Excerpt from Scabies

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Background

Scabies is a common parasitic infection of global proportion. Worldwide, an estimated 300 million cases occur annually. The arthropod Sarcoptes scabiei var hominis causes an intensely pruritic and highly contagious skin infestation, which affects males and females of all socioeconomic stratas and all ethnic groups.

Scabies has been reported for more than 2500 years. Aristotle discussed "lice in the flesh," which resulted in vesicles, and Celsus recommended sulfur mixed with liquid pitch as a remedy for the disease. However, the disease was first ascribed to the mite by Giovan Cosimo Bonomo in 1687. It was the first human disease recognized to be caused by a specific pathogen.

Pathophysiology

The scabies mite is an obligate parasite and completes its entire life cycle on humans. Other variants of the scabies mite can cause infestation in other mammals such as dogs, cats, pigs, ferrets, and horses, and these variants can infest human skin as well. However, they are unable to reproduce in humans and only cause a transient dermatitis.

The S scabiei var hominis mite that infects humans is female and can just be seen with the naked eye (0.3-0.4 mm long). The male is about one half this size. The male fertilizes the female on human skin and then dies. Newly mated females burrow into human skin, using proteolytic enzymes to dissolve the stratum corneum of the epidermis. The mite has 4 pairs of legs and tracheal breaths and thus does not penetrate deeper than the outer layer of the epidermis.

The female deposits eggs in the burrows, and then the eggs incubate and hatch after 3-5 days (range up to 8 d). About 90% of the hatched mites die, but those that survive go through various molting stages and reach maturity after a little more than 2 weeks. The female adults, who never leave their burrows, die after 1-2 months.

In a classic scabies infection, anywhere from 5-15 mites (range, 3-50) live on the host. Little evidence of infection exists during the first month (range, 2-6 wk), but after 4 weeks and with subsequent infections, a delayed-type IV hypersensitivity reaction to the mites, eggs, and scybala (packet of feces) occurs. The time required to induce immunity in primary infestations probably accounts for the latent period of 4 weeks of asymptomatic infection. In reinfestation, the sensitized individual may develop a reaction rapidly (within hours). The resultant skin eruption, and its associated intense pruritus, is the hallmark of classic scabies.

Crusted, or Norwegian scabies (so named because the first description was from Norway in the mid 1800s), is a distinctive and highly contagious form of scabies. In this variant, hundreds to millions of mites infest the host individual, who is usually immunocompromised, elderly, or physically and/or mentally disabled and impaired. Extensive, widespread, crusted lesions appear with thick, hyperkeratotic scales over the elbows, knees, palms, and soles. Serum immunoglobulin E (IgE) and immunoglobulin G (IgG) levels are extremely high in these patients, yet the immune reaction does not seem to be protective. Cell-mediated immunity in classic scabies demonstrates a predominantly CD4 T-cell infiltrate in the skin, while one study suggests a CD8 predominance in crusted scabies.

Atypical infestations may also befall the very young (neonates).

Frequency and epidemiology

While many accounts of the epidemiology of scabies suggest that epidemics or pandemics occur in 30-year cycles, this may be an oversimplification of its incidence. These accounts coincided with the major wars of the 20th century. Because it is not a reportable disease, and data are based on variable notification, the incidence of scabies is difficult to ascertain. Indeed, while epidemics have been reported (1919-1925, 1936-1949, 1964-1979), it is clearly an endemic disease in many tropical and subtropical regions. Prevalence rates are extremely high in aboriginal tribes in Australia, in Africa, in South America, and in other developing regions of the world. Incidence in parts of Central America and South America and in one Indian village approach 100%. In parts of Bangladesh, the number of children with "the itch" exceeds the number with diarrheal and respiratory diseases combined.

Worldwide, the prevalence of scabies has been estimated at 300 million cases annually, although this figure may be an overestimate. In the United States and in other developed regions around the world, scabies occurs in epidemics in nursing homes, hospitals, long-term care facilities, and other institutions. It is seen frequently in the homeless populations but occurs episodically in other populations as well. No recent published data are available on its incidence in the United States. In one epidemiologic study in the United Kingdom, scabies was shown to have a higher frequency of occurrence in winter months than in summer months, and it more commonly affected women and children. In this study, the disease was found to be more prevalent in urban regions.

While scabies appears to be more common in the younger population, it certainly occurs in all ages, all ethnic groups, all socioeconomic levels, and in both sexes. It is not directly related to hygiene, but it is associated with poverty and crowding.

Mode of transmission

Mites are unable to fly or jump. They crawl at a rate of 2.5 cm/min. While the mite's life cycle occurs completely on its host, they are able to live on bedding, clothes, or other surfaces at room temperature for about 48 hours while remaining capable of infestation and burrowing. At temperatures below 20°C S scabiei are immobile, although they can survive such temperatures for extended periods.

Transmission is predominantly through direct skin-to-skin contact, and for this reason, scabies has been considered a sexually transmitted disease. Indirect contact through fomites such as infested bedding or clothing is possible, although not usual. However, the greater the number of parasites on a person, as in crusted scabies, the more likely that indirect contact will abet transmission of the disease.

Mortality/Morbidity

Classic scabies is primarily a nuisance. However, it can indirectly lead to long-term morbidity. Scabies and other parasitic skin diseases can lead to long-term colonization of skin lesions by group A streptococci. Several studies have demonstrated a correlation between poststreptococcal glomerulonephritis (PSGN) and scabies. Conversely, in one World Health Organization sponsored study in the Solomon Islands, an intervention of mass chemotherapy lead to a decrease of scabies by 96% and a parallel drop in an indicator of renal disease. In remote Aboriginal communities in Australia where scabies is endemic, the repeated infestations appear to be related to the extremely high levels of renal failure and rheumatic heart disease observed in the communities.

While the microbiology of secondary bacterial infection in scabies lesions probably changes based on geographic location, one study demonstrated that the predominant aerobic and facultative bacteria recovered from lesions were Staphylococcus aureus, group A streptococci, and Pseudomonas aeruginosa. Multiple anaerobes were recovered as well, suggesting polymicrobial colonization of lesions.

Other complications of scabies include impetigo, furunculosis, and cellulites. The staphylococci and/or streptococci in the lesions can lead to pyelonephritis, abscesses, pyogenic pneumonia, sepsis, and death.

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