Envenomation

Written By: Katherine Hogan and Annette Dorfman
Albany Medical Center, Albany, New York

Edited By: Lorraine Thibodeau
Albany Medical Center, Albany, New York

Objectives

  • Recognize the signs and symptoms of dangerous envenomations
  • Discuss the importance of proper wound care principles when treating victims of envenomations
  • Understand the indications of antivenom therapy and the complications associated with its use
  • Review the potential pitfalls in the misdiagnosis of spider bites

Thousands of envenomations occur each year in the United States involving snakes, spiders, scorpions and marine life. Consequently, many patients present for evaluation and treatment of confirmed or suspected envenomations. A clinician’s careful assessment of the history, signs and symptoms is important in guiding therapy and avoiding misdiagnosis.

Snakes

A Southern copperhead snake, a type of pit viper. Note the heat-sensing organ, located in the pit between the eye and mouth.

There are thousands of species of snakes, and approximately 15% are reported to be venomous. Approximately 5-6 deaths occur each year in the U.S. as a result of snake envenomation. The highest number of deaths is reported in North Carolina, Texas, and Arkansas, but recent data show increasing venomous snake bites in the Southwestern United States.

Western Diamondback Rattlesnake

The largest family of venomous snakes is Viperidae, which includes the subfamily of Crotalinae, or pit vipers. Rattlesnakes, copperheads, and cottonmouths are examples. Pit vipers usually have a triangular head, elliptical pupils, fangs, and a characteristic ‘single row’ arrangement of their caudal plates. There may be an associated rattle at the very end of the caudal plate. However, the most important characteristic is the presence of bilateral “pits,” or heat-sensing organs, located between the eye and the nostril on each side.

Elapidae, the other venomous snake family, is comprised of cobras, and coral snakes, as well as many species of Australian snakes. In this family, the only snake indigenous to the United States is the coral snake. A popular rhyme can be applied to help identify coral snakes, but only those found in the U.S.: “Red on yellow, kills a fellow. Red on black, venom lack.”

A Texas coral snake (“Red on yellow, kill a fellow”).

A harmless milk snake (“Red on black, venom lack” or “friend of Jack”)

Spiders

Black widow spider – note the red hourglass

Over 40,000 species of spiders have been identified and many more are not yet classified. Most bites are minor and cause only local irritation, yet clinicians must be aware that some spider bites can cause significant morbidity and rarely, mortality. In the United States, the black widow (Latrodectus spp.) and brown recluse (Loxosceles spp.) constitute the majority of medically significant envenomations. The black widow species is found throughout the U.S. These spiders are identified by their shiny black bodies and red markings. The most common feature is a red hourglass on their ventral aspect.

Brown recluse spider – note the violin-shaped marking

The brown recluse spider (Loxosceles reclusa) inhabits the South and Midwest. They may be colloquially referred to as ‘fiddleback spiders’ due to the dark fiddle [or violin) shaped marking on their dorsal aspect.

Scorpions

An Arizona scorpion

Scorpions are members of the class Arachnida and, therefore, are related to spiders. They have four pairs of legs and a long curling tail with a stinger at the end. Scorpions primarily inhabit the Southwestern U.S. Most species are not dangerous to humans, however, the bark scorpion (C. exilicauda formerly sculpturatus) found in Arizona and New Mexico harbors a poisonous venom.

Jellyfish & Stingrays

Cyanea jellyfish – common along the New England coast during summer months

Jellyfish envenomate their victims using stinging cells called nematocysts (cnidocysts). Stingrays, on the other hand, are feared because their long tails are equipped with one or more dagger-like, venomous spines. Various species inhabit the coastal waters of the United States.

An Atlantic stingray

Classic Presentation

The classic presentation of an envenomation depends upon the type of bite or sting the patient has suffered.

A patient presenting with a snake bite often witnesses the envenomation and may, at first, experience a sense of impending doom. However, they are often able to provide a good history, describing the bite in detail, and hopefully, characteristics of the snake as well. Envenomations may cause a combination of local and/or systemic effects. Individuals may also have different responses to specific venoms, consequently, even bites that appear to have minimal local effects may have devastating systemic consequences. This is more likely after a coral snake bite than any other type.

Coral snake venom blocks neuromuscular transmission at acetylcholine receptor sites. Often, the first sign is ptosis followed by progressive neuromuscular weakness. The patient may develop slurred speech, fasiculations, drowsiness, weakness, and trouble breathing. The cause of death in these cases is usually progressive paralysis of the respiratory muscles.

In contrast, a patient bitten by a pit viper will initially have localized edema and pain, which usually spreads proximally and involves the entire extremity. After a few hours, vesicular lesions and bullae, often hemorrhagic, may develop. The patient may then experience systemic effects such as nausea, weakness, muscle fasiculations, changes in taste sensation (metallic), and sensory changes, involving the mouth, fingers, and toes. Viper bites disrupt the coagulation cascade and alter vascular permeability. This may ultimately lead to pulmonary edema and refractory shock. Some pit vipers also cause neurotoxicity and respiratory failure.

When obtaining the history from a patient with a suspected spider envenomation, definitive species identification is frequently difficult. Puncture marks are uncommon and the bite may not even be felt. Therefore, recognition of the ‘toxindrome’ – the signs and symptoms associated with a medically significant spider bite – may be more useful.

A neurotoxic venom is released by the black widow spider (Latrodectus spp.). Latrodectism is characterized by pain (local, radiating, regional) that increases over the course of an hour and may radiate proximally along the affected limb to the trunk. Muscle spasm may occur as well. Systemic effects are less common, but may include hypertension, agitation, fever, paresthesias, fasiculations and cardiac effects. Widow spider bites occur in dark/dry places during the warmer months. A cytotoxic or necrotic venom is released by the brown recluse spider (Loxosceles spp.). Loxoscelism encompasses a spectrum of local and systemic effects caused by the toxin. Bites can vary from mild, self-limiting erythema to large, necrotic ulcerations. Local pain and burning are common. Systemic symptoms are rare, but may include hemolysis, disseminated intravascular coagulation and renal failure.

Envenomations from scorpions may present with a range of clinical findings. There is often burning pain, pruritus and paresthesias at the site of the sting. Severe systemic symptoms of the Arizona bark scorpion include catecholamine (restlessness, coma, and convulsions) and cholinergic effects (bradycardia, hypotension, salivation, lacrimation, and diarrhea).

Marine envenomations bordering the United States are typically minor in nature and are characterized by local pain, erythema and edema with a range of severity. Jellyfish leave linear sting contact marks, occasionally in a cross-hatched pattern. People are often stung when they inadvertently brush past a jellyfish or step on a stingray. The spine of a stingray is sharp enough to cause penetrating trauma and carries a cytotoxic venom as well. In addition to localized pain, bleeding, and erythema, systemic symptoms such as hypotension and shock, may occur.

It is important to remember that many of these envenomations are puncture wounds. Thus, in addition to the venom-specific effects, the patient is also prone to infections from bacterial flora that may enter the wound, such as anaerobes and tetanus.

Initial Actions and Primary Survey

The first priority in the evaluation of envenomations includes the initial assessment of a patient’s airway, breathing and circulation. If the toxin has caused respiratory compromise then intubation may be needed. The affected extremity should be immobilized and slightly compressed, to prevent the spread of venom. A tourniquet is not indicated, and may cause more harm. In addition, ice water immersion, suction, and incision of the wound are also contraindicated. All constrictive clothing, jewelry, and accessories should be removed.

How do I make the diagnosis?

The key starting point in treating a definite or suspected envenomation is establishing the diagnosis. If the bite or sting was witnessed and a clear history is available, the diagnosis is relatively simple. However, many envenomings are unwitnessed and an accurate diagnosis is difficult to establish. The timing of the bite (if witnessed) and the onset and progression of symptoms can help confirm a diagnosis. Local species and their specific envenomation profiles should be taken into account when making a diagnosis.

Diagnostic Testing

Snake envenomation is a clinical diagnosis, made by history and physical exam. Usually the victim can describe the snake, and it is helpful if a picture is taken. There are no serum tests to determine the type of snake venom present. A complete blood count, chemistry, coagulation profile (including fibrinogen, PT, aPTT) and CPK should be drawn. A Foley catheter may be placed to monitor urine output and to obtain a urinalysis for myoglobin.

Laboratory testing can be crucial in determining the severity of systemic symptoms in patients presenting with snakebites, but less so in those with bites from other organisms. Patients exhibiting severe local or systemic effects from spider bites should have basic lab work drawn, including complete blood count, chemistry panel and liver function tests. CPK may be followed in cases of severe muscle spasm/rigidity due to a black widow spider.

Imaging is rarely needed for diagnostic purposes. Patients injured by marine life may require x-rays to look for spine remnants from stingrays, scorpion fish or sea urchins.

Treatment

For most bites and stings, appropriate first aid is the mainstay of treatment. Removal of spines, stingers, tentacles and fangs using proper protective gear is the first step. Washing the affected site, local wound care and applying ice to reduce swelling are essential actions. The affected extremity should be closely examined for edema and signs of compartment syndrome, while, at the same time, the patient’s hemodynamic status and blood pressure should be monitored. If there is concern for compartment syndrome, surgical consultation for emergent fasciotomy is prudent. Additional treatments include parenteral analgesia and tetanus prophylaxis.

Next, the treating physician should decide if antivenom is indicated. In general, any patient who displays moderate to severe signs and symptoms after a venomous snake bite is a candidate for treatment. Antivenom indications include increased swelling and erythema beyond the local bite wound and any disruption of the coagulation cascade. If a coral snake bite is suspected, the patient should immediately receive antivenom, even before the onset of symptoms. Physicians should contact their Poison Control Center for specific antivenom availability and guidance (1-800-222-1222).

Once the decision is made to administer antivenom, the patient must be closely monitored for anaphylaxis. Two IV lines should be established. Oxygen, ECG monitor, and resuscitation equipment should be in the room.

Spider bites from U.S. species are not likely to cause envenomation that requires urgent administration of antivenom. However, children, the elderly and patients with serious co-morbidities bitten by black widow spiders may benefit from early antivenom therapy. Antivenom indications include increasingly severe pain, proximal spread of muscle spasm/rigidity, increased sweating and hypertension. Antivenom is not used routinely for spider envenomations in the United States because of concern for anaphylaxis. The risks and benefits need to be weighed before antivenom treatment is initiated. It is advisable to obtain early consultation with Poison Control for specific antivenom availability and recommendations (1-800-222-1222) For the brown recluse spider, no antivenom is available in the United States and no specific treatment is supported by the evidence

Additional therapies include benzodiazepines for muscle spasm associated with latrodectism. For necrotic ulcers or skin damage related to brown recluse spiders, regular and intense wound care may be needed.

Stings from the Arizona bark scorpion may require antivenom therapy if there are signs of systemic neurotoxicity. The antivenom currently lacks FDA approval and patients should be counseled regarding the risk of anaphylaxis and other potential side effects. The antivenom is only available in Arizona.

Marine envenomations should be treated according to the type of injury inflicted. Local symptoms may require analgesia or a regional nerve block. Most jellyfish stings should first be treated by inactivating the nematocysts with vinegar or acetic acid. Submerging the affected area in hot water provides effective analgesia as well. Stingray envenomations should be soaked in very hot water as soon as possible

Disposition

If a snake is confirmed to be non-venomous and no signs of envenomation are present, the patient can be safely discharged after an eight hour observation period from the time of initial bite. Most other patients should be hospitalized on telemetry monitoring. Envenomations necessitating the administration of antivenom require admission to an Intensive Care Unit (ICU) and may require ventilatory support, intravenous vasopressors, and/or further invasive monitoring.

Once symptoms of a spider bite are under control, patients may be discharged. They should be cautioned to seek medical attention for increased pain or systemic effects that warrant treatment. Patients who receive antivenom therapy should be admitted to an ICU as noted above.

Most scorpion stings are minor and local wound care is sufficient. However, if an Arizona bark scorpion sting is suspected, the patient should be monitored in an ICU setting regardless of the administration of antivenom.

All patients who have received antivenom therapy should be educated at discharge about the possibility of delayed serum sickness and its associated symptoms.

Marine envenomations are typically self-limiting and the patient may be discharged after appropriate wound care and pain control have been provided.

Pearls and Pitfalls

Following envenomations, immobilization and splinting of the extremity is indicated. Other actions that are unhelpful and discouraged include arterial tourniquets, incision and/or suction of the wound, venom extraction devices, cryotherapy, oxidizing substances or proteases.

Consultation should be made with a specialist experienced in the treatment of envenomations. Physicians should contact their Poison Control Center (1-800-222-1222) for assistance.

The muscle rigidity of black widow spiders may be severe. If it involves the abdominal musculature, the envenomation may be mistaken for an acute abdomen. To distinguish the two, abdominal rigidity associated with a black widow spider will have little or no associated tenderness.

Delayed localized tissue necrosis 4 months after a brown recluse spider bite

Many skin lesions are incorrectly attributed to spider bites which lead to delays in diagnosis and treatment. Methicillin-resistant Staphylococcus aureus (MRSA), for example, is a condition often mistaken for a spider bite, and loxoscelism in particular. MRSA occurs widely in the community, especially in areas of group living (i.e. nursing homes, prisons, military barracks, and summer/sports camps). Lacking history of a bite, other causes of skin ulceration should be considered, including infectious, inflammatory, vascular, and neoplastic etiologies. More rigorous work up of skin lesions will likely generate etiologies more credible than spider envenomation.

Selected References

Braitberg G, Segal L. Spider bites. Aust Fam Physician. 2009;38(11):862-67.

Gold BS, Barish RA, Dart RC: North American snake envenomation: Diagnosis, treatment, and management.Emerg Med Clin North Am 2004; 22:423.

Otten, EJ. Venomous Animal Injuries. In Rosen’s Emergency Medicine Concepts and Clinical Practice, 6th ed. 2006:894-903.

Vetter RS, Isbister GK. Medical aspects of spider bites. Annu Rev Entomol. 2008;53:409-29.

Weinstein SA et al. Envenomations: an overview of clinical toxinology for the primary care physician. Am Fam Physician. 2009;80(8):793-802.