Chapter 52 Spider Bites

Leslie V. Boyer, Greta J. Binford, Janice A. Degan

Spiders and Their Venoms

Spiders number nearly 42,000 described species and are found in all habitats except the open sea.^38,^64,^225,²²⁷ They are carnivorous predators with important ecologic roles in most terrestrial ecosystems. Many are capable of wind-borne dispersal (ballooning), which has led to colonization of even the most isolated land masses on Earth.

As with ticks, mites, scorpions, and other arachnids, the spider’s body consists of an abdomen and an unsegmented cephalothorax (prosoma) with chelicerate jaws, pedipalps, and four pairs of legs (Figure 52-1). They are distinct from other arachnids in that they have no abdominal segmentation and the male pedipalpal tarsi are modified as secondary genitalia.²⁷³ In addition, spiders have venom that is produced in a gland in the anterior prosoma (Figure 52-2) and delivered through a cheliceral fang. On the abdomen, they have silk-producing glands and a set of spinnerets.

FIGURE 52-1 External anatomy of an araneomorph spider. A, Dorsal. B, Ventral (legs omitted).

FIGURE 52-2 Longitudinal section of the prosoma. E, endosternite; ESO, esophagus; P, pharynx; S, sucking stomach; SEG, supraesophageal ganglion; SUB, subesophageal ganglion.

(Modified from Foelix RF: Biology of Spiders, ed 2, Oxford, UK, 1996, Oxford University Press. With permission.)

The overwhelming majority of spider species is carnivorous, so eating involves the challenge of capturing live prey. Prey capture is a multistep process in which spiders must find prey, ensnare it, immobilize it, and digest it externally before the liquefied meal can be consumed. Venoms are a central tool in this process, and the primary function of venom in spiders is prey capture. They are also, but more rarely, used in defense. For their predatory function, venoms are complex mixtures of neurotoxic and proteolytic peptides, proteins, and biogenic amines.^1,^15,^107,^202,²⁷³ The venom of a single spider can have between 200 and 1000 distinct chemicals.⁸⁹ In the past decade there has been a burst of research interest in spider venoms because they contain vast stores of unknown chemical compounds with potential application in research, pharmacology, or as insecticides. We direct the curious to Arachnoserver,³³⁶ a database of sequences, structural, and molecular target information of all published spider venom toxins.

Transcriptomic and proteomic analyses are beginning to reveal patterns of complexity and variation in venom composition within and among species. We do not attempt to review the full literature of characteristics of spider venoms, but rather focus on what is known (and unknown) about venoms and their effects on humans. Most toxins in spider venoms have great target specificity, acting selectively on arthropods, vertebrates, or other groups, including some with mammalian-specific activity.^1,^126,²²¹ Venom composition varies widely across spider species, and variation may exist within species between genders and among geographically isolated populations.^136,²⁰⁰ Venom potency also varies within individuals, both seasonally and developmentally. Despite this tremendous venom diversity, only a few dozen spider lineages are considered harmful to humans because (1) most others have an insufficient quantity of venom, (2) the toxins do not affect mammals, or (3) the fangs cannot penetrate human skin. In a few species, although laboratory evidence suggests potential mammalian toxicity, human envenomations have not been reported, perhaps because of the rarity of encounters between spiders and humans in some habitats. Table 52-1 lists an assortment of spider families, including those with species that have been reported to bite humans.

TABLE 52-1 Spiders of Clinical Interest

In addition to the direct action of venom toxins on humans, there are other ways in which spiders are suspected or known to be clinically relevant. Occasionally, spider digestate may infect wounds created by spider bites, which could in theory affect clinical appearance, although no direct evidence substantiates this suspicion. Similarly, mechanical punctures of human skin by spider fangs may introduce bacteria into wounds; however, this is another suspicion that has yet to be empirically substantiated. Outside of venom and bite mechanics, some theraphosids (tarantulas) produce urticating hairs that irritate skin or mucous membranes of animals or humans. Exposure to these may result from direct contact with a spider or from indirect exposure to materials, such as clothing, that may carry loose hairs.

General Assessment and Treatment of Spider Bites

Awareness of the differential diagnosis is crucial for management of any patient presenting for evaluation of “spider bite,” because the offending creature is rarely observed and identified. In general, it is outside of normal biologic activity for spiders to bite humans, except in defense. A defensive bite risks the spider’s life and tends to occur only when its life is threatened by being crushed. Thus, true spider bites are much less common causes of skin lesions than are insect bites or cutaneous infections. Furthermore, no pathognomonic clinical signs prove the diagnosis without retrieval and identification of a spider that was seen actually biting.²⁵⁴ Diagnosis of arachnidism without direct evidence can lead to inappropriate treatment and inadequate consideration of more severe underlying medical issues.^162,^165,^168,^247,²⁹⁶ Therefore, the medical history, physical examination, and laboratory evaluation of putative spider bites must usually take into consideration an alternative etiology. Treatment plans should include careful follow-up and patient counseling to manage any uncertainties in the final diagnosis.

The differential diagnosis of a local lesion may include fungal, bacterial, and viral infections, especially herpes simplex and zoster; the vesiculobullous diseases; arthropod-borne infectious diseases (e.g., Lyme disease); other bites and stings; foreign body reactions; and systemic conditions that predispose to focal skin lesions (e.g., diabetes mellitus, leukemia, lupus erythematosus). First-aid interventions by the patient may mask an otherwise benign process by superimposition of trauma, burns, or chemical irritation. Systemic signs and symptoms may require differentiation from the effects of snake or scorpion neurotoxin, pesticide toxicity, sepsis, meningitis, hemolytic anemia, or acute abdomen, depending on the circumstances.

The medical history should include details of the bite circumstances to demonstrate consistency with expected spider habitat and behavior. This includes location indoors or outside, time of day or night, and human activity at the time of injury. The victim should attempt to recall the appearance of the involved arthropod. If it was believed killed through garments or bedclothes, an attempt should be made to retrieve its remains. Crushed spider parts can be examined and identified by arachnologists or entomologists at many universities and museums. Until identified, spiders may be preserved in 70% to 80% ethanol. The evolution of subsequent wound and systemic symptoms should be noted, along with modifiers, including home treatment and underlying health of the patient. If the local geography lacks species consistent with the suspected pathophysiology, the recent travel histories of household contacts should be considered.

Physical examination includes particular attention to the bite site, as well as general assessment for systemic effects. Local findings of importance include anatomic location (spiders are more likely to bite defensively at sites where clothing binds tightly; thin skin is more readily envenomed than callous skin) and number of separate lesions (multiple bites suggest parasitic insect bite rather than spider bite). Central punctae, vesicles, or erosions should be noted, as well as the pattern of peripheral changes, including erythema, pallor, hemorrhage, induration, tenderness or anesthesia, and local lymphatic involvement. Systemic findings, depending on the species involved, may include changes in vital signs, diaphoresis, generalized rash, facial edema, gastrointestinal distress, muscle fasciculations, spasm or tenderness, or altered mental status.

The laboratory evaluation for envenomation is usually simple, seldom requiring more than a complete blood count and urinalysis. Assessment for other elements of the differential diagnosis, however, may be much more elaborate. Depending on circumstances, this may include viral, bacterial, or fungal culture; Lyme disease titer; radiography of the abdomen or of the injured part; stool test for occult blood; electrocardiogram; or skin biopsy.

General supportive measures are the mainstays of therapy for most spider bites. These include basic local hygiene, tetanus prophylaxis, analgesics, hydration, and surgical follow-up if indicated for debridement and management of extensive necrotic lesions. Corticosteroids are of unproved benefit and are generally not indicated. Antibiotics, although not of value for simple venom injury, are prescribed when bacterial cellulitis cannot be eliminated from the differential diagnosis. Specific measures, including antivenom, for treatment of envenomation by particular spider species are discussed later in this chapter.

Guide to Spider Diversity and Identification

This chapter provides more information on spider diversity than do most reviews in order to (1) inform the reader of the immense diversity of spiders that are medically insignificant and thus emphasize the rarity of spiders known to cause medical problems; (2) emphasize the need for accurate species identification, particularly in reporting of cases for publication or teaching; and (3) facilitate accurate identification of spiders that are caught in the act of biting by directing professionals to proper identification keys. Table 52-1 includes species worldwide that are currently known or suspected to be medically noteworthy, even minimally, either through the effect of bites or urticating hairs. Geographic range and the most recent systematic work on each group are included. Box 52-1 lists the genera of serious medical significance for each continent. For effective communication, groups of spiders must be recognized by the same name worldwide. This chapter uses the current official nomenclature.

Of the three spider suborders, two contain clinically significant species: Mygalomorphae and Araneomorphae. Mygalomorphs include the baboon spiders or tarantulas, trapdoor spiders, purse-web spiders, mygalomorph funnel-web spiders, and several other groups that lack common names. Most spiders are araneomorphs. These include jumping spiders, orb-weaving spiders, widow spiders, wolf spiders, and fiddleback spiders (see Table 52-1). The most conspicuous characteristics that distinguish these groups are the orientation of the chelicerae (jaws) and the number of book lungs. Spider fangs are located on the chelicerae, which open sideways in the Araneomorphae and move diagonally in the Mygalomorphae, requiring the latter to rear back for a downward, snakelike strike (Figures 52-3 and 52-4). Mygalomorphae have two pairs of book lungs, whereas most Araneomorphae have only one pair. Lung slits, which open into the book lungs, are easily visible in a ventral view of the anterior abdomen (see Figure 52-1). Characteristics that distinguish families, genera, and species include eye number and pattern, numbers of tarsal claws, and details of genitalia structure.

FIGURE 52-3 Movement of the chelicerae in mygalomorphs (A) and araneomorphs (B).

(Modified from Foelix RF: Biology of Spiders, ed 2, Oxford, UK, 1996, Oxford University Press. With permission.)

FIGURE 52-4 Fangs of mature male tarantula (Aphonopelma species).

(Courtesy Michael Cardwell and Associates, 1992.)

Although many spider species are geographically localized (e.g., Atrax robustus in Australia, Phoneutria nigriventer in Brazil), some, such as the widow spiders (Latrodectus), are found worldwide (see Table 52-1). Others, such as members of the Loxosceles genus, are widely distributed in more than one continent, and still others, such as Tegenaria agrestis, appear to have naturalized in specific geographic regions distant from their point of ecological origin. Because of the resulting worldwide species diversity and overlap, the remainder of this chapter is structured according to spider taxonomy rather than isolated species or geographic location.

Suborder Mygalomorphae

Mygalomorphs make up less than 10% of all spider species. They are found worldwide, with greatest abundance and diversity in tropical regions. Tarantulas (Theraphosidae) are the most famous mygalomorphs and include the largest spiders known, reaching up to 10 cm (4 inches) in body length. Most mygalomorphs are smaller, some less than 1 mm (0.004 inch) in adult body length. Most live multiple years (some up to 20), and females continue to molt after reaching adulthood. They have diverse habits but typically live in silk-lined burrows or silken tubes. For the purpose of prey capture, mygalomorph silk is only slightly sticky relative to araneomorph silk and is generally used to signal the presence of prey rather than to ensnare them. Once prey are detected, spiders run out of their retreat, seize prey in their jaws, and return to the retreat to feed. Individuals wander when dispersing, and males leave their retreats in search of females during mating season.

Family Theraphosidae: Tarantulas and Baboon Spiders

Theraphosidae is the largest mygalomorph family with respect to both numbers of species and sizes of the largest individuals (Figure 52-5). Approximately 800 described species are found on all continents, with the greatest abundance and diversity in tropical regions. They mature in 3 to 9 years and can live for 15 to 25 years. Individuals live in burrows, with trip-line threads extending from the entrance. These are sometimes located in abandoned rodent burrows or hollow trees. Theraphosids have dense tufts of specialized hairs on their tarsi (feet) that enable them to climb on smooth surfaces and may aid in prey capture. They have two tarsal claws, eight closely grouped eyes, and two pairs of spinnerets. As a group, they are found mainly in tropical and subtropical areas.

FIGURE 52-5 Theraphosidae.

(Courtesy Gita Bodner.)

Confusingly, the word tarantula was first applied to Lycosa tarentula, a species of European spider actually belonging to the wolf spider family, or Lycosidae, which are properly classified within the suborder Araneomorphae, described later in this chapter. In the United States, the term tarantula usually refers only to the large spiders of the family Theraphosidae, suborder Mygalomorphae.

Grammostola mollicoma is the largest tarantula known, with a body length of 7 to 10 cm (3 to 4 inches) and leg spread of 21 to 27 cm (8.3 to 10.6 inches).^42,¹¹¹ Harpactirella lightfooti, the baboon spider or bobbejaan-spinnekop, is a mygalomorph spider found in South Africa. Body length is 3 cm (1.2 inches); the cephalothorax is brown with a yellowish border.

Venom

In the last few years our understanding of tarantula venoms has increased because of analyses of the venom transcriptomes and proteomes of multiple species.¹⁵⁷ In the United States, Rheochostica henzi and members of the genus Aphonopelma (Figure 52-6) have venom containing hyaluronidase, nucleotides, and polyamines.^48,^58,^172,^259,²⁶⁰ Polyamines are thought to act as neurotransmitters and increase venom effectiveness, particularly with respect to paralysis of insect prey.²⁷³ Hyaluronidase is postulated to be a spreading factor, and the nucleotide adenosine triphosphate (ATP) potentiates the major effects of the venom on mice. Both venoms cause rapid, irreversible necrosis of skeletal muscle when injected intraperitoneally into mice.²²³ Dugesiella (Rheochostica) venom was found to have a necrotoxin with several similarities to sea snake venoms.¹⁷² In comparison, the venom of Scodra griseipes, an African tarantula, includes higher-molecular-weight (greater than 25,000 daltons) proteins and enzymes plus lower-weight polypeptides (4000 to 9000 daltons); the second group is believed to contain polypeptide neurotoxins.⁵⁶ S. griseipes venom toxins have an effect on mammalians, but this species is not known to be clinically relevant. Recent work suggests that venom chemotaxonomy may be a useful method of nondestructive species recognition, at least within the Brachypelma genus.⁸⁸

FIGURE 52-6 Mature female Aphonopelma iodium.

(Courtesy Michael Cardwell and Associates, 1997.)

Urticating Hairs

Several genera of tarantulas, including Haplopelma, Lasiodora, Grammostola, Acanthoscurria, and Brachypelma, possess urticating hairs irritating to skin and mucous membranes. These genera are located throughout the western hemisphere, with many species indigenous to the United States. When one of these spiders is threatened, it rubs its hind legs across the dorsal surface of its abdomen and flicks thousands of hairs toward the aggressor. These barbed hairs can penetrate human skin, causing edematous, pruritic papules. Itching may persist for weeks. There are four morphologic types of urticating hairs. Tarantulas within the United States possess only type I hairs, which do not penetrate the skin as deeply as do type III hairs. Type II hairs are incorporated into the silk web retreat and are not thrown off by the spider. Type III hairs can penetrate up to 2 mm into human skin; this is the type of hair most likely to cause inflammation. They are typically found on Mexican, Caribbean, and Central and South American species. Type IV hairs, which belong to the South American spider Grammostola, are able to cause inflammation of the respiratory tract in small mammals. Rats and mice have been reported to die of asphyxia within two hours after exposure to the hairs.^67,³²⁸

Clinical Presentation

Despite the presence in venom of components toxic to rodent nerves and skeletal muscle, most tarantula bites result in only mild to moderate local symptoms in humans. A few can cause more severe pain and swelling, numbness, or lymphangitis. In a series of nine cases in Australia, by either Selenocosmia species or Phlogiellus species, no major effects occurred. Local pain was the most common effect, followed by puncture marks or bleeding. Severe pain occurred in four of seven patients, where severity of pain was documented. Mild systemic effects occurred in one case.¹⁴⁹ Species of tarantula implicated as causes of human envenomation include those in genera Mygalarachne (formerly Sericopelma) of Panama, Pterinochilus of Africa, Aphonopelma of Mexico and the United States, Pamphobaeteus of South America, Euathlus of Costa Rica, Theraphosa of French Guyana, Grammostola of Colombia, Selenocosmia and Phlogiellus of Australia, Poecilotheria of India, Lampropelma of Thailand, Lasiodora of Brazil, and Avicularia of Central America and the southwestern United States. Envenomation usually involves immediate pain at the bite site, occasionally followed by some redness and swelling, and is usually without necrosis or serious sequelae.^42,^48,^58,^109,²⁶⁰

A paucity of reports in the peer-reviewed medical literature hinders proper characterization of occasional tarantula neurotoxicity, particularly in cases (e.g., Poecilotheria) that are reported anecdotally and in the pet trade, to provoke severe neuromuscular symptoms Although no fatalities have been reported, localized pain followed by emesis, weakness, and collapse have been noted after envenomation by Harpactirella lightfooti, the baboon spider of South Africa.^42,^129,²¹³

Urticating hairs may cause intense inflammation, which may cause pruritus for weeks. Individuals who handle tarantulas may unwittingly transfer urticating hairs from hand to eye, causing keratoconjunctivitis or ophthalmia nodosa. Keratoconjunctivitis has been described after handling of a Thailand black tarantula, Haplopelma minax. Fine intracorneal hairs were noted at examination, and inflammation settled quickly with topical corticosteroid treatment; at a 36-month follow-up, the eye was normal.³⁷

More severe ophthalmic complications occurred in two cases after handling Chilean rose tarantulas, Grammostola cala. In these victims, initial findings were similar, with intracorneal hairs and keratoconjunctivitis, but progressive panuveitis followed, with corneal granulomas, iritis, cataract, hyalitis, and chorioretinitis apparently related to migration of hairs through the media of the eye. The differences in outcome with exposure to the two species may result from differences in hair morphology, which may also explain differences in other reports of ophthalmic injury from tarantula or caterpillar hair exposure.³⁷ Similar cases of ophthalmia nodosa have been described.^24,¹⁷¹ Sheth and colleagues,²⁶⁸ in London, reported another case of ocular inflammation after handling a pet Chilean rose tarantula. A 9-year-old boy presented with new onset of painful red right eye, initially thought to be trauma-induced 2 weeks earlier. The left eye was unaffected and visual acuity was within normal limits. Slit-lamp examination showed right eye injection with multiple hairs at all levels of the cornea with associated opacities, and moderate anterior uveitis. In addition, hair-like vitreous lesions and peripheral full-thickness retinal infiltrates were seen. Topical therapy with corticosteroids and antibiotics was initiated and follow-up continued with long-term tapering topical steroids. Hair removal was not attempted because of the large number and depth of hairs. Eighteen-month follow-up revealed full resolution of inflammation, normal visual acuity, and no complications (Figures 52-7 and 52-8).

FIGURE 52-7 Cornea showing multiple tarantula hairs with associated inflammatory infiltrate and scarring.

(From Atkins JA, Wingo CW, Sodeman WA, et al: Necrotic arachnidism, Am J Trop Med Hyg 7:165, 1958.)

FIGURE 52-8 Color photograph of retina showing punched-out full-thickness inflammatory lesions associated with embedded tarantula hairs.

(From Atkins JA, Wingo CW, Sodeman WA, et al: Necrotic arachnidism, Am J Trop Med Hyg 7:165, 1958.)

Treatment

Theraphosid bite management is symptomatic. Elevation and immobilization of the extremity and oral analgesics may help reduce pain. All bites should receive local wound care and appropriate tetanus prophylaxis. Urticating hairs can be removed from skin with repeated application and removal of sticky tape, followed by copious irrigation if necessary; exposed eyes should be irrigated primarily. Topical or systemic corticosteroids and oral antihistamines may also be useful for urticating hair reactions.

Family Hexathelidae: Funnel-Web Mygalomorphs

The family Hexathelidae includes 11 genera and approximately 74 species, which are currently known from the Old World and Chile.²³⁷ The funnel webs that typify members of this group are silk-lined tubular retreats that extend into a protected space, such as a burrow in the ground or a hole in a tree. Sheets of silk radiate from the retreat and signal the presence of prey. These webs superficially resemble webs of araneomorph spiders in the family Agelenidae. Distinguishing characteristics of the spiders include a shiny carapace; long, spiny sensory hairs on the legs; and paired claws lacking claw tufts on the tips of the feet, with teeth lining the medial claw.

Clinically significant hexathelid spiders are species of Atrax (Figure 52-9) and Hadronyche. Technically, taxonomists now consider Atrax and Hadronyche species as members of only one genus.²³⁷ To avoid confusion in the literature, species names still use either Atrax or Hadronyche, depending on their original names. We discuss these species as a cohesive taxonomic group. Among these, the species A. robustus is best known and most carefully studied. Atrax and Hadronyche species, all of which are believed to be dangerous, have been described in southern and southeastern Australia, Tasmania, Papua New Guinea, and the Solomon Islands. As a group, they prefer cool, moist coastal and mountainous regions.^122,^124,²⁶⁹

FIGURE 52-9 Funnel-web spider (Atrax species) wearing an engagement ring.

(Courtesy Sherman Minton, MD.)