Section 25. Insect Bites and Infestations

Pediculosis capitis is an infestation of the scalp by the head louse, which feeds on the human scalp and neck and deposits its eggs on the hair.

Synonyms Head lice, louse, nit.

Epidemiology

Age Children > adults.

Gender F > M.

Prevalence 12 million school children annually in the United States. Ten percent of children worldwide.

Incidence Common.

Race Caucasian, Asians > blacks.

Epidemics In schools.

Season Summer.

Etiology Pediculus humanus capitis (2-mm, six-legged, wingless insect; Fig. 25-1).

Pathophysiology

Head lice are transmitted from person to person via shared hats, caps, brushes, combs, or head-to-head contact. The pediculus humanus capitis female lays approximately 10 ova per day gluing their eggs to the hair within 1 to 2 mm of the scalp. The ova hatch in 10 days, the louse emerges as a nymph, reaches its adult form 10 days later, and has a life span of 30 days. Lice have anterior mouthparts that attach and feed on blood five times a day, the louse cannot survive for more than 3 days off the human head. Majority of patients have a population of <10 head lice.

History

Humans contract lice by sharing brushes, hats, close head-to-head contact, etc. The scalp louse deposits nits on the hair next to the scalp, and scalp hair grows 0.5 mm daily (thus, the presence of nits 15 cm from the scalp indicates that the infestation is approximately 9-month-old). New viable eggs have a creamy-yellow color; empty eggshells are white. The infestation first manifests as severe pruritus of the back and sides of scalp. Crusts and secondarily impetiginized lesions are common and may extend onto neck, forehead, face, and ears. In extreme cases, the scalp can become a mass of matted hair, lice, nits, and purulent discharge called plica polonica.

Physical Examination

Skin Findings

Type Lice, nits (1-mm eggs; Fig. 25-2), macules, papules, excoriations.

Sites of Predilection Scalp: occipital, postauricular regions.

Differential Diagnosis

The differential diagnosis of pediculosis capitis includes hair casts, dried hairspray or gel, dandruff (epidermal scales), impetigo, seborrheic dermatitis, and tinea capitis.

Laboratory Examinations

Dermatopathology Bite site will show intradermal hemorrhage and a deep, wedge-shaped infiltrate with eosinophils and lymphocytes.

Microscopic Examination Shows lice and/or nits adherent to hair.

Wood’s Light Nits will fluoresce pearly white and are not movable.

Management

Head lice is best treated with pediculicides: permethrin, lindane, malathion, or mercuric oxide ointment. The pyrethroids are neurotoxic to lice and current management recommends topical 1% or 5% permethrin applied to scalp and washed off after 10 minutes followed by combing nits out with a fine-toothed comb. The treatment should be repeated 8 to 10 days later. If nits and eggs are still present, pyrethrin/pyrethroid-resistant lice may be present and two applications of 0.5% or 1% malathion should be used combine with fine-toothed combing.

Lindane should be avoided in children, because there is potential CNS toxicity associated with overuse or accidental ingestion. Home remedies (such as oils, kerosene, formic acid, and vinegar) are also NOT recommended. All other family members should be checked for asymptomatic lice, because epidemics start in the family, not the school. Floors, play areas, and furniture should be vacuumed. Clothes and bedding should be washed and dried on high heat. Combs and brushes should be soaked in pediculicide for 15 minutes and should not be shared between family members until infection is cleared.

Pediculosis pubis is an infestation of hairy regions, most commonly the pubic area but at times the hairy parts of the chest, axillae, and upper eyelashes. It is manifested clinically by mild-to-moderate pruritus.

Synonyms Crabs, crab lice, pubic lice, phthiriasis.

Epidemiology

Age 14 to 40 years.

Gender M > F.

Incidence Common.

Etiology Phthirus pubis (1-mm, six-legged insect, crablike in appearance; Fig. 25-3).

Pathophysiology

The pubic louse lives exclusively on humans and they are transmitted by close physical contact (crowded living conditions, sharing a bed, sharing towels, sexually transmitted). The P. pubis female lays 1 to 2 ova per day, has a lifecycle of 35 days and cannot live off the host for more than 1 day. They remain stationary with their embedded mouth parts in the skin and their claws grasping a coarse hair (pubic, perianal, axillary, eyelashes, eyebrows, facial hair) on either side (Fig. 25-4).

History

Pediculosis pubis may be asymptomatic or pruritic for months. With excoriation, the lesions may become tender with enlarged regional (inguinal, axillary) lymph nodes. There may also be maculae caeruleae (tâches bleues), slate-gray, 1-cm macules on trunk or legs, because of the breakdown product of heme from the louse saliva.

Physical Examination

Skin Findings

Type Lice, eggs (nits), macules, papules, excoriation, crust.

Color Red, blue (maculae caeruleae).

Distribution Pubic, axillary areas > trunk. Children: eyelashes, eyebrows.

General Findings

LAD

Differential Diagnosis

The differential diagnosis for pediculosis pubis includes tinea, folliculitis, and scabies.

Laboratory Examinations

Dermatopathology Bite site will show intradermal hemorrhage and a deep, wedge-shaped infiltrate with eosinophils and lymphocytes.

Microscopy Lice and nits may be identified with hand lens or microscope.

Management

Head lice is best treated with pediculicides: permethrin, lindane, malathion, or mercuric oxide ointment. The treatment of choice for pubic lice is topical lindane, 5 minutes to the infested and adjacent hairy areas for infested adolescents or adults. Lindane should be avoided in children, because there is potential CNS toxicity associated with overuse or accidental ingestion. In children, topical 1% or 5% permethrin can be applied to the area for 10 minutes. If nits and eggs are still present, resistant lice may be present and 0.5% or 1% malathion should be used.

All sexual contacts should be checked and treated for pubic lice. Clothes and bedding should be washed and dried on high heat to kill ova and parasites. Pediculosis of the eyelashes can be treated with petrolatum bid to eyelashes and mechanical removal.

Pediculosis corporis is caused by the body louse that lives on bedding and clothing but intermittently infests humans to feed.

Synonym Body lice.

Epidemiology

Age All ages.

Gender M = F.

Incidence Common.

Etiology Pediculus humanus corporis (2 - to 6 mm, six-legged louse).

Pathophysiology

Pediculus humanus corporis live on clothing, crawling onto the human host only to feed, predominantly at night. The adult female body louse has a life cycle of 35 days, lays 15 eggs a day on clothing seams and can survive 10 days without a blood meal. On an average, 20 lice can be found on a person with an infestation.

History

Pediculosis corporis is transmitted via contaminated clothing or bedding. The louse feeds transiently on the skin causing a pruritic eruption, deposits nits on clothes that can remain viable for weeks. Furthermore, the louse can carry rickettsial organisms (typhus, trench fever) and transmit them to the human host while feeding.

Physical Examination

Skin Findings

Type Macule, papule, urticaria, lice, nits (Fig. 25-5).

General Findings

Usually, none present.

Differential Diagnosis

The differential diagnosis of pediculosis corporis includes eczema and scabies.

Laboratory Examinations

Dermatopathology Bite site will show intradermal hemorrhage and a deep, wedge-shaped infiltrate with eosinophils and lymphocytes.

Microscopic Examination Lice, nits can be visualized on infested clothes.

Wood’s Light Nits will fluoresce pearly white.

Course and Prognosis

Undiagnosed pediculosis corporis can lead to worsening pruritus and increased risk of transmission to unaffected population.

Management

Because the body louse is rarely on the person’s body, treatment is directed more toward patient education. Clothes and bedding should be washed in hot water and then dried on high heat to kill lice and ova.

Scabies is an infestation by the mite Sarcoptes scabiei, characterized by severe pruritus and transmitted by close proximity to the infested person.

Insight

Although resistance to several of the antiparasitics exists and may be growing, many cases of so-called resistant scabies infestation turn out not to be scabies at all.

Synonym Seven-year-itch, mange.

Epidemiology

Age In children who are younger than 5 years, young adults (sexually transmitted), bedridden elderly patients.

Gender M = F.

Incidence Common. In 300 million cases annually worldwide.

Season Fall, winter.

Etiology S. scabiei var hominis (400 μm in size).

Pathophysiology

The scabies mite can only survive and replicate on human skin and are transmitted by close proximity. Once on the human host, the scabies mite lays eggs under the skin as it burrows. The ova hatch in 5 days, reach maturity in 2 weeks and then die after 2 months. The mites and eggs can remain alive for 1 to 2 days on clothes or bedding.

History

One month after exposure, the infested person begins with intractable itching. For pruritus to occur, there must be sensitization to the S. scabiei mite. The widespread rash is usually caused by less than 10 mites jumping from site to site on the body. In neonates, immunosuppressed or debilitated patients with more severe infestation, mites can exceed hundreds in number (Norwegian scabies).

Physical Examination

Skin Findings

Type Burrows, vesicles (Fig. 25-6), excoriations, scale, or crust.

Color Gray or flesh-colored, black dot (the mite).

Distribution Fingers, wrists, elbows, umbilical area, genital area, feet.

Variants

• Nodular Scabies: 10 mm, red, nodular lesions on the scrotum, back, feet (Fig. 25-7).
• Norwegian Scabies: widespread mite infestation causing a crusted generalized rash.

Differential Diagnosis

The differential diagnosis of scabies includes impetigo, papular urticaria (insect bite), psoriasis, contact dermatitis, atopic dermatitis, or pediculosis.

Laboratory Examinations

Dermatopathology Scabetic burrow, mite, or feces in the stratum corneum. Dermis shows a diffuse eosinophilic infiltrate.

Scabies Prep (Fig. 25-8). A scraping can reveal mites (400-μm round mites with protruding legs), eggs (100-μm oval particles), or feces (10-μm small oval particles).

Course and Prognosis

Scabies pruritus persists for weeks even after adequate treatment, and nodular lesions can take months to years to resolve.

Management

A scabicide such as permethrin 5% cream should be applied from neck to toes, left on for 8 hours, and then washed off. Other topical options include lindane, benzyl benzoate, crotamiton, or sulfur. All family members and close contacts should be treated simultaneously and the treatment can be repeated in 1 week to ensure successful eradication. Residual pruritic symptoms and nodules are typical and do not indicate persistent infection. For infants and young children with whole-body involvement, permethrin, 10% crotamiton, or 6% to 10% sulfur ointment should be applied to all body surface areas. For pruritus, emollients, antihistamines, and topical steroids can be used for symptomatic relief. Bedding and clothing should be machine washed in hot water and machine dried on high heat. Infested articles can also be stored in an airtight garbage bag for more than 72 hours so the mites will die.

Papular urticaria is an immunologic-mediated reaction to insect bites, characterized by an intensely pruritic eruption occurring at the bite sites hours to days after the bites. Of note, patients are often unaware of having been bitten.

Insight

Three factors can contribute to the bizarre occurrence of only one child being affected by papular urticaria in a family: (1) Some individuals are more attractive to certain insects; (2) Individuals have different levels of reactivity to bites; (3) Some patients are very aware of the lesions while others will not notice them unless pointed out.

Epidemiology

Age Children > adults.

Incidence Extremely common.

Race Asians > other races.

Season Spring, summer.

Etiology Bites from mosquitoes, gnats, fleas, mites, bedbugs, etc.

Pathophysiology

Papular urticaria follows an insect bite and is a type I hypersensitivity reaction triggered by injected foreign antigens in a patient who is sensitive.

History

At the site of a bite, pruritus and erythema occur a few hours after the bite and persist for 1 to 2 weeks or longer. Affected individuals are often not aware of being bitten by insects. Additionally, some family members do not react to bites or are not as highly exposed as the children. Reactions in infants on first exposure may be mild but upon subsequent exposure, young children (aged 1–5 years) have a more severe response, then the reactions subside as the child grows older.

Physical Examination

Skin Findings

Type Macules, papules (Fig. 25-9), vesicles, excoriations, scars.

Color Red.

Shape Round, domed.

Arrangement Usually, in groups of three (“breakfast, lunch, and dinner”).

Distribution Legs > arms > trunk.

Differential Diagnosis

The differential diagnosis of papular urticaria includes allergic contact dermatitis, especially to plants such as poison ivy or poison oak.

Laboratory Examinations

Dermatopathology Edema, spongiosis, dermal inflammation with eosinophils.

Course and Prognosis

Papular urticaria is self-limited, but may take weeks to month to clear.

Management

The treatment of papular urticaria is symptomatic with oral antihistamines, antipruritics, or topical steroids. Preventative measures include insect repellents such as DEET (in children or adults) or citronella when outdoors as well as flea/tick control on indoor pets.

Fire ant stings are caused by ants of the Solenopsis species and are characterized by burning, painful stings that flare immediately and then vesiculate with purulent fluid of the puncture site.

Epidemiology

Age Children > adults.

Gender M = F.

Incidence Common, especially in endemic areas.

Geography South America, Southern United States.

Etiology Fire ants (Solenopsis saevissima, Solenopsis richteri, and Solenopsis invieta)

Pathophysiology

Fire ants grasp the host with their mandibles and then pivot injecting venom with their abdominal stinger in a circular pattern. The venom contains hemolytic factors (triggering mast cells to release histamine) and allergenic proteins.

History

Fire ants will swarm on a person and bite ferociously, leaving hemorrhage puncta at the site of puncture. The initial reaction is a wheal flaring around the bite site followed by vesiculation. The vesicle becomes cloudy after 10 hours and may last for several days before crusting and may scar. Systemic fever, GI distress, urticaria, and respiratory symptoms are common and become more severe with successive attacks.

Physical Examination

Skin Findings

Type Macule, papule, wheal, vesicle (Fig. 25-10).

Color Red, pink (Fig. 25-11).

Size 1 to 3 cm.

Distribution Legs, torso, arms.

Differential Diagnosis

The differential diagnosis of fire ant bites includes other bites, viral exanthema, or VZV.

Laboratory Examinations

Dermatopathology Perivascular lymphocytic infiltrate with neutrophils and some eosinophils, later vesiculation with neutrophils and epidermal necrosis, dermal edema.

Course and Prognosis

Once bitten, the patient can feel quite ill, and the skin is very itchy and painful at the sites of bites. Systemic reactions can be quite severe (bronchospasm, laryngeal edema, hypotension) and even life-threatening, especially in small children with numerous bites. These reactions tend to increase in severity with subsequent attacks, and fatal anaphylaxis reactions are becoming more common.

Management

Local application of cool compresses, topical analgesics, or topical steroids can alleviate the burning, stinging pain. A papain solution of one-fourth meat tenderizer and four parts water, applied topically, can also relieve the itch and burn. Systemic antihistamines, steroids, or epinephrine may be needed in severe cases with signs of respiratory distress. Patients with a known allergy to fire ant bites should carry an epinephrine pen or a sublingual tablet of isoproterenol for immediate use.