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  In addition to any focused or antidotal therapy available, aggressive symptom-based supportive care is important for all envenomations.

  
  
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  Knowledge of local venomous species may be helpful, although be aware that patients may have contact with nonlocal or exotic venomous animals.

  
  
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  North American venomous bites are rarely unprovoked.

  
  
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  Contact your local poison control center (1-800-222-1222) for assistance with diagnosing and managing all envenomations.

In 2010, there were more than 60,000 calls made to United States Poison Centers related to bites and envenomations. Although there are many venomous animal species in North America, a majority of these calls involved insects (including bees, wasps, hornets, and ants), arachnids (including spiders and scorpions), and snakes. From information provided in the 2010 Annual Report of the American Association of Poison Control Centers’ National Poison Data System, there were a total of 5 fatalities related to all bites or envenomations and approximately 2,500 instances of antivenin being given.

The clinical presentations of the various forms of venom exposure vary greatly and are dependent on multiple factors including the species of the animal, the amount of venom delivered, and potential baseline medical problems in the envenomed patient. Patients presenting with an animal envenomation may therefore display a variety of symptoms ranging from local reaction to a bite or sting to generalized yet nonspecific effects (eg, vomiting, headache, hypertension) or toxin-specific findings (eg, paralysis or coagulopathy). This chapter focuses on the presentation, evaluation, and treatment of 2 of the most clinically relevant North American envenomations: snakes and spiders.

Snakes

Venomous snakes found in North America are most easily divided into their 2 families: Elapidae and Viperidae (subfamily Crotalinae). The majority of venomous snakebites occurring yearly in North America are caused by snakes in the Crotalinae subfamily, which includes rattlesnakes (genus Crotalus), copperheads, and cottonmouths (genus Agkistrodon). Less than 5% of venomous snakebites are from the Elapidae family, which includes the coral snake. Fewer still may be from bites by exotic, nonnative snakes usually being kept as pets.

Venomous snakes found natively in North America are generally nonpredatory to humans. Bites, therefore, take place on provocation of the snake—either intentional or accidental. These bites are typically located on extremities, but particularly troublesome cases have been reported in which venomous bites have involved the face, neck, or tongue. The vast majority of venomous snakebites occur in young men, with an appreciable association with alcohol intoxication. Children are also at a higher risk for being bitten by a venomous snake.

There are a few characteristics that can help identify a North American snake as being part of the Crotalinae subfamily. These snakes have vertical slit-like pupils, long fangs, and a triangular head. This subfamily is also referred to as “pit vipers” because they have heat-sensing pits located on their heads just behind the nostrils and in front of the eyes (Figure 65-1). Crotaline venom contains a combination of chemicals that cause primarily local tissue damage and hematologic effects.

Elapidae native to North America are the coral snakes. These snakes, found mostly in the Southeast United States (particularly Florida and Texas), have a characteristic color pattern that distinguishes them from the similar-appearing but nonvenomous scarlet king or milk snake. People often remember this pattern difference by reciting the rhyme, “Red on yellow kills a fellow. Red on black, friend of Jack” (Figure 65-2). Elapid venom has a curare-like neurotoxic effect and is said to be one of the most potent North American venoms. However, multiple characteristics of the snake make clinically significant bites from these snakes rare. They tend to reside in remote unpopulated areas and even if confronted will attempt to flee before biting. Unlike the crotalids, the elapids’ fangs are short and unlikely to penetrate thick clothing or shoes. After biting, elapids will remain attached and “chew” on their victim to inject the venom. Although this makes it more difficult for these snakes to deliver a clinically significant amount of venom, it also makes it more difficult to clinically assess a patient with a potential bite, as there may not be bite or fang marks in a patient who has had a potentially life-threatening envenomation.

Spiders

Like snakes, there are 2 major groups of spiders that cause medically significant envenomations in North America: the black widow (genus Latrodectus) and the brown recluse (genus Loxosceles). It is difficult to estimate the true incidence of spider bites, because the history of a bite is oftentimes unreliable, with many patients and physicians reasoning that a rash, abscess, or cellulitis originated from a spider bite when no spider was seen. Additionally, with the possible exception of the female black widow spider, the general public has difficulty distinguishing medically relevant spiders from those that are generally benign. That being said, bites from brown recluse and black widow spiders can be deadly in extreme circumstances and can unquestionably cause substantial morbidity and pain.

There are 5 species of black widow spiders found in the United States. These spiders are medium-sized, typically black colored, and have species-specific ventral markings. The female Latrodectus mactans has the characteristic red hourglass ventral marking and has a larger body and fangs than her male counterpart, making the female more likely to cause envenomation (Figure 65-3). The clinical effects of a black widow spider envenomation in humans are thought to be caused by the neurotoxin α-latrotoxin.

There are 2 major species of recluse spiders found in the United States: the Loxosceles reclusa (brown recluse) and Loxosceles deserta (desert recluse). The brown recluse is found primarily in the southern and midwestern United States, and the desert recluse’s range is in the southwestern portion of the country. North American recluse spiders are brown to gray-colored with dark dorsal markings that have a violin pattern, giving it its other names, fiddleback or violin spider (Figure 65-4). The toxin in recluse spider bites is complex, but is thought to contain proinflammatory and necrosis-inducing substances similar to phospholipase D and hyaluronidase.