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Snake Envenomation, Moccasins
Updated: Jul 24, 2008
Introduction
Background
Moccasins are new world pit vipers (family Viperidae, subfamily Crotalinae), which may be identified by a heat-sensing pit anteroinferior to each eye, elliptical pupils, a triangular head, and undivided subcaudal scales.
Snake envenomations, moccasins. Copperhead (Agkistrodon contortrix). Photo courtesy of Sean Bush, MD.
Moccasins comprise the genus Agkistrodon, which includes the cottonmouth (Agkistrodon piscivorus) and copperhead (Agkistrodon contortrix) in the southeastern United States; the cantil (Agkistrodon bilineatus) in Mexico and Central America; the mamushi (Agkistrodon blomhoffii), Siberian pit viper (Agkistrodon halys), and Central Asian pit viper (Agkistrodon intermedius) in central and northeastern Asia; and the Malayan pit viper (Calloselasma rhodostoma) and hundred-pace snake (Deinagkistrodon acutus) in southeastern Asia.
Snake envenomations, moccasins. Cottonmouth or water moccasin (Agkistrodon piscivorus). Photo courtesy of Sean Bush, MD.
Pathophysiology
Envenomation occurs when the moccasin injects venom via hollow movable fangs located in the anterior mouth. The effects of moccasin envenomation are generally similar to rattlesnake envenomation. However, in most cases, moccasin envenomation is less serious than envenomation by rattlesnakes. For further discussion of more severe pit viper envenomation, see Snake Envenomations, Rattle.
Moccasin venom is complex, with nearly 50 identified components. These can be broken down into 4 major categories:
- Proteolytic enzymes that directly destroy tissue, as happens in digestion of prey animals
- Inflammatory mediators, including histamine- and bradykinin-like factors, that cause pain, erythema, swelling, and occasionally distributive shock
- Fibrinolytic enzymes that cleave fibrin into ineffective D-dimers, resulting in coagulopathy
- Antiplatelet factors that cause thrombocytopenia
Although neurotoxic factors can be detected in moccasin venom, clinically significant neurotoxicity does not occur with envenomation by copperheads or cottonmouths.
Frequency
United States
Approximately 5,000 snakebites are reported to poison centers each year. Of the venomous snakebites for which the species is known, moccasins are responsible for 42%. The vast majority of these (86%) are copperhead envenomations. In portions of the southeastern United States, copperheads account for 85% of all reported snake envenomations.
International
An estimated 300,000-400,000 venomous snakebites occur per year worldwide. The proportion of these caused by Agkistrodon species is not known.
Mortality/Morbidity
The American Association of Poison Control Centers (AAPCC) has reported only one death from moccasin envenomation since its first annual report in 1983. Prospective studies of morbidity from moccasin envenomation have not been conducted. However, in two retrospective studies of copperhead victims, patients missed a median of 2 or 6 weeks of work.
Sex
Incidence of snakebite is higher in males than in females.
Age
Young adults are bitten most commonly.
Clinical
History
- Most bites occur on the extremities. Upper extremity bites predominate in males and are often associated with deliberate handling of the snake.
- Pain around the bite site
- Swelling
- Nausea, vomiting, or diarrhea
- Syncope, near syncope
- Co-intoxication with alcohol is common and may affect the patient's judgment and ability to comply with therapy.
Physical
Although moccasin envenomation usually is associated with less severe local effects than rattlesnake envenomation, severe envenomations do occur.
- Fang marks are common, but they may be absent.
- Hemorrhagic vesicles may be present at the envenomation site.
- Tenderness surrounding bite site is almost always present.
- Local edema
- Use a pen to mark and time the border of advancing edema and tenderness often enough to gauge progression.
- Circumferential measurements of the envenomated extremity, repeated over time, may help differentiate swelling that is progressing (getting worse) from swelling that has stabilized (improving or not getting worse) but for which the leading edge is moving proximally because of limb elevation (see Media file 4). Measurements of the contralateral (uninvolved) extremity may be useful for comparison.
Wound measurement in snakebites. Courtesy of Carolinas Poison Center.
- Rapidly progressive swelling is usually indicative of a more severe envenomation.
- Erythema at the bite site, proximal to the bite site, and along patterns of lymphatic drainage
- Ecchymosis
- Bullae
- Bleeding into the tissues of the bite site is common. Systemic bleeding is uncommon in moccasin envenomations, occurring in less than 5% of copperhead envenomations.
- Tachycardia is common and due to pain, anxiety, and third-spacing of fluids due to inflammation.
- Hypotension is uncommon and usually due to intravascular volume depletion.
Causes
A large percentage of bites are considered intentionally interactive—they occur when the snake is handled, kept as a pet, or abused. Many bites are associated with ethanol use.
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References
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Further Reading
Keywords
moccasin snake bite , moccasin snake envenomation , pit viper snake bite, moccasin venom, snake bite, Agkistrodon genus, cottonmouth, Agkistrodon piscivorus, copperhead, Agkistrodon contortrix, cantil, Agkistrodon bilineatus, mamushi, Agkistrodon blomhoffii, Siberian pit viper, Agkistrodon halys, Central Asian pit viper, Agkistrodon intermedius, Malayan pit viper, Calloselasma rhodostoma, hundred-pace snake, Deinagkistrodon acutus
