Introduction
Background
Insects are arthropods of the class Insecta. Insects have an adult stage characterized by a hard exoskeleton, 3 pairs of jointed legs, and a body segmented into head, thorax, and abdomen. Insects comprise the most diverse and numerous class of the animal kingdom and include numerous species of praying mantis, dragonflies, grasshoppers, true bugs, flies, fleas, bees, wasps, ants, lice, butterflies, moths, and beetles. The number of species is estimated at between 6 and 10 million, with more than a million species already described. Insects represent more than half of all known living organisms and potentially represent more than 90% of the differing life forms on Earth. Hence, human contact with insects is unavoidable. Exposure to biting or stinging insects or to their remains can range in severity from benign or barely noticeable to life threatening.
Yellowjacket wasp. Image courtesy of CDC.
Louse, Pediculus humanus, dorsal view after feeding on blood. Most lice are scavengers, feeding on skin and other debris found on the host's body, but some species feed on sebaceous secretions and blood. Image courtesy of CDC.
Differentiating between insect bites and stings
Many patients confuse an insect bite with a sting and may use the terms interchangeably. A bite is usually from mouth parts and occurs when an insect is agitated to defend itself or when an insect seeks to feed. Bites from mosquitoes, fleas, bed bugs, and mites are more likely to cause itching than pain.
Anopheles albimanus mosquito feeding on human host. Image courtesy of CDC.
A bedbug, Cimex lectularius, feeding on human blood. The bedbug's name comes from its preferred habitat, including mattresses, sofas, and other furniture. Image courtesy of CDC.
A stinging apparatus is usually a sharp organ of offense or defense, especially when connected with a venom gland, and adapted to inflict a wound by piercing, as the caudal sting of a scorpion. The stinger is typically located at the rear of the animal. Animals with a stinger include bees, wasps, hornets, and scorpions. Most stinging insects are of the order Hymenoptera, which includes ants, bees, and wasps. Hymenoptera stings result in more fatalities than stings or bites from any other arthropod. Wasps can bite and sting at the same time (See Wasp Stings).
Since their introduction into the southern United States in the 1920s, imported fire ants anchor themselves with their mandibles and subsequently inflict a sting. Fire ants often pivot or re-anchor themselves only to sting again and again resulting in a sensation of fire at the site. When fire ants swarm, they often position themselves on their victim and sting simultaneously in response to an alarm pheromone released by one or several individuals. Immobilized or elderly patients can become rapidly covered by swarms of these ants, resulting in severe stings and even death (See Fire Ant Bites).
Bites by classes Insecta
This article is limited to bites by insects and not arachnids. Stings by members of the order Hymenoptera and order Scorpionida are discussed in other articles, as are bites of venomous arachnids in the class Arachnida (spiders) and bites of the order Acarina (mites and ticks).
Injuries from exposure to millipedes (class Diplopoda), centipedes (class Chilopoda), and caterpillars (order Lepidoptera) also are discussed in other articles (see Differentials); however, many of the principles that guide diagnosis and treatment of insect bites also apply to bites and stings of these other organisms.
Exotic insects
While illness related to insect exposure in a particular locale may be easily recognizable, the emergency physician must also be aware of more exotic insect-related diseases as humans travel to more remote areas of the country and the world. Additionally, exotic insects are often kept as pets (sometimes illegally) or can be encountered in shipments of foreign origin.
Anaphylactic shock
Anaphylactic shock is the most notable immediate risk associated with insect exposures. Hypersensitivity to otherwise harmless insect saliva, venom, body parts, excretions, or secretions can cause systemic responses in some individuals. Diagnosing the early phases of a systemic allergic reaction preceding anaphylactic shock is of paramount importance in treating any patient in whom insect exposure is suspected. Severe anaphylaxis can be fatal in as little as 10 minutes.
The reoccurrence rate is 40-60% for insect stings. Hence, the patient should be instructed on how to avoid future exposure to the causative agent, if possible. A prescription and clear instructions on the use of an epinephrine autoinjector should be provided to patients when the risk of another reaction is judged to be substantial.1
Diseases transmitted by insect bites
The need to be aware of diseases transmitted by insect bites is crucial; Lyme disease, transmitted by ticks, and malaria, transmitted by mosquitoes, are discussed in other articles (see Tick-borne Diseases, Lyme; Malaria).
Chagas disease, increasingly found in the desert southwest and in persons residing in or traveling to Central America or South America, should be considered, particularly when the bite site is on the soft skin of the periorbita or lips.2 Because this infection may produce an acute and chronic illness with notable morbidity and mortality, especially in pediatric patients, clinicians should maintain a high index of suspicion (see Trypanosomiasis).
Mosquito and tick-borne encephalitides such as those produced by the eastern equine virus or the West Nile virus also should be considered in patients presenting with meningismus (see Encephalitis).
Of note, some illnesses transmitted by insects do not produce symptoms until long after the infecting bite. In South America and parts of Africa, blackflies (Simuliidae) are responsible for transmission of onchocerciasis. This illness is also known as river blindness and eventually can produce blindness years after the initial infection. This disease is extremely rare in the United States. Chagas disease, a leading cause of cardiomyopathy in the world, may present latently as well. Obtaining a history of international travel is important because this information can lead to a diagnosis that would otherwise be overlooked. Determining the destination, time of year, length of stay, and time since travel are all important pieces of history to obtain.
Exposure to arthropods may produce dermatitis, cellulitis, urticaria, or blistering unrelated to biting or stinging. Some species of moths, caterpillars, centipedes, beetles, and spiders have urticating hairs or secretions that can cause cutaneous irritation. For further information, please refer to the respective articles on these exposures (see Differentials).
An uncommon occurrence in North America is myiasis by fly larvae. Fly larvae enter the host through varying mechanisms ranging from oviposition of live, burrowing larvae on the host, on or near open wounds, to attachment to other bloodsucking insects. While not generally the result of an insect bite, myiasis can produce pustules and lesions similar to insect bites. These lesions generally contain one or more developing fly larvae. Severe cases of myiasis can cause seizures.
While plant-eating phytophagous insects can bite in self-defense, their bites generally are not purposeful. This article is limited to discussion of organisms that bite to feed on blood or to catch prey.
Relatively harmless insects
Cockroaches have been reported to bite humans, but their bite generally is harmless. Continued repeated exposure to their remains and feces poses a greater health threat, such as increased incidence of asthma, especially in inner cities, and their remains and feces are possible vectors for transmission of viral and bacterial diseases.
Earwigs generally are harmless insects that have earned an unpleasant reputation. This may be because of their depiction in popular culture, such as in the television series, "The Night Gallery." Although they appear to have a large pincer on the posterior abdomen, it is not capable of rendering anything more serious than a mild pinch. Additionally, and contrary to popular belief, they do not routinely enter the human ear canal and parasitize humans. Cockroaches are much more likely to be found lodged in a patient's auditory passage.
Pathophysiology
Mouthparts of biting insects can be classified into 3 broad groups: piercing, sponging, and biting. Tremendous diversity exists in the morphology of these groups. Insects discussed in this article generally are nonvenomous, yet many species inject saliva while biting. Their saliva may aid in digestion, inhibit coagulation, increase blood flow to the bite, or anesthetize the bite locus. Most lesions are the result of the victim's immune response to these insect secretions. In the case of Chagas disease, the infective organism is transmitted via the feces of a reduviid bug, which enters through the bite site when the wound becomes pruritic and is scratched. Most insect bites are minor and can result in superficial puncture wounds to the skin. Horseflies feed with a large scissorlike proboscis that can cause a relatively deep and painful wound.
Frequency
United States
In the United States, the American Association of Poison Control Centers reported 42,620 cases of exposures to insects in 2007. Just more than 200 of these were listed as resulting in moderate or major reactions. A moderate reaction is defined as signs or symptoms that were more pronounced or systemic, whereas a major reaction is life-threatening or lead to significant residual disability. Fatalities among these exposures are rarely reported to poison centers and usually result from allergic reactions to Hymenoptera stings. Because of underreporting, these numbers are only a glimpse as to what is actually occurring.3
International
Reliable statistics are not available for insect bite exposures because most cases are not reported and do not require hospital care.4 A study in tropical Zimbabwe, where biting insects are common, found that 1.5% of hospital admissions were related to insect exposure, including both bites and stings. A vast majority of these were arachnid or Hymenoptera related.
Mortality/Morbidity
Mortality associated with insect bites is from hypersensitivity reactions, either anaphylactic (immunoglobulin E [IgE] mediated) or anaphylactoid (non-IgE mediated), or from complications resulting from infection. Reliable figures on incidence and prevalence are not available. Estimates of mortality from insect-provoked anaphylaxis in the United States range from 50-150 persons annually. In Arizona, for example, death from reduviid-associated anaphylaxis has been reported as a leading cause of death from insect exposure. Worldwide, the greatest morbidity and mortality associated with insect bites are due to Anopheles species mosquito bites resulting in infection with malaria (see Malaria).4
Sex
No sexual preference known.
Age
No age preference known.
Clinical
History
Most patients are aware of insect bites when they occur or shortly thereafter, but because it is such a common occurrence, the exposure is typically dismissed unless a severe or systemic reaction occurs.
- Reactions to bites may be delayed due to the host being asleep or because the saliva of some micropredators may contain an anesthetic secreted to allow uninterrupted blood-feeding.
- Patients who present with a history of homelessness or of staying in homeless shelters may have an exposure to organisms such as bedbugs.5,6,7 Alternatively, patients with impairment from mental illness may also be susceptible to infestation with insect parasites.
- Exposure to feral animals or even to domesticated animals, such as livestock or house pets, may predispose patients to exposure to biting insects such as fleas, bedbugs,8,6,7 or lice.
- Types of reactions
- In a local reaction, the patient may complain of discomfort, itching, moderate or severe pain, erythema, tenderness, warmth, and edema of tissues surrounding the site. Although it may involve neighboring joints, local reactions cause no systemic symptoms.
- In a severe local reaction, complaints include generalized erythema, urticaria, and pruritic edema. Severe local reactions increase the likelihood of serious systemic reactions if the patient is exposed again at a later time.
- In a systemic or anaphylactic reaction, the patient may complain of localized symptoms as well as symptoms not contiguous with the bite location. Symptoms can range from mild to fatal. Early complaints typically include generalized rash, urticaria, pruritus, and angioedema. These symptoms may progress, and the patient may develop anxiety, disorientation, weakness, gastrointestinal disturbances (eg, cramping, diarrhea, vomiting), uterine cramping in women, urinary or fecal incontinence, dizziness, syncope, hypotension, stridor, dyspnea, or cough. As the reaction progresses, patients may experience respiratory failure and cardiovascular collapse.
- Delayed reactions may appear 10-14 days after a sting. Symptoms of delayed reactions resemble serum sickness and include fever, malaise, headache, urticaria, lymphadenopathy, and polyarthritis.
Physical
- Without a clear patient history, diagnosis of an insect bite can be difficult since the initial response may be limited to erythema, local pain, pruritus, or edema.
- Wheals and urticaria are common initial signs and generally appear within a few minutes of the bite. Unfortunately, many dermatologic conditions also produce similar cutaneous signs and may confound the diagnosis.
- Identification of the insect responsible for the bite may be possible by examining the location, number, pattern, and sequelae of the bite(s).
- Physical examination elements indicating a systemic reaction include the following:
- Cardiovascular - Hypotension, orthostasis
- Cutaneous - Urticaria, wheals, angioedema, blood at bite site, pruritus
- Respiratory - Tachypnea, stridor, wheezing, bronchospasm
- Gastrointestinal - Hyperactive bowel sounds
More on Bites, Insects |
 Overview: Bites, Insects |
| Differential Diagnoses & Workup: Bites, Insects |
| Treatment & Medication: Bites, Insects |
| Follow-up: Bites, Insects |
| Multimedia: Bites, Insects |
| References |
| Further Reading |
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Further Reading
Clinical guidelines
The diagnosis and management of anaphylaxis: an updated practice parameter.
American Academy of Allergy, Asthma and Immunology - Medical Specialty Society
American College of Allergy, Asthma and Immunology - Medical Specialty Society
Joint Council of Allergy, Asthma and Immunology - Medical Specialty Society. 1998 Jun (revised 2005 Mar). 41 pages. NGC:004211
Consultation and referral guidelines citing the evidence: how the allergist-immunologist can help.
American Academy of Allergy, Asthma and Immunology - Medical Specialty Society. 2006 Feb. 29 pages. NGC:005003
The practice of travel medicine: guidelines by the Infectious Diseases Society of America. Infectious Diseases Society of America - Medical Specialty Society. 2006. 96 pages. NGC:005086
Clinical trials
Cause of Unexplained Anaphylaxis
Population Pharmacokinetics of Benznidazole in Children With Chagas Disease
Immunogenicity, Safety and Interchangeability of Two Tbe Vaccines Administered According to a Conventional Schedule in Children
Keywords
insect bite, bug bites, Insecta, Hymenoptera, Arachnida, anaphylactic shock, Lyme disease, Chagas disease, trypanosomiasis, tick-borne encephalitides, blackflies, Simuliidae, onchocerciasis, river blindness, dermatitis, cellulitis, urticaria, myiasis, fly larvae, human botflies, NewWorld screwworms, Old World screwworms, Wohlfahrtia flies, Tumbu flies, fly maggots, delusional parasitosis, plant-eating phytophagous insects, cockroach bite, earwigs, reduviid bug, horsefly bites, Anopheles mosquito, malaria, angioedema
