Chiggers, Trombicula alfreddugesi, are unique in that only the six-legged larvae, and not the eight-legged adults or nymphs, affect humans and animals. Found most commonly in the southern United States, the larvae are found in vegetation, and attaches to those who pass by. Chigger infestation is self-limited, usually seen in the summer and fall, and presents as intensely pruritic, grouped, edematous, erythematous 1–2 mm papules or papulovesicles, with a hemorrhagic punctum at sites of constricted clothing (elastic in socks, edges of underwear, belts at the waist) [15]. In boys and men, chiggers can also be found on the penis, causing “summer penile syndrome” with symptoms of penile swelling, pruritus, and dysuria. The larvae inject an irritating substance into the skin, causing the pruritus, before falling off the skin or being scratched off. Secondary infection may develop.

As the name suggests, grain mites are found in grain and straw. Bites from these mites result in intensely pruritic macules, papules, vesicles, or pustules followed by urticarial wheals without burrows. Fever and purpura may develop in more severe cases. Avian mites (also known as fowl mites or bird mites) are rarely found on humans, but instead are found in nearby birds, nests, clothing, and bedding. Two different genera of avian mites cause accidental skin eruptions, termed gamasoidosis in humans: Dermanyssus and Ornithonyssus. Eruptions are typically a widespread pruritic papular dermatitis with a morbilliform appearance, with possible vesicles and urticarial wheals without burrows. Eruptions from either type of mite are self-limited.

Demodex mites, Demodex folliculorum and Demodex brevis, are normally found in human pilosebaceous units of the head and neck. In some susceptible individuals, they may cause a pruritic folliculitis on the face and/or trunk comprised of both papular and pustular lesions, occasionally with a perioral affinity resembling rosacea. Demodex often may affect the eye as well, leading to blepharitis, chalazions, and other findings. This eruption is more common in adults and, in children, may suggest immunocompromise.

Ticks are round arachnids with specialized mouth parts for sucking blood. There are three categories of ticks: hard ticks (Ixodidae), soft ticks (Argasidae), and Nuttalliellidae. Only hard and soft ticks bite humans; however, hard ticks transmit the majority of diseases because they can remain attached to human skin, whereas soft ticks do not. Most tick bites are painless, and transmission of diseases usually result only after 24 h of tick attachment. Tick-borne diseases often have specific vectors, and more than one disease can be transmitted at a time. Ixodes scapularis and I. pacificus transmit Lyme disease, babesiosis, human granulocytic ehrlichiosis (human anaplasmosis), and in Europe, may cause viral encephalitis. The Dermacenter ticks, D. variabilis (the American dog tick) and D. andersoni (the Rocky Mountain wood tick), are major vectors for Rocky Mountain Spotted Fever in the eastern and western United States, respectively. D. andersoni also transmits Colorado tick fever, Q fever, and tularemia. Amblyomma americanum (the lone star tick) is a vector for human monocytic ehrlichiosis. Ornithodoros soft ticks transmit borrelial relapsing fever. Ixodes ticks tend to bite the torso; Amblyomma ticks, the lower legs, buttocks and groin; Dermacentor ticks, the head, neck, and upper trunk.

Patients may present with the offending tick still attached, often thinking that it is a new mole, as the attached tick slowly engorges itself with blood before falling off up to 2 weeks later. Hypersensitivity reactions to bites may cause erythematous papular, nodular, bullous, and ulceronecrotic lesions that are usually pruritic. A classic sign of tick bites is the “comet” sign, where many bites spread from initial points in the distal areas of the ankles and legs [23].

Spiders are found worldwide and play an important role in controlling insect populations. Spiders tend to bite out of self-defense and bites are common, though the offending spider is usually not identified. The two most clinically relevant species in the United States are the black widow (Latrodectus mactans) and brown recluse (Loxosceles reclusa) species and will be discussed here.

The black widow spider is a black- or brown-colored spider with a reddish hourglass visible on the ventral abdomen, and is found throughout the North American continent and Cuba. It has a potent depolarizing neurotoxin, alpha-latrotoxin, and female spiders are more dangerous due to their larger size. A black widow spins webs in cool, dark places and tends to bite only if threatened. Bites appear as two red marks with surrounding edema, and are normally seen on exposed skin, buttocks, or genitalia. The bites result in severe pain within minutes, and over the course of hours, chills, vomiting, muscle cramps, abdominal rigidity, and partial paralysis may ensue. A morbilliform eruption may sometimes be present, and several reports of resulting priapism exist in the literature [24]. Children may develop profuse sweating and become more agitated and irritable. Most black widow bites run a self-limited course, though a small minority of bites may be lethal.

Many species of Loxosceles spiders can be found throughout the world. Loxosceles reclusa, the brown recluse spider, is found hidden in dark, dusty areas, such as attics and basements, predominantly in the Southeastern, Midwestern, and Southwestern United States. It is a brown spider with a dark-brown, violin-shaped mark on the dorsal cephalothorax measuring around 1 cm in size. The primary toxic enzyme is sphingomyelinase D, which has numerous hematologic effects, including red blood cell lysis and skin necrosis due to neutrophil activation. Other enzymes found include alkaline phosphatase, esterase, ATPase, and hyaluronidase. The spider tends to bite only in self-defense, and these slow-healing bites are usually found on the extremities. Mild cases result in a self-resolving urticarial reaction. Classically, a bite causes localized pain, pruritus and erythema within 6 h and progressively becomes more painful, ulcerated, and necrotic over the subsequent 18 h. Lymphangitis and gangrene may develop over the following week, occasionally together with a generalized petechial or morbilliform eruption, as well as fever, chills, nausea, and arthralgias. Serious systemic findings are rare, but may include necrotizing fasciitis, severe hemolysis, renal insufficiency, pulmonary edema, disseminated intravascular coagulation, and shock.

Scorpions are nocturnal, tropical arachnids with a stinger containing venom at the end of a curved, elongated tail. They are found worldwide, and in the United States, Centruroides exilicauda is the most common species, found most commonly in the southern states. Scorpions tend to hide in dark places and sandboxes, and sting only by accident or in self-defense. Stings deposit two toxins, a hemolytic toxin and a neurotoxin, typically leading to pain and paresthesias often out of proportion to the erythema and edema seen, accompanied by tachycardia and hypertension. Locally, other cutaneous findings that may be seen include petechiae, purpura, bullae, necrosis, and lymphangitis. The deposited neurotoxin may potentially cause salivation, vomiting, colicky abdominal cramps, psychomotor agitation, convulsions, cardiac arrhythmias, acute pulmonary edema [23] and, rarely, respiratory paralysis and death. In general, children are at higher risk for severe envenomation, as most fatalities from scorpion stings are reported in children younger than 10 years old [31, 32]. Notably, stings from the Egyptian scorpion lead to death from respiratory failure in 50 % of children.

Lice (or louse, if singular) are wingless, `flat-bodied insects that have been infesting humans worldwide for thousands of years. Three types of lice are clinically important—head lice (Pediculus humanus var. capitis), body lice (Pediculus humanus var. corporis), and pubic lice, also known as head crab or crab lice (Phthirus pubis). Lice are obligated to live off human blood, and measure 1–4 mm in size. Head and body lice are elongated and similar in appearance, whereas crab lice resemble a miniature version of the food.

In children, head lice cause the most common of the lice infestations, and are most frequently seen in children ages 3–12 and their parents. It affects all races, though African-American children are less often infected. Head lice usually cannot survive away from the human host more than 36 h. Adult lice are approximately 1–3 mm in size and feed on the scalp every 4–6 h. Females live for approximately 1 month and lay five to ten eggs daily. Eggs, called nits, are 0.8 mm in size, and viable nits are usually found cemented by a proteinaceous matrix to the hair within 0.6 mm of the scalp, but can be farther away in warmer climates. Nits develop into adult lice over the course of 2 weeks. Bites from head lice are painless, and the hallmark of disease, intense scalp pruritus, usually manifests within 2–6 weeks of infestation. The diagnosis is made by identifying the louse or nits in the scalp. Erythematous macules, papules, excoriations, scaliness or secondary bacterial infection can sometimes be seen on the scalp. Cervical or suboccipital lymphadenopathy may also be present.

Of note, there is increasing resistance to some of these therapies, particularly permethrin and lindane. No therapies are reliably ovicidal, so repeat treatment in 1 week is recommended for all of the following. This also helps to fight growing resistance, which has already been reported with permethrin, pyrethrins, lindane, and malathion. Until safety is established for the following therapies, only mechanical removal of lice and nits is recommended for children under 2 years old, except for permethrin 1 % cream, which is FDA approved down to 2 months of age [3, 4, 13, 41–53].

The body louse measures 2.4–4 mm in length and is most often found in areas of poor hygiene, poverty, and overcrowding. Unlike the other two types of lice, body lice are vectors of several human diseases, including epidemic typhus (Ricketssia prowazekii), relapsing fever (Borrelia recurrentis), trench fever, and bacillary angiomatosis (Bartonella quintana). The louse is rarely found on human skin, but rather, resides in the seams of clothing, where its nits can also be found, and it survives for up to 1 month away from the host. Bites from the body louse are usually painless, and result in erythematous or copper-colored macules, pinpoint papules or wheals, with a hemorrhagic punctum. Excoriations, secondary bacterial infections, furuncles, and lymphadenopathy may be found. Unlike scabies, the hands and feet are not typically involved.