Most insect bites are presented in form of puncture wounds, papule formation, localized urticaria and pruritus. These symptoms typically subside within a few hours or days. Both immunodeficiency and secondary infection after scratching may provoke more severe local symptoms. Patients infected with HIV, those suffering from mast cell disorders or lymphoproliferative diseases may present with tissue necrosis due to insect bites. Lymphadenopathy and fever may be observed. Scratching may lead to secondary bacterial infection or allow transmission of trypanosoma.

Systemic reactions are very rare, but may consist in:

• Anaphylaxis. If previous exposure to insect allergens led to mast cell sensitization, an insect bite may trigger a type I hypersensitivity reaction manifesting in severe urticaria, pruritus, vomiting, respiratory symptoms, tachycardia, hypotension and possibly anaphylactic shock.
• Serum sickness. Characteristic symptoms are malaise, fever, chills, lymphadenopathy, arthalgia and abdominal pain [9].
• Papular urticaria. Patients suffering from papular urticaria typically present with chronic skin lesions and pruritus that tend to disappear in winter but that often reappear next summer. Papules may form at a considerable distance from the causative insect bite [10].

Patients currently treated with beta-sympatholytics and angiotensin-converting enzyme inhibitors seem to be at higher risks of systemic reactions.

Most insect bites don't require any additional diagnostic measures.

If patients present uncommon symptoms like tissue necrosis, these should guide the physician's decision regarding further diagnostic measures. They may be carried out to identify potentially underlying immunodeficiency.

Thorough anamnesis and query for possible exposure to insects may be of great help to identify the etiological agent in patients presenting with serum sickness or papular urticaria.

With regards to anaphylaxis, immediate treatment takes priority over diagnostic workup. However, patients that show severe adverse reactions to insect bites are considered to have higher risks of anaphylaxis upon getting bitten again. Therefore, the causative species and/or allergen should be identified if possible. Significant advances have been made regarding diagnostic measures such as skin tests to prove hypersensitivity to determined proteins [4] [11].

Local application of ice and possibly antihistamines may help to reduce inflammation and pruritus. The latter is of major importance to avoid scratching and subsequent secondary infection, particularly in children. Systemic treatment is not necessary in uncomplicated cases of insect bites.

In contrast, tissue necrosis and systemic symptoms do require special treatment, possibly even surgery. While antihistamines and corticosteroids is occasionally used to treat serum sickness and papular urticaria, they are routinely administered to patients suffering from anaphylaxis. Additionally, these patients may require application of adrenaline. Depending on adrenaline-mediated effects, aggressive intravenous fluid therapy to compensate for hypotension and administration of beta-sympathomimetics to induce bronchodilation may be necessary. Early intubation and oxygen supply is recommended to avoid asphyxia due to bronchospasm.

Prognosis is generally excellent. Most insect bites are little more than transient nuisances; however, severe reactions to insect bites may be associated with significant morbidity and mortality. Anaphylaxis, tissue necrosis and secondary bacterial infection may pose serious threats to human health and require immediate medical attention. The risk of an anaphylactic reaction is significantly increased in patients presenting underlying mast cell disorders [8].

As has been mentioned above, mosquitos, fleas, bugs and lice shall serve as examples for insects that may inflict bites in this article.

• Mosquitos pertain to the order of Diptera and constitute the family Culicidae. Female mosquitos dispose of mouthparts forming a proboscis that allows for penetration of human skin and sucking of blood.
• Fleas form a rather small order within the insect class, the Siphonaptera. Similar to mosquitos, they are able to pierce human skin and feed on blood.
• Bugs, or true bugs, pertain to the order Hemiptera. Of medical importance are bed bugs (family Cimicidae) and assassin bugs (family Reduviidae). They pierce and suck using their proboscis.
• Lice form the order Phthiraptera and have retractable mouthparts that are also adapted to piercing and sucking.

With the exception of the oceans, there is no place on earth that's free from insects. They can be found on every continent, even in Antarctica. However, insect populations are generally larger in hot and humid climate zones. In temperate climates, the risk of insect bites is higher during the warm summer months.

Many insect species inhabit certain geographic regions and thus, only locals and travelers may be bitten by a particular type of insect. While this also limits exposure to pathogens transmitted by insects, expansion of vectors may put additional human populations at risk. Such phenomena have been observed in mosquitos transmitting malaria and leishmaniasis, for instance [2] [3].

People pertaining to any race, gender and age group may be bitten by insects. Overall morbidity and mortality are very low, but serious complications may be observed even in patients who haven't been diagnosed with hypersensitivity previously. Such reactions may be fatal [4].

Insects either bite to access possible food sources or to defend themselves and their offspring. While biting, they expose their host or enemy to proteins contained in their saliva [5], to anticoagulants injected in order to feed more easily on blood, to local anesthetics to avoid being removed while feeding, and possibly to pathogens they serve as vectors for. With regards to the former, all kind of proteins may act as allergens and severe reactions to insect bites are almost exclusively mediated by the immune system of the patient. Usual reactions to insect bites are also evoked by the immune system and mainly comprise urticaria and pruritus as characteristic symptoms of a type I hypersensitivity reaction. This may cause the patient to scratch, which, in turn, may lead to secondary infections.

If healthy individuals are bitten by insects, the main risk for complications rests in transmission of pathogens. A plethora of viruses, bacteria and protozoa may be transmitted by biting insects. Some pathogens use distinct insect species as an unspecific carrier, while others have to complete developmental stages within determined insects.

• Mosquitos are known to transmit all kinds of pathogens. They serve as vectors for viruses (e.g., those viruses causing yellow fever, Dengue fever, West Nile fever), bacteria (mainly Enterobacteriaceae) and protozoa (e.g., Dirofilaria immitis [6], Leishmania spp., Plasmodium spp.).
• Although fleas are rarely considered when talking about vector-borne diseases today, they are of major historical importance as vectors of Yersinia pestis, causative agent of the bubonic plague. Of note, this disease is still not eradicated [7], but antibiotics have considerably reduced its dreadfulness. Fleas may also transmit Rickettsia spp. that cause typhus.
• While bed bugs are not known to transmit infectious diseases, assassin bugs serve as vectors for Trypanosoma cruzi. This protozoan parasite causes Chagas disease and is actually transmitted via the feces of the bug, but infection is largely facilitated if the patient scratches bite sites.
• Similar to fleas, presence of lice has been associated with bouts of typhus. Lice may also serve as vectors for Borrelia recurrentis and Bartonella quintana, etiological agents of relapsing fever and trench fever, respectively.

Insect bites may be prevented by minimizing exposure. Appropriate measures are particularly recommended to people living in or traveling to geographical regions where insects transmit diseases like malaria and Chagas disease. Preventive measure comprise:

• Wearing of long-sleeved clothes.
• Use of window screens and/or mosquito nets.
• Use of repellents. Insect species may be resistant to certain formulations. Up-to-date information should be consulted before deciding for any particular repellent.
• Prolonged stays in high-risk areas such as those close to stagnant waters should be avoided. This restriction may apply only to certain times of the day and year.

In order to avoid secondary infections of insect bites, patients should refrain from scratching.

Patients with a medical history of adverse reactions to insect bites should carry adrenaline pens to allow for immediate countermeasures in case of anaphylaxis. In severe cases, desensitization should be considered [12]. Spontaneous desensitization may occur while growing up [5].

Insects constitute a huge class of animals that may comprise as much as ten million species - some experts estimate even higher numbers. Animals as distinct as beetles, flies, wasps, bees, ants, moths and butterflies are all insects. They pertain to the phylum of arthropods, like arachnids do, but both classes can easily be distinguished by counting legs: Insects have six legs, arachnids have eight. Of course, there are many other morphologic and functional differences between insects and arachnids, but this method should suffice for non-zoologists.

An insect's body consists of head, thorax and abdomen, and is sustained by a hard exoskeleton. Upon closer inspection, antennae, eyes or ocelli and mouthparts may be recognized on the head. The insect's mouthparts are often used to illustrate evolution: While most insect species dispose of labrum, mandibles, maxillae, labium and hypopharynx, these organs have been largely modified to adapt to distinct forms of nutrition. Insects may chew, bite, suck, sponge, lick and pierce with their mouthparts.

Any damage inflicted with an insect's mouthparts may be referred to as an insect bite. Because mosquitos pierce and suck with their mouthparts, they cause insect bites. Other insects that may invoke lesions by using their mouthparts are fleas, bugs and lice. Insects may bite to access food sources or to defend themselves.

On the other hand, evolution brought forward a wide variety of insects that dispose of specialized organs to inject venom into the body of a potential enemy, e.g., the sting of bees and wasps. These organs are usually supported by an insect's abdomen. Although insect bite and insect sting are often used interchangeably, they are clearly different entities from a zoological point of view. Of note, some species, like wasps may bite and sting. Most humans who encounter wasps are stung, not bitten, though.

Some insect bites are associated with papule formation, transient pruritus and/or localized pain, others are not even noticed by the person who got bitten [1]. However, certain patient groups have high risks of severe, possibly lethal complications. These patient groups comprise people who present hypersensitivity reactions after exposure to insect allergens as well as immunocompromised patients who may develop local reactions or systemic symptoms.

Insects constitute a huge class of animals that comprises several millions of species. In general, an insect's body consists of head, thorax and abdomen, whereby three pairs of legs can be observed on the thorax. Indeed, the fact that insects have six legs and arachnids have eight may be used to distinguish animals pertaining to either one taxonomic class.

The insect's head carries antennae, primitive eyes and mouthparts. Evolution brought forward a wide variety of species whose mouthparts are adapted to pierce human (or other species') skin and to suck blood. From a zoological point of view, this is an insect bite. Considering that definition, mosquitos, fleas, bugs and lice bite.

In contrast, other insects have a sting which is typically located towards the rear end of the animal and which has evolved to inject venom into the body of a possible enemy. These insects sting. While there is a clear difference between an insect bite and an insect sting, some species - wasps, for instance - may bite and sting.

In most cases, insect bites are minor nuisances that cause urticaria and pruritus, symptoms that usually subside within a few hours or days. However, biting insects may also transmit serious diseases, e.g., malaria, leishmaniasis and Chagas disease. Thus, prevention of insect bites does not only aim at avoiding formation of itchy papules but is of major importance for anyone living in or traveling to geographical regions where the aforementioned diseases are endemic.

Severe adverse reactions to insect bites are rarely observed. Immunodeficient individuals may present with tissue necrosis due to an insect bite. Other patients may suffer an allergic reaction and develop life-threatening anaphylaxis, though insect stings are much more likely to induce anaphylaxis than insect bites.

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