Ectoparasites, Cutaneous Parasites, and Cnidarian Envenomation

Etiology

Scabies is caused by the nearly microscopic human “itch mite” Sarcoptes scabiei var. hominis . Scabies causes intense pruritus, generally developing several weeks after infestation, with itching that worsens at night. Scabies is spread by skin-to-skin contact and rarely via contaminated clothing, bedding, or other fomites. The scabies mite lives for about a day away from a human host.

Mites burrow into the skin's outer layer and lay 2-4 eggs along the burrow path each day. A single female can lay up to 40 eggs. The time from ovum to mature mite is about 10-14 days. This cycle can repeat indefinitely, though some people can clear the infestation through their own immune response.

Epidemiology

Scabies occurs worldwide and is epidemic in much of the tropics. People of all ages are affected. Prevalence in the developing world is estimated at 10% in the general population and up to 50% among children. Crowding is a likely important factor, because scabies is highly contagious through skin-to-skin contact.

Clinical Features

In most people, lesions and pruritus develop several weeks after infestation due to a delayed-type hypersensitivity immune response to the mites, ova, saliva, and/or feces. Secondary papules, pustules, and/or vesicles are often present. The distribution of lesions includes finger webs, flexor surfaces of the wrists, axillae, breasts, umbilicus, genitals, buttocks, and feet. Burrows are 3-5 mm, threadlike, linear lesions seen typically in the finger webs, on the wrists, and on the glans penis. Secondary bacterial infection is common, and impetigo occurring in the aforementioned distribution suggests scabies.

In temperate climates, the face and scalp are spared except in infants and the elderly. In the tropics, the scalp and face may be involved; secondary infection is more frequent; and burrows are often absent. Crusted scabies (formerly called “Norwegian scabies”), in which thousands to millions of mites are present on an immune-compromised host, may occur anywhere on the body, although an acral distribution is common. Hyperkeratotic plaques and crusts predominate, resembling psoriasis. Burrows are obscured by overlying crusts, and, surprisingly, pruritus is often mild or absent.

Diagnosis

Identification of burrows in a typical distribution can make a clinical diagnosis. But burrows are not always present and are less common in tropical climates. Exposure history and typical distribution of lesions can also be used to make a clinical diagnosis.

A definitive diagnosis of scabies is made by identification of the mite, eggs, or feces in skin scrapings. Take scrapings from burrows whenever possible: with a scalpel blade coated with mineral or immersion oil, scrape the lesions firmly enough to cause pinpoint bleeding. Place the scrapings on a slide with a coverslip and then examine under a low-power objective lens. Sensitivity is dependent on experience; failure to find evidence of mites cannot rule out scabies.

Treatment

All household members and intimate contacts should be treated simultaneously, even if asymptomatic, to avoid reinfestation.

• Permethrin 5% cream offers a high cure rate and minimal toxicity. It is safe for infants >2 months old. Apply 30-50 mL from the neck down, leave 8-12 hours, then wash. One treatment is generally effective, but it can be reapplied after 5-7 days.

• Lindane 1% is a second-line topical treatment but is banned in some countries due to neurotoxicity presenting as seizures and neuromuscular rigidity, usually affecting children or elderly patients. It is applied like permethrin. Avoid using in infants and young children; it is not recommended for women who are pregnant or nursing.

• Ivermectin is an oral alternative with increasing popular use and is well tolerated. A single dose of 200 µg/kg body weight is as effective as lindane, and two doses, 2 weeks apart, is as effective as permethrin.

• An alternative treatment for infants <3 months includes 10% crotamiton cream, applied on two consecutive nights and washed off 48 hours after the second treatment. A second treatment in 2 weeks may be given.

• 5-10% sulfur ointment is commonly used in Africa and South America; it is safe for infants and is the treatment of choice for pregnant women.

Clothing and linens should be washed in hot water at the time of treatment. Nonwashable clothing should not be worn for 3-5 days. Household fumigation is not necessary.

Etiology

Head lice (pediculosis capitis) and body lice (pediculosis corporis) are caused by the human louse Pediculus humanus , a light-gray insect, 3-4 mm long, that feeds on blood . The head and body subspecies are practically identical, with distinct patterns of infestation. All forms of lice require a human host to survive and would die after 2-3 days without feeding.

Female head lice live on the scalp for 3-4 weeks and lay about 10 eggs, or nits, a day. Nits are deposited on the base of the hair shaft and incubate for 7-12 days; female nymphs mature in roughly another 10 days. The total number of adult lice is usually less than 10 at any time, and the infestation is primarily at the back of the head. Body lice live in the seams of clothing and move to the body transiently to feed.

Phthirus pubis lice, or pubic lice, are 1-2 mm and brown in color. Pubic lice live on pubic hair but may be found on any body hair and on eyelashes and eyebrows. Nits take about a week to hatch and another 2-3 weeks to mature.

Epidemiology

Head lice are found worldwide, are common among children across socioeconomic classes, but are associated with poor hygiene and crowding when present on adults.

Body lice occur worldwide in conditions of poor hygiene, especially where clothing is not changed and washed regularly. Transmission occurs through close body contact or sharing infested clothing. Lice may live for several days on clothing or bedding, but routine laundering will kill them. Body lice are vectors for typhus ( Rickettsia prowazekii ), trench fever ( Bartonella quintana ), and relapsing fever ( Borrelia recurrentis ).

Sometimes called crabs, pubic lice are primarily transmitted sexually and also endemic worldwide. In regions where pubic hair shaving is common, this condition has become very unusual.

Clinical Features

Head lice cause pruritus of the scalp, and scratching leads to excoriations. Further complications include furunculosis or impetigo. Scalp pyoderma should prompt an inspection for head lice.

The hair is often matted or lusterless, and regional lymph nodes may be enlarged. Light-gray adults and nymphs can be seen crawling among the hair and nits are usually apparent on examination. Nits are 0.5-mm ovals cemented to individual hairs where the shaft emerges from the scalp and grow out with the hair. Nits are initially translucent near the base of the hair shaft, where they are first laid, but after they hatch, when they have grown about 1 cm from the scalp, they appear white.

Body lice produce generalized pruritus, with excoriations usually worse over the back, shoulders, and arms. In contrast to scabies and head lice, the head, hands, and feet are spared. Typical lesions are excoriated papules with or without secondary bacterial infection. Lice and nits are not found on the body but may be seen in clothing.

Pubic lice cause pruritus from the umbilicus to the mid-thighs, most severely in the pubic area. Finding the tiny lice or nits may require a careful search. Often, no skin lesions are present, but excoriations or small bluish macules, called maculae caeruleae, are seen occasionally. These are thought to be hemosiderin deposition from bite trauma.

Diagnosis

For travelers in tropical areas, it is important to distinguish nits from white and black piedra, a fungal infection that can appear similar to nits ( Chapter 38 ). White piedra loosely adheres to hair shafts, as opposed to nits, which are strongly attached. White piedra tends to affect the axillae, groin, and face more often than the scalp. Black piedra commonly occurs in the scalp and facial hair, can be similarly sized to nits, and is strongly attached to the hair. Visualization with a KOH preparation and microscopic inspection will reveal characteristic septate hyphae, distinguishing black piedra from lice.

Lice are diagnosed by finding the louse or nits. The use of a hand lens may aid diagnosis.

Treatment