Insect bites and stings

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Overview

Insect bites and stings occur when an insect is agitated and seeks to defend itself through its natural defense mechanisms, or when an insect seeks to feed off the bitten person.

Pathophysiology

Insects inject formic acid, which can cause an immediate skin reaction often resulting in redness and swelling in the injured area.

Diagnosis

History and Symptoms

The sting from fire ants, bees, wasps and hornets are usually painful, and may stimulate a dangerous allergic reaction called anaphylaxis for at risk patients. Wasps can bite and sting at the same time.

Differentiating insect bites from other diseases

Different rash-like conditions can be confused with insect bites and are thus included in its differential diagnosis. The various conditions that should be differentiated from insect bites include:

Disease	Features
Impetigo	It commonly presents with pimple-like lesions surrounded by erythematous skin. Lesions are pustules, filled with pus, which then break down over 4-6 days and form a thick crust. It's often associated with insect bites, cuts, and other forms of trauma to the skin.
Insect bites	The insect injects formic acid, which can cause an immediate skin reaction often resulting in a rash and swelling in the injured area, often with formation of vesicles.
Kawasaki disease	Commonly presents with high and persistent fever, red mucous membranes in mouth, "strawberry tongue", swollen lymph nodes and skin rash in early disease, with peeling off of the skin of the hands, feet and genital area.
Measles	Commonly presents with high fever, coryza and conjunctivitis, with observation of oral mucosal lesions (Koplik's spots), followed by widespread skin rash.
Monkeypox	The presentation is similar to smallpox, although it is often a milder form, with fever, headache, myalgia, back pain, swollen lymph nodes, a general feeling of discomfort, and exhaustion. Within 1 to 3 days (sometimes longer) after the appearance of fever, the patient develops a papular rash, often first on the face. The lesions usually develop through several stages before crusting and falling off.
Rubella	Commonly presents with a facial rash which then spreads to the trunk and limbs, fading after 3 days, low grade fever, swollen glands, joint pains, headache and conjunctivitis. The rash disappears after a few days with no staining or peeling of the skin. Forchheimer's sign occurs in 20% of cases, and is characterized by small, red papules on the area of the soft palate.
Atypical measles	The symptoms commonly begin about 7-14 days after infection and present as fever, cough, coryza and conjunctivitis. Observation of Koplik's spots is also a characteristic finding in measles.
Coxsackievirus	The most commonly caused disease is the Coxsackie A disease, presenting as hand, foot and mouth disease. It may be asymptomatic or cause mild symptoms, or it may produce fever and painful blisters in the mouth (herpangina), on the palms and fingers of the hand, or on the soles of the feet. There can also be blisters in the throat or above the tonsils. Adults can also be affected. The rash, which can appear several days after high temperature and painful sore throat, can be itchy and painful, especially on the hands/fingers and bottom of feet.
Acne	It is typical of teenagers, usually appears on the face and upper neck, but the chest, back and shoulders may have acne as well. The upper arms can also have acne, but lesions found there are often keratosis pilaris, not acne. The typical acne lesions are comedones and inflammatory papules, pustules, and nodules. Some of the large nodules were previously called "cysts"
Syphilis	It commonly presents with gneralized systemic symptoms such as malaise, fatigue, headache and fever. Skin eruptions may be subtle and asymptomatic It is classically described as: Non-pruritic bilateral symmetrical mucocutaneous rash Non-tender regional lymphadenopathy Condylomata lata and Patchy alopecia.
Molluscum contagiosum	The lesions are commonly flesh-colored, dome-shaped, and pearly in appearance. They are often 1-5 millimeters in diameter, with a dimpled center. Generally not painful, but they may itch or become irritated. Picking or scratching the lesions may lead to further infection or scarring. In about 10% of the cases, eczema develops around the lesions. They may occasionally be complicated by secondary bacterial infections.
Mononucleosis	Common symptoms include low-grade fever without chills, sore throat, white patches on tonsils and back of the throat, muscle weakness and sometime extreme fatigue, tender lymphadenopathy, petechial hemorrhage and skin rash.
Toxic erythema	It is a common rash in infants, with clustered and vesicular appearance.
Rat-bite fever	It commonly presents with fever, chills, open sore at the site of the bite and rash, which may show red or purple plaques.
Parvovirus B19	The rash of fifth disease is typically described as "slapped cheeks," with erythema across the cheeks and sparing the nasolabial folds, forehead, and mouth.
Cytomegalovirus	The common symptoms include sore throat, swollen lymph nodes, fever, headache, fatigue, weakness, muscle pain and loss of appetite.
Scarlet fever	It commonly includes fever, punctate red macules on the hard and soft palate and uvula (Forchheimer's spots), bright red tongue with a "strawberry" appearance, sore throat and headache and lymphadenopathy.
Rocky Mountain spotted fever	The symptoms may include maculopapular rash, petechial rash, abdominal pain and joint pain.
Stevens-Johnson syndrome	The symptoms may include fever, sore throat and fatigue. Commonly presents ulcers and other lesions in the mucous membranes, almost always in the mouth and lips but also in the genital and anal regions. Those in the mouth are usually extremely painful and reduce the patient's ability to eat or drink. Conjunctivitis of the eyes occurs in about 30% of children. A rash of round lesions about an inch across, may arise on the face, trunk, arms and legs, and soles of the feet, but usually not on the scalp.
Varicella-zoster virus	It commonly starts as a painful rash on one side of the face or body. The rash forms blisters that typically scab over in 7-10 days and clears up within 2-4 weeks.
Chickenpox	It commonly starts with conjunctival and catarrhal symptoms and then characteristic spots appearing in two or three waves, mainly on the body and head, rather than the hands, becoming itchy raw pox (small open sores which heal mostly without scarring). Touching the fluid from a chickenpox blister can also spread the disease.
Meningococcemia	It commonly presents with rash, petechiae, headache, confusion, and stiff neck, high fever, mental status changes, nausea and vomiting.
Rickettsial pox	The first symptom is commonly a bump formed by a mite-bite, eventually resulting in a black, crusty scab. Many of the symptoms are flu-like including fever, chills, weakness and muscle pain but the most distinctive symptom is the rash that breaks out, spanning the person's entire body.
Meningitis	It commonly presents with headache, nuchal rigidity, fever, petechiae and altered mental status.

Treatment

Medical Therapy

Stingers in the skin should be removed by scraping it away with a fingernail or credit card edge. Baking soda paste should then be applied for 20 minutes to neutralize the formic acid. Bites from mosquitoes, fleas and mites are more likely to cause itching than pain.

Related Chapters

• Bee sting
• Schmidt Sting Pain Index