Mosquito Bites

Etiology

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 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.[1]

Epidemiology

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.[2] What is known is that mosquito-borne disease presents a tremendous disease burden, infecting 700 million and causing a million deaths every year.[3]

Some small studies have shown a possible predilection for males over females and adults over children.[4] However, there have been no large trials to date to confirm this as fact.

Pathophysiology

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.[5]

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.[6] 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.[7] In real life, the time to desensitization may be prolonged due to efforts to avoid exposure and 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.[5]

History and Physical

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.

Evaluation

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.[5]

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.

Treatment / Management

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, 0.3 mg/kg of loratadine was also found to be effective. In some studies, starting medications prophylactically before exposure can prevent symptoms.[8][9](B3)

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.

Differential Diagnosis

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.[10] Additionally, cellulitis following mosquito bites is a frequent finding after the patient has been excoriating the area.

Prognosis

Local reactions from mosquito bites are usually self-limiting and do not last more than ten days. However, mosquito-borne illnesses have greater morbidity, and their prognosis depends on the causative agent.

Complications

There are a few rare complications that must be noted. Anaphylaxis is very rare but must be considered and treated promptly.[11] 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.[12]

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.[13]