Michael James Burns
This chapter reviews the recognition and clinical management of bites and infestations associated with arthropods that are members of Class Insecta (Hexapoda) (insects) and Class Arachnida, subclass Acari(ticks and mites). Exposure to these arthropods may result in minor annoying local reactions, systemic allergic reactions, secondary bacterial infections, and the transmission of a host of serious systemic infectious diseases.
BIOLOGY, GENERAL DESCRIPTION, AND CLINICAL PRESENTATION
Arthropods are invertebrates having an exoskeleton, segmented bodies, jointed appendages, and bilateral symmetry (1). Insects have six legs and three distinct body regions—head, thorax, and abdomen—and include cockroaches, termites, lice, the true bugs (bedbugs, wheel bugs, assassin or kissing bugs), fleas, flies (which include mosquitoes), beetles, ants, wasps, bees, and others. Ticks and mites have eight legs as adults and nymphs, with one disk-shaped body region and no true head. Tick and mite larvae have six or fewer legs. Adult ticks are pea-size, whereas nymphal ticks and adult mites are about the size of a grain of sand or smaller. The arthropods most important to humans are hematophagous, meaning they must feed on warm-blooded vertebrates for blood or tissue fluid, with mouthparts that are designed for probing and sucking blood and tissue fluids and saliva that contains substances that counteract host hemostasis.
Skin lesions from arthropod bites include wheals, papules, vesicles, and blisters which are produced by hypersensitivity reactions to salivary components or venom (1,2). Secondary bacterial infections can occur, and late cutaneous allergic responses may occur in atopic individuals and be confused with many infectious and noninfectious conditions.
BEDBUGS
Bedbugs (Cimex lectularius) are blood-sucking reddish-brown wingless insects about 5 mm long (adults), oval and flattened, smell like rotten raspberries, and can survive for long periods without feeding (1,3,4). The nymphal stages are smaller and also take blood meals. Except when venturing out to bite, they remain hidden on and in mattresses and in cracks and crevices of walls and floors near beds. Feeding is painless, occurs at night, and takes only 5 to 10 minutes, during which anticoagulants and vasodilators are injected. They often leave black specks on sheets and mattresses, a mix of feces, and heme. Common in tropical and underdeveloped countries, bedbug infestations have increased greatly in North America and Europe, not only in homeless shelters and places of overcrowding but in apartments, single-family homes, and upscale hotels. They are not known to transmit any disease. Bites may produce allergic reactions or no reaction at all. The typical presentation consists of extremely pruritic red papules or vesicles, sometimes painful, often in a linear or clustered pattern, most concentrated on skin that is exposed while sleeping. Anaphylaxis and urticarial and bullous lesions may occur. The rash is usually responsive to topical corticosteroids and oral antihistamines. Bedbugs are extremely difficult to eradicate. Resistance to insecticides is common. Excessive use of insecticides for treating bed bug infestations, usually pyrethroids and pyrethrins, their use contrary to label directions, failure to wash or change pesticide-treated bedding, and inadequate notification of pesticide application can cause human toxicity in the United States. The most frequent manifestations of toxicity are headache, dizziness, respiratory tract irritation, dyspnea, nausea, and vomiting (5).
CONENOSE OR KISSING BUGS
Triatomine insects (Hemiptera, Reduviidae), commonly known as conenose, kissing, assassin, or reduviid bugs, are 1 to 3 cm long, brown or black, and good fliers (1,6). They are widely distributed in the western and southwestern United States, Mexico, and Central and South America. Usually found in the wild, they can adapt to live in cracks and holes of substandard dwellings and come out at night to feed on human blood. Their bites are usually painless, but allergic reactions to their bites can result in papular or nodular lesions and even anaphylaxis. They have been reported to be a common cause of anaphylaxis of unknown etiology in children living in endemic areas of the United States (7). Some species can produce intensely painful bites with marked swelling at the bite site. Several members of this group (Triatoma, Rhodnius) transmit the protozoan parasite Trypanosoma cruzi (causative agent of American trypanosomiasis or Chagas disease) to humans. These insects are plentiful in southern Arizona and southern California and a high proportion in these regions have been found to be infected with T. cruzi (8). Wheel bugs (Arilus cristatus) are true bugs that bite humans only in self-defense (1). They are 3 cm long and gray, have a small narrow head and a cogwheel-like crest on their thorax, and are found in the southern United States. The bite causes immediate intense pain with swelling and inflammation, but they do not transmit any diseases. Blister beetles are large plant-feeding insects, commonly found in the southeastern, central, and southwestern United States, whose body fluids produce large painless blisters within a few hours after contact with live or dead beetles (1). Mild tingling or burning can occur prior to onset of blistering. The blisters are usually not serious and resolve within several days. Ingestion of beetles or their products may cause vomiting, abdominal cramps, and diarrhea.
CATERPILLARS AND CENTIPEDES
Some caterpillars have urticating hairs or spines that secrete a toxin when exposed to human skin, causing a burning sensation, swelling, numbness, or intense stabbing pain (1). Other caterpillars may cause a self-limited, pruritic, papular dermatitis after the skin comes in contact with hairs of the caterpillars. Outbreaks of caterpillar dermatitis have been reported among children at childcare facilities, and are frequently misdiagnosed by physicians as varicella, scabies, flea bites, mosquito bites, contact dermatitis, viral exanthems, and methicillin-resistant Staphylococcus aureus MRSA infections (9). Because the hairs or spines can remain embedded in the skin, repeated application of adhesive tape at the site to remove the spines may be needed, in addition to cold compresses, topical corticosteroids, and oral antihistamines.
Centipedes (Class Chilopoda), which are not insects or arachnids, can cause a painful “bite,” which can become red and swollen (1). The bite is produced and injected by sharp claws on their first pair of legs, which have attached venom glands. Treatment includes washing with soap and water, applying cool compresses, and oral analgesics, with anecdotal benefit after immersing the area in hot water.
FLEAS
Fleas feed exclusively on blood (1). The cat flea (Ctenocephalides felis), which also infests dogs and other mammals, is the predominant flea species in North America. Fleas, which are wingless and cannot fly, are easily recognized by their jumping behavior. Fleabites in humans cause allergic reactions and result in clusters or irregular groups of several to a dozen or more where the flea has explored and probed. In the allergic person, a pruritic wheal forms around each probe site within 5 to 30 minutes. Indurated papules and occasionally vesicular or bullous lesions develop within 12 to 24 hours, accompanied by intense itching, and may last a week or more. Fleas can transmit plague (Yersinia pestis), murine/endemic typhus (Rickettsia typhi and Rickettsia felis), and cat-scratch disease (Bartonella henselae). Children who ingest fleas containing the larval stage of the dog and cat tapeworm, Dipylidium caninum,may become infected with the adult tapeworm. The small female chigoe flea, Tunga penetrans, burrows into the skin of persons living or traveling in tropical and subtropical areas of North and South America, the West Indies, and Africa, producing a disorder known as tungiasis. It grows to about 1 cm within 2 weeks and lays eggs beneath the dermis. It commonly infests the legs and feet, including the areas between the toes or under the toenails, and produces intense inflammation and itching. One sees a black dot in the center of a pustule. These are surgically removed by widening the cavity and removing the flea intact.
FLIES, GNATS, MOSQUITOES
Black flies are smaller than mosquitoes, travel in large swarms, and are vicious biters (1). Allergic reactions to the bites result in red itchy papules and nodules with surrounding swelling and occasionally systemic allergic reactions with fever, wheezing, and diffuse urticaria. Deer and horse flies are large flies whose bites produce deep, painful wounds that often become secondarily infected (1). Bite lesions are solitary or scattered, not grouped or linear. Local and systemic allergic reactions can occur. Stable fly bites produce small papular lesions. Biting midges, sometimes called “no-see-ums” or “biting gnats,” are fierce biters and are so small that they can pass through ordinary screens. Their bites produce small papular lesions with red halos and wheals. Deer flies can transmit tularemia, but black, horse, and stable flies and biting midges do not transmit any diseases in North America.
Mosquito bites can result in allergic reactions, producing multiple, scattered lesions that are not linear or grouped (1). A wheal and flare may be seen 30 minutes after the bite, and delayed onset of pruritic papules is common. Mosquitoes are the most important arthropod vectors of disease in the world because of their transmission of malaria, filariasis, yellow fever, dengue, West Nile, and other encephalitis viruses (10,11).
MAGGOTS
Myiasis is an invasion or infestation by maggots, the larvae of two-winged flies (Diptera), and is most commonly found in debilitated or comatose persons, the homeless, alcoholics, those with peripheral vascular disease, or young children (1). It occurs when the female fly lays eggs near a wound or when the fly deposits freshly hatched larvae directly in a wound. The larvae of some fly species are able to penetrate skin and invade normal tissues, whereas others are capable of living only on the surface of the wound. Although usually occurring in wounds, myiasis can also occur in the nose, mouth, sinuses, eyes, anal region, vagina, and bladder. Myiasis is not contagious from person to person, as fly larvae are not capable of reproduction. Transmission occurs only via an adult female fly. Physical removal of each individual maggot, often surgically or by use of a large suction catheter, is the most effective treatment (1,12). If large numbers of maggots are found in a wound, soaking the area with vegetable oil followed by irrigation and then individual removal of remaining larvae may be effective. There are no medications or chemicals that will dislodge maggots. When exposed to a chemical insult, maggots retract into the wound, making it more difficult to remove them. Furuncular myiasis is produced by the larva of the human botfly (found in Mexico and Central and South America) and by the Tumbu fly (found in tropical Africa). The larvae burrow into host tissue, producing painful pruritic cutaneous nodules called warbles, each with a central opening or punctum. The person sometimes feels something moving. Visitors may acquire this disorder while traveling and return home before the maggot completes its development. These lesions are easily mistaken for staphylococcal boils. Furuncular myiasis is treated by surgical removal of the intact larvae or by applying petroleum jelly, suffocating the maggot, which may come wriggling out backward. “Bacon therapy” is also effective; the punctum is covered with raw meat, and in a few hours, the larvae migrate into the meat and are then easily extracted.
LICE AND MITES
Lice are tiny, elongate, soft-bodied, light-colored, wingless insects that feed on blood (1,13,14). Three species infest humans: (1) the human body louse (Pediculus humanus corporis), (2) the human head louse (Pediculus humanus capitis), and (3) the pubic or crab louse (Phthirus pubis). The body louse lives on clothing, lays its eggs (nits) on clothing, and leaves the clothing only to take a blood meal. It can transmit epidemic typhus (Rickettsia prowazekii), trench fever (Bartonella quintana), and louse-borne relapsing fever (Borrelia recurrentis). The head louse and pubic louse are not known to transmit any diseases. All lice are transmitted from person to person by close body contact, possibly by sharing of hats, combs, and brushes (head louse) or by sharing clothing (body louse). Humans infested with lice often develop itchy red papules with excoriated areas that can become secondarily infected. The adult lice are often difficult to see, but the nits of the head louse and pubic louse can be found attached on shafts of hair near the base. Head louse infestation is most common in white schoolgirls and affects all social and economic backgrounds. Pubic lice can sometimes be found in the eyebrows and eyelashes, in axillary hair, on the coarse trunk hairs of males and, rarely, in the scalp. Bluish-gray macules, known as blue spots, may sometimes be seen on the skin in pubic lice infestation. People infested with pubic lice often have other sexually transmitted diseases.
Mites look like tiny small hairy ticks, with one disk-shaped body segment and no true head. Many species bite humans that cause skin lesions from allergic reactions but the mites do not live on humans or their clothing (1). Chiggers is a condition caused by bites by the larvae of the chigger (or harvest) mite (Family Trombiculidae) (1). These mites are found in grasses and scrubs, especially at forest edges. Chigger mite larvae, which are tiny (0.2 mm long); round; and red, yellow, or orange in color, bite humans and inject saliva that dissolves host tissue, then feed on the dissolved tissue. Pruritic macules and wheals develop within 3 to 6 hours, followed by intensely pruritic papules and pustules at 10 to 16 hours. Lesions are often linear or grouped. Numerous other species of mites will bite humans, causing painful pruritic red papules. Persons bitten by chicken mites and straw itch mites may have thousands of lesions. The tiny itch mite Pyemotes herfsi has recently caused outbreaks affecting thousands of people in Kansas, Nebraska, and Missouri; a person may have hundreds of bites resulting in a severe pruritic papular dermatitis. The house mouse mite transmits rickettsialpox (Rickettsia akari) to humans in the United States. Unlike the scabies mite, none of these mites live on humans.
SCABIES
Scabies, an infestation of the epidermis by Sarcoptes scabiei var. hominis, which spends its entire life on human skin, is transmitted by close body contact (15). The fertilized female mite, too small to see with the naked eye, burrows into the epidermis, and lays two to three eggs daily for 30 days (1). Larvae hatch in 3 to 4 days and migrate to the skin surface. Maturation from egg to adult takes about 10 to 14 days. The mites feed on cells of the stratum corneum. Their secretions and excretions sensitize the host and produce irritation and itching with pruritic crusted red papules, papulovesicles, pustules, and weeping eczematous lesions. Transmission is by direct skin-to-skin contact. Transmission by infested bedding, towels, or clothing is uncommon except in the case of crusted (hyperinfestation) scabies. The incubation period after infestation until symptoms appear is about 3 to 6 weeks for a first infestation but may be much shorter (as little as 48 hours) in recurrent infestations. A person may be very contagious before any symptoms or lesions even occur. There are 5 to 20 mites on the body in a typical infestation, usually found in burrows on the hands, wrists, elbows, knees, beltline, penis, the breasts of females, and the feet of children, but they are especially common in the webs of the fingers and the folds of the wrists. In infants, lesions are frequently found on the scalp, face, palms, and soles. Burrows are often difficult to find. Itching commonly occurs in locations where there are no mites present. Extensive excoriations and secondary bacterial infections can cause puzzling skin lesions. Diagnosis is made by gentle scraping of the burrows with a scalpel blade covered with mineral oil. The mineral oil and scraped material are transferred to a glass slide, a coverslip is applied, and the slide is examined under a microscope for mites, eggs, or feces. Persons who are immunocompromised (e.g., AIDS) or debilitated (especially elderly or institutionalized persons) may develop crusted hyperinfestation also called “Norwegian” scabies, in which hyperkeratotic crusted nodules and plaques become widespread on the body. Millions of mites may be present in this condition, making it extremely contagious. It is easily misdiagnosed as eczema or another condition. Other closely related scabies mites infest domestic animals, especially dogs (sarcoptic mange). These mites can bite humans and produce pruritic papular lesions without burrows. They cannot complete their life cycle on humans, so no specific anti-scabies treatment is needed (1).
TICKS
Among the biting arthropods, ticks are second only to mosquitoes in the number of pathogens transmitted to humans. There are four stages in the life cycle: egg, larva, nymph, and adult. Even in highly endemic areas, ticks are not evenly distributed. Ixodes scapularis is most likely to be found in brushy areas at the interface between fields and forests (forest boundaries), whereas Dermacentor variabilisprefers meadows and roadsides. They “quest” for host animals by climbing vegetation (few inches to 3 ft) and passively wait for a host to pass by. Ticks cannot fly, jump, or swim. Larvae, nymphs, and adult females (but not adult males) increase in size enormously during feeding. They are capable of withstanding environmental stresses, and adults may live for several years. The soft-bodied ticks feed and engorge within a few minutes. The hard-bodied ticks attach for many days; once attached, feeding (and transmission of disease) may not begin for several days. When feeding, they excrete large amounts of saliva and feces containing pathogens. Tick bites are usually painless, and there is often no reaction at the bite site after the tick is removed or drops off. Sometimes, a hypersensitivity reaction to the bite occurs, or a red nodule may persist at the bite site for weeks to months (2).
I. scapularis (deer tick or black-legged tick), transmits Lyme disease (Borrelia burgdorferi), anaplasmosis (Anaplasma phagocytophilum, a rickettsia-like organism), babesiosis (Babesia microti, a malaria-like protozoan), and Powassan disease (a type of viral encephalitis) to humans (1,16). Lyme disease is most common in the northeastern and upper Midwest United States because the primary host for the tick’s nymphal stage in these regions, the white-footed mouse, is capable of high rates of spirochetemia and is an excellent transmitter to ticks. The nymph, not the adult tick, most often transmits Lyme disease to humans; the nymphs are more likely to remain attached undetected for long periods of time because of their small size (about the size of a poppy seed or the head of a pin). Adult ticks, which primarily feed on deer and are the size of an apple seed, are also capable of transmitting infection but are more easily seen and removed before disease transmission occurs. Lyme transmission requires >24 hours of attachment for nymphs and 36 hours for adults. Ixodes pacificus (western black-legged tick), found in the western United States, transmits Lyme disease (but infection rates in the tick are very low owing to their feeding behavior) and anaplasmosis. Both nymphal I. scapularis and I. pacificus ticks in the southern and western United States have very low rates of infection with the Lyme disease organism (1). They feed primarily on lizards, which are incapable of infecting ticks, and they rarely bite humans. Lyme transmission from I. pacificus requires >96 hours of attachment for nymphs. D. variabilis (American dog tick) transmits Rocky Mountain spotted fever (Rickettsia rickettsii) in the eastern United States, with disease transmission requiring >24 hours of attachment (1,16). Only adults feed on humans. Only 1% to 5% of ticks in an endemic area are infected with the rickettsial organism. This tick also transmits tularemia (Francisella tularensis) and can cause tick paralysis (owing to a neurotoxin secreted by the tick). For tick paralysis to develop, the tick must remain attached for at least 5 to 6 days. Dermacentor andersoni (Rocky Mountain wood tick) transmits Rocky Mountain spotted fever in the western United States, tularemia, and Colorado tick fever (Coltivirus) and can cause tick paralysis. Amblyomma americanum (Lone Star tick), whose primary host is the deer, transmits tularemia, ehrlichiosis (Ehrlichia chaffeensis, a rickettsia-like organism), and STARI (“Southern tick-associated rash illness”), a newly described Lyme-like disease in the southeastern United States, whose cause is undetermined (1,16). People disrobing or lying on the ground in heavily infested wooded areas may be attacked in great numbers by larval A. americanum ticks, causing hundreds of pruritic papules (17). Amblyomma maculatum (Gulf coast tick), common in the southeastern United States, has been reported to transmit Rickettsia parkeri, a newly described spotted-fever-group rickettsia producing a mild febrile illness accompanied by multiple eschars and a maculopapular eruption (1,16). The brown dog tick, Rhipicephalus sanguineus, transmits Rocky Mountain spotted fever in eastern Arizona (16) Q fever (Coxiella burnetii) can be acquired from inhalation of dead tick parts or feces in dust.
Ixodes, Dermacentor, and Amblyomma spp. are all hard-bodied ticks, and prolonged attachment is required for feeding and disease transmission. Hard ticks will remain attached for 7 to 14 days before dropping off (1).
In contrast, soft-bodied Argasidae ticks of the genus Ornithodoros bite, feed, and drop off within a few minutes and prefer animal burrows or manmade animal or human shelters (1,16). Nymphs and adults transmit tick-borne relapsing fever (Borrelia hermsii and other spp.) in rodent-infested cabins in the high mountains of the western United States, Canada, and Mexico. Humans are bitten when they sleep in infested cabins.
Other tick species in the United States bite humans but do not transmit disease (1). Ticks in other parts of the world transmit a host of diseases that are important to travelers, including rickettsial diseases such as Boutonneuse fever and African tick bite fever, and arboviruses causing encephalitis and hemorrhagic fever (11).
Table 374.1 lists the biting arthropods of medical importance in North America, their geographic distribution, and the diseases they transmit.
TABLE 374.1
Diseases Transmitted by Biting Arthropods in North America
DIFFERENTIAL DIAGNOSIS
Reactions to arthropod bites are difficult to diagnose, as many patients may be unaware or cannot remember any exposure. The pruritic papules, vesicles, pustules, nodules, and bullae resulting from allergic reactions to arthropod bites may easily be misdiagnosed as allergic or irritant reactions to drugs or chemicals, atopic or contact dermatitis, Herpes simplex, staphylococcal and other bacterial infections, or cutaneous fungal infections (1,2). Multiple linear papules or vesicles should suggest bites by bedbugs or chigger mites, whereas irregular groups of papules and vesicles are typical for fleas and chiggers. Lesions caused by mosquitoes are multiple and scattered, not linear or grouped. A deer or horse fly bite that becomes secondarily infected and forms an ulcer could be misdiagnosed as a brown recluse spider bite. Arthropod bites and infestations should also be considered in the differential diagnosis of any patient complaining of itching. “Insect bites” should generally not be diagnosed upon the basis of lesions alone. Scabies should be considered in the differential diagnosis of patients presenting with scattered pruritic papules, especially when found on the hands, wrists, elbows, and genital areas. Crusted “Norwegian” scabies should be considered in the differential diagnosis for immunosuppressed or debilitated patients presenting with subacute or chronic widespread hyperkeratotic plaques or eczematous lesions, even in the absence of pruritus. Scabies can easily be misdiagnosed as eczema, contact dermatitis, psoriasis, fungal infection, adverse drug reaction, senile pruritus of the elderly, “pruritus of AIDS,” and many other conditions.
An arthropod-borne infection should always be considered in the differential diagnosis of persons presenting with an acute febrile illness (10). The patient should be questioned about travel to endemic areas or outdoor exposures. Lyme disease or rickettsial infections may but not always, have a characteristic rash (11,16). Tick paralysis, which may be confused with Guillain–Barré syndrome, botulism, or spinal cord compression, presents initially with restlessness, irritability, paresthesias, and ataxia followed by rapidly progressive ascending flaccid paralysis, with a normal sensory examination, a clear sensorium, and lack of fever. Tick paralysis is especially likely in children and in girls, in whom the ticks are often concealed in long hair at the nape of the neck. Patients with pubic lice should be screened for sexually transmitted diseases.
Patients suffering from delusions of “parasitosis” have an unwarranted belief that live organisms, such as mites or insects, are present on the body (18,19). The typical patient is a well-educated, responsible, hard-working woman who may have an excessive preoccupation with cleanliness and has a single delusion and no other mental disorder. This belief often persists for many years despite numerous physician visits and repeated treatments of the house with pesticides and the patient by medication, usually physician-prescribed. Typical characteristics of the “bugs” include that they change color; they jump; they appear and disappear while being watched; they enter the skin and reappear; they invade the hair; nose and ears; they infest the patient’s hair, and can be combed or shaken onto a sheet or towel; and they sometimes come out of common household items (toothpaste, cosmetics) or teeth after brushing. Family members often support the patient but are never afflicted. Patients typically bring in samples of the supposed pest for examination, which consist of small bits of lint, cloth, dandruff, or scabs. These patients should be referred to a dermatologist to definitively rule out an infestation and to a psychiatrist, although the patients will generally not accept any suggestion that this is “all in their head.” An entomologist may also need to be consulted (1). Delusions of parasitosis known as “cocaine bugs” may also be seen in persons abusing cocaine and amphetamines and in patients with schizophrenia, depression, anxiety disorders, and neurologic disorders.
ED EVALUATION
The emergency physician should be familiar with the biting insect, mite, and tick fauna in the local area. A complete history should include travel, outdoor activities, any known exposures to biting arthropods, and medications already used as these could affect or change the appearance of lesions (1). Patients complaining of itching or papules in the scalp or genital area should be asked about household or sexual contacts that may have head lice or pubic lice and should be examined carefully for adult lice and nits attached to the hair shafts. The skin should be examined carefully for evidence of bites, allergic reactions, secondary bacterial infections, burrows of scabies, or rashes typical for infections transmitted by arthropods.
KEY TESTING
• Skin scrapings for scabies
• Low power microscopic examination of suspected lice
• Serologic testing and stains for various virus, rickettsia, bacteria, and protozoa causing systemic illness
• Skin examination for embedded ticks
ED MANAGEMENT
Pruritic skin lesions should be treated with cold compresses, soothing lotions, topical corticosteroids, and systemic antihistamines. Systemic allergic reactions may require treatment with epinephrine, intravenous fluids, systemic corticosteroids, and antihistamines. Secondary bacterial infections, often seen with deer or horse fly bites or with excoriated mosquito or flea bites, are usually caused by S. aureusor Streptococcus pyogenes and should be treated with appropriate antimicrobial agents. A wound culture should be obtained for serious local infections, and empiric therapy for MRSA should be considered, and tetanus immunization should be given if needed. Patients who present with undifferentiated fever with or without generalized rash, rash without fever, or signs of encephalitis after an arthropod bite may require investigation for one of the many viral, rickettsial, or bacterial infections transmitted by these creatures (10,11).
Resistance of lice to topical pediculicides has increased dramatically (13,14). The Centers for Disease Control and Prevention recommends treatment of head lice only if live lice are detected or if nits are found attached to hair shafts less than one-quarter inch from the scalp (14). Of the topical agents available for treatment, malathion lotion 0.5% (not recommended in children younger than 2 years of age), and ivermectin lotion 0.5% (not recommended for children younger than 6 months of age), are the most effective; lindane is the least effective; and permethrin and pyrethrins are intermediate in effectiveness (20,21). If pyrethrins (combined with piperonyl butoxide) or permethrin lotion 1%, both available without prescription in the United States, are used, a second application should be done 7 to 10 days later. A second application of malathion may be needed 7 days later. Use of a fine-toothed comb on wet hair is recommended to remove nits. Lindane, which is banned in California, should be avoided because of neurotoxicity, high resistance rates, and the potential for contaminating the environment. Two other topical agents, benzyl alcohol lotion 5%, and spinosad suspension 0.9%, available by prescription only in the United States, are also effective, but repeat treatments may be necessary. Oral ivermectin (not FDA-approved for this indication) is an alternative when topical treatments fail (22). Mechanical removal of lice with repeated wet combing every 3 to 4 days by parents is an alternative to malathion but is not nearly as effective. “Natural” herbal and chemical products marketed for treatment of lice are not effective and may be unsafe. Bedding, clothing, and towels used by the infested person in the 2 days prior to treatment should be washed and dried at high temperatures, dry cleaned, or kept in sealed plastic bags for 2 weeks. Household contacts of patients with head lice and sexual partners of those with pubic lice should be examined for lice and nits every 2 to 3 days and treated only if crawling lice or nits within one-quarter inch of the skin are found. Many schools routinely exclude children for head lice, but this practice is not recommended by the American Public Health Association.
Scabies should be treated with permethrin 5% cream applied to all areas of the body from the neck down and washed off after 8 to 14 hours, usually with a second treatment 1 week later recommended (15). Topical crotamiton 10% cream is safe for newborn babies and infants but is less effective than permethrin. Lindane-resistant scabies is now widespread in the United States. Ivermectin 200 μg/kg orally in a single dose for adults is effective in resistant cases, with a second dose 2 weeks later usually recommended (15,21,23). It is not FDA-approved for this indication, but its use is recommended by the Centers for Disease Control and Prevention, and by The Medical Letter, as an alternative agent. Crusted (“Norwegian”) scabies in debilitated or immunocompromised patients should be treated with topical keratolytic agents followed by repeated doses of total-body permethrin cream and oral ivermectin. An alternative treatment for pregnant and lactating women and children younger than 2 months is precipitated sulfur 6% in petrolatum, but there is little evidence for efficacy or safety. Itching may continue for up to 4 weeks after curative therapy and is not an indication for repeat treatment. Bedding, clothing, and towels used in the previous 48 to 72 hours should be washed and dried at high temperatures or dry cleaned or stored in sealed plastic bags for the same period of time.
The best method for preventing infection with tick-borne pathogens is to avoid exposure to ticks. If exposure is unavoidable, measures to reduce the risk of infection include the use of protective clothing and tick repellents, checking the entire body daily for ticks, and prompt removal of attached ticks. To remove an attached adult tick, the tick should be grasped with blunt curved forceps, narrow tweezers, or gloved fingers as close to the point of attachment as possible, then pulled firmly and steadily in the direction of attachment. Some back-and-forth motion may be needed, but the tick should not be twisted. If the tick’s head breaks off in the skin, use tweezers to remove it. Disinfect the bite site and wash hands thoroughly. This method is most effective for the large adults of Dermacentor and Amblyomma tick species but may not be effective for adult Ixodes ticks or larval ticks of any species, which are much smaller. Application of viscous lidocaine 2% may be effective for these smaller adults or larval ticks. Application of isopropyl alcohol, petroleum jelly, a hot match, fingernail polish, or use of sharp forceps should be avoided, as this may cause the tick to salivate or regurgitate and increase the potential for disease transmission. Two other anecdotal methods of tick removal have been suggested: using a disposable razor to shave off the tick’s body without squeezing the tick, and using cryospray with liquid nitrogen to instantly freeze the tick, then wiping it off or removing it in toto, but these methods and application of viscous lidocaine have not been subjected to clinical trials (24). An entomologist should be contacted if it is important to identify a tick or other biting arthropod that has bitten a person. The Centers for Disease Control and Prevention website on ticks at www.cdc.gov/ticks is an excellent resource on ticks found in the United States, including color pictures, geographic distributions, diseases transmitted by ticks, tick avoidance, tick removal methods, and tick life cycles.
Although some practicing physicians routinely prescribe antimicrobial agents to persons who have been bitten by ticks in areas highly endemic for Lyme disease, most experts believe that routine antimicrobial prophylaxis or serologic testing after a tick bite is not necessary (1,25). The risk of acquiring Lyme disease or any other tick-borne infection after a tick bite is quite low, even in highly endemic areas. Doxycycline 200 mg orally in a single dose for adults may be effective in preventing Lyme disease in a highly endemic area if the tick is at least partially engorged or attached for greater than 72 hours. The Infectious Diseases Society of America recommends prophylaxis with single-dose doxycycline 200 mg for nonpregnant adults and children at least 8 years of age (4 mg/kg for children; maximum, 200 mg) only when all of the following are met: (a) The tick can be identified as an adult or nymphal I. scapularis tick that is likely to have been attached for >36 hours based on the degree of tick engorgement with blood or confidence about the time of exposure to the tick; (b) prophylaxis can be started within 72 hours of the time that the tick was removed; (c) the local rate of infection of ticks with B. burgdorferi is >20%; and (d) doxycycline is not contraindicated (25). Infection of >20% of ticks with B. burgdorferi is found in parts of New England, the mid-Atlantic states, Minnesota, and Wisconsin but is rare in other locations in the United States. Although amoxicillin is an effective treatment for early Lyme disease and is the preferred agent for pregnant women and children younger than age 8, there is no evidence that it is an effective preventive agent after a tick bite. There is no evidence that antimicrobial treatment is effective in preventing other tick-borne infections in the United States, and it is not recommended.
Tick paralysis is treated supportively. A rapid and complete recovery occurs after removal of the tick.
CRITICAL INTERVENTIONS
• Recognize that serious allergic reactions including anaphylaxis may be the result of arthropod bites, even when no arthropod was seen.
• Check the skin and scalp for ticks in patients with rapidly progressive ascending flaccid paralysis, particularly children and those with a normal sensory examination, clear sensorium, and no fever.
• Treat secondary bacterial infections with appropriate antimicrobials, with consideration for community-associated MRSA.
• Institute infection control measures and offer treatment to those who have had close unprotected contact with patients with scabies.
• Consider an arthropod vector-borne disease in the differential diagnosis of an acute febrile illness in the appropriate circumstances, regardless of whether the patient has a rash or remembers a bite.
DISPOSITION
Almost all patients suffering from arthropod bites can be discharged unless they have life-threatening anaphylaxis or serious secondary bacterial infections of skin lesions or are suspected to have a serious arthropod-borne systemic infectious disease or tick paralysis. Patients should be advised to avoid tick-infested areas. If exposure is unavoidable, the use of protective clothing, wearing light-colored clothing (making it easier to spot ticks), daily inspections of the entire body with prompt removal of ticks, and application of tick and insect repellants to skin and clothing are recommended (1). Permethrin aerosol spray is very effective when used on clothing and tents but should not be applied to the skin. DEET is the most effective insect repellent that can be applied to the skin, and it repels a variety of mosquitoes, chiggers, ticks, fleas, and biting flies (1). Picaridin is an alternative effective insect repellent. Persons who have been bitten by ticks should be advised to promptly seek medical attention if they develop a fever or a skin lesion at the site of the bite up to 30 days later.
Common Pitfalls
• Failure to obtain a history of travel or exposure to environments where biting arthropods are present.
• Failure to recognize that an acute febrile illness, with or without generalized rash, may be caused by an arthropod vector-borne infection.
• Failure to screen for sexually transmitted diseases in persons with pubic lice.
• Misdiagnosis of scabies as eczema, contact dermatitis, fungal infection, psoriasis, nonspecific pruritus or other conditions, and failure to treat household contacts of patients with scabies.
• Routinely prescribing antibiotics to patients with reported tick bites.
REFERENCES
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