INTRODUCTION

Section 2 of 11  

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• Clinical
• Differentials
• Workup
• Treatment
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Background

Insect bites are an unpleasant fact of life in most parts of the world. Bite reactions typically present as intensely pruritic erythematous papules that commonly are excoriated. Vesicular and bullous reactions are not uncommon, and large pseudolymphomatous nodules may occur. Systemic reactions to the insect order Hymenoptera (bees, hornets, wasps, yellow jackets, ants) include fatal anaphylaxis.

Bites are not merely a nuisance; arthropods commonly serve as disease vectors. Malaria, leishmaniasis, onchocerciasis, filariasis, and rickettsial diseases remain primary public health problems in many parts of the world. Control of arthropod vectors and personal protection with repellents are important steps in controlling the spread of arthropod-borne disease.

Pathophysiology

The physical insult of an arthropod bite or sting causes little injury. Instead, lesions occur as a result of the body's immune response to antigens introduced by the bite or sting. The time course of insect bite reactions reflects the immune mechanism involved. Immediate hivelike skin lesions reflect hypersensitivity to the bite, mediated by immunoglobulin E (IgE). Delayed pruritic papules, nodules, and vesicles usually become symptomatic within 48 hours following the bite. They are manifestations of a delayed hypersensitivity (type IV cell-mediated immunity) to antigens introduced during the bite.

Less commonly, lesions occur as a result of toxins introduced by the bite or sting (eg, brown recluse spider bites). The extensive tissue necrosis that can follow a brown recluse bite is the result of endothelial injury mediated by neutrophils. Sphingomyelinase D is the toxin responsible for initiating the neutrophilic reaction. Hyaluronidase contained within the venom dissolves dermal ground substance and allows the toxin to spread. After envenomation, necrosis usually spreads in a dependent fashion as hyaluronidase dissolves the gelatinous barrier of mucopolysaccharides within the skin and the force of gravity causes the toxin to spread.

Frequency

United States

Bites and stings are a problem throughout the United States. In the Midwest and along parts of the East Coast, mosquitoes and biting flies as well as ticks account for most bites. In arid areas, including much of the southwest and parts of California, flying insects are less common, and crawling arthropods are the primary cause of bites and stings. Arthropod vectors commonly spread disease.

Rocky Mountain spotted fever (RMSF) is carried by a variety of ticks. The highest incidence of disease transmission is noted on the East Coast of the United States, especially North Carolina. Dermacentor variabilis, a hard tick with a highly ornate scutum, is the primary vector of the disease in the United States.

Formerly exotic diseases, such as West Nile fever and Dengue fever, are now found in parts of the United States and are carried by mosquito vectors.

In parts of south Texas, endemic typhus is carried by opossums and is spread by cat fleas that feed on the opossums and then bite humans.

Lyme disease, babesiosis, and human anaplasmosis (human granulocytic ehrlichiosis) are carried by the Ixodes species of hard ticks. In parts of the northeastern United States, 90% of Ixodes scapularis ticks carry the Lyme disease spirochete.

Fire ants are ferocious stingers that swarm and sting in great numbers when their mounds are disturbed. They occur throughout the warmer areas of the southern United States and are predicted to spread as far north as the lower Mid-Atlantic states. One positive aspect concerning the spread of fire ants is the observation that tick populations decline dramatically when fire ants are present. Ticks lay their eggs directly on the ground where fire ants forage in great numbers, consuming most tick eggs.

Other arthropod-borne diseases in the United States include tularemia (transmitted by Dermacentor and Amblyomma tick and Chrysops deer fly species) and cat-scratch disease (which appears to be spread by cat fleas).

Liponyssoides mouse mites spread rickettsial pox in New York City. Viral encephalitis is spread by mosquitoes in many parts of the country.

International

Throughout the world, arthropods are important vectors of disease.

Arthropods have been implicated as vectors in most of the great epidemics that have afflicted humankind. As an example, plague is spread by fleas. In nature, reservoirs of disease exist in the rodent population, and disease is spread sporadically to humans by the bites of rat fleas. In the crowded cities of Europe, the human flea probably was at least partially responsible for the spread of the disease.

Worldwide, malaria remains a huge public health problem. Anopheles mosquitoes are the vectors of this protozoan disease.

Sandflies are important vectors of diseases such as leishmaniasis and bartonellosis. Leishmaniasis commonly presents as an indolent poorly healing ulcer on an exposed area. In areas of the New World, especially Brazil, leishmaniasis caused by Leishmania braziliensis may recur as espundia, with massive destruction of the central face and nasopharynx. Old world leishmanial ulcers caused by Leishmania tropica do not carry a risk of late mucocutaneous disease but may be associated with mild visceral disease. In the Mediterranean region, Leishmania infantum causes a form of visceral leishmaniasis much milder than the severe visceral leishmaniasis found in India.

Bartonellosis is found in the Peruvian Andes and is of interest because it is the oldest described form of bacillary angiomatosis. A closely related family of Bartonella bacteria causes verruga peruana (Peruvian bacillary angiomatosis), cat-scratch disease, bacillary angiomatosis of AIDS, and endocarditis in the homeless. In addition to the sandfly, lice and fleas are important vectors for these diseases. The varied manifestations of these diseases may result partially from interactions between the organism and vector, ie, bartonellosis transmitted by fleas appears to cause a different disease than bartonellosis transmitted by lice. In refugee populations, important louse-borne diseases include trench fever and typhus.

Rickettsial fevers in Europe and Africa are carried by Rhipicephalus, Ixodes, and Dermacentor tick species. Scrub typhus in Asia is transmitted by Trombidiidae chigger mites. Dengue fever and yellow fever are spread by Aedes mosquitoes. Onchocerca volvulus, a worm transmitted by black flies of the family Simuliidae, is a primary cause of blindness in Africa. Elephantiasis in Africa and East Asia is caused by filarial worms transmitted by Culex, anopheline, aedean, and mansonian mosquitoes.

Mortality/Morbidity

The importance of arthropods as vectors of disease cannot be overstated. Lyme disease, RMSF, and mosquito-borne encephalitis are major public health problems in the United States. In Asia, Africa, and Latin America, arthropod-borne diseases remain a greater problem.

Race

Little data are available on racial differences in insect bite reactions. Clearly, certain individuals are more attractive to biting arthropods, but these differences appear to relate to body heat, body odor, the use of fragrances, and carbon dioxide excretion and do not appear to be race related.

Differences in individual reactions to arthropods are a reflection of immune status, not race. Dark-skinned races are more likely to be deficient in the enzyme glucose-6-phosphate dehydrogenase (G-6-PD). Inherited deficiencies of this enzyme make dapsone therapy for brown recluse spider bites hazardous.

Sex

Little evidence exists that sex is responsible for differences in reactions to arthropod bites.

Age

Children may be more susceptible to the effects of black widow spider bites. The American Academy of Pediatrics recommends that diethyltoluamide (DEET) concentrations not exceed 30% in insect repellent products used on children and that the lowest effective concentration should be used.

CLINICAL

Section 3 of 11  

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History

Accurate identification of the arthropod can be important in making decisions concerning prophylactic antibiotic treatment. As a public health measure, arthropod populations of medical importance are monitored.

Patients usually are not well trained at identifying ticks and spiders; therefore, it is helpful if the physician is better trained.

Physical

Insect bites typically appear as pruritic papules, grouped in areas where the bites occur. For instance, bedbug bites are most common on the head and neck, while fleabites usually occur on the legs. A linear arrangement of 3 bites in a row is commonly a sign of bedbug (Cimex) bites. Cimex species commonly infest birds' nests and bats' roosts. After seasonal migrations of their preferred hosts, bugs wander into houses in search of alternate food sources.

Vesicular and bullous bite reactions are common. Intense pruritus and a distribution in exposed areas suggest the diagnosis of a bullous bite reaction.

Causes

Many arthropods bite and sting. Accurate identification of the offending arthropod is critical in brown recluse and black widow spider bites, Centruroides exilicauda and the closely related Centruroides sculpturatus scorpion stings, and when considering antibiotic prophylaxis for an arthropod-borne disease.

• Spiders: The brown recluse is a small brown spider with a large leg span and a dark brown "violin case" pattern on the cephalothorax. The black widow is a large shiny black spider with a red hourglass on the ventral abdomen.
• C exilicauda (formerly C sculpturatus): This scorpion, considered the only lethal scorpion in the United States, is found in Arizona and northern Mexico. It is a small tan-to-buff scorpion with long tail segments and a blunt thorn on the tail. "Racing stripes" may be present.
• Ticks: Hard ticks have a hard dorsal scutum and anterior-facing mouthparts. They live free in the environment and take long blood meals during each of 3 life stages. Soft ticks lack a scutum and have retroverted mouthparts. They live in close association with an animal host and take frequent blood meals from the host.
• • Ixodes hard ticks are vectors for Lyme disease, babesiosis, and human granulocytic ehrlichiosis. I scapularis is a small brown tear-shaped tick with an inornate scutum. The legs usually are black, and an anterior anal groove is present on the ventral side.
  • Dermacentor hard ticks include both Dermacentor andersoni and D variabilis. They have a highly ornate scutum. D andersoni is found in the Rocky Mountains and D variabilis in most of the rest of the United States, especially on the East Coast. These ticks carry RMSF and tularemia.
  • Amblyomma americanum is a hard tick with long mouthparts and an ornate scutum with a single iridescent white spot in the female and horseshoe-shaped markings in the male. These ticks are vectors for RMSF, tularemia, and ehrlichiosis.
  • Rhipicephalus sanguineus, the brown dog tick, has an inornate scutum, brown legs, and short anterior mouthparts attached at a widened diamond-shaped base. These ticks transmit RMSF, ehrlichiosis, and boutonneuse (rickettsial) fever.
  • Ornithodoros is the major soft tick that serves as a disease vector. These ticks have retroverted mouthparts, lack a scutum, and are vectors of borrelial relapsing fever.

DIFFERENTIALS

Section 4 of 11  

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Other Problems to be Considered

Bullous bite reactions may mimic immunobullous diseases. In particular, scabies may mimic bullous pemphigoid and yield false-positive immunofluorescence results.

WORKUP

Section 5 of 11  

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Lab Studies

• Radioallergosorbent test can confirm sensitivity to Hymenoptera.
• Serologic tests can help in the diagnosis of arthropod-borne diseases. In one study, Amblyomma americanum ticks from 9 states were found to harbor Ehrlichia chaffeensis, Ehrlichia ewingii, Borrelia lonestari, or Rickettsia amblyommii in up to 84% of ticks. Lice carry trench fever caused by Bartonella quintana, epidemic typhus caused by Rickettsia prowazekii, and relapsing fever caused by Borrelia recurrentis. Fleas carry plague, murine typhus, and cat scratch disease.
• Blood counts and assessment of coagulation and fibrin split products help evaluate disseminated intravascular coagulation (DIC) after brown recluse spider bites.

Procedures

• Tick extraction is accomplished best by grasping the tick near the head and pulling or twisting the tick gently from the skin. Take care not to squeeze or break the tick. A plastic device termed a tick nipper is effective.

Histologic Findings

Arthropod bites and stings typically produce a wedge-shaped perivascular infiltrate rich in lymphocytes and eosinophils. Lymphocytes can be large and atypical and mimic lymphoma. Bullous bite reactions create an inflammatory subepidermal bulla.

TREATMENT

Section 6 of 11  

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Medical Care

Typically, bites are managed with topical antipruritics, such as camphor and menthol, and with topical corticosteroids.

• Treatment of stings: Symptomatic treatment alone may be required unless anaphylaxis occurs.
  • Consider desensitization in patients with a documented history of anaphylaxis. Desensitization improves quality of life. Therefore, patients with a history of anaphylaxis should be referred to an allergist.
  • Advise these patients to carry an injectable source of epinephrine (adrenaline).
  • Antihistamines may benefit patients some but are not a substitute for epinephrine.
  • Two available forms of injectable epinephrine are (1) the EpiPen, which is an autoinjector that injects an intramuscular dose of 0.3 mL of epinephrine 1:1000, and (2) an EpiPen Jr that delivers a similar dose of epinephrine 1:2000.
• Treatment of bites: Most insect bites respond to topical or intralesional corticosteroids.
  • Topical steroids vary in strength.
  • The most potent corticosteroids are superior in efficacy but present a risk of atrophy and systemic absorption if used for extended periods (usually > 3 wk).
  • Topical antipruritics (eg, camphor, menthol) can be helpful.
  • Topical anesthetics also can help. For example, pramoxine has a long history of safe use and little tendency to cause allergic reactions.
• Spider bites: Reactions to spider bites depend on the type of spider, the age or size of the patient, and amount of venom injected. Most spider bite reactions are similar to other arthropod bites and require no specific treatment.
• • Brown recluse spider bites can result in extensive necrosis of skin, soft tissue, and muscle.
    • Venom contains a toxin that causes neutrophils to damage the endothelial cells of blood vessels.
    • Thrombosis of the vessel and necrosis of tissue ensue.
    • Minor-appearing wounds may be associated with systemic absorption of toxin with diffuse endothelial injury and with disseminated intravascular coagulation (DIC).
    • Apply ice to brown recluse bites.
    • Use surgical debridement conservatively.
    • Antibiotics may help if resulting wounds become infected.
    • Use of systemic steroids and dapsone has yielded inconsistent and often disappointing results. Randomized controlled data in a rabbit model suggest dapsone has no beneficial effect. Intralesional triamcinolone may be of some help.
  • Black widow bites cause abdominal pain that resembles an acute abdomen.
    • If the bite is recognized, unnecessary abdominal surgery can be avoided.
    • Narcotic analgesia, intravenous calcium gluconate, benzodiazepines, and antivenin can help management.
• Antibiotic prophylaxis after tick bites: Prophylactic antibiotic use for the prevention of tick-borne disease is controversial.
  • Prophylactic antibiotics can prevent arthropod diseases, such as scrub typhus and RMSF, if the dose is sufficient and the antibiotic is administered at the appropriate time. Recent evidence suggests that single dose prophylaxis may also be possible for Lyme disease.
  • In RMSF, premature administration of antibiotics may delay the onset of disease; however, a single dose administered shortly before the expected onset of disease can prevent the disease.
  • In Lyme disease prophylaxis, weigh the risks and cost of antibiotic therapy against the risks of disease and cost and efficacy of existing therapies for established disease. In highly endemic areas, the risk-benefit ratio occasionally may favor prophylactic treatment. This is true especially if the tick is identified accurately as a Lyme disease vector (usually, tear drop–shaped Ixodes tick with brown inornate scutum), and the tick is heavily engorged, suggesting a long period of attachment.
  • Because the timing of single-dose prophylaxis is difficult, the author recommends a 10-day course of oral tetracycline. In general, the prescription can be provided and the patient can fill the prescription at the first sign or symptom of disease.
  • Give special consideration to prophylactic treatment of pregnant women. Weigh the cost and risks of therapy for both mother and fetus against the cost and risks of untreated disease in both.

Surgical Care

Surgical care is indicated only for brown recluse spider bites and consists only of conservative debridement.

Consultations

Consultation with a medical entomologist or knowledgeable physician is important when dealing with arthropod vectors of disease. State health or agriculture departments can be helpful resources, as can local universities and US military facilities.

MEDICATION

Section 7 of 11  

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See Treatment for a full discussion of bite reactions, treatment of spider bites, treatment of vector-borne disease, and antibiotic prophylaxis following tick bites. Topical steroids, antipruritics, and anesthetics are the most prescribed treatments for most insect bites.

Drug Category: Topical corticosteroids

Have anti-inflammatory properties and cause profound and varied metabolic effects. In addition, these agents modify the body's immune response to diverse stimuli. Class I steroids (eg, clobetasol) can be used. In pediatric patients, usually it is best to use a milder corticosteroid, such as hydrocortisone, hydrocortisone butyrate (Lacti-Care-HC, Westcort), fluticasone (Cutivate), or desonide (Tridesilon, DesOwen).

Drug Name	Hydrocortisone (Lacticare HC, Westcort, Dermacort, CortaGel, Cortaid)
Description	Adrenocorticosteroid derivative suitable for application to skin or external mucous membranes. Has mineralocorticoid and glucocorticoid effects resulting in anti-inflammatory activity.
Adult Dose	Apply sparingly to affected areas bid/qid
Pediatric Dose	Apply as in adults
Contraindications	Documented hypersensitivity; viral, fungal, and bacterial skin infections
Interactions	None reported
Pregnancy	C - Safety for use during pregnancy has not been established.
Precautions	Prolonged use, application over large surface areas, application of potent steroids, and occlusive dressings may increase systemic absorption of corticosteroids and may cause Cushing syndrome, reversible HPA-axis suppression, hyperglycemia, and glycosuria

Drug Name	Menthol (Rhuli gel)
Description	Generally safe and effective for symptomatic relief.
Adult Dose	Apply sparingly to affected area
Pediatric Dose	Apply as in adults
Contraindications	Documented hypersensitivity
Interactions	None reported
Pregnancy	C - Safety for use during pregnancy has not been established.
Precautions	For external use only

Drug Category: Topical anesthetics

Stabilize the neuronal membrane and prevent the initiation and transmission of nerve impulses, thereby producing the local anesthetic action.

Drug Category: Sympathomimetics

For anaphylaxis.

Drug Category: Antipruritics

Relieve itch associated with bites.

FOLLOW-UP

Section 8 of 11  

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Further Inpatient Care

• Observe patients with minor-appearing wounds following a brown recluse spider bite for several days to monitor the development of DIC. When necrosis is minor, toxin is absorbed systemically, resulting in diffuse endothelial damage and the possibility of DIC.
• Most scorpion stings, including those of C exilicauda (formerly C sculpturatus), result only in local symptoms. Children appear more prone to severe systemic reactions, which usually occur on the day of envenomation.

Deterrence/Prevention

• Antibiotic prophylaxis after tick bites is controversial. Clearly, prevention of bites is a better means of preventing disease. Personal protection using arthropod repellents and permethrin applied to clothing is highly effective. Tick populations can be reduced dramatically by clearing leaf debris from wooded areas. Hard ticks are sensitive to desiccation and require a layer of leaf debris into which they can descend to rehydrate. Removing leaf debris causes many ticks to die of dehydration and also provides for more effective control of ticks using smaller amounts of insecticide. If a tick bite occurs despite these efforts, weigh the risks and cost of antibiotic prophylaxis against the risk of acquiring disease from the tick bite (see Treatment).
• Prevention of bites and stings: Screen doors and windows create a barrier between people and the outside world. Venturing outside the home environment subjects people to bites and stings.
• Prevention of stings
• • Avoiding perfumes and bright clothing can reduce the risk of bee and vespid stings.
• • Spiders and scorpions commonly are found in shoes, rubbish, woodpiles, attics, and garages. When disturbing these environments, look before touching boxes or containers.
  • Advise those who live in areas where scorpions are common to shake out shoes before putting them on.
• Prevention of bites
• • Many topical repellents have been studied, including N,N-diethyl-3-methylbenzamide (formerly diethyltoluamide, or DEET), picaridin, other piperidines and piperazines, dimethyl phthalate, dimethyl carbate, ethyl hexane diol, butopyronoxyl, and benzyl benzoate.
  • Of these agents, DEET consistently has shown the most reliable repellent effect against a wide range of arthropods. Picaridin is also effective against mosquito bites.
  • DEET is available in concentrations up to 100%. Local skin irritation, including blistering reactions, can occur but appears to be less common with lower concentrations. Systemic toxicity, including fatal reactions, has been reported.
  • The American Academy of Pediatrics recommends concentrations of DEET no higher than 30% for children and that the lowest effective concentration be used. Several 10% DEET lotions currently are available. Long-acting slow-release formulas offer excellent protection with lower concentrations of DEET, reducing the risk of reactions from systemic absorption.
  • The US military uses a 30% long-acting DEET formula that has demonstrated excellent efficacy against a wide range of arthropods. The same formula has been available in the United States as Ultrathon and Amway HourGuard, but it may be difficult to find. Consumer products now tend to have lower concentrations of DEET, more suitable for children.
  • Permethrin, an insecticide, is sold as a spray tick repellent. Permethrin spray leaves a residue in clothing that lasts for several wash cycles and is effective against a wide range of North American ticks. North African camel ticks have shown resistance to permethrin, which actually increases the attachment of these ticks by triggering a pheromonelike response. This phenomenon has not been described in North American ticks. Permethrin also can be used to treat screens, sleeping bags, tents, and bedding.
• Flea control measures
• • Lufenuron is an effective oral agent for preventing flea infestation in cats and dogs. A long-acting injectable preparation of lufenuron currently is available.
  • Topical fipronil is effective against tick and flea infestation.
  • For treating an infested environment, boric acid, growth regulators (eg, pyriproxyfen), and insecticides (including organophosphates and carbamates) have proven effective.

Patient Education

• Patients with anaphylactic reactions must carry injectable epinephrine. Be sure that patients are thoroughly familiar with the use of the injector before leaving the healthcare facility.
• Advise patients that the prevention of bites and stings is possible.
• For excellent patient education resources, visit eMedicine's Blood and Lymphatic System Center and Bites and Stings Center. Also, see eMedicine's patient education articles Insect Bites, Allergy: Insect Sting, Malaria, Black Widow Spider Bite, Brown Recluse Spider Bite and Ticks.

MISCELLANEOUS

Section 9 of 11  

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Medical/Legal Pitfalls

• Failure to prescribe epinephrine for a patient who reports symptoms of immediate hypersensitivity or anaphylaxis to arthropod venom can result in death. Instruct patients in the use of bee sting kits, and advise patients to carry the kit with them.

Special Concerns

• Give special consideration to prophylactic treatment of pregnant women. Weigh the cost and risks of therapy for both mother and fetus against the cost and risks of untreated disease in both.

MULTIMEDIA

Section 10 of 11  

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• Follow-up
• Miscellaneous
• Multimedia
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Media file 1:  Amblyomma americanum ticks are identified by an ornate scutum and long anterior mouthparts.
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Media file 2:  Rhipicephalus ticks have an inornate scutum.
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Media file 3:  Dermacentor andersoni has a highly ornate scutum.
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Media file 4:  Cat fleas are identified by genal and pronotal combs that resemble a mustache and mane of hair. The frons (forehead) is sloping.
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Media type:  Photo

REFERENCES

Section 11 of 11

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• Multimedia
• References

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Article Last Updated: Feb 28, 2007