Snakebites

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Overview

Types of Poisonous Snakes

• Two families of venomous snakes are native to the United States. The vast majority are pit vipers, of the family Crotalidae, which include rattlesnakes, copperheads and cottonmouths (water moccasins). Pit vipers get their common name from a small "pit" between the eye and nostril that allows the snake to sense prey at night. The vipers are generally considered to be the most advanced family of snakes since they possess a very sophisticated venom delivery system. Large tubular fangs are placed in the front of the mouth and they are hinged, allowing them to be folded back when not in use.
• About 99 percent of the venomous bites in this country are from pit vipers. Some--Mojave rattlesnakes or canebrake rattlesnakes, for example--carry a neurotoxic venom that can affect the brain or spinal cord. Copperheads, on the other hand, have a milder and less dangerous venom that sometimes may not require antivenin treatment.
• The other family of domestic poisonous snakes is Elapidae, which includes two species of coral snakes found chiefly in the Southern states. The Elapidae contains some of the world's most dangerous snakes including cobras, mambas and sea snakes. In North America, three species of elapids are found, the Eastern and Western coral snakes and the Yellow bellied sea snake. The coral snakes are relatively small snakes that spend most of their time underground. Their primary food is other snakes. Despite their small size and small fangs, their venom is extremely toxic. Coral snakes have small mouths and short teeth, which give them a less efficient venom delivery than pit vipers. People bitten by coral snakes lack the characteristic fang marks of pit vipers, sometimes making the bite hard to detect.
• Though coral snakebites are rare in the United States--only about 25 a year by some estimates--the snake's neurotoxic venom can be dangerous. A 1987 study in the Journal of the American Medical Association examined 39 victims of coral snakebites. There were no deaths, but several victims experienced respiratory paralysis, one of the hazards of neurotoxic venom.
• Some nonpoisonous snakes, such as the scarlet king snake, mimic the bright red, yellow and black coloration of the coral snake. This potential for confusion underscores the importance of seeking care for any snakebite (unless positive identification of a nonpoisonous snake can be made).
• The bites of both pit vipers and coral snakes can be effectively treated with antivenin. But other factors, such as time elapsed since being bitten and care taken before arriving at the hospital, also are critical.

Epidemiology and Demographics

Every state but Maine, Alaska and Hawaii is home to at least one of 20 domestic poisonous snake species. About 8,000 people a year receive venomous bites in the United States; nine to 15 victims die. Some experts say that because victims can't always positively identify a snake, they should seek prompt care for any bite, though they may think the snake is nonpoisonous. Some deaths are sudden, however in fact it is uncommon to die within four hours of a snake bite. In the 1900’s, the untreated death rates were as high as 40% to 50%. Improved supportive treatment and the availability of effective antivenoms has reduced this considerably.

History and Symptoms

• The bite site is usually painless. It may have classical paired fang marks, but this is not the most common picture. Often there are just a few lacerations or scratches, and sometimes these may be painless or go unnoticed. Bruising, bleeding, and local swelling may be present, but significant local tissue destruction is uncommon.
• Regional lymphadenopathy may be marked, even with non-venomous snake bites, and is not by itself an indication for the administration of antivenom. It may contribute to abdominal pain in children.
• The usual sequence of systemic symptom development:

  • (<1hr) Headache, irritability, photophobia, nausea, vomiting, diarrhea, confusion; coagulation abnormalities; occasionally sudden hypotension with loss of consciousness.
  • (1-3 hrs) Cranial nerve paralysis (ptosis, diplopia, dysphagia etc), abdominal pain, hemoglobinuria, hypertension, tachycardia, hemmorrhage.
  • (>3hrs) Limb and respiratory muscle paralysis leading to respiratory failure, peripheral circulatory failure with pallor and cyanosis, myoglobinuria, eventually death.

• This sequence of events is highly variable. Some bites, even apparently trivial ones, have been associated with acute deterioration over a five minute period leading to death. This may occur as soon as 30 minutes to an hour after the original bite. Acute, severe cardiac depression may be the mechanism for sudden death.
• Paralysis, when it occurs, usually commences with cranial nerves, then skeletal muscle, then the muscles of respiration. In small children or with highly venomous snake bites it may happen much more quickly.
• Major bleeding disturbances are rare although the development of coagulopathies and a DIC-like picture are relatively common. Thromboctopenia and hemolysis may occur. About 20% of patients who die after snake bite have cerebral hemmorrhages.
• Muscle destruction from myolytic toxins is not uncommon and may not be associated with muscle tenderness; it may lead to renal failure and should be specifically looked for, because early treatment with antivenom will reduce its severity.
• Snake bite should always be considered in any case of unexpected confusion or loss of consciousness following outdoor activities in snake country. Prognosis depends on the type of snake and the quantity of venom injected. An angry snake and multiple bites is associated with greater venom volumes.

Risk Stratification and Prognosis

Recovery is usually complete, though the patient usually develops a sensitivity to equine immunoglobulin. If the patient develops serum sickness, the severity is reduced by steroid administration (eg. prednisolone 1mg/kg every 8 hours) until resolution occurs. A course of steroids is recommended in all patients who receive polyvalent antivenoms.

Treatment

Medical Management of Snake Bites

• Only 1 in 20 snake bites require active emergency treatment or the administration of antivenom. Medical management depends on the degree of systemic envenomation and the type of venom.

Critically ill patients

• Maintain immobilization, splint and bandage until the situation is under control.
• Support airway, breathing and circulation.
• Intubate and ventilate with 100% Oxygen if airway or respiration fail.
• Obtain toxicology consultation
• Give antivenom immediately. Intravenous epinephrine should be given only for life-threatening hypotension or anaphylaxis - its use has been associated with cerebral hemorrhage.
• Volume expansion may be necessary.
• Severe coagulation disturbances, electrolyte abnormalities, and muscle damage leading to acute renal failure are likely.
• Repeat antivenom as clinically indicated.
• General management as for less seriously ill patients as well (see below).

Less seriously ill patients

• No signs of systemic spread
• Admit to ICU for non-invasive monitoring, strict bedrest and full head injury observations (wake hourly).
• Leave bandages in place.
• Obtain toxicology consultation
• Obtain appropriate antivenoms and venom detection kit.
• Obtain intravenous access.
• Take blood for group and X-match, coagulation screen (including fibrinogen levels, and tests for DIC), full blood count, electrolytes and calcium, creatinine kinase and arterial blood gases. Perform ECG. Repeat at appropriate intervals.
• Collect urine for microscopy to detect hematuria and for free protein, hemoglobin and myoglobin measurement. Record urine output. Freeze the first sample for venom detection.
• Draw up drugs in case of anaphylaxis to antivenom.
• When ready, cut a hole over the wound site, inspect and take swabs for use with the venom detection kit.
• Once the results of the venom detection kit are known, slowly and progressively remove the bandages.

  • If systemic symptoms ensue:
  • Re-apply bandages and give antivenom as clinically indicated.

• Ensure the patient is well hydrated (to reduce the risk of acute renal failure due to rhabdomyolysis).
• Repeat blood tests, ECG, etc at clinically relevant intervals.
• Correct abnormal coagulation; look out for disseminated intravascular coagulation (heparin probably contra-indicated in DIC from snake bite).
• Analgesia and sedation - be cautious.
• Correct hypotension, if present, with volume expansion and vasopressors (exclude occult bleeding).
• Watch for development of renal failure - monitor urine output and composition.
• Tetanus prohylaxis is recommended.
• Usually, if there are no signs of envenomation four hours after removal of the bandages, and if repeat blood tests taken at that time are normal, then it is probable that significant envenomation has not occurred. If laboratory tests are not available, 12 to 24 hours is a reasonable period of observation.

First Aid for Snake Bites- What NOT to do

Though U.S. medical professionals may not agree on every aspect of what to do for snakebite first aid, they are nearly unanimous in their views of what not to do. Among their recommendations:

• No ice or any other type of cooling on the bite. Research has shown this to be potentially harmful.
• No tourniquets. This cuts blood flow completely and may result in loss of the affected limb.
• No electric shock. This method is under study and has yet to be proven effective. It could harm the victim.
• No incisions in the wound. Such measures have not been proven useful and may cause further injury.
• Do NOT wash the area of the bite. It is extremely important to retain traces of venom for use with venom identification kits!
• Stop lymphatic spread - bandage firmly, splint and immobilize. The lymphatic system is responsible for systemic spread of most venoms. Significant lymphatic spread of the venom can be arrested by complete immobility of the affected part and the application of a firm bandage (as firm as you would put on a sprained ankle) over a folded pad placed over the bitten area. The bandage should be applied immediately, starting at the bitten area and then extending from the periphery towards more central parts of the body. Bites to the head, neck, and back are a special problem - firm pressure should be applied locally if possible.
• Immobility is best attained by application of a splint, reassurance and immobilization (eg, putting the patient on a stretcher).
• Removal of the bandage will be associated with rapid systemic spread. Hence ALWAYS wait until the patient is in a fully-equipped medical treatment area before bandage removal is attempted.

Pharmacotherapy

Acute Pharmacotherapies

• Antivenoms

• Antivenom should be given to all patients who exhibit signs of systemic spread. One ampule (50ml of 17% protein) should neutralize the average venom yield from milking a snake of that species, and is usually enough for all but the most severe envenomations. Severe bites may require much more (up to 20 times the recommended dose!). If the situation allows, antivenoms should be given slowly (over half an hour, diluted in an IV fluid). A test dose may be advisable, particularly following prior exposure to equine protein.
• Antivenoms are prepared from horse serum. The risk of anaphylaxis is very low (less than 1% even for polyvalent antivenoms), but is increased in people who have had prior exposure to horses, equine tetanus vaccines, and a general allergic history. This increased risk is much more common in people aged 50 years or more. About 4% of all administrations are associated with minor reactions.
• Pre-treatment with a non-sedating anti-histamine (ie, promethazine 0.25 mg/kg), subcutaneous adrenaline (0.25mg for adults, 0.01mg/kg for children), and IV steroids (hydrocortisone 2mg/kg) is still recommended, although severe reactions are rare. In general the risk from the snake toxins is much greater than the risk of administering the antivenom.
• If an antivenom is administered, ALWAYS advise the patient of the possibility of delayed serum sickness (up to 14 days later). This is characterized by fever, rash, generalized lymphadenopathy, aching joints and renal impairment. The likelihood of developing this depends on the volume of antivenom required. It occurs in about 10% of patients who are given polyvalent antivenoms. Treatment with steroids is usually all that is needed. Shelf life is 3 years when stored in a refrigerator. Antivenoms should not be frozen.

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