Lecture 9: Ectoparasitic Infestations

Pediculus humanus capitis

Head lice

Found on the head, eyebrows & eyelashes

Primarily spread by direct contact with hair of infested person

Three stages: egg (nit), nymph, adult

Nit cemented to base of hair shaft nearest to scalp

Nymph: hatches from egg; immature form (must feed!)

Tickling sensation

Itching (allergic reaction)

Irritability & difficulty sleeping

Sores from scratching

Nits within 6 mm of scalp → active infestation

Nits > 6 mm of scalp → previous infestation

Treatment: Pediculocides

Pediculus humanus humanus

Body Lice

Live and lay eggs on clothing & bedding; only move to skin to feed

Spread by direct contact & through contact w/ infested clothing, bedding,

Three stages: egg (nit), nymph, adult

Eggs usually in clothing or bedding

Itching and rash

Scratching → sores

Years of infestation → thickened, hyper-pigmented skin, especially around midsection

Infectious Diseases: Epidemic (louse-borne) typhus

Pthirus pubis

Pubic Lice

Typically on pubic hair; sometimes on other course hair (eyelashes, beard)

Most common in young, sexually active people

Usually spread through sexual contact

Three stages: egg (nit), nymph, adult; eggs attached to hair shafts

Treatment: Pediculocides

Fleas

Not very host-specific

Many flea species will feed on human blood → small, red, pruritic lesions

Papular urticaria (hypersensitivity reaction) in young children

Scratching → secondary bacterial infection

Bubonic plague

Chigoe Flea

Sandy environments

Adult female burrows into the skin usually on feet, often under nails & between toes (can live several weeks)

Severe inflammation & ulceration can occur

Secondary infection w/ tetanus or gangrene

Flies

Not very host-specific

Painful/irritating bites disrupt normal behaviors (e.g., sleeping)

Inflammatory reaction at bite site → pruritic local erythema & edema

More severe local reactions may occur, e.g., blistering or bullous lesions

Papular urticaria

Systemic allergic reactions- anaphylaxis

Secondary bacterial infection

Cutaneous Myiasis

Infestation of the Skin by Fly Larvae

Most species only cause painful lesions

Fatal cases rare

Secondary bacterial infections can occur

Larvae need to be surgically removed

Bed bugs

Painless bites

Skin reactions vary from no reaction to bullous skin lesions to papular urticaria

Typical reaction: red papule or wheal (often itchy)

Not effective disease vectors

Kissing bugs

Painless bites

Local reactions vary from no reaction to erythema & edema to hemorrhagic bullous lesions

Allergic reactions (including anaphylaxis) well known

Vectors for Chagas disease

Mites

Demodex folliculorum – live in hair follicles (mostly on face)

D. brevis – live in sebaceous (oil) glands—(primarily facial)

Prevalence of infestation increases with age

Typically cause no harm (commensal symbiosis)

Demodex folliculitis: rosacea-like, inflammatory papules and pustules on the face and neck

Acaricides (permethrin) or oral ivermectin

Scabies mite

Sarcoptes scabiei var hominis

Burrows into the upper layer of skin

Feeds on cells & tissue fluid

Usually spread through direct, prolonged, skin-to-skin contact

Itching (esp. at night) & rash, often on hands, wrists, elbow, knee, penis, breast, shoulder blade

Tiny burrows on skin

Crusted (Norwegian) scabies: Vesicles & thick crusts over skin; itching may be minimal or absent; found in elderly & immunocompromised patients

Treatment: Scabicide

Chigger mites

Larval stages parasitize vertebrates

Larvae found in grassy/shrubby areas

Larvae attach to host, inject proteolytic saliva, feed for 2–10 days, drop off & molt

Chigger dermatitis: red, itchy papules at bite sites

Chiggers vector scrub typhus

Hard ticks

Hardened dorsal shield (scutum)

Feed till engorged (hours–days)

Afterward, larvae/nymphs molt, adults reproduce

Larval and nymphal stages leave host to molt

Three-host ixodid ticks vector viral, bacterial, and protozoan diseases

Allergic reactions to bites

Sensitization of patient to allergens in red meat

Tick paralysis from tick salivary neurotoxins

Bacterial: Lyme disease, Rocky Mountain spotted fever

Soft ticks

Lack dorsal scutum – look leathery

Feed rapidly (< 1 hour) & repeatedly

Leave host between meals

Soft tick relapsing fevers

Allergic reaction to bites