# Parasitic and Ectoparasitic Infestations

## Scabies

### Etiology and Transmission

Scabies is caused by **Sarcoptes scabiei var. hominis**, an obligate human parasite. Transmission occurs via prolonged skin-to-skin contact, typically requiring 15 to 20 minutes of sustained contact. Fomite transmission is rare except in crusted scabies. The female mite burrows into the stratum corneum and lays 2 to 3 eggs per day, completing a lifecycle of approximately 30 days. The incubation period is 3 to 6 weeks for primary infestation but only 1 to 3 days for re-infestation in a sensitized host.

### Clinical Features

**Classic scabies** is intensely pruritic, characteristically worse at night. It presents with papules, vesicles, and burrows in the web spaces, wrists, elbows, axillae, periumbilical area, genitalia, and buttocks. In infants, the distribution is atypical, with involvement of the palms, soles, face, and scalp. Nodular scabies produces persistent pruritic nodules on the scrotum, axillae, and groin, representing a hypersensitivity reaction. **Crusted (Norwegian) scabies** presents with hyperkeratotic plaques harboring thousands to millions of mites, typically in immunocompromised, elderly, or neurologically impaired patients. It is highly contagious and may present with minimal pruritus, often being misdiagnosed as psoriasis or eczema.

### Diagnosis

**Dermoscopy** reveals the "jet with contrail" sign, in which a triangular dark structure (the mite head) is followed by a trailing burrow, with sensitivity of 83 to 91 percent in experienced hands. **Skin scraping with mineral oil** allows visualization of mites, eggs, or scybala (fecal pellets) on microscopy. The **burrow ink test** involves applying ink to a suspected burrow and wiping with alcohol; ink tracks along the burrow, confirming its presence.

<image>Dermoscopic image showing the classic "jet with contrail" sign of a scabies mite within its burrow</image>

<image>Clinical photograph of interdigital web space burrows and papules in classic scabies</image>

### Treatment

| Treatment | Regimen | Indications / Notes |
|---|---|---|
| Permethrin 5% cream | Apply neck down, wash off 8-14 hrs; repeat in 1-2 wk | First-line; emerging resistance reported |
| Oral ivermectin | 200 mcg/kg; repeat in 1-2 wk | Preferred for crusted scabies (with topical permethrin); not FDA-approved <15 kg |
| Sulfur 5-10% in petrolatum | Apply nightly x 3 nights | Safe in pregnancy and infants <2 months |

**Permethrin 5% cream** is first-line, applied from the neck down and washed off after 8 to 14 hours, with a repeat application in 1 to 2 weeks. Emerging resistance has been reported in some populations. **Oral ivermectin** at 200 mcg/kg, repeated in 1 to 2 weeks, is preferred for crusted scabies (combined with topical permethrin and keratolytics) but is not FDA-approved for children under 15 kg. **Sulfur 5 to 10% in petrolatum** is safe in pregnancy and infants under 2 months. Environmental measures include washing bedding and clothing in hot water at 60 degrees Celsius, drying on high heat, and sealing non-washable items in plastic bags for 72 hours. All close contacts must be treated simultaneously regardless of symptoms.

## Pediculosis

### Head Lice (Pediculus humanus capitis)

Head lice are most common in school-age children and are not a marker of poor hygiene. Diagnosis requires visualization of live lice or viable nits within 6 mm of the scalp. Treatment options include permethrin 1% (over-the-counter), malathion 0.5%, ivermectin 0.5% lotion, and spinosad. Wet combing with a fine-toothed nit comb is an important adjunct. "No-nit" school policies are not evidence-based, according to AAP recommendations.

### Body Lice (Pediculus humanus corporis)

Body lice live in the seams of clothing and serve as vectors for Rickettsia prowazekii (epidemic typhus), Bartonella quintana (trench fever), and Borrelia recurrentis (relapsing fever). Treatment focuses on improved hygiene and laundering clothing; topical pediculicides are rarely needed.

### Pubic Lice (Pthirus pubis)

Pubic lice have declining prevalence, associated with pubic hair grooming trends. They can infest eyelashes (pediculosis palpebrarum), treated with petrolatum or ivermectin. All cases should be screened for other STIs.

<image>Microscopic image of Pediculus humanus capitis adult louse and nit attached to hair shaft</image>

## Cutaneous Larva Migrans

Cutaneous larva migrans is caused by hookworm larvae (most commonly Ancylostoma braziliense) penetrating the skin. The exposure history typically involves walking barefoot on sandy beaches contaminated with dog or cat feces in tropical regions (Caribbean, Southeast Asia, Africa). The clinical presentation is a serpiginous, erythematous, pruritic tract advancing 1 to 2 cm per day, usually on the feet, buttocks, or hands. The condition is self-limited over weeks to months, but treatment hastens resolution. **Ivermectin** 200 mcg/kg as a single dose is preferred, with albendazole 400 mg daily for 3 days as an alternative. Topical thiabendazole can be used for limited disease. Cryotherapy is not effective and causes unnecessary tissue damage.

## Leishmaniasis

### Cutaneous Leishmaniasis

Cutaneous leishmaniasis is caused by Leishmania species transmitted by sandfly bites (Phlebotomus and Lutzomyia species). Old World species include L. major (wet/rural), L. tropica (dry/urban), and L. aethiopica. New World species include the L. mexicana complex and the L. braziliensis complex, the latter carrying risk of mucocutaneous disease. Clinically, a painless papule at the bite site evolves into a nodule with central ulceration forming a "volcanic crater" over weeks to months. "Chiclero ulcer" refers to ear involvement from L. mexicana. Sporotrichoid spread is possible. Diagnosis relies on tissue smear (Giemsa stain showing amastigotes in macrophages), PCR, and culture on NNN medium.

Treatment depends on species and risk of mucocutaneous disease. Observation is appropriate for limited L. major infection, which may self-resolve. Intralesional sodium stibogluconate is used for localized disease. Systemic options include liposomal amphotericin B, miltefosine (oral), and pentavalent antimonials. L. braziliensis must always be treated systemically due to the risk of mucocutaneous disease.

<image>Clinical photograph of cutaneous leishmaniasis showing a well-demarcated ulcer with raised indurated borders on the face</image>

## Other Parasitic Infestations

### Tungiasis

Tungiasis, caused by Tunga penetrans (sand flea), is endemic in sub-Saharan Africa and South America. It presents as a painful nodule with a central black dot on the feet and is treated by surgical extraction.

### Myiasis

Myiasis is caused by larvae of Dermatobia hominis (botfly) or Cordylobia anthropophaga (tumbu fly), presenting as a furuncle-like lesion with a central pore. The larva can be forced to emerge by occluding the pore with petroleum jelly.

### Cercarial Dermatitis (Swimmer's Itch)

Cercarial dermatitis results from avian schistosome cercariae penetrating human skin, causing pruritic papules at entry sites. The condition is self-limited and managed symptomatically with antihistamines and topical corticosteroids.

## Clinical Pearls

Always examine the interdigital web spaces and genitalia when scabies is suspected; a burrow on the penile shaft in a young male is nearly pathognomonic. Crusted scabies is the great masquerader and should be considered in any immunosuppressed patient with treatment-resistant "psoriasis" or "eczema." Post-scabetic pruritus can persist 2 to 4 weeks after successful treatment due to ongoing hypersensitivity and does not necessarily indicate treatment failure. In cutaneous larva migrans, the larva is typically 1 to 2 cm ahead of the visible track, so biopsy of the trailing edge will miss the organism. Any New World cutaneous leishmaniasis case with L. braziliensis must receive systemic therapy to prevent devastating mucocutaneous disease (espundia).

## References
- Andrews' Diseases of the Skin, 13th Edition
- Chosidow O. Scabies. N Engl J Med. 2006;354(16):1718-1727
- Engelman D, et al. The 2020 International Alliance for the Control of Scabies Consensus Criteria for the Diagnosis of Scabies. Br J Dermatol. 2020;183(5):808-820
- Handler MZ, et al. Cutaneous and mucocutaneous leishmaniasis: Differential diagnosis, diagnosis, histopathology, and management. J Am Acad Dermatol. 2015;73(6):911-926
- Heukelbach J, Feldmeier H. Scabies. Lancet. 2006;367(9524):1767-1774