.¹ A review specifically of copperhead bites in West Virginia described similar outcomes and noted that the peak effects of envenomation were not present until longer than 4 hours after the bite.²
• Local tissue destruction rarely contributes to long-term morbidity. Occasionally, skin grafting is required to close a defect from fasciotomy, but wounds requiring fasciotomy to reduce compartment pressures from muscle edema are infrequent.
• Data gathered in a 5-year retrospective chart review from the University of Tennessee Medical Center at Knoxville (UTMCK), a level-I trauma center, focused on 25 bites. Of these, 4 required fasciotomy and 2 subsequently needed split-thickness skin grafting. The average length of stay was 3.2 days. No deaths occurred, and morbidity was limited to the local wounds.

Race

White males account for 76% of the victims.

Sex

• National studies report a 9:1 male-to-female ratio.
• UTMCK studies report a 2.1:1 male-to-female ratio.

Age

• National studies report 50% of patients were aged 18-28 years. UTMCK studies report 25% were aged 18-28 years, with a mean of 29.5 years.
• National studies report 95% of bites were located on an extremity, especially the hand. UTMCK studies report 96% of bites were located on an extremity, 56% to the hand.
• National studies report a seasonal occurrence of 90% from April to October. UTMCK studies report 100% occurrence from April to October (May: 1 bite out of 25 cases; June to August: 19 bites out of 25 cases; and September to October: 5 bites out of 25 cases).
• In the pediatric population, most snakebites occurred in school-aged children and adolescents around the perimeter of the home during the afternoon in summer months. The most frequent wound sites were the lower limbs.³

Clinical

History

History usually can be obtained from the patient. Most cases result from attempting to handle snakes, so the genus usually is known. Knowledge of indigenous fauna also is important.

The time elapsed since the bite is a necessary component of the history. This allows assessment of the temporal effects of the bite to determine if the process is confined locally or if systemic signs have developed.

• Obtain a description of the snake or capture it, if possible, to determine its color, pattern, or the existence of a rattle.
• Most snakes remain within 20 feet after biting.
• Assess the timing of events and onset of symptoms. Inquire about the time the bite occurred and details about the onset of pain. Early and intense pain implies significant envenomation.
• Local swelling, pain, and paresthesias may be present.
• Systemic symptoms include nausea, syncope, and difficulty swallowing or breathing.
• Determine history of prior exposure to antivenin or snakebite.
• Determine history of allergies to medicines because antibiotics may be required.
• Determine history of comorbid conditions (eg, cardiac, pulmonary, and renal disease) or medications (eg, aspirin, anticoagulants such as warfarin [Coumadin] or GPIIb/IIIa inhibitors, beta-blockers).

Physical

Follow the established routine for a complete comprehensive examination.

• Vital signs, airway, breathing, circulation
• Fang marks or scratches (determine coral snakebite pattern by expressing blood from the suspected wound)
• Local tissue destruction
  • Soft pitting edema that generally develops over 6-12 hours but may start within 5 minutes
  • Bullae
  • Streaking
  • Erythema or discoloration
  • Contusions
• Systemic toxicity
  • Hypotension
  • Petechiae, epistaxis, hemoptysis
  • Paresthesias and dysesthesias - Forewarn neuromuscular blockade and respiratory distress (more common with coral snakes)

Causes

• In the United States, more than 40% of victims put themselves in danger by either handling pets or attempting to capture reptiles in the wild. The popularity of keeping exotic species has increased the number of envenomations by nonnative species.
• UTMCK data support this by reporting that 15 of 25 patients were bitten handling snakes; 2 of these were involved in religious ceremonies.

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