Mosquito bite allergies, also termed hypersensitivity to mosquito bites, are excessive reactions of varying severity to mosquito bites. They are allergic hypersensitivity reactions caused by the non-toxic allergenic proteins contained in the saliva injected by a female mosquito (male mosquitos do not take blood-meals) at the time it takes its blood meal, and are not caused by any toxin or pathogen. By general agreement, mosquito bite allergies do not include the ordinary wheal and flare responses to these bites although these reactions are also allergic in nature. Ordinary mosquito bite allergies are nonetheless detailed here because they are the best understood reactions to mosquito bites and provide a basis for describing what is understood about them. Mosquito bite allergies are informally classified as 1) the skeeter syndrome, i.e., severe local skin reactions sometimes associated with low-grade fever; 2) systemic reactions that range from high-grade fever, lymphadenopathy, abdominal pain, and/or diarrhea to, very rarely, life-threatening symptoms of anaphylaxis; and 3) severe and often systemic reactions occurring in individuals that have an Epstein-Barr virus-associated lymphoproliferative disease, Epstein-Barr virus-negative lymphoid malignancy, or another predisposing condition such as eosinophilic cellulitis or chronic lymphocytic leukemia. The term papular urticaria is commonly used for a reaction to mosquito bites that is dominated by widely spread hives. Here, papular urticaria is regarded as a symptom of mosquito bite allergy manifested in individuals with one of the other mosquito bite allergies, but particularly in those associated with eosinophilic cellulitis. Mosquitos belong to the biological order of Diptera (which includes all two-winged insects), suborder Nematocera, family Culicidea. There are >3,500 different mosquito species with the Aedes and Culex genera being common in North America. It is assumed that any species of mosquito that causes an ordinary mosquito bite reaction in humans is capable of causing mosquito bite allergies. In addition to mosquitoes, the Diptera order includes numerous other types of biting insects such as midges (e.g. sand flies) and gnats. Bites by the latter insects or possibly some other insects may cause reactions that are mechanistically and clinically similar to those seen with mosquito bites. Mosquito bite allergies occur more often where insect bites are frequent. Consequently, cases (as well as various other allergic disorders) are more prevalent in tropical climates, underdeveloped areas, and areas dominated by poverty. That is, not only climate but also cultural and socioeconomic conditions play roles in facilitating the development and prevalence of diverse allergic diseases, including mosquito bite allergies.
Ordinary reactions
Presentations The typical reaction to mosquito bites involves the development of an itchy wheal that may contain a central red dot and is surrounded by splotchy redness. This "immediate reaction" occurs at some time during the first 20 minutes following the bite. Within hours of the bite, a "delayed reaction", in which the wheal evolves into a papule develops and then dissipates over the next few days or weeks. However, there is a wide variability in the type of reaction that individuals mount in response to these bites. The initial mosquito bites in previously unexposed individuals do not cause a skin reaction but do initiate the development of antibodies and/or lymphocytes that are directed against the allergens in mosquitoes' saliva. These individuals thereby become sensitized and reactive to subsequent mosquito bites. After repetitive mosquito bites, individuals may become less sensitive or completely insensitive to the bites in the natural process of allergen desensitization. Individuals therefore progress through five stages in which the type of reaction to a mosquito bite depends on the number of their previous bite exposures and levels of acquired sensitization and desensitization to these bites. The 5 stages an individual may undergo in reacting to repetitive mosquito bites are:
Stage I: Previously unexposed individuals have no immediate or delayed reaction. Stage II: Shortly after their initial exposure, individuals become partly sensitized and therefore develop a delayed but no immediate reaction. Stage III: Following further exposures, individuals become fully sensitized and therefore have an immediate reaction followed by a delayed reaction. Stage IV: After >2–20 years of repetitive exposures, individuals may become partially desensitized and therefore have an immediate but no delayed reaction. Stage V: After many further exposures during the 2–20 years of repetitive exposures, individuals may become fully desensitized and therefore show no reaction. In a study of 41 Canadian adults experimentally exposed to mosquito bites, 11 individuals exhibited no reaction, 23 individuals exhibited immediate followed by delayed reactions, 6 individuals exhibited only immediate reactions, and 1 individual exhibited only a delayed reaction. Overall, 70-90% of individuals experience an immediate reaction, and 55-65% experience a delayed reaction to mosquito bites. Individuals also vary in the severity of their reactions to mosquito bites. Most individuals show a "small reaction" in which a 2–10 mm (0.1-0.4 inches) diameter wheal evolves into a similarly sized papule that dissipates over several days. About 2.5% of individuals (based on self reports) show a "large reaction" in which the wheal is much larger than 10 mm (it may exceed 3 cm, i.e. 1.2 inches) in diameter and evolves into an extensive lesion that has black and blue discolorations due to subcutaneous bleeding, blistering, and/or necrosis. The latter reactions, which appear to be caused by the development of an inflammatory Arthus reaction at the site, may be very painful.
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