Walter R. Wilson MD
James M. Steckelberg MD
Ectoparasitic infestation is extremely common worldwide and virtually every human who resides in nonpolar climates is bitten repeatedly by arthropods. Specific infectious diseases transmitted by arthropod bite are reviewed elsewhere in this book. This chapter describes ectoparasitic infestations and toxic reactions to arthropod stings and bites. There is considerable overlap between infestations and bites because ectoparasites either bite or burrow under the skin of their human host.
ECTOPARASITIC INFESTATION
Essentials of Diagnosis
General Considerations
Ectoparasites are arthropods or helminths that infest the skin or hair of humans or mammals and feed on blood, serum, or tissue. Ectoparasites attach to skin by mouth parts or burrow beneath skin where they may inflict local tissue injury or necrosis, cause hypersensitivity reactions, or inject pathogens or toxins into the host. Table 91-1 lists common ectoparasitic infestation in humans. Box 91-1 summarizes the signs and symptoms associated with the most common types of infestation.
Because ticks attach and feed on their blood meal painlessly, their presence is often not readily detected by their host. Tick secretions produce local reactions, fever, or paralysis. The local reactions include small pruritic, papulonodular lesions that may vary in size from a few millimeters to several centimeters, and an eschar may develop. Tick-induced fever and constitutional symptoms disappear within 24–48 h after removal of the tick.
Tick paralysis is an uncommon illness, which is thought to result from toxins found in tick saliva, most often in dog or wood ticks. Children, especially those with long hair where the tick may reside undetected, are most often those affected by tick paralysis. Ascending flaccid paralysis begins 5–6 d after attachment and progresses over several days, resulting in diminished deep-tendon reflexes and cranial-nerve palsies. Sensation remains intact and spinal fluid analysis is normal. Untreated, respiratory paralysis that requires endotracheal intubation may occur. Removal of the tick results in rapid improvement and complete recovery usually within hours to several days. Ticks should be removed by gentle retraction with tweezers or forceps.
Itching and rash occur as a result of hypersensitivity to the mite excreta deposited in its burrow. Scratching kills the burrowing mite, but symptoms persist as a result of infestation elsewhere. Patients complain of intense pruritus, often worse at night or after a hot shower or bath. A lymphocytic, eosinophilic infiltrate develops around the burrow resulting in a dark wavy line 5–15 mm in length that ends in a small bleb that contains the female mite. Lesions are most commonly located on the wrists, elbows, scrotum, and skin folds and along belt and clothing lines. The face, neck, arms, and soles are usually spared. Papulovesicular eczematous plaques and pustules may develop in untreated patients.
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Table 91-1. Common ectoparasitic infestation in humans. |
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Crusted or Norwegian scabies results from hyperinfestation with thousands or millions of mites and usually occurs in patients with AIDS or other immunocompromised diseases. Crusted scabies is characterized by the presence of thick keratotic crusts, diffuse erythema, scaling, and dystrophic rashes—a condition resembling psoriasis. The characteristic burrow may not be visible in crusted scabies, and pruritus is often minimal or absent. Patients with AIDS and encrusted scabies are at risk of cellulitis and bacteremia resulting from skin fissures.
Transmission of scabies results from close person-to-person contact and is facilitated by crowded unsanitary conditions and sexual promiscuity. Outbreaks of scabies occur in households, hospitals, nursing homes, daycare centers, prisons, mental institutions, and chronic-care facilities. Medical-care personnel are at risk of acquisition. Patients with crusted scabies are highly contagious for others, including health care personnel, and should be placed in strict isolation until treated. Transmission of scabies via sharing of contaminated clothing or bedding is unlikely, because the mite does not survive > 18–24 h without its host.
The diagnosis of scabies should be considered in individuals who have pruritus and rash in the characteristic distribution. Burrows should be unroofed and the scrapings examined microscopically for the characteristic mite or its eggs and fecal pellets. Biopsy or scraping of skin lesions in patients with crusted scabies should be examined microscopically.
Chiggers or other mites commonly infest humans and human habitats. The mouse mite is the vector of rickettsial pox in urban areas of the United States; trombiculid mites transmit scrub typhus in Southeast Asia. The larvae of trombiculid mites reside on grass and other plants and attach to human hosts by contact. The mites penetrate and burrow beneath the skin producing an intense pruritic papule ≤ 2 cm in diameter, which vesiculates, pustulates, and ulcerates. Scratching kills the mite.
House mites (Dermatophagoides spp.) are extremely common worldwide. They reside in furniture, carpets, curtains, and elsewhere in the house and feed on shed human dander. Their allergens cause asthma, rhinitis, eczema, and other hypersensitivity conditions in sensitized adults with known house dust allergies. A positive skin test may suggest the diagnosis.
Three species of lice infest humans and feed daily on blood. Their bite results in an intensely pruritic local lesion. In sensitized individuals, a generalized maculopapular, urticarial rash is common and may be associated with fever and constitutional symptoms.
Head lice (Pediculus humanus var. capitis) are transmitted by human-to-human contact or by shared headgear, brushes, or combs. The prevalence is highest among school children or institutionalized individuals.
Body lice (P. humanus var. corporis) transmit epidemic typhus, relapsing fever, and trench fever. Body lice inhabit bed or body clothing and emerge to feed on humans. The body louse cannot survive more than a few hours without a human host. Transmission of body lice is the result of human-to-human contact or sharing a bed or body clothing and is facilitated by overcrowding and poor sanitation such as may occur among homeless individuals or during war or natural disasters.
The pubic louse (Phthirus pubis) primarily infests the hair in the pubis but may also infest the eyelashes, axillary hair, or other hair. This louse is most often transmitted by sexual contact. Blepharitis occurs with eyelash infestation.
The diagnosis of pediculosis is confirmed by the identification of lice or their eggs (nits) attached to the hair or clothing.
Furuncular myiasis results from human bot fly (Dermatobia hominis) larvae. In Central and South America, the adult female bot fly captures a mosquito or other blood-sucking insect and deposits eggs on the abdomen of the insect. When the carrier insect ingests a blood meal, the eggs are deposited on the skin surface and hatch, and the larvae penetrate the skin. The African tumva fly (Cordylobia anthropophaga) deposits eggs on wounds or clothing, which hatch on contact with skin and penetrate the skin. Furuncular myiasis results in a characteristic pustular lesion with a central breathing pore that emits bubbles under water. The larvae may be expressed, or surgical excision may be required.
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BOX 91-1 Signs and Symptoms of Ectoparasitic Infestation |
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Differential Diagnosis
Ectoparasitic infestation must be discriminated from other conditions that cause pruritic, maculopapular, pustulovesicular, or ulcerative dermatitis such as chronic contact dermatitis or neurodermatitis. Tick paralysis must be differentiated from Guillain-Barré syndrome, botulism, or polio. Crusted or Norwegian scabies resembles chronic eczema or psoriasis. Creeping dermal myiasis resembles cutaneous larva migrans caused by Ancylostoma braziliense.
Treatment
The treatment for ectoparasitic infestation is shown in Box 91-2. Secondary bacterial infections should be treated with appropriate antimicrobial therapy. Tetanus toxoid should be administered as needed in patients with significant cutaneous wounds, such as those caused by myiasis, leeches, or other infestation.
Prognosis
The prognosis in treated individuals is excellent and is influenced by the transmission of ectoparasite-specific infections, such as plague, typhus, and other serious bacterial infections.
Prevention
Prevention of tick infestation is facilitated by avoiding brushy vegetation, wearing protective clothing, tucking the trouser cuffs inside socks, and spraying clothing with 0.5 permethrin or an application of repellents containing N, N-diethyl-m-toluamide (DEET). Health care workers who treat patients with crusted scabies should wear protective gowns and gloves. Close contacts with patients with scabies should be treated even if they are asymptomatic. Bedding and clothing should be washed with hot soapy water. Combs and brushes belonging to patients infested with lice should be disinfected in hot water (65°C) for 5–15 min or soaked in insecticide for at least one hour. Clothing and bedding may be deloused by washing in hot water and drying in a clothing dryer at 65°C for 30–45 min or by fumigation with insecticide. Thorough cleaning of carpets, furniture, curtains, and other fabrics may reduce house mites. Fleas may be killed by spraying or dusting their nesting sites with insecticide. Myiasis risk may be reduced by good hygiene, including washing with soap and water after horseback riding or fly exposure. Openly draining wounds should be cleaned and dressed regularly. Wearing protective clothing reduces the risk of infestation by land or aquatic leeches.
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BOX 91-2 Treatment of Ectoparasitic Infestation |
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ARTHROPOD STINGS & BITES
Essentials of Diagnosis
General Considerations
Arthropods are ubiquitous pests. The most common arthropod bite of humans is by mosquitoes. Mosquitoes are the vectors for many life-threatening infectious diseases, such as malaria, yellow fever, and viral encephalitis. These infections are discussed elsewhere in this book (see Chapters 80 and 89). This chapter reviews arthropod stings and bites that envenomate or produce local trauma. Table 91-2 lists the common arthropods that sting or bite humans. Box 91-3 summarizes the signs and symptoms associated with arthropod stings and bites.
The order Hymenoptera includes aphids (bees and bumblebees), vespids (wasps, hornets, and yellow jackets), and ants. Their venoms contain numerous peptides and enzymes that cause localized and systemic reactions. Although the stings are painful and death may occur as a result of multiple stings, most fatalities are the result of hypersensitive anaphylactic reactions. Honeybees and bumblebees attack when their colony is disturbed, lose their stinger during envenomation, and then die. African honeybees (killer bees) were introduced into Brazil in the 1950s, have migrated progressively northward, and now inhabit the southern United States. These bees are highly aggressive, swarm, and may attack without provocation, often inflicting a large number of stings before the victim can escape. Rarely, human death has resulted from killer bee attacks.
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Table 91-2. Common arthropods that sting or bite humans. |
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Common vespids include yellow jackets, which are distinguished by bright yellow and black abdominal bands, and hornets, which are usually black, brown, or grayish color. Vespids feed on decaying vegetative material, garbage, and especially foods and liquids containing sugar. Vespids become aggressive in late summer and fall when most stings occur. Vespids do not lose their stingers during attack.
The compositions of venom from aphids and vespids vary among species. Toxins include histamine, acetylcholine, kinens, and numerous enzymes such as hylauronidase phospholipases. Uncomplicated stings cause immediate pain, wheal, and flare reaction and local edema, which subside over a few hours. Multiple stings (100–500) may cause constitutional symptoms, hypotension, generalized edema, rhabdomyolysis, hemolysis, and death. Approximately 0.5%–5% of individuals in the United States have immediate type hypersensitivity and are at risk for mild to life-threatening anaphylaxis.
Ant bites are extremely common in the United States and worldwide. Fire ants are aggressive, red-colored ants that inhabit the southern United States. They excavate areas in fields and urban areas and, when disturbed, mobilize huge numbers of attacking ants that may inflict thousands of stings on a single person. Many other species of ants inhabit the United States, and all that are large enough and capable of biting humans when provoked. Ant venom contains proteins, enzymes, and cytotoxic hemolytic piperidines. The bite is followed by immediate pain, wheal, and flare and then by the formation of a pustule, which ulcerates and may become secondarily infected. Anaphylactic reactions to ant stings occur in ~ 0.5%–2% of sensitized individuals.
The brown recluse spider (L reclusa) and at least four other species of Loxosceles are distributed throughout the United States and abroad. The brown recluse spider is a species that most commonly bites humans in the United States. This shy spider is 7–20 mm in body length and 2–4 cm in leg span. It has a characteristic dark violin-shaped spot on its dorsal surface. The spider emerges at night to hunt. Usually, it inhabits dark areas, such as basements, closets, attics, clothing, bed clothing, and other similar areas. Bites occur when the spiders are threatened or are involuntarily pressed against the skin such as may occur during sleep or while dressing. Initially, the bite is usually painless or may cause mild stinging. Complex toxins include proteolytic enzymes that cause necrosis. Of these, sphingomyelinase B is the most potent and results in vascular thrombosis and extensive local necrosis. Within hours after the bite, the area becomes painful and pruritic with induration surrounded by a zone of pale ischemia surrounded by erythema. Most lesions resolve spontaneously within a few days. In severe cases, the central area becomes hemorrhagic and necrotic with the formation of a bulla. The bulla ulcerates to form an eschar that leaves a large area of necrosis. Healing may take months to years and may require skin grafting. Rarely, severe systemic reactions including hemolytic anemia and renal failure may occur.
The female widow spider bite envenomates a potent neurotoxin. The black widow (L mactans) is widely distributed throughout the United States (except Alaska) and is most prevalent in the southeast. The spider measures ~ 1 cm in body length and 5 cm in leg span, is shiny black, and has a red-orange hourglass on the ventral abdomen. The black widow webs are most often located in dark places, such as garages, barns, and outdoor privies. The spider bites when the web is disturbed or the spider is provoked. The initial bite may be painless or feel like a pinprick. Two small erythematous red fang marks are visible. The venom does not produce local necrosis. Alpha-latro toxin is the most active toxin, and it binds irreversibly to nerves and causes release and depletion of acetylcholine, norepinephrine, and other neurotransmitters. Painful muscle cramps develop in the area ~ 30–60 min after the bite. Severe pain, muscle cramps, and rigidity may become generalized and may be accompanied by tachycardia, hypertension, muscle weakness, urinary retention, and, in extreme cases, paralysis, respiratory arrest, and rhabdomyolysis. Pain and muscle rigidity usually begin to subside within 12–24 h but may recur over the next days to weeks before resolving.
Tarantulas are large, hairy, nonaggressive spiders that have become popular pets. When threatened, they may bite their owners. Their bite is similar to that of a bee sting, causing localized pain and swelling.
C sculpturatus inhabits the southwestern United States and northern Mexico. Scorpions are ~ 7 cm in length and are yellow-brown. Envenomation is with a neurotoxin that affects sodium channels, resulting in nerve excitation. Symptoms include pain and hyperesthesia at the bite, which may progress to cranial and skeletal nerve hyperexcitability, muscle cramps and twitching, increased lacrimation, salivation, and hypertension. In severe cases, respiratory paralysis, rhabdomyolysis, and death may occur. Symptoms maximize within 6 h and usually subside within 1–2 d, but pain and paresthesias may persist for weeks.
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BOX 91-3 Signs and Symptoms of Arthropod Stings and Bites |
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Differential Diagnosis
The pustular ulcerative lesions resulting from ant bites resemble impetigo in children. Severe abdominal pain and abdominal muscle rigidity caused by a black widow spider bite must be differentiated from acute abdominal syndromes such as pancreatitis or peritonitis. However, in black widow spider bites, the abdomen is nontender to palpation. The muscle twitching and cramping from black widow spider and C sculpturatus scorpion bites may be confused initially with early tetanus. The increased salivation and hyperexcitability after C sculpturatus envenomation may resemble rabies.
Treatment
Treatment of arthropod stings and bites is outlined in Box 91-4. Administration of antihistamines, topical corticosteroids, and antimicrobial agents for secondary bacterial infection are general therapeutic measures for symptomatic arthropod stings. Anaphylaxis is treated by the administration of subcutaneous or, in severe cases, intravenous epinephrine, fluid resuscitation, bronchodilators, and, if necessary, endotracheal intubation and vasopressors. Patients with a history of severe allergy or anaphylaxis associated with insect stings should carry commercially available kits to treat anaphylaxis and other serious reactions when engaging in activities that place them at risk of arthropod exposure. Desensitization injections reduce the risk of recurrent anaphylaxis in hypersensitized individuals for bee and hornet stings.
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BOX 91-4 Treatment of Arthropod Stings or Bites |
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Prognosis
The prognosis for most individuals after arthropod stings is excellent. Most deaths result from anaphylaxis. Deaths are rare in treated patients envenomated by brown recluse spiders, black widow spiders, or C sculpturatus scorpions.
Prevention
Spraying insecticides in dark areas inhabited by spiders reduces the risk of exposure. When camping outdoors, individuals should check their shoes, backpacks, and sleeping bags for scorpions.
REFERENCES
Goddard J: Physicians Guide to Arthropods of Medical Importance. CRC Press, 1993.
Maguire JH, Spielman A: Ectoparasite infestations and arthropod bites and stings. In Fauci AS et al: Harrison's Principles of Internal Medicine, 14th ed. 1998