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Merck Manual

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Spiders and Scorpions


Charles M. Hendrix

, DVM, PhD, Department of Pathobiology, College of Veterinary Medicine, Auburn University

Last full review/revision Jul 2013 | Content last modified Jul 2013

Spiders of medical importance in the USA do not inflict particularly painful bites, so it is unusual for a spider bite to be suspected until clinical signs appear. It is also unlikely that the spider will remain in close proximity to the victim for the time required for signs to develop (30 min to 6 hr). Almost all spiders are venomous, but few possess the attributes necessary to cause clinical envenomation in mammals—mouth parts of sufficient size to allow penetration of the skin and toxin of sufficient quantity or potency to result in morbidity.

The spiders in the USA that are capable of causing clinical envenomation belong to two groups: widow spiders (Latrodectus spp) and brown spiders (mostly Loxosceles spp).

Widow Spiders:

Widow spiders usually bite only when accidental skin contact occurs. The most common species is the black widow spider, Latrodectus mactans, characterized by a red hourglass shape on its ventral abdomen. In the western USA, the western black widow, L hesperus, predominates, while the brown widow, L bishopi, is found in the south, and the red widow, L geometricus, is found in Florida.

Venom from these spiders is one of the most potent biologic toxins. The most important of its five or six components is a neurotoxin that causes release of the neurotransmitters norepinephrine and acetylcholine at synaptic junctions, which continues until neurotransmitters are depleted. The resulting severe, painful cramping of all large muscle groups accounts for most of the clinical signs attributed to envenomation.

Unless there is visual confirmation of a widow spider bite, diagnosis must be based on clinical signs, which include restlessness with apparent anxiety or apprehension; rapid, shallow, irregular respiration; shock; abdominal rigidity or tenderness; and painful muscle rigidity, sometimes accompanied by intermittent relaxation (which may progress to clonus and eventually to respiratory paralysis). Partial paresis also has been described.

An antivenin (equine origin) is commercially available but is usually reserved for confirmed bites of high-risk individuals (very young or very old). Symptomatic treatment is usually sufficient but may require a combination of therapeutic agents. Calcium gluconate IV (10 mL of a 10% solution is the usual human dose) is reportedly helpful. Meperidine hydrochloride or morphine, also given IV, provides relief from pain and produces muscle relaxation. Muscle relaxants and diazepam are also beneficial. Tetanus antitoxin also should be administered. Recovery may be prolonged; weakness and even partial paralysis may persist for several days.

Brown Recluse Spiders:

There are at least 10 species of Loxosceles spiders in the USA, but the brown recluse spider, L reclusa, is the most common, and envenomation by it is typical for the genus. These spiders have a violin-shaped marking (or “fiddleback”) on the spider’s dorsum, the cephalothorax, although it may be indistinct or absent in some species. In the northwestern USA, the unrelated spider Tegenaria agrestis reportedly causes a clinically indistinguishable dermonecrosis in people and presumably in domesticated animals. Brown recluse spider venom has vasoconstrictive, thrombotic, hemolytic, and necrotizing properties. It contains several enzymes, including a phospholipase (sphingomylinase D) that attacks cell membranes. Pathogenetic mechanisms of the characteristic dermal necrosis are poorly understood, but activation of complement, chemotaxis, and accumulations of neutrophils affect (or amplify) the process.

A history of a bite by a brown recluse spider is useful but rare. A presumptive diagnosis may be based on the presence of a discrete, erythematous, intensely pruritic skin lesion that may have irregular ecchymoses. Within 4–8 hr, a vesicle develops at the bite wound, and sometimes a blanched zone surrounds the erythematous area, producing a “bull’s-eye” appearance to the lesion. The central area sometimes appears pale or cyanotic. The vesicle may degenerate to an ulcer that, unless treated in a timely manner, may enlarge and extend to underlying tissues, including muscle. Sometimes, a pustule follows the vesicle and, on its breakdown, a black eschar remains. The final tissue defect may be extensive and indolent and require months to heal. However, medical authorities claim that not all brown recluse spider bites result in severe, localized dermal necrosis.

Systemic signs sometimes accompany brown recluse spider envenomation and may not appear for 3–4 days after the bite. Hemolysis, thrombocytopenia, and disseminated intravascular coagulation are more likely to occur in cases with severe dermal necrosis. Fever, vomiting, edema, hemoglobinuria, hemolytic anemia, renal failure, and shock may result from systemic loxoscelism.

In known spider bites, early treatment can be successful, but unfortunately, many cases are not recognized until cutaneous necrosis has become extensive; treatment at that stage is less rewarding but is still of value. Immediate application of cold packs is beneficial, and if administered early, corticosteroids protect against cutaneous necrosis by stabilizing cell membranes and suppressing chemotaxis. Corticosteroids also tend to protect against systemic involvement. Radical excision has been advocated, but its value is questionable. Dapsone, an inhibitor of leukocyte function, which is frequently used in the treatment of leprosy, is currently considered the drug of choice for brown recluse spider bites. In people, it is administered at 100 mg, bid, for 14–25 days. Broad-spectrum antibiotics are useful in preventing secondary infection, and tetanus immunoprophylaxis should be considered.


Tarantulas (many assorted genera and species) are common throughout the USA and South America. They are increasingly being kept as household pets throughout the world. Along with many other exotic arthropods, they are frequently sold in exotic pet stores.

Tarantulas should not be mixed with or allowed contact with other domesticated pets. Fine hairs (setae), present over much of the tarantula’s abdomen, may be discharged (or cast off) by the tarantula as a defense mechanism against larger creatures. If these fine hairs make contact with unprotected skin, they can produce contact allergic reactions. Care must be taken to keep these fine hairs from entering the eye, particularly the cornea, because they can be quite difficult to remove. In addition, the bite of the tarantula can be fatal to dogs or cats, which are very susceptible to tarantula venom. Safety of the tarantula is another reason to keep these spiders separated from other pets, because a dog or cat can easily injure or kill a pet tarantula.


Most of the scorpions found throughout the USA possess posterior abdominal stingers that connect to venom glands. The stinger and its associated venom can be used both as mechanisms of self-defense and of predation. For the most part, the stings of these scorpions are considered to be innocuous in most domesticated mammalian species, because the amount of venom is too minute or the venom has very little pharmacologic potency. The sting of these arthropods is analogous to an insect sting/bite, with pain and swelling at the site of the injury.

Relative to pharmacologically potent scorpion stings in domesticated animals, there are two geographic scenarios of veterinary clinical importance. The first involves Centruroides sculpturatus, commonly known as the Arizona bark scorpion. This venomous arthropod can be found in all counties of Arizona, into western New Mexico, southern Utah, southern Nevada to the Las Vegas vicinity, California along the Colorado River (where it is rare), and down to the area of the state of Sonora, Mexico. It has been reported to produce envenomation in dogs; however, its sting is similar to that of the venomous hymenopterans, producing local pain and swelling with the possibility of associated hypertension. Most animals recover without a problem, but more severe reactions are possible.

The second scenario involves myriad geographic locales (eg, desert-like and jungle ecosystems) throughout the world where many venomous scorpions abound. People with penchants for hiking the world should be warned to be wary of visits to these areas, and traveling with canine companions in such scenarios should be strongly discouraged.

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