Chapter 34 Arthropod bites and stings
1. What are arthropods? Are most arthropods harmful to humans?
2. Describe various ways arthropods injure humans. What insect or arachnid would cause the injury?
3. How do you diagnose a bite or sting?
The annoyance of the mosquito or the immediate pain of the bee sting rarely poses a problem of recognition. Diagnostic problems arise when the arthropod is not seen or felt but leaves a nonspecific papular rash with redness, pain, itching, or swelling. A careful history, common knowledge of local arthropod populations, and a physical examination with a high index of suspicion is necessary. Arthropod bites may present with itchy, red papules in a particular distribution or pattern that suggests an area of potential arthropod exposure. For example, grouped bites around an exposed ankle suggest a nonflying arthropod such as a flea, mite, chigger mite, or bedbug. Mysterious bites are often secondary to a zoonotic source, which includes exposures to pets, pests such as rats and mice, and nesting birds or bats near the home or workplace. Consultation with a veterinarian or entomologist is extremely beneficial in unusual cases. A biopsy of the skin lesion often will show perivascular dermatitis with eosinophils.
4. Why are some bites and stings extremely painful or dangerous while others are simply itchy, red, irritating papules?
5. One person on a hike is “eaten alive” by mosquitos while his companions are not bothered at all. Do ectoparasites such as mosquitos, ticks, fleas, and mites bite randomly?
No. Most arthropods are attracted to their hosts by a number of physical and chemical stimuli such as warmth, composition of sweat, carbon dioxide, vibrations, and odor. On the other hand, lipids derived from the degenerative epidermal cells often have a repellent quality to them. Thus, to arthropods, some human hosts are far more attractive than others.
6. Why are bite reactions to ectoparasites so much different in different people?
The severity of bug bite reactions depends most on the immunologic status of the patient. In general, a severely symptomatic eruption may result from only a few bites in an immunologically sensitized person, while many bites may produce no symptoms at all in a person with an acquired immunologic tolerance.
• Stage 2: After a 2-week period of exposure, a delayed red papule begins within 24 hours after a bite and subsides within a week.
• Stage 3: An immediate wheal after a bite, followed by a delayed papule 24 hours later, becomes the clinical presentation.
• Stage 5: With continual exposure, immunologic tolerance begins, and there is no reaction after even prolonged exposure to insect bites; that is, with the immediate as well as the delayed response is lost.
Dahl MV: Cutaneous reactions to arthropod bites and stings, Clin Cases Dermatol 3:11–16, 1991.
7. How do you treat bee stings?
The best treatment for stings is prevention. Exposure to areas of wildflowers, dandelions, or clover fields should be avoided. Always wear shoes. Brightly colored clothing, flowery designs, and the use of colognes, perfumes, and scented soaps should be minimized. When confronted by agitated bees or wasps, avoid rapid movements and either stand still or withdraw very slowly to prevent further aggravation.
The venom-containing barbed stinger, if still present at the site of the sting, should be removed by gently scraping the skin horizontally with a dull knife or credit card (Fig. 34-1). Stinger removal with forceps compresses the venom gland, forcing more venom into the skin, and should be avoided. Symptomatic care with rest, elevation, and ice to the area are helpful. Antihistamines may also be useful. Early signs of systemic toxicity or allergic reactions should be noted.
9. How do you treat anaphylactic syndrome from a bee sting?
If a reaction begins, treatment includes subcutaneous injection of epinephrine, 1:1000 in aqueous solution. This may be repeated in 15 to 20 minutes. In addition, intravenous diphenhydramine and cortisone as well as oxygen, fluids, vasopressors, and bronchodilators may also be used as needed. Since most fatalities occur within the first hour, early intervention by the allergic patient with a self-administered epinephrine injection may prevent a reaction from developing. Emergency treatment kits may be prescribed and carried at all times (Ana-Kit by Hollister-Stier [Spokane, WA], or EpiPen/EpiPen Jr by Dey Pharma [Basking Ridge, NJ]). Education and sting avoidance and evaluation for venom immunotherapy should be considered.
10. What are the unique characteristics of fire ants and their sting?
The fire ant, accidentally introduced into the United States in the 1930s from Brazil, now occupies most of the Southeast, limited only by cold winter weather in the north and the deserts of the Southwest. When the ants are provoked, they may attack en masse, administering up to several thousand stings to a victim. The sting is actually initiated by the ant biting the flesh and then pivoting slightly and repeatedly stinging the victim to form an annular eruption of sterile pustules and sting reaction sites. The sting site develops a large wheal and flare punctuated by a small sterile pustule, which ulcerates and potentially scars. The large erythematous wheal will then gradually desquamate over the next couple of days, and the sting site resolves normally. The unusual fire ant venom, a piperidine alkaloid, has become a leading cause of anaphylactic reaction to venomous animals in the United States. Treatment of the ant stings is symptomatic. Select patients with severe local or generalized sting reactions may be considered for immunotherapy.
11. What species of spiders are medically important?
The brown recluse (Loxosceles reclusa) and the female black widow (Latrodectus mactans) (Fig. 34-2) are the most serious spiders in the United States. The hobo spider (Tegenaria agrestis) in the Pacific Northwest has been reported to produce dermonecrotic arachnidism, a serious necrotic reaction in the skin induced by the venom. However, whether or not this spider produces dermonecrotic arachnidism is controversial; some authorities believe that the reported bites have come from other causes. As many as 16 other species may bite humans, causing a small amount of pain or even necrosis at the site, but these reactions are transient, self-limiting, and require little attention. Most species are either too small or the biting mouthparts will not penetrate the human skin and, therefore, inject an insignificant amount of venom.
12. How do you diagnose and treat the black widow spider bite?
The female black widow spider is 10 to 15 mm long and has a black, globose abdomen, often with a red hourglass figure on the ventral surface. This spider is found in all 48 contiguous states. The venom is a potent neurotoxin.
Figure 34-2. Female black widow spider demonstrating the characteristic hour glass on the ventral surface. (iStock_6222259)
Elston DM: What’s eating you? Latrodectus mactans (the black widow spider), Cutis 69:257–258, 2002.
13. How do brown recluse spider bites present?
Unlike the black widow, the brown recluse produces a dermonecrotic toxin that can cause severe necrosis of the skin as well as a hemolytic toxin that causes severe, even life-threatening, hemolysis. The brown recluse is a shy, reclusive, brown spider with a dark brown violin-like marking on the dorsum of the cephalothorax, hence the name fiddleback spider. The spider is located largely in the Midwest but has been reported along the East Coast, in Texas, and in California. The bite commonly occurs when a person is cleaning old storage rooms or woodpiles outdoors, where the spider resides. Often, the spider is not seen or recovered at the time of the bite. The initial bite may be painless but is usually sharp and stinging for 6 to 8 hours and then is replaced by dull, aching pain that gradually increases in intensity and is accompanied by itching. The bite then begins to show a central blue color (impending necrosis), a surrounding white area (a vasospasm and ischemia), and a peripheral red halo (inflammation). Extension of this reaction demonstrates a remarkable gravitational spread to dependent areas. Over the next 2 to 4 days, the extent of the necrosis will be known. If a larger area of necrosis develops after a few days, a rare but serious and life-threatening systemic reaction to the hemolytic toxin may occur. Therefore, observation for hematologic changes such as red blood cell hemolysis or disseminated intravascular coagulopathy (DIC) should be observed.
14. How should brown recluse spider bites be treated?
First aid measures are important and can be remembered by the mnemonic RICE. Rest, ice, compression, and elevation of the bite site decrease blood flow, temperature, and enzymatic activity of the dermonecrotic toxin. General wound care, tetanus toxoid, antibiotics such as erythromycin or cephalexin for secondary infection, and observation for systemic hematologic problems may be necessary until the wound heals. Although dapsone and corticosteroids have been used to treat brown recluse spider bites, the use is controversial because some animal studies and nonrandomized human series have not demonstrated a beneficial effect. As serious as these bites can be, it is important to remember that most bites are inconsequential and heal without any problem. In those rare patients with hemolytic anemia, blood transfusions and intravenous fluid replacement is indicated.
Key Points: Dermonecrotic Arachnidism
1. The brown recluse spider (Loxosceles reclusa) is the most common cause of dermonecrotic arachnidism.
2. The hobo spider (Tegenaria agrestis) in the Pacific Northwest has been reported to produce dermonecrotic arachnidism, although this is controversial.
15. What is scabies?
Scabies is a contagious infestation caused by a mite that affects humans and other mammals, such as dogs, cats, horses, cows, pigs, and others. There are approximately 40 known species of scabies. Sarcoptes scabiei var. hominis is the etiologic agent of human scabies and is a full-time ectoparasite living, eating, and breeding on the human host. The female mates, burrows into the upper epidermis, lays her eggs, and eventually dies after 1 month. The mite and its feces cause severe pruritus.
16. How is scabies characterized clinically?
• The rash has a distinctive distribution involving the interdigital webs of the hands, the volar wrists, extensor elbows, axillary areas, central abdomen, genitalia, buttocks, and anterior thighs. The papular lesions affecting the shaft and glans penis as well as the scrotum are almost diagnostic. Facial lesions in adults are absent. Difficult cases include the very young, who may have pustules on the face or scalp.
• The rash consists of pruritic papules, but a diagnostic linear burrow consisting of a very fine scale is often seen in the interdigital web area or on the volar wrists (Fig. 34-3A). Nodular lesions commonly occur on male genitalia (Fig. 34-3B).
• The diagnosis is confirmed by scraping a small linear scaly burrow to reveal the female mite, her eggs, or fecal material under the microscope. Recovery of the diagnostic mite on scraping is often difficult in the excessively clean patient or patients that have been partially treated. Less commonly the diagnosis is established by visualizing the mite on a skin biopsy (Fig. 34-4).
17. What is Norwegian scabies?
Norwegian (crusted) scabies is a massive infestation with inordinate numbers of scabies mites due to inadequate host response. The large numbers of organisms cause a hyperplastic growth of the epithelium. Scratching may be absent because itching is variable. The condition is seen in debilitated patients, those with mental illness, sensory neuropathy, paresis, and immunosuppressed patients such as those with acquired immune deficiency syndrome, post-transplantation, or undergoing topical or systemic corticosteroid therapy. Topical treatment with permethrin and keratolytics is difficult, so oral ivermectin (200 μg/kg) has been used with good success.
Figure 34-4. Biopsy of scabies demonstrating mite in burrow.
(Courtesy of James E. Fitzpatrick, MD.)
Tran L, Siedenberg E, Corbett S: Crusted (Norwegian) scabies, J Emerg Med 22:285–287, 2002.
18. What agents are used to treat scabies?
Sixty grams of 5% permethrin cream should be applied at night from the neck down with special attention to the subungual areas on the fingernails and the genitalia as well as the waistline. The cream should be removed in the morning and the clothes and linen from the night before should be thoroughly washed in hot water and dried. This entire procedure should be repeated in 4 to 5 days. All members of the family should be treated to eliminate asymptomatic, incubating cases. A shortened two-hour application may be used in pregnancy, if indicated.
Karthikeyan K: Treatment of scabies: newer perspectives, Postgrad Med J 81:7–11, 2005.
Bolognia JL, Jorizzo JL, Rapini R Petal: Dermatology, Spain, 2008, Mosby-Elsevier.
19. Discuss the three varieties of lice that affect humans.
The head louse (Pediculus humanus var. capitus) is 2 to 4 mm long with three pairs of legs that are of equal length. The body is dorsoventrally flattened. The entire life cycle is spent in the scalp hair. The visible eggs or nits are deposited on the hair shaft, singly and close to the scalp. Pruritus of the scalp with secondary infection is common. Associated cervical and occipital lymphadenopathy is common. Head lice are more common in school-aged children, especially young females with longer hair (Fig. 34-5).
20. How should lice infestations be treated?
Head and pubic lice infestations are treated similarly. Due to resistance and safety, a 5% permethrin cream is applied 8 to 12 hours overnight, the treatment is repeated in one week (with occlusion), and the scalp (within ¼ inch) is observed for viable nits. Ovide (malathion 0.5%) is applied 8 to 12 hours (approved for individuals over 6 years of age) and the treatment is repeated in 7 days. Finally, oral ivermectin 200 to 250 mg/kg is given on day 1 and day 8. As with scabies, children should get less than 15 kg, and pregnant/lactating women should probably not get oral ivermectin. Removal of the nits with a specially designed, fine-toothed comb ensures that the infestation is clear. All close contacts should also be treated. Body lice are eliminated by thorough cleaning of the clothes or changing them for new ones. The patient needs symptomatic relief only and a fresh change of clothes.
Elston DM: Drugs used in the treatment of pediculosis, J Drugs Dermatol 4:207–211, 2005.
21. How do flea infestations typically present?
Only the adult flea is a parasitic bloodsucker, while the immature, wormlike, legless, eyeless larvae scavenge for food for the next 1 to 6 months before pupating. The adult fleas are small, approximately 3 mm in size, and streamlined with no neck or waist. They are laterally compressed, taller than broad. Bristles, combs, and claws are used to cling to their host and move briskly through the hair. They are wingless but have powerful rear legs to reach the ankles of their prospective hosts (Fig. 34-6).
Steen CJ, Carbonaro PA, Schwarz RA: Arthropods in dermatology, J Am Acad Dermatol 50:819–842, 2004.
22. How do you treat a flea infestation?
Treatment of the patient is symptomatic. The pet should be treated with insecticidal powders, shampoos, and dips. For each adult flea found on the animal, there are perhaps hundreds of adults, eggs, larvae, and pupae in the bedding material of the pet. Apply strict environmental controls: Vacuum the pet’s sleeping areas throughout, wash or destroy the bedding, and apply an insecticide containing a residual pyrethroid and a growth regulator hormone to the area.
23. How is Cheyletiella infestation recognized in humans? In animals? How does it compare to canine scabies?
Cheyletiella species are free-living, nonburrowing, ectoparasitic mites of dogs, cats, and rabbits. The mite is about the same size as the scabies mite but can be easily differentiated by the presence of pincher-like palps tipped with strong claws used for grasping fur (Fig. 34-7). The eggs are attached to the hair shaft of the animal. The most common sources of the infestation are long-haired cats and new puppies. Animals are often asymptomatic, but a white dandruff-like scale on their backs and necks is often seen on close examination. Small, yellow-white scales which are the mites themselves, are also seen, and hence the disease is called “walking dandruff.”
Figure 34-7. Cheyletiella mite resembles scabies in size but is identified by the hooklike palps anteriorly.
Other animals with mite infestations that affect humans include the following:
24. Where do bedbugs live? What do their bites look like?
The human bedbug, Cimex lectularius, is a reddish-brown, wingless insect resembling the size and shape of a ladybug that has been stepped on (Fig. 34-8A). They dine alone at night—rapidly and painlessly—but live gregariously during the day in dark closets, behind wallpaper, or under furniture and are not usually seen. Once thought to be associated only with unclean housing, bedbugs can be found in the most pristine homes and may be passively brought in on luggage, clothing, or secondhand furniture. Relatives of the human bedbug may be associated with bats or birds that live in or nearby the home.
The pruritic bites are often multiple and grouped into a “breakfast, lunch, and dinner” pattern (Fig. 34-8B). Treatment of the patient is symptomatic, but fumigation of the home is necessary to get rid of the pest. With a good description of the bug, patients are often able to recover one to confirm the diagnosis.
Thomas I, Kihiczak GG, Schwartz RA: Bedbug bites: a review, Int J Dermatol 43:430–433, 2004.
Kolb A, Needham GR, Neyman KM, et al: Bedbugs dermatologic therapy 22:347–352, 2009.
25. Do species of bedbugs that parasitize other animals bite humans?
Yes. Close relatives of the human bedbug often inhabit the nests of birds or bats (Fig. 34-9) that are located in the attic or eaves of homes. They may bite humans, particularly if the natural host leaves the nest. In fact, bird or bat bugs are much more common than human bugs.
26. What are zoonotic dermatoses?
Zoonotic dermatoses, also called “cryptic bites,” are caused by arthropods that infest animals. These include:
• Nesting birds: Swallows, sparrows, pigeons, starlings on or near the house; bites begin when the birds have fledged. Swallow bugs (Oeciacus vicarus, C. adjunctus) are not rare
27. What are the kissing, or assassin, bugs?
Assassin bugs are triatomids, a subfamily of the reduviids, of the order Hemiptera, or “true bugs,” and are generally adapted to nocturnal, rapid (3 to 5 minutes), painless feedings on blood. They live in rural settings, finding refuge in animal burrows, poorly constructed buildings, and occasionally drawn toward lighted homes at night in the southwest United States, Central America, and South America. The bites form red papules that can lead to allergic sensitization and moderately severe itching that last for weeks. Later, large patches of urticaria may develop, as well as systemic anaphylactic reaction.
28. Why are kissing bugs important?
In Central and South America, many triatomids, particularly the kissing bug, or binchugas, are infected with the etiologic agent of Chagas’ disease (American trypanosomiasis), which is spread to humans from contamination of broken skin with the bug’s feces. This triatomid has the unusual feature of defecating immediately after a blood meal so that the itch-induced scratching will contaminate the bite wound with the fecal material containing infectious organisms. It is a leading cause of heart disease in Central and South America and accounts for the death of a large number of young adults in South America. The organism, T. cruzi, also affects the esophagus, colon, and nervous system.
31. How does Lyme disease present?
The classic presentation is that of a person—in spring or early summer, living in an endemic area—with a history of a tick bite associated with fever and an annular, spreading rash called erythema chronicum migrans. Unfortunately, not all components are always present. As many as 50% of people with Lyme disease never develop the important sign of erythema chronicum migrans, and another 50% of the people do not recall a tick bite.
In addition to constitutional symptoms of general malaise, aches and pains, and fever, the disease may be complicated by cardiac, neurologic, and arthritic symptoms. Treatment in the early phase consists of antibiotics such as amoxicillin, erythromycin, or, most likely, doxycycline for a 3-week period of time. If Lyme disease progresses on to a more chronic problem, or there is evidence of neurologic symptoms, it is often necessary to use ceftriaxone in order to cross the blood–brain barrier.
33. How do tick bites affect humans? How are ticks removed once they are attached to the skin?
The tick bite may cause a foreign body reaction, allergic reaction to salivary proteins, reaction to a toxin (e.g., tick paralysis), or, more importantly, infectious disease carried by the tick (Fig. 34-10). Prompt removal of an attached tick may prevent transmission of potential tick-borne infectious disease. Suffocation techniques with petrolatum, heating with matches, or application of irritants should be avoided. Removal of the attached tick should be done by gentle, steady traction by blunt forceps or glove-protected fingers that grip the tick near the head’s attachment. Direct contact with the tick should be avoided to prevent contact with infectious organisms. The site and date of the tick removal should be recorded for future observations. The tick can be saved for identification and possible analysis for infectious disease.
Figure 34-10. Attached feeding tick with an allergic host response manifesting as erythema and induration.
(Courtesy of the Fitzsimons Army Medical Center teaching files.)
Gammons M, Salam G: Tick removal, Am Fam Physician 15:643–645, 2002.
34. How do you prevent tick bites?
• Wear protective clothing, including long-sleeved shirts and pants tucked into the socks. The clothing should be white or light-colored to facilitate close and frequent inspection for ticks.
36. What are the most effective insect repellents?
The most effective repellent for the prevention of bites of mosquitos, chiggers, blackflies, midges, and fleas is DEET. An 8% to 10% concentration is adequate for children and a 20% to 50% concentration for adults. A new preparation, extended-duration topical arthropod repellent (Editar), provides a slow release and is marketed by 3M as Ultrathon and by Amway as Hour Guard. Permethrin aerosol (Permanone) applied to clothing is the best tick repellent.
| INFECTIOUS DISEASE | VECTOR |
|---|---|
| Anaplasmosis (human granulocytotropic) | Hard ticks |
| Arboviruses (including yellow fever, dengue, encephalitis) | Mosquitos, ticks |
| Babesiosis | Hard ticks |
| Boutonneuse fever (tick-bite fever) (Rickettsia conorii) | Rabbit flea |
| Chagas disease | Triatomid (kissing) bugs |
| Colorado tick fever | Hard ticks |
| Ehrlichiosis, monocytotropic (Ehrlichia ewingii) | Hard ticks |
| Endemic relapsing fever (Borrelia duttoni) | Soft ticks |
| Epidemic relapsing fever (Borrelia recurrentis) | Human body lice |
| Epidemic typhus (R. prowazekii) | Human body lice |
| Filariasis (Wuchereria bancrofti, Brugia malayi) | Mosquitos |
| Leishmaniasis (Leishmania spp.) | Phlebotomid flies |
| Loiasis (Loa loa) | Tabanid flies |
| Lyme disease (Borrelia burgdorferi) | Hard ticks |
| Malaria (Plasmodium spp.) | Mosquitos |
| Murine typhus (R. mooseri) | Rat fleas, lice |
| Onchocerciasis (Onchocerca volvulus) | Black flies |
| Plague (Yersinia pestis) | Rat fleas |
| Q fever (Coxiella burnetii) | Hard ticks, fleas |
| Rickettsial pox (R. akari) | Mouse mites |
| Rocky Mountain spotted fever (R. rickettsii) | Hard ticks |
| Scrub typhus (R. tsutsugamushi) | Mites (chiggers) |
| Trypanosomiasis, African sleeping sickness | Glossina (tsetse) flies |
| West Nile fever | Mosquitos |
Adapted from Braunstein, WB: Ectoparasites. In Mandell GL, Bennett JE, Dolin R, editors: Principles and practice of infectious diseases, ed 6, Philadelphia, 2005, Elsevier, pp 3301–3302.
Elston DM: Insect repellents: an overview, J Am Acad Dermatol 38:644–645, 1998.
