Introduction
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
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.
More on Insect Bites |
 Overview: Insect Bites |
| Differential Diagnoses & Workup: Insect Bites |
| Treatment & Medication: Insect Bites |
| Follow-up: Insect Bites |
| Multimedia: Insect Bites |
| References |
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Further Reading
Keywords
bites, repellents, arthropod bites, malaria, leishmaniasis, onchocerciasis, filariasis, rickettsial diseases, Rocky Mountainspotted fever, RMSF, West Nile fever, dengue fever, endemic typhus, lyme disease, babesiosis, ehrlichiosis, tularemia, cat-scratch disease, viral encephalitis, pruritic erythematous papule, mosquitoes, biting flies, ticks, Dermacentor variabilis, D variabilis, Dermacentor andersoni, D andersoni, cat fleas, Ixodes species, Ixodes scapularis, I scapularis, fire ants, Dermacentor tick, Amblyomma tick, Amblyomma americanum, Chrysops deer fly, Liponyssoides mouse mite, rickettsial pox, viral encephalitis, plague, rat flea, Anopheles mosquito, sandfly, bartonellosis, Leishmania braziliensis, Leishmania tropica, Leishmania infantum, visceral leishmaniasis, bacillary angiomatosis, lice, trench fever, typhus, rickettsial fever, Rhipicephalus tick, Rhipicephalus sanguineus, Ixodes tick, scrub typhus, Trombidiidae chigger mite, yellow fever, Aedes mosquito, Onchocerca volvulus, elephantiasis, Culex mosquito, mansonian mosquito, mosquito-borne encephalitis, diethyltoluamide, DEET, pruritic papules, bedbug bites, fleabites, Cimex species, brown recluse spider bite, black widow spider bite, Centruroides exilicauda, C exilicauda, Centruroides sculpturatus, C sculpturatus, scorpion sting, boutonneuse fever, Ornithodoros, borrelial relapsing fever
