Insect and other arthropod bites

INTRODUCTION — The bites of insects and other arthropods may be a minor nuisance or may lead to serious medical problems, including transmission of insect-borne illnesses and severe allergic reactions. Reactions to the bites of mosquitoes, ticks, black flies, horse and deer flies, sand flies, stable flies, biting midges, fleas, centipedes, and biting mites are discussed in this topic review. Infectious diseases transmitted by mosquitoes, ticks, fleas, kissing bugs, and sand flies are discussed in detail separately.

●Insect bites are different from insect stings. Stings involve the injection of venom into the victim and may cause reactions ranging from local irritation to life-threatening anaphylaxis. The medical consequences of the most common insect stings are reviewed separately. (See "Bee, yellow jacket, wasp, and other Hymenoptera stings: Reaction types and acute management" and "Stings of imported fire ants: Clinical manifestations, diagnosis, and treatment" and "Scorpion envenomation causing neuromuscular toxicity (United States, Mexico, Central America, and Southern Africa)".)

●Spider bites are mentioned briefly here and discussed in greater detail separately. (See "Diagnostic approach to the patient with a suspected spider bite: An overview" and "Bites of recluse spiders" and "Widow spider bites: Clinical manifestations and diagnosis" and "Widow spider bites: Management".)

●Contact with caterpillars and moths can also cause dermatitis and even allergic reactions in humans in the absence of a bite or sting, as discussed separately. (See "Lepidopterism: Skin disorders secondary to caterpillars and moths".)

OVERVIEW OF ARTHROPOD BITES — Most arthropods are insects, but the phylum Arthropoda also includes spiders, centipedes, and crustaceans.

Host (human) factors — There is some evidence that specific host factors may predispose individuals to increased arthropod biting. Analyses of the distributions of transmission rates of insect-borne infections indicate that, typically, 20 percent of the host (human) population is responsible for approximately 80 percent of disease transmission by blood-sucking insects [1]. Possible factors that attract mosquitos specifically include blood type, metabolic rate, amount of CO₂ released, body temperature, clothing types and colors, and varying amounts of volatile organic compounds (VOCs) emanating from human skin [2-5]. In addition, the less defensive the host, the higher the probability that the insect will be successful in achieving the desired high quantity of blood needed for its meal. Thus, the presence of ketones or lactic acid, signaling a malnourished, weakened, or physically exhausted host, serves as an attractant. Whether specific dietary components alter host attractiveness is largely unexplored, although one study showed that eating bananas may increase attraction of certain Anopheles mosquitoes [6].

Mechanisms of injury — Arthropod bites consist of punctures made by the mouthparts of the offending organism. The word "bite" probably should be restricted in meaning to purposeful biting by a species for catching prey or blood feeding and not to accidental biting by plant-feeding (phytophagous) insects. Phytophagous and predaceous insects sometimes "bite" in self-defense, piercing the skin with their proboscis, but the injury is actually just a simple stab wound and rarely has noticeable consequences.

Arthropod bites result in minimal mechanical injury to human skin. Lesions instead result from the host's immune reactions to the arthropod's salivary secretions or venom. In addition, atopic individuals may develop hypersensitivity to antigens found in arthropod saliva.

Arthropod saliva is injected while feeding for a variety of reasons:

●Lubrication of mouthparts to aid insertion

●Increased blood flow at the bite site

●Anesthesia at the bite site

●Interference with blood coagulation

●Suppression of host immune and inflammatory responses

●Enhanced digestion

Mouthpart types — There are two methods of obtaining blood by arthropods. Some groups, such as mosquitoes, bed bugs, kissing bugs, and sucking lice, obtain blood directly from capillaries or small veins: a method called "solenophagy." Others, such as ticks, horse flies and deer flies, black flies, and biting midges, obtain their blood by lacerating blood vessels and feeding from the resulting pool of blood: a method termed "telmophagy."

There are several types of insect mouthparts that can be generally grouped into three broad categories: piercing-sucking, chewing, and sponging. Within these categories, there are all sorts of adaptations and specializations among the various insect orders (figure 1). Piercing-sucking mouthparts, especially the bloodsucking types, are most important in human bites, while chewing (common in beetles and cockroaches) and sponging mouthpart types are of little significance to human health.

Arthropods that bite humans — Arthropods that commonly bite humans include [7]:

●Mosquitoes (see "Malaria: Epidemiology, prevention, and control", section on 'Mosquito life cycle')

●Ticks (see "Evaluation of a tick bite for possible Lyme disease")

●Kissing bugs (see "Reactions to bites from kissing bugs (primarily genus Triatoma)")

●Bed bugs (see "Bedbugs")

●Black flies

●Horse and deer flies

●Sand flies

●Stable flies

●Biting midges

●Fleas

●Centipedes

●Biting mites

●Chiggers (see "Chigger bites")

●Lice (see "Pediculosis capitis" and "Pediculosis corporis" and "Pediculosis pubis and pediculosis ciliaris")

●A small number of types of spiders (see "Diagnostic approach to the patient with a suspected spider bite: An overview" and "Bites of recluse spiders" and "Widow spider bites: Management" and "Widow spider bites: Clinical manifestations and diagnosis")

TYPES OF REACTIONS — Insect bites may result in several types of local reactions, papular urticaria, or systemic allergic reactions. Rarely, other forms of systemic reactions occur, such as serum sickness [8].

Local reactions — The normal reaction to an insect bite is an inflammatory reaction at the site of the punctured skin, which appears within minutes and consists of pruritic local erythema and edema. A single punctum may be visible. Dual puncta can be seen with spider bites and centipede envenomations. Symptoms usually subside within a few hours. Local reactions are caused by irritant substances concentrated in insect saliva (eg, anticoagulants such as factor Xa inhibitors, digestive enzymes such as amylases and esterases, agglutinins, and mucopolysaccharides) [9]. In some cases, a local reaction is followed by a delayed skin reaction consisting of local swelling, itching, and redness.

Treatment — Insect bites and local reactions should be washed with soap and water. Reduction of local edema may be induced with cooling (ice or cold pack). Topical creams, gels, and lotions, such as those containing calamine or pramoxine, can be helpful in reducing pruritus, if necessary. However, routine use of topical anesthetic and antihistamine preparations should be avoided because they can sensitize the skin following sun exposure and induce allergic contact sensitivity [10].

Minimally sedating oral antihistamines (eg, cetirizine or fexofenadine, once or twice daily) may be helpful for patients with troublesome itching and are preferred over sedating agents, particularly in small children. However, the sedating agent hydroxyzine (hydrochloride or pamoate; 10 to 25 mg every four to six hours, as needed) may be helpful for controlling persistent pruritus in adults, perhaps largely due to the sedating effects. H1 and H2 antihistamines (eg, famotidine) may be used concurrently.

Concurrent use of oral H1 antihistamines and topical antihistamines applied over large surface areas should be avoided because this combination can cause systemic anticholinergic toxicity and topical antihistamines can induce contact hypersensitivity from application of the medication, precluding future systemic administration of the H1 antihistamine.

Dramatic local swelling and induration can be reduced with a brief course of oral glucocorticoids, although this should be reserved for severe cases.

Unusual local reactions

●Uncommonly, local reactions evolve to become vesicular (picture 1), bullous (picture 2 and picture 3), or necrotic (picture 4) [11,12]. Patients should maintain good hygiene and avoid scratching to prevent infection.

•Bullous reactions have been reported with bed bug and other arthropod bites (see "Bedbugs"). Bullae should be kept clean as they heal, and, if lesions persist, they may be treated with topical corticosteroids [12]. They may warrant additional workup to rule out other bullous disorders. If the arthropod bite has resulted in longstanding bullous lesions with tissue eosinophilia, the diagnosis of eosinophilic cellulitis should be considered [13].

•Necrotic lesions most commonly result from the injection of venom, which occurs with some spider bites and centipede bites. Treatment of spider bites is discussed elsewhere. (See "Bites of recluse spiders", section on 'Treatment'.)

●Immunocompromised patients and those with certain lymphoproliferative disorders (eg, acquired immunodeficiency syndrome, Epstein-Barr virus-related NK lymphocytosis) may develop severe local reactions [11,14-18]. In such patients, lesions can progress to become necrotic or may be accompanied by systemic symptoms, including lymphadenopathy and fever. However, bite-like lesions may also arise spontaneously in patients with these disorders, a condition called eosinophilic dermatoses of hematologic malignancy (EDHM) [19]. (See 'Differential diagnosis' below.)

Papular urticaria — Papular urticaria is a hypersensitivity disorder in which insect bites, most often those of fleas, mosquitoes, or bed bugs, lead to recurrent and sometimes chronic, itchy papules on exposed areas of skin (eg, arms, lower legs, upper back, scalp) [20,21]. The 0.5 to 1 cm lesions may be urticarial at the start of the syndrome but become persistent and papular and/or nodular with time (picture 5 and picture 6).

Papular urticaria is reported predominantly in young children (typically 2 to 10 years of age). The diaper/underwear areas, genital, perianal, and axillary areas are spared [22]. The diagnosis of papular urticaria is made clinically, although there may be a delay between the inciting bite(s) and the onset of lesions, or insect bites may not have been noticed at all [22]. Usually, only one child in a family is affected, a clue that infestation at home is unlikely. New lesions may appear sporadically, and renewed itching may reactivate older lesions, leading to a chronic and cycling disorder that may last from months to years.

Treatment — Management of papular urticaria includes selective and limited use of nonsedating antihistamines for pruritus, midpotency topical corticosteroids applied to individual lesions, and reassurance, as the disorder eventually resolves spontaneously [22].

Systemic allergic reactions — Systemic allergic reactions to insect bites are uncommon but have been described in response to the bite of Triatoma (kissing bugs), mosquitoes, ticks, black flies, deer flies, horse flies, and centipedes [23-28]. Reactions to Triatoma are reviewed in more detail separately. (See "Reactions to bites from kissing bugs (primarily genus Triatoma)".)

Patients with mast cell disorders may present with severe systemic allergic reactions following insect bites [29,30]. Reactions can result following insect bites, stings of Hymenoptera insects, and a variety of other triggers, which cause widespread activation of mast cells with prominent flushing and hypotension. In patients with hypotension in response to an insect bite or sting, a serum total tryptase can be obtained to screen for mast cell disorders. Patients with tryptase levels above 5 to 8 ng/mL are at risk for severe and/or systemic reactions to Hymenoptera and insects, even in the absence of systemic mastocytosis [31,32]. (See "Mast cell disorders: An overview" and "Mastocytosis (cutaneous and systemic) in adults: Epidemiology, pathogenesis, clinical manifestations, and diagnosis".)

Treatment — Anaphylaxis associated with insect bites needs to be treated promptly with epinephrine. Patients who have experienced systemic reactions should be supplied with an epinephrine autoinjector and instructed in how and when to use it. (See "Anaphylaxis: Emergency treatment", section on 'Discharge care' and "Prescribing epinephrine for anaphylaxis self-treatment".)

Referral to an allergy specialist should be facilitated whenever possible. Allergy specialists are able to assess the patient's clinical history to assure that the correct trigger for the allergic reaction has been identified and, in some cases, perform confirmatory testing. For patients who have suffered anaphylaxis, allergists are able to provide effective training in the self-injection of epinephrine.

Other systemic reactions — Occasionally, patients develop systemic reactions of uncertain pathogenesis to insect bites that cause minimal symptoms in most individuals. As an example, a Japanese report described two patients with local skin reactions, fever, fatigue, nausea, anorexia, and hepatosplenomegaly in response to mosquito bites [33]. These sporadic reactions likely result from factors unique to the patient.

DIFFERENTIAL DIAGNOSIS — Several dermatologic disorders can present with flattened lesions or scattered inflammatory papules that may resemble arthropod bites.

●Folliculitis – Superficial folliculitis is characterized by small, follicularly based inflammatory papules and pustules (picture 7A-B). Cultures taken from pustular lesions are useful for identifying causative organisms. (See "Infectious folliculitis".)

●Herpes zoster – Very limited cases of herpes zoster involving just a few papules or vesicles can mimic insect bites. When the clinical presentation is uncertain, laboratory confirmation of varicella zoster virus is indicated, and polymerase chain reaction (PCR) testing can be performed on a lesion of any stage. (See "Epidemiology, clinical manifestations, and diagnosis of herpes zoster", section on 'Approach to diagnosis'.)

●Lymphomatoid papulosis – Lymphomatoid papulosis is an uncommon, chronic, recurrent skin disorder that presents with red-brown papules or nodules that typically persist for several weeks (picture 8). Lesions are often asymptomatic and may be crusted, necrotic, or hemorrhagic. Progression to mycosis fungoides, anaplastic large cell lymphoma, or Hodgkin lymphoma occurs in a minority of patients. The diagnosis is confirmed via skin biopsy. (See "Lymphomatoid papulosis".)

●Eosinophilic dermatoses of hematologic malignancy (EDHM) – Lesions resembling insect bites may arise spontaneously in patients with a variety of hematologic malignancies, with most reports in patients with chronic lymphocytic leukemia [14]. Proposed diagnostic criteria involve persistent and pruritic papules, plaques, nodules, or blisters with eosinophilic infiltration on histopathology and no other identifiable cause of tissue eosinophilia. (See "Eosinophilic cellulitis (Wells syndrome)", section on 'Differential diagnosis'.)

●Pityriasis lichenoides et varioliformis acuta (PLEVA) – PLEVA is a rare, benign skin disease that presents with recurrent crops of inflammatory papules (picture 9). Lesions may demonstrate ulceration, vesiculation, pustulation, or crusting. Skin biopsies assist with diagnosis. (See "Pityriasis lichenoides et varioliformis acuta (PLEVA)".)

●Erythema multiforme – Raised, targeted lesions, often symmetrical, sometimes with a central blister on the limbs, can be confused with arthropod bites (picture 10). (See "Erythema multiforme: Pathogenesis, clinical features, and diagnosis".)

●Drug use – Chronic abusers of amphetamine and cocaine may experience formication (a feeling that ants are crawling on the skin) and can present with self-inflicted skin lesions from an imaginary infestation (picture 11). (See "Methamphetamine: Acute intoxication".)

●Delusional infestation – Patients with delusional infestation (also called delusions of parasitosis) believe that they are being bitten by imaginary insects or mites. The skin may be excoriated and scabbed from efforts to remove the offending insects. (See "Delusional infestation: Epidemiology, clinical presentation, assessment, and diagnosis" and "Treatment of delusional infestation".)

SPECIFIC ARTHROPOD BITES

Mosquitoes — The most commonly occurring insect bites are inflicted by mosquitoes, of the family Culicidae (order Diptera). The genera Anopheles, Culex, Aedes, and Psorophora are usually responsible for human bites. Only female mosquitoes feed on blood (picture 12). Mosquitoes may be found near their breeding sites, which include a wide variety of stagnant water sources, since they need an aquatic environment to complete their life cycle. (See "Malaria: Epidemiology, prevention, and control", section on 'Mosquito life cycle'.)

Local reactions — Local pain, pruritus, and erythema are typical after mosquito bites (picture 13). Common reactions include an immediate wheal-and-flare response that peaks at approximately 20 minutes and/or an indurated pruritic papule that peaks at two to three days and resolves over the ensuing days to weeks [34]. Sometimes, large indurated lesions may occur (picture 14).

Some people, particularly young children, can develop very dramatic swelling surrounding the site of the bite, heat, redness, itching, and pain, which may be accompanied by low-grade fever [35]. This has been termed "Skeeter syndrome" and can be mistaken for and treated as cellulitis [36]. However, large local reactions develop within hours after a bite, while cellulitis develops over days. In many patients, these exaggerated local reactions improve with age, presumably due to natural desensitization, and can be managed with prophylactic antihistamines during the summer months [37,38]. (See "Allergic reactions to mosquito bites", section on 'Differential diagnosis'.)

There are reports of more severe local reactions including ecchymotic, vesiculated, blistering, bullous, and Arthus reactions, which may linger for weeks [34].

Patients with Epstein-Barr virus-associated lymphoproliferative disorders may develop necrotic skin lesions at the site of mosquito bites [18,39-42].

Systemic allergic reactions — Rarely, patients can develop classical anaphylaxis in response to mosquito bites, presenting with some combination of generalized urticaria, angioedema, wheezing, vomiting, hypotension, loss of consciousness, or other manifestations of anaphylaxis [34,35,43]. (See "Allergic reactions to mosquito bites".)

One case series of four patients with severe anaphylactic reaction after mosquito bites reported that all were ultimately found to have a diagnosis of systemic mastocytosis [44]. (See "Mastocytosis (cutaneous and systemic) in adults: Epidemiology, pathogenesis, clinical manifestations, and diagnosis".)

Disease transmission — Diseases routinely transmitted by mosquitoes in the United States include West Nile virus, St. Louis encephalitis, Eastern equine encephalitis, and La Crosse encephalitis. Transmission of chikungunya and Zika may also occur. (See "Chikungunya fever: Epidemiology, clinical manifestations, and diagnosis" and "Zika virus infection: An overview" and "St. Louis encephalitis" and "Arthropod-borne encephalitides".)

Diseases transmitted by mosquitoes worldwide include malaria, yellow fever, dengue and dengue hemorrhagic fever, lymphatic filariasis, chikungunya, and other arboviruses. (See "Laboratory tools for diagnosis of malaria" and "Yellow fever: Epidemiology, clinical manifestations, and diagnosis" and "Chikungunya fever: Epidemiology, clinical manifestations, and diagnosis" and "Arthropod-borne encephalitides" and "Dengue virus infection: Clinical manifestations and diagnosis" and "Lymphatic filariasis: Epidemiology, clinical manifestations, and diagnosis".)

Mosquitoes do not transmit human immunodeficiency virus (HIV) infection, since the virus neither survives nor replicates well in mosquitoes, and blood from one meal is not flushed into the next host [45,46].

Prevention — Guidelines on vector control on a global level, issued by the World Health Organization [47], focus on reducing or eliminating larval habitats. On an individual level, patients should be instructed to apply insect repellents that contain active ingredients such as N,N-diethryl-3-methylbenzamide (DEET), picaridin (known as KBR 3023 and icaridin outside of the United States), IR3535, oil of lemon eucalyptus, para-menthane-diol (PMD), or 2-undecanone [48]. Additional protective measures include the use of protective covers such as screens on windows and netting over beds [9]. (See "Prevention of arthropod and insect bites: Repellents and other measures", section on 'Mosquitoes'.)

Specific allergen immunotherapy may play a role in preventing subsequent reactions in those rare cases where the individual experiences anaphylaxis to mosquito bites. Some studies have reported the potential role of allergen immunotherapy, using either whole body mosquito extracts or recombinant salivary antigens, as a means of desensitizing patients who have experienced anaphylactic or delayed reactions to mosquito bites [34,49]. This therapy is not widely employed, as the available commercial extracts are whole body mosquito extracts, which contain few salivary proteins and therefore provide an unpredictable outcome and response. The salivary antigens that have been identified are all derived from Aedes aegypti but may have crossreactivity with other mosquito species. The use of immunotherapy with recombinant salivary allergens is only available in a few centers [34].

Ticks — Ticks have multiple life stages in which they may bite people (picture 15). The primary concern with tick bites is disease transmission, although tick paralysis may occur from an attached tick, and (rarely) allergic reactions to their bites may happen, and some tick bites appear to sensitize patients to allergens that can later cause food (specifically red meat) allergy. (See "Allergy to meats", section on 'The role of ticks in red meat allergy'.)

Tick paralysis — The salivary neurotoxins of various tick species can cause a disease known as tick paralysis, which is rare but has been reported, most often in Australia and North America [50-52]. It typically begins after a tick has been attached and feeding for four to seven days. Initial symptoms often include paresthesias and a sense of fatigue and weakness and can progress to unsteady gait, followed by ascending and then respiratory paralysis. The patient should be examined and any ticks removed immediately. Tick paralysis is important to recognize because it can be fatal, although, if diagnosed promptly, it can be cured with the combination of tick removal and supportive care. (See "Tick paralysis".)

Disease transmission — Ticks can transmit several infectious diseases:

●Lyme borreliosis (ie, Lyme disease) is one of the most frequently reported tick-borne human diseases. It is caused by the spirochete Borrelia burgdorferi, which is transmitted by the bite of infected Ixodes ticks. I. scapularis, the deer tick, is the primary vector of Lyme disease in the Eastern and North-Central United States (picture 16), while I. pacificus is the vector in the Western United States (picture 17). Other Ixodes species are involved in Europe and Asia. (See "Evaluation of a tick bite for possible Lyme disease" and "Epidemiology of Lyme disease".)

●Rocky Mountain spotted fever (RMSF) in the Eastern and South-Central United States is primarily transmitted through the bite of Dermacentor variabilis (the American dog tick) (picture 18). Dermacentor andersoni (the Rocky Mountain wood tick) is the primary vector in the Mountain states west of the Mississippi River (picture 19). The brown dog tick, Rhipicephalus sanguineus has been found to transmit the agent of RMSF [53]. (See "Biology of Rickettsia rickettsii infection".)

●Human ehrlichiosis and tularemia may be transmitted by the lone star tick (Amblyomma americanum) (picture 20), as well as I. scapularis. (See "Human ehrlichiosis and anaplasmosis" and "Tularemia: Clinical manifestations, diagnosis, treatment, and prevention".)

●Babesiosis is most commonly transmitted by I. scapularis. (See "Babesiosis: Microbiology, epidemiology, and pathogenesis".)

Allergy — Bites of some ticks can cause rare systemic allergic reactions immediately after the bite itself, which have been reported with bites from the Ixodes tick, I. holocyclus (the Australian paralysis tick), and Argas reflexus (the European pigeon tick) [54-57]. Salivary proteins of ticks are thought to be allergenic [54].

The bites of other ticks have been implicated in sensitizing patients to a carbohydrate determinant, galactose-alpha-1,3-galactose (also called alpha-gal), which is also found in the drug cetuximab and some red meats, resulting in allergic reactions upon exposure to these substances instead of to the tick bites directly [58]. In the United States, A. americanum (lone star tick) is thought to be the sensitizing tick, while I. ricinius is implicated in Europe and I. holocyclus in Australia [59,60]. Patients with alpha-gal sensitization can react to cetuximab (an epidermal growth factor receptor [EGFR] inhibitor) upon the first exposure because cetuximab is a human-murine chimerized immunoglobulin monoclonal antibody, produced in mammalian cell cultures, which contains the alpha-gal moiety in the Fab region of the molecule [61]. The reactions can be severe, including anaphylaxis [62]. Alpha-gal-related reactions to the ingestion of red meat is unusual in its delayed onset (up to four to six hours after ingestion). (See "Allergy to meats", section on 'Meats and monoclonal antibodies (cetuximab)' and "Infusion-related reactions to therapeutic monoclonal antibodies used for cancer therapy", section on 'Cetuximab'.)

Prevention — The only known form of prevention of tick bites is avoidance of the tick. Suggestions for avoidance include: wear protective clothing, use repellents that contain DEET or permethrin, avoid areas of high vegetation including tall grass and leaf litter, bathe or shower within two hours of coming indoors to assist in washing off crawling ticks, carefully examine gear and pets for ticks, and tumble-dry clothes in a dryer for one hour on high heat [63]. The only known prevention of anaphylaxis to red meat as a result of sensitization through the tick bite is to avoid ingesting red meat [58]. (See "Allergy to meats".)

Flies — Species of biting flies are capable of inducing allergic reactions and/or transmitting infectious diseases.

Reactions — Black flies, horse and deer flies, stable flies, sand flies, and biting midges have been reported to induce systemic allergic and inflammatory reactions.

●Black flies, sometimes called buffalo gnats or turkey gnats, are vicious biters (picture 21 and picture 22 and picture 23). Black fly bites have been implicated in allergic reactions of intensely itching papular urticaria, angioedema (picture 24), anaphylaxis, and in a late-onset systemic syndrome characterized by fever, leukocytosis, lymphadenitis, and papular lesions [64-66]. Black flies occur near fast-flowing streams, creeks, and rivers worldwide. In North America, some of the worst problems occur in the Northeastern United States and Canada.

●Horse flies and deer flies occur worldwide and inflict painful bites with their slashing/lapping mouthparts (picture 25 and figure 1) [67]. Delayed or necrotic reactions may develop (picture 26) [9]. In addition, there have been several reports of systemic anaphylaxis from their bites [24,68].

●Sand flies are small, delicate, mosquito-like flies that inflict painful bites (picture 27) [69]. These bites can also be pruritic but usually do not enlarge. Sand flies occur worldwide (including the United States) but are notorious pests in the Middle East and tropics.

●Stable flies resemble house flies but have a piercing, bayonet-like proboscis capable of biting through clothing (picture 28). They can be significant medical and veterinary pests in temperate and tropical areas and especially along beaches [67].

●Biting midges (genus Culicoides) are found worldwide. They are tiny, gray/black, winged insects less than 3 mm in length (picture 29). The term "no-see-um" comes from the fact that people often feel the sharp, burning bite but are unable to see the midge. Females require blood in order to produce mature eggs and typically feed at dawn and dusk. Coastal areas and marshlands are preferred breeding habitats [70,71]. Bites may result in small welts or localized allergic reactions in sensitive individuals [72,73]. In a Taiwanese series of 220 subjects, local reactions included immediate (ie, within one hour), immediate followed by delayed, and isolated-delayed reactions consisting of pruritic papules or vesicles that could persist for weeks to months [72]. A small number of patients had systemic symptoms of fever and lymphadenopathy. Seasonal exposure to midges (especially nonbiting midges, family Chironomidae) may cause respiratory symptoms [74].

Biting midges are widespread but only transmit diseases to humans in certain geographical areas. Important examples include Oropouche and Simbu viruses in Africa and South America [75] and filarial worms of the Mansonella species in South and Central America as well as parts of Africa and the Caribbean. (See "Loiasis (Loa loa infection)".)

Disease transmission — Flies can serve as vectors of infectious diseases in geographic regions that support transmission of specific pathogens:

●In Africa and Latin America, black flies in the genus Simulium (particularly S. damnosum) are vectors of onchocerciasis [76]. (See "Onchocerciasis".)

●In West and Central Africa, the Chrysops fly, also known as the tabanid fly, transmits Loa loa. (See "Loiasis (Loa loa infection)".)

●Also in Africa, the bite of a tsetse fly (Glossina spp) can transmit human African trypanosomiasis, or sleeping sickness. (See "Human African trypanosomiasis: Epidemiology, clinical manifestations, and diagnosis".)

●Sand flies are capable of transmitting sand fly fever, bartonellosis, and leishmaniasis. Of these, leishmaniasis is by far the most widespread and clinically important [77]. (See "South American bartonellosis: Oroya fever and verruga peruana" and "Cutaneous leishmaniasis: Clinical manifestations and diagnosis" and "Visceral leishmaniasis: Epidemiology and control".)

●Nonbiting fly species such as house flies and other filth flies with sponging mouthparts (figure 1) are involved in mechanical disease transmission [78]. House flies have been implicated in transmission of enteric infections in settings where facilities for clean water and hygienic practices are limited [79]. (See "Pathogenic Escherichia coli associated with diarrhea" and "Shigella infection: Epidemiology, clinical manifestations, and diagnosis".)

●Myiasis is the infestation of human tissues by a variety of bot fly and other fly larvae. Myiasis may be obligate (such as with screwworm flies) or facultative (opportunistic infestations due to common blow flies). Obligate myiasis is usually seen in patients or travelers returning from tropical areas, whereas cases of facultative myiasis can occur anywhere, especially in hospitals and nursing homes. (See "Skin lesions in the returning traveler", section on 'Myiasis'.)

Prevention — Biting flies can be deterred by DEET and para-menthane-diol (PMD). Additionally, the sand fly can be repelled by picaridin (known as KBR 3023 and icaridin outside of the United States). (See "Prevention of arthropod and insect bites: Repellents and other measures".)

Ongoing studies examining efficiency of traps for biting midges suggest that ultraviolet (UV) baited traps outperform CO₂-baited traps for most Culicoides species, with UV black light seeming to be more effective than UV LED light [80,81].

Fleas — Fleas (order Siphonaptera) inflict bites that are often inconsequential and ignored (picture 30). They can, however, result in local reactions or even eschars, pustules, or necrotic lesions. (See "Clinical manifestations, diagnosis, and treatment of plague (Yersinia pestis infection)", section on 'Clinical manifestations'.)

Reactions — Flea bites are usually papules arranged in a nonfollicular pattern but can induce papular urticaria (picture 31 and picture 32) [82,83]. (See 'Papular urticaria' above.)

Prevention — Fleas can be deterred by DEET and para-menthane-diol (PMD). (See "Prevention of arthropod and insect bites: Repellents and other measures".)

Centipedes — Centipedes are found in moist and usually warm climates worldwide, and, although they occur in the continental United States, human envenomations are typically reported from Asia, Indonesia, India, Hawaii, South America, and Australia (picture 33) [84-89].

Reactions — Centipedes inject venom, which they use to immobilize prey through a modified first pair of legs (not the mouth). Centipedes are largely nocturnal, and most human bites occur during the night. Bites are usually painful, and localized erythema and edema are common. Many patients are bitten on the extremities, and two small puncture marks may be visible [85].

In many cases, symptoms are transient and do not require treatment. However, in a series of 94 patients presenting for emergency care in Sao Paulo, one-third required treatment for persistent or severe symptoms (usually pain) [89].

Most centipede bites resolve without complications, although various sequelae are reported, including local infection and necrosis [88], myocardial infarction [86,87], rhabdomyolysis with kidney failure [90], and allergic reactions, including anaphylaxis [28,91]. In the largest retrospective series, which included 245 patients presenting to an urban hospital in Bangkok over 10 years, bites were accompanied by urticarial rash in six percent, anaphylaxis in five percent, and fever in four percent [92].

Treatment — For patients who do present for post-bite care, reported therapies include systemic analgesics, antihistamines, application of ice packs, immersion of the affected body part in hot water, and local injection of anesthetics [88,89,92-94]. In a prospective study of 60 patients, subjects were randomized to one of three interventions: application of ice packs (15 minutes), hot water immersion (43 to 45°C [109.4 to 113°F] water bath for 15 minutes), or injection of ketorolac 30 mg [93]. Each of the therapies reduced pain, although the authors concluded that ice packs were the most practical and least invasive and noted that 3 of 22 patients assigned to hot water immersion had worsening of symptoms.

Mites — Mites are tiny arachnids that display incredible diversity in form and habitat (picture 34). Many species of mites may bite people, including chigger mites (see "Chigger bites"); rat, chicken, and fowl mites; straw itch mites; scabies mites; cheyletid mites; bird mites, and others [95]. With the exception of scabies mites and follicle mites (Demodex spp), all cases of biting mites are self-limiting and will subside after removing the source of infestation (ie, they do not take up residence and live on humans).

Reactions — Bites from mites are generally small, erythematous papular lesions that are intensely pruritic. They may have a wheal appearance or appear pustular, vesicular, or, rarely, bullous. They can become crusted and need to be monitored for progression to secondary infection. Both chigger bites (picture 35) and scabies are generally localized or grouped, though scabies have burrows as a typical finding (picture 36). Treatment of infestations is reviewed separately. (See "Scabies: Epidemiology, clinical features, and diagnosis" and "Infectious folliculitis", section on 'Demodex folliculitis'.)

Prevention — People complaining about mites biting them should be evaluated for exposure to mite-infested areas and a detailed history obtained to rule out exposure to infested animals or (certain) grains and food products. In addition, persons claiming to be infested with mites may be suffering from delusions of parasitosis [96]. Chiggers can be deterred by DEET or by wearing knee-high rubber boots when outdoors in infested areas. (See "Prevention of arthropod and insect bites: Repellents and other measures".)

Spiders — Spider bites are discussed in detail separately. (See "Diagnostic approach to the patient with a suspected spider bite: An overview" and "Bites of recluse spiders" and "Widow spider bites: Clinical manifestations and diagnosis" and "Widow spider bites: Management".)

INFORMATION FOR PATIENTS — UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." The Basics patient education pieces are written in plain language, at the 5^th to 6^th grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10^th to 12^th grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.

Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient education" and the keyword(s) of interest.)

●Basics topic (see "Patient education: Insect bites and stings (The Basics)")

●Beyond the Basics topic (see "Patient education: Bee and insect stings (Beyond the Basics)")

SUMMARY

●Spectrum of reactions to insect bites – Most insect bites cause local inflammatory reactions that subside within a few hours without complications. However, more severe local symptoms, papular urticaria, systemic allergic reactions, and transmission of a disease-causing pathogen are also possible. (See 'Introduction' above and 'Types of reactions' above.)

●Mosquitos – Mosquito bites can cause varying degrees of local swelling, papular urticaria in children, and rare systemic allergic reactions, including anaphylaxis. Several pathogens are transmitted by mosquitoes in the United States, including the vectors for West Nile virus, St. Louis encephalitis, Eastern equine encephalitis, La Crosse encephalitis, Dengue virus, and Zika virus. (See 'Mosquitoes' above.)

●Ticks – Tick bites are mainly of concern because ticks can cause tick paralysis and transmit infectious pathogens such as Lyme disease, Rocky Mountain spotted fever, ehrlichiosis, and anaplasmosis. The lone star tick has been identified as a vector for an acquired food allergy that presents as delayed anaphylaxis after red meat consumption. (See 'Ticks' above.)

●Flies – Various biting fly species are capable of inducing systemic allergic reactions and/or transmitting infectious diseases. (See 'Flies' above.)

●Fleas – Flea bites are usually only a nuisance, but children can develop papular urticaria, and bites can transmit infectious diseases such as plague and murine typhus. (See 'Fleas' above.)

●Centipedes – Centipedes are found in moist and usually warm climates worldwide and inject venom, which they use to immobilize prey, through their first pair of legs (not the mouth). Most bites occur at night and, aside from pain and erythema, resolve with only rare complications. Centipedes do not transmit disease. (See 'Centipedes' above.)

●Mites – Many species of mites may bite people, although only scabies mites and follicle mites (Demodex spp) take up residence and live on humans. Bites of mites are generally small, erythematous papular lesions which are intensely pruritic. They can become crusted and need to be monitored for progression to secondary infection. (See 'Mites' above.)

ACKNOWLEDGMENT — The UpToDate editorial staff acknowledges Mariana C Castells, MD, PhD, who contributed to earlier versions of this topic review.