Mosquitoes are the insect that causes the most insect bites worldwide. Belonging to the Culicidae family, they are naturally occurring everywhere except Antarctica. Although the bites themselves are relevantly benign, mosquitoes are vectors for an increasing number of diseases and pose a very real burden on public health.
There are three clinically significant genera of mosquitoes, Anopheles, Culex, and Aedes, which are usually responsible for human bites. Mosquitoes are generally found near standing bodies of water as this environment is necessary for the completion of their life cycle. Only the larger female mosquitoes actually bite, as they need blood nourishment for egg production. Each genus of mosquito has its distinctive patterns of feeding. Daytime feeders are attracted by dark-colored clothing, but as they near their host, they use olfactory cues, carbon dioxide, and lactic acid, to hone in. The consensus thought is that certain sweat glands, fragrances, and even alcohol make people more attractive to mosquitoes. One small study showed that individuals who drank alcohol attracted more mosquitoes.
It is unknown how many individuals are affected by mosquito bites annually as most go unreported. A recent American Association Of Poison Control Centers study had only 252 reported cases. What is known is that mosquito-borne disease presents a tremendous disease burden, infecting 700 million and causing a million deaths every year.
Some small studies have shown a possible predilection for males over females and adults over children. However, there have been no large trials to date to confirm this as fact.
The reaction after a mosquito bite is believed to be mostly from toxins located in their saliva. Unlike other biting insects, they do not leave stingers or fangs in the skin of victims. These toxins usually serve the purpose to aid in feeding via vasodilating, anticoagulant and antiplatelet properties. However, in sensitized patients, there will be a local reaction.
The thinking is that a bite in unsensitized individuals will cause no observable reaction, although some may have a small pruritic papule. However, once sensitized, individuals will go through a spectrum of presentations. A 1940 paper described five stages of mosquito bite reactions. Initially, they will only have a delayed response with a pruritic wheal that forms around the bite peaking at around 24 hours. After more bites from the same species, they develop an immediate reaction, peaking at 30 minutes, in addition to this delayed response. With time this progresses to only an immediate reaction and later no reaction at all; this is species-specific due to varying antigens in the mosquitoes’ saliva. One study was able to induce desensitization in healthy volunteers by subjecting them to 100 bites every two weeks for ten months. In real life, the time to desensitization may be prolonged due to efforts to avoid exposure and with seasonal changes.
Immediate reactions are postulated to be due to IgE mediated type 1 reactions as they correlate with serum levels. When this IgE binds to mosquito saliva proteins, mast cells are triggered, causing a release of histamines and leukotrienes responsible for the wheal, itch, and warmth of immediate reactions. IgG and T-cells are involved in Type 4 delayed reactions. It is thought that IgG also triggers an immune complex type 3 reaction, responsible for the case reports of serum sickness-like symptoms in some case reports.
A thorough history and physical is all that is needed to diagnose mosquito bites. Typically, patients endorse the sensation of a bite before the onset of skin findings. The actual bite is painless, especially compared to other insects. Usually, wheals and papules are round and have a central bite mark. However, severe excoriations may obscure physical findings.
Although work has been done to create mosquito extracts for diagnostic testing, currently available extracts have limited use and a varying amount of antigen. At this point, the standard for clinical trials is the mosquito bite test. However, the limitation of this technique is the availability of disease-free laboratory raised mosquitoes.
Most reactions are mild and self-limiting, making testing unnecessary. Also, by the time someone has an evaluation for possible mosquito-borne illness, their skin findings would have likely already resolved.
There have been a few small volunteer trials on medications to relieve the symptoms of mosquito bites. Today it is suggested that cetirizine (10 mg by mouth) can reduce itching and swelling as a result of bites. In the only pediatric-focused study, 0.3mg/kg of loratadine was also found to be effective. In some studies, starting medications prophylactically before exposure can prevent symptoms.
There is a need for more clinical trials to explore the efficacy of topical glucocorticoids and leukotriene receptor antagonists. In other insect bites, topical glucocorticoids have demonstrated effectiveness and thus have recommendations for use for more severe mosquito bites. In severe and substantial reactions it may be prudent to start oral glucocorticoids, as one would with other urticarias.
In general, the diagnosis of insect bites is obvious; however, careful consideration should be given to avoid misdiagnosing other pruritic skin conditions.
Mosquito bites are frequently misdiagnosed as bacterial cellulitis; this is especially true in the very young, who may have more exaggerated reactions to bites. A general rule of thumb is that mosquito bites develop over hours, while cellulitis develops over days. Additionally, cellulitis following mosquito bites is a frequent finding after the patient has been excoriating the area.
Local reactions from mosquito bites are usually self-limiting and do not last more than ten days. However, mosquito-borne illness has greater morbidity, and their prognosis depends on the causative agent.
There are a few rare complications that must be noted. Anaphylaxis is very rare but must be a consideration and treated promptly. Skeeter syndrome can develop in young or immunocompromised patients. The characteristic of this syndrome is the development of fever and large areas of erythema hours after the bite. The rapidity of symptoms is what makes cellulitis an unlikely culprit.
Hypersensitivity to mosquito bites (HMB), has been mostly described in East Asian journals; this occurs in patients with chronic EBV infections and presents as an exaggerated reaction to mosquito bites. These symptoms include bulla formation, skin necrosis, high fever, and malaise. The patient will usually recover from these episodes, but may later develop a malignant histiocytosis that may prove fatal.
Preventing mosquito bites is the most important therapy, as it prevents not only the local skin reaction but also any mosquito born illnesses. Patients are advised to wear clothes that cover their arms and legs and to avoid the most active time for mosquitos, dusk, and dawn.
In many countries in the developing world, mosquito nets are used to protect from bites at night. In fact, “la mosquetera” is a mainstay gift of baby showers in Latin America. If nets are permethrin treated, it increases their effectiveness; however, caution should be used in infants to avoid toxicity.
DEET (diethyltoluamide) is the most commonly recommended and studied mosquito repellant to date. It works by creating an offensive odor and foul taste barrier that discourages biting. Although it has an excellent safety protocol with proper use, care must be taken to not leave on overnight or apply to faces; this is especially important in children who are at higher risk for toxicity. Picaridin is a new repellant, with comparable efficacy to DEET, but without the unpleasant order.
Prior to the introduction of DEET, citronella was the premier insect repellant. However, in head to head trial, it appears it only confers one hour of protection, versus the 8 hours of protection provided by DEET.
Global eradication programs including spraying of bodies of water and eliminating small pools of water are thought to have controlled mosquito-borne illnesses, especially in the United States.
The bites of mosquitoes may be a minor problem or may lead to serious medical conditions including mosquito-borne illnesses and severe allergic reactions. In recent years Dengue, West Nile Virus, Chikungunya and Zika virus have had increasing incidence in the United States. It is imperative for healthcare workers (the nurse practitioner, primary care provider) to be up to date on not only how to treat local reactions, but how to prevent bites and diminish the impact of the next big epidemic. The team should work together in diagnosing and treating the patient, as well as in providing patient education.
Identifying and treating mosquito bites and their sequelae is best addressed as part of an interprofessional team approach including primary care physicians, infective disease specialists, mid-level practitioners, and nursing (including nurses with specialty training in infectious disease control) to provide optimal patient care.
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