Although nearly 40,000 species of spiders have been described worldwide, medically significant envenomations have been described in only a few dozen (Table 211–3). Spiders are carnivores, and venom probably evolved for paralyzing prey. The vast majority of spiders pose little harm to humans because their venom-injecting fangs are too small to penetrate human skin, the amount of venom injected is too little to produce toxicity, or the venom itself has little effect on mammalian cells. Even if a reaction is elicited, it is often local, and systemic toxicity is confined to a few specific species (Table 211–3 and Figure 211–1).
Medically Important Spider Bites and Treatment
||Download (.pdf) Table 211–3
Medically Important Spider Bites and Treatment
|Spider ||Bite Features ||Complications ||Treatment |
|Loxosceles: brown recluse spider, corner spider (worldwide distribution) ||Painless bite, usually firm erythematous lesion that heals with little or no scar over days to weeks ||Occasional hemorrhagic blister at 24 h; dermatonecrosis; systemic effects rare, mostly in in children, at 24–72 h ||No validated treatments |
|Widow spider: black widow, redback, button spider (worldwide distribution); ||Pinprick bite; pain can spread to entire extremity; target lesion 1–2 cm ||Acetylcholine and norepinephrine release; muscle cramps extending to trunk, back, and abdomen; hypertension, tachycardia ||Latrodectus antivenom, species specific; derived from horse serum; Fab antivenom available in Mexico |
|Armed spider: banana spider (Central and South America) ||Intense pain at bite site ||Severe pain, sympathetic and parasympathetic effects; priapism; vertigo, visual disturbances ||Antivenom available in Brazil |
|Funnel-web spider (Australia) ||Severe pain, with wheal and erythema at site; very rapid envenomation ||Parasympathetic effects, muscle fasciculation; pulmonary edema; cerebral edema; death can occur within minutes ||Compressive elastic bandage; funnel-web spider antivenom |
|Tarantula (worldwide) ||Painful bite with local erythema and edema ||Barbed hairs can penetrate cornea and conjunctivae; contact dermatitis from hairs ||Ophthalmology consult for red eye and pain |
Range of recluse (genus Loxosceles) spiders in the United States.
SPIDERS CAUSING NECROTIC ARACHNIDISM (LOXOSCELES)
Loxosceles are brown spiders that have a worldwide distribution. Native species exist in the United States (Figure 211–1), and of these, Loxosceles reclusa (the brown recluse spider) occupies the largest geographic area and accounts for the majority of significant envenomations. In South America, particularly Brazil, Loxosceles laeta and Loxosceles intermedia account for most significant envenomations. Envenomation outside of endemic areas is unusual.18 Loxosceles spiders are nocturnal; are shy; are found both indoors and outdoors in dark, dry areas such as basements, closets, and woodpiles; and may bite when threatened. A pigmented, violin-shaped pattern on the cephalothorax of the brown recluse is often present (Figure 211–2). However, this characteristic is considered unreliable and often misinterpreted. Loxosceles species are most accurately identified by their eye pattern, which consists of six paired eyes (one anterior pair and two lateral pairs).18 Most other U.S. spiders have eight eyes arranged in two rows of four. The venom of the brown recluse contains multiple enzymes, including hyaluronidase and sphingomyelinase D, which is the major enzyme responsible for necrosis. Significant necrotic wounds are rare but possible through neutrophil activation, platelet aggregation, and thrombosis. Although both local and systemic complications of Loxosceles envenomation have been well described, the perceived threat of the brown recluse far exceeds its actual danger. For more information about recluse spiders see http://spiders.ucr.edu.
Close-up look at the characteristic fiddle-shaped back marking on the brown recluse spider (Loxosceles reclusa).
Bites by Loxosceles spiders are described as initially painless, which often prohibits possible identification of the spider. The most common manifestation of a bite is a mild erythematous lesion that may become firm and heal with little or no scar within several days or weeks. Occasionally, a more severe local reaction occurs, beginning with mild to severe pain several hours after the bite, accompanied by localized erythema, pruritus, and swelling. A hemorrhagic blister then forms, surrounded by vasoconstriction-induced blanched skin (Figure 211–3). By day 3 or 4, the hemorrhagic area may become ecchymotic, which leads to the "red, white, and blue" (erythema, blanching, and ecchymosis) sign. The ecchymotic area may become necrotic, with eschar formation by the end of the first week. The necrotic, slowly healing ulcers may not reach maximum size for many weeks after envenomation and can occasionally result in a significant cosmetic defect requiring skin grafting.
Early brown recluse spider bite (approximately 8 hours old) with a violaceous center surrounded by a faint spreading erythema. [Photograph by Lawrence B. Stack, MD. Reproduced with permission from Knoop KJ, Stack LB, Storrow AB, Thurman RJ: The Atlas of Emergency Medicine, 3rd ed, © 2009 by McGraw-Hill, Inc., New York.]
Although significant systemic effects are not uncommon after bites of L. laeta, the predominant South American species, they rarely occur after bites of the brown recluse, the predominant U.S. species. Systemic effects, the hallmark of which is hemolysis, are seen more often in children and typically occur 24 to 72 hours after the bite. Other effects include nausea, vomiting, fever, chills, arthralgias, thrombocytopenia, rhabdomyolysis, hemoglobinuria, and renal failure. Disseminated intravascular coagulation and death are extremely rare.
Correct diagnosis of a brown recluse envenomation without definitive spider identification is difficult. Although the presence of a consistent clinical picture in an endemic area is suggestive, it is likely that a myriad of infectious and noninfectious conditions are misdiagnosed as brown recluse bites.18 In patients who are suspected of having been bitten and who exhibit signs and symptoms of envenomation, obtain a CBC, BUN and creatinine, and coagulation profile. Assays to detect envenomation have been used in research, but a commercial test is not currently available.
Treatment of a possible necrotic spider bite should include the usual supportive measures, including pain medication. Antibiotics are indicated if signs of infection exist. However, secondary infections are uncommon. Arrange follow-up for serial wound evaluation. If ulceration develops, surgical debridement is delayed until clear margins are established, often 2 to 3 weeks after the bite.
Patients with systemic symptoms following a bite warrant hospitalization. Various treatments have been advocated for brown recluse spider bites, including antihistamines, antivenom, colchicine, dapsone, hyperbaric oxygen, surgical excision, steroids, and topical nitroglycerin. None of these therapies have clear benefit, and most wounds from the brown recluse are self-limiting and heal without any medical intervention. Administration of the leukocyte inhibitor dapsone continues to be advocated by some despite lack of supporting research and known adverse effects, including hemolysis and methemoglobinemia. Early antivenom administration after envenomation is efficacious in animal models.19 However, the delayed time in which patients typically present after envenomation, the current inability to definitively identify an envenomation, and the inability to prognosticate the development of dermatonecrosis, which in itself is rare, greatly limits its potential use.
An equine-derived antivenom is commonly used in Brazil; however, its efficacy is unclear. In the United States, there is no commercially available Loxosceles antivenom.
HOBO SPIDER (ERATIGENA AGRESTIS; FORMERLY TEGENARIA AGRESTIS)
A native of Europe and central Asia, the hobo or northwestern brown spider is now found in the Pacific Northwest of the United States and southern British Columbia. Hobo spiders are brown with gray markings and have a 7- to 14-mm body length and a 27- to 45-mm leg span. They live in moist, dark areas such as woodpiles and basements.
Little documentation supports the occurrence of necrosis from hobo spider bites.18 In its native European habitat, it is not considered poisonous to humans, and venom analysis comparing European to U.S. species has confirmed no unique differences. Confirmed bites have demonstrated localized erythema, itching, pain, and swelling.20
There is no diagnostic test for hobo spider envenomation, and there is no proven treatment.
WIDOW SPIDERS (LATRODECTUS)
Latrodectus or "widow" spiders have a worldwide distribution. In the United States, the black widow is the most well known, although of the five Latrodectus species found commonly in the United States, only three (Latrodectus mactans, Latrodectus variolus, and Latrodectus hesperus) are actually black. Other varieties may be predominantly brown (Latrodectus geometricus) or red (Latrodectus bishopi). An orange-red hourglass-shaped marking characterizes many of the Latrodectus species (Figure 211–4). Female spiders are relatively large, with a body size ranging up to 1.5 cm in length and leg spans of 4 to 5 cm. The male spider is approximately one-third the size of the female and lighter in color, and his bite cannot penetrate human skin. Black widow spiders are found most often in woodpiles, basements, garages, and sheds. Latrodectus will aggressively defend her web, particularly when guarding her eggs. Most black widow bites occur between April and October and are usually seen on the hands and forearms.
Black widow spider (Latrodectus mactans) with offspring. Note characteristic hourglass marking on abdomen. [Photograph by Lawrence B. Stack, MD. Reproduced with permission from Knoop KJ, Stack LB, Storrow AB, Thurman RJ: The Atlas of Emergency Medicine, 3rd ed, © 2009 by McGraw-Hill, Inc., New York.]
The black widow spider injures its victim and its prey with highly potent venom. The most active component of the venom is α-latrotoxin, which acts through both calcium-dependent and calcium-independent pathways leading to receptor stimulation, pore formation, and ultimately massive release of neurotransmitters (predominantly acetylcholine and norepinephrine).21 Acetylcholine release accounts for neuromuscular manifestations, and norepinephrine release accounts for the cardiovascular manifestations.
Most Latrodectus bites are felt immediately as a pinprick sensation at the bite site, followed by increasing local pain that may spread quickly to include the entire bitten extremity. Erythema appears approximately 20 to 60 minutes after the bite. In many bites, a small, <5-mm erythematous macule develops that may evolve into a larger target lesion with a blanched center and surrounding erythema (Figure 211–5). The clinical syndrome is called latrodectism. Victims frequently complain of muscle cramp–like spasms in large muscle groups, although physical examination of the "cramping" extremity rarely reveals rigidity. The pain often increases progressively, becomes generalized, and can involve the trunk, back, and abdomen. Localized diaphoresis near the site of envenomation can be seen. Severe abdominal wall musculature pain and cramping are well described. Hypertension and tachycardia are common, and systemic symptoms include headache, nausea, vomiting, diaphoresis, photophobia, and dyspnea. Rarely reported complications include atrial fibrillation, myocarditis, priapism, and death. The pain with envenomation can be severe and intermittent and, if untreated, often lasts for a day. Occasionally, symptoms may persist for several days.
Black widow spider bite on the knee. [Photograph by Gerald O'Malley, DO. Reproduced with permission from Knoop KJ, Stack LB, Storrow AB, Thurman RJ: The Atlas of Emergency Medicine, 3rd ed, © 2009 by McGraw-Hill, Inc., New York.]
Because an immediate pinprick sensation is usually reported with Latrodectus bites, it is common for the offending spider to be identified. In the absence of a witnessed bite, a clinical diagnosis can be made based on characteristic symptoms and signs. There is no confirmatory laboratory test.
Cleansing of the bite site is reasonable. Pain and muscle spasms can generally be controlled with liberal doses of opioids and benzodiazepines.22 Although IV calcium has been advocated to relieve symptoms, a retrospective review of patients with Latrodectus envenomation indicated that this treatment is ineffective.22 For severe envenomations, admission may be required for continued analgesia. The most effective therapies for severe envenomation are parenteral opioids and Latrodectus antivenom.
Administration of Latrodectus antivenom often causes rapid resolution of symptoms and can significantly shorten the course of illness. Even in severely symptomatic cases of Latrodectus envenomation, patients can often be discharged from the ED after a short observation period when antivenom is administered. Successful treatment of latrodectism with antivenom has been described even with administration 90 hours after envenomation.23 Antivenin Latrodectus mactans is not contraindicated in pregnancy.24
Latrodectus antivenom is produced in at least three countries with specificity for indigenous species: Redback (Latrodectus hasselti) Spider Antivenom (CSL Ltd., Melbourne, Australia), Button Spider Antivenom (South African Vaccine Producers Institute, Edenvale, South Africa), and Antivenin Latrodectus mactans (Merck & Co., Inc., Whitehouse Station, NJ). It is likely that the antivenom to one species would be clinically effective in treating the bites of the others. Indications, amount, and route of administration vary according to product. Antivenin Latrodectus mactans and Button Spider Antivenom are administered IV, and Redback Spider Antivenom is typically administered by IM injection. An Australian-based study using Redback Spider Antivenom found minimal clinical difference between IV and IM routes.25
Latrodectus antivenom is derived from horse serum, and hypersensitivity reactions are possible. Redback antivenom is the most frequently used antivenom in Australia, and adverse reactions are rare. Until recently one death from anaphylaxis had been reported after administration of Antivenin Latrodectus mactans in the United States. Analysis attributed this to the antivenom being administered undiluted via IV push to an asthmatic patient with known allergies to multiple medications. Slow administration of diluted Antivenin Latrodectus mactans has generally been considered safe.26 However, two additional cases of anaphylaxis to Antivenin Latrodectus mactans have been described despite dilution and slow administration.27,28 In one of these cases, the anaphylaxis resulted in cardiac arrest, and despite successful resuscitation, the patient later died.27 Such cases should be kept in mind, particularly because latrodectism itself is rarely life-threatening. A F(ab)2 antivenom [Antivenin Latrodectus Equine Immune F(ab)2 (Analatro)], expected to be less immunogenic and safer than whole-antibody products, is currently commercially available in Mexico. At the time of writing, it is not approved in the United States.29
ARMED SPIDERS (PHONEUTRIA)
The armed spiders are found throughout South America and Costa Rica. The majority of clinically important bites have been described in Brazil. The spiders are solitary, nocturnal, do not construct a web, and possess potent neurotoxic venom. They have been reported to hide in banana bunches during shipping and can bite workers handling these bananas at their destination. When threatened, they assume a characteristic aggressive position by raising their four front legs, displaying their fangs, bristling their leg spines, and moving position to continually face their threat. The best-known armed spider, Phoneutria nigriventer (banana spider), is large, with a body size up to 3.5 cm and leg length up to 6 cm. P. nigriventer venom contains a mixture of potent neurotoxins that produce CNS, spinal cord, and autonomic effects.
Most P. nigriventer bites produce no significant symptoms. Significant envenomation produces local symptoms (severe pain) followed by sympathetic stimulation (tachycardia, hypertension), parasympathetic hyperactivity (nausea, vomiting, diaphoresis, salivation), spinal cord impairment (priapism), and CNS effects (vertigo, visual changes). Children and the elderly are at highest risk for serious envenomation. Pulmonary edema, shock, and death are rare.30 Most healthy adults recover in 1 to 2 days.
In most cases, supportive care is adequate. Local anesthetic infiltration at the bite site can control pain. A polyvalent antivenom (Instituto Butantan, São Paulo, Brazil) is available for cases of severe envenomation from P. nigriventer.
FUNNEL-WEB SPIDERS (ATRAX/HADRONYCHE)
Funnel-web spiders are arguably the most deadly spiders in the world. Fortunately, they exist in a localized geographical location in eastern Australia, and no fatalities have been reported since the introduction of antivenom in 1981.31 The spiders are so named because they construct a cylindrical web that extends into a recess, such as a burrow in the ground or a hole in a tree.
Funnel-web spiders have a shiny black body and long fangs, and females can grow up to 4 cm in body length. Females stay close to their webs, but the smaller and more aggressive males tend to wander, especially during the summer following a rain. Atrax venom contains a potent mixture of neurotoxins with neuromotor and autonomic effects.
Atrax bites may result in local reaction with immediate pain, followed by wheal formation and surrounding erythema. Later, localized sweating and piloerection may be observed. The vast majority of Atrax bites do not result in significant envenomation or systemic toxicity. The onset of severe envenomation is rapid and unlikely to begin after 2 hours.31 Symptoms and signs of systemic toxicity include perioral paresthesias, parasympathetic hyperactivity (nausea, vomiting, diaphoresis, salivation, lacrimation, bronchorrhea), neuromuscular stimulation (muscle fasciculation, tremors, spasms, weakness), and CNS toxicity (altered level of consciousness). Death after Atrax robustus envenomation has been reported as a result of cardiac arrest, hypotension, or pulmonary failure occurring between 15 minutes and 3 days after a bite.
To reduce venom absorption and systemic toxicity from a bite on an extremity, apply a compressive elastic bandage to the entire length of the limb, and splint the extremity to prevent movement.32 Immobilize the victim and transport promptly to the hospital.
The specific treatment for systemic toxicity is Funnel-Web Spider Antivenom (CSL Ltd., Melbourne, Australia). If the patient has signs of systemic toxicity upon arrival or develops them after the compressive elastic bandage is carefully removed, antivenom should be administered until symptoms improve. Supportive therapy for hypotension (IV fluid), bronchorrhea (atropine), tremors and agitation (benzodiazepines), and hypertension and tachycardia (β-blockers) may be necessary, but antivenom is the only therapy known to consistently improve survival.
Tarantulas are large, hairy spiders belonging to the family Theraphosidae that are popular as pets. The hairs found on the abdomen of most species of tarantulas in North and South America resemble a velvety covering and are used defensively. When threatened, tarantulas may flick these hairs a short distance with their two back legs. Although North American tarantula hairs rarely penetrate human skin, the hairs can imbed deeply into the conjunctiva and cornea and can cause inflammation in all levels of the eye, from conjunctiva to retina. Patients who manifest a red eye and pain after handling a tarantula should be examined to determine if offending barbed hairs are present in the cornea or conjunctiva. Although hairs are sometimes easily seen on slit-lamp examination, they may at times be very difficult to detect. Therapy includes surgical removal of the hairs and topical application of steroids to control inflammation. Ophthalmia nodosa is a granulomatous, nodular reaction that can occur in cases of ocular exposure to tarantula hairs.33 Patients may also develop a diffuse contact dermatitis from indirect exposure to hair while cleaning a tarantula cage. Bites from tarantulas are typically painful, with local erythema and edema, and some patients describe local joint stiffness following bites on nearby areas. Systemic symptoms other than fever are unusual.
Yellow sac spiders (Cheiracanthium) are medium-sized, typically yellow spiders that have a worldwide distribution. A few species are commonly found in homes. The most common symptoms of a bite are local sharp pain. Minor erythema, swelling, and pruritus may occur at the bite site. Dermatonecrotic lesions are rare.34
Wolf spiders (Lycosa) are small- to medium-sized (3- to 5-mm body length) ground-dwelling spiders with a worldwide distribution. The venom produces local pain and occasionally induration and erythema, but no systemic symptoms and no skin necrosis.
Jumping spiders (family Salticidae) are typically small (<15 mm), brightly colored, and very active spiders with a worldwide distribution. A bite may produce pain, swelling, pruritus, and erythema with resolution in 2 days.
Daddy long-legs spiders (family Pholcidae) are common cellar and outbuilding dwellers along the Pacific coast and in southwestern deserts. There are no case reports of human envenomation.