Scabies _- Life Cycle, Pathogenesis, Types, Signs & Symptoms, Risk Factors, Diagnosis & Treatment

Human scabies is an intensely pruritic skin infestation caused by the host specific mite, Sarcoptes scabiei hominis. Scabies is a global public health problem, affecting persons of all ages, races, and socio economic groups. The disease remains common primarily because of diagnostic difficulty; Inadequate treatment of patients & their contacts; And improper environmental control measures. Prevalence of scabies is higher in children, and sexually active individuals than in other persons.
Transmission of scabies primarily occurs through direct skin to skin contact, and for this reason, it is commonly seen among sexually active individuals. Less frequently, the disease can spread by indirect contact through fomites, such as infested bedding or clothing. A person infested with mites can spread the disease even if he or she is asymptomatic. And there may be a prolonged period between the primary infection & symptom onset.Show more

Human Scabies mite is an obligate parasite, that completes its entire life cycle on humans. And only the female mite infects the human. It is large enough to be seen with the naked eye. The mite has four pairs of legs; And it crawls at a rate of 2.5 cm per minute. It is unable to fly or jump. The mite does not penetrate the superficial layer of the epidermis, the stratum corneum. It is able to survive on bedding, clothes, or other surfaces at room temperature for abut 2 to 3 days. At temperatures below 20 degree Celsius, the mite is unable to move. However, it can survive such temperatures for extended periods.

This image shows the complete life cycle of the scabies mite. The female mite lays eggs inside the burrow she made within the stratum corneum, as you can see in this picture. Then the eggs hatch within 2 to 3 days to form larvae, which have 3 pairs of legs. Then the larvae mature in to nymphs, which have 4 pairs of legs. Finally, the larger nymphs become adults. Mating takes place only once, and the female is fertile for the rest of her life. Mature female makes a serpentine burrow in the stratum corneum, using proteolytic enzymes in order to lay eggs to start a new cycle.
There are 2 main types of scabies, classic scabies; And crusted scabies. Classic scabies is the predominant type, and typically 10 to 15 mites live in the host. There is little evidence of infection exists during the first month. However, after 4 weeks & with subsequent infections, a delayed type hypersensitivity reaction occurs against mites, eggs, larvae, & feces. With re infestation, the sensitized individual may develop a rapid reaction. The resultant skin eruption & associated intense pruritis are the hallmarks of classic scabies.
Crusted scabies is a distinctive, and highly contagious form of the disease. In this variant, hundreds to millions of mites infest the host individual, who is usually immunocompromised, elderly, or physically or mentally impaired.
Risk factors for scabies include, young age; Presence of many children in the household; Poor housing; Sharing clothes & towels; And irregular use of showers.

Patients complain of intense pruritis that is worse at night. Lesions are distributed predominantly in the following areas in adults. Flexor aspects of the wrists. Inter digital web spaces of the hands. Axillae. Elbows. Waist. Buttocks. And genitalia. Pruritic papules & vesicles on the scrotum & penis in men, and areolae in women are highly characteristic. Infants & young children may develop lesions diffusely, but unlike in adults, lesions are common on the face, scalp, neck, palms, & soles. In immunocompromised individuals, all cutaneous sites are susceptible for lesions.
Physical examination findings include primary & secondary lesions. Primary lesions are the first manifestations of the infestation, and typically include small papules, vesicles, and burrows. Burrows are a pathognomonic sign that represents the intra epidermal tunnel created by the moving female mite. They appear as serpiginous, thread like elevations in the superficial epidermis, ranging from 2 to 10 mm in length. These may not be readily apparent, and must be actively sought. A black dot may be seen at one end of the burrow, indicating the presence of a mite. High yield locations for burrows include the following. Webbed spaces of the fingers. Flexor surfaces of the wrists. Elbows. Axillae. Belt line. Feet. Scrotum in men. And Areolae in women.

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