Diagnosis and Treatment

A potassium hydroxide (KOH) examination of scales from his abdomen reveals definite scabietic elements (Figure 3), confirming the diagnosis of scabies. Images of these findings are shown to the patient who, until that moment, was doubtful of the diagnosis. He blames the family dog until he is told that human scabies is species-specific, as are canine mites, which means the source has to be human.

Figure 3. Microscopic findings of scabies. Photo courtesy of Joseph Monroe, MPAS, PA.

The patient has misgivings about treating the whole family, as his wife and children are asymptomatic. This is quite common, as many patients do not begin to itch for weeks or even months after being infested with scabies. Based on the timeline, the clinician has to  assume that all other family members are infested as well.1

Because the children are more than 10 years of age, all family members are treated at the same time with topical permethrin cream and oral ivermectin 200 µg/kg. Both treatments are repeated 7 to 10 days later.2,3


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Before starting treatment, the family is advised to change bedding and to vacuum well around beds and sofas. There is no need to have the house treated by exterminators, as the organisms can only live for up to 72 hours off of the body.4

The patient is told to notify the children’s close friends and relatives in case scabies was already passed on to them. Failure to comply with this recommendation can cause treatment failure.

After the diagnosis, the patient recalls that he has visited a household several times for his job. He recalls that the young children would usually sit on or near him on a sofa, often for extended periods. He notifies that family of his diagnosis and is told that the family members were all diagnosed and successfully treated for scabies recently.

Etiology of Scabies

The organism responsible for scabies is the ectoparasite Sarcoptes scabiei var. hominis, a 0.3- to 0.5-mm mite.5,6 A member of the phylogenetic class Arachnida, scabies have a 10- to 14-day life cycle and live their entire life in the stratum corneum. Once laid in the stratum corneum, the eggs reach adulthood in 1 to 2 weeks. Humans develop an allergic reaction to the mite, eggs, and excreta (Figure 4).7,8

Figure 4. The life cycle of Sarcoptes scabiei, the causal agent of scabies.8 Credit: CDC/Alexander J. da Silva, PhD; Melanie Moser.

Epidemiology

Scabies is estimated to affect more than 200 million people worldwide each year, with the highest occurrence reported in the low- and middle-income strata of tropical and undeveloped countries.9,10

Although scabies is highly contagious, transmission requires prolonged skin to skin contact, such as from sexual exposure or from the hands and arms of infested individual.6 Children often acquire scabies from other children, bringing it home to pass on to other family members. Transmission by fomites is rare and seen primarily in crusted scabies, formerly termed Norwegian scabies.6

Pathophysiology

As the gravid scabies females burrow into the stratum corneum, feeding on tissue juices, they leave behind droppings (scybala), dead or dying mites, and other scabietic elements (eggshell fragments), provoking a vigorous immune reaction. In patients naive to this antigen, itching that can take weeks to months to develop. In persons previously exposed, itching can begin in hours to days after exposure. Once the itching begins, it is typically unremitting.11

The most common visible lesions suggestive of scabies on children are clear fluid-filled vesicles or bullae typically located in areas with thin, tender skin, such as between the fingers and on volar wrists. Occasionally, these vesicles will be at the terminus of a linear skin disturbance, which represents the burrows created by the mites. In adults, in addition to volar wrists and interdigital areas, scabies also affects the waistline, anterior axillae, and areolae, as well as the penile shaft and glans, rarely affecting hairy areas (Figure 5).5-6,8 Penile lesions appear as red edematous papules on the shaft or glans and are highly corroborative of the presence of scabies.6

Figure 5. Body areas commonly affected by scabies.8 Credit: CDC/Alexander J. da Silva, PhD; Melanie Moser.

In contrast to adults, infants can develop scabies from head to foot, with impressive vesiculation and accompanying impetiginization. It should be noted that scabies is often diagnosed in the presenting patient by finding it on other family members in the examination room, babies and older children being prime examples.12

Diagnosis of Scabies

Scabies is one of the most frequently misdiagnosed causes of severe, unremitting itching.13 One way to avoid misdiagnosis is microscopic confirmation of scabies, by examination of skin scrapings at 10× magnification. Considered the gold standard for scabies diagnosis, its use is limited by the availability of a microscope and the skill required to use it. 14

Traditionally, clinicians have been advised to look for and scrape burrows, but these are often missing or difficult to detect. If necessary, consider vigorously scraping tiny clear fluid-filled vesicles on volar wrists, between fingers, and on children’s feet and hands, and examining under the microscope with KOH 10% solution.13

Scrapings can be examined immediately after collection, often quickly finding scabietic elements. Clinicians may or may not see the entire intact organism under the microscope because the sample may be disrupted by the scraping process, but if positive, scabietic elements will likely be seen, including eggs, eggs shells, scybala (Figure 6), or parts of adult scabies organisms (Figure 7).

If negative, the slide can be left on the microscope and looked at again throughout the day, often finding scabietic elements once the KOH solution has had a chance to clear the field of keratinous material in which scabies can reside. If the slide is still negative for scabietic elements, the patient can be retested within 1 to 2 weeks.

Before treatment, clinicians must rule out other conditions as part of the differential diagnosis, including eczema, xerosis, and contact/irritant dermatitis, among many other conditions (Table 1).5

Table 1. Differential Diagnosis of Scabies5

Chronic pruritus in the elderly
Delusions of parasitosis  
Eczema
Hodgkin lymphoma
Infantile acropustulosis
Irritant vs contact dermatitis
Lice infestation
Prurigo nodularis
Renal failure (especially in dialysis patients)
Seborrheic dermatitis
Xerosis

A dermatoscope with 10× magnification can also be used to diagnose scabies. The organism appears as a tiny triangular gray dot at the end of a burrow. However, burrows are not that common, despite what is traditionally taught. Moreover, the dermatoscope cannot detect diagnostic scabietic elements, which are often the only diagnostic findings.13

As this case illustrates, the presentation of scabies varies significantly from case to case. This patient had a somewhat unusual presentation of an eczematoid rash on his trunk, which can be triggered by an allergic reaction to scabietic elements. This so-called ID reaction occurs with many contact dermatoses and infections, such as poison ivy and fungal infections. The itchy penile papules were also highly suggestive of scabies and were all but pathognomonic for that diagnosis, even when found on other family members.15

Treatment of Scabies

Scabies can be difficult to treat, despite the availability of highly effective treatments.16

There is conflicting data regarding the need to combine ivermectin and permethrin 5% cream or lotion. Ivermectin has not been approved for use in treating scabies but is considered relatively safe when used as directed in proper doses having been used in humans for many years with success and safety for various parasitic diseases, such as river blindness (Onchocerca volvulus), as well as for scabies.17

In addition, studies report that permethrin monotherapy is just as effective as combination therapy, however, the experience of the author has not borne out that contention.18,19 When permethrin is not available, 10% to 25% benzoyl benzoate and 2% to 10% precipitated sulfur have proven to be safe and effective.

Other medications that have been used in the past include malathion and lindane, both of which are considered potentially toxic.20,21 Moxidectin is closely related to ivermectin, but has a longer half-life. It is approved for the treatment of river blindness. This drug has been given to animals as a single dose for scabies, killing mites as they hatch, with studies suggesting that this agent is safe, effective, and well-tolerated as a single dose.22 Though further studies in humans are indicated, given the rise in resistance levels of human scabies to available agents, especially permethrin, it would appear that moxidectin holds considerable promise as an alternative scabicide.

Common Mistakes in Scabies Management

Several factors may account for treatment failures in scabies (Table 2). Patient and family education plays a crucial role in achieving optimal treatment outcomes, especially in terms of the need to comply with treatment instructions and identification of possible sources of reinfestation.

Table 2. Potential Causes of Scabies Treatment Failures

FactorNotes
Empirical misdiagnosisUse of subjective impressions to diagnose dermatologic conditions is common, especially in primary care settings, and can lead to misdiagnosis
Failure to identify and adequately treat all contactsThis is especially true of family members who doubt the diagnosis, especially if they are asymptomatic
Inadequate treatment of patientThe most common mistake is treating only once, which allows a new crop of mites to hatch 7-10 days later
Failure to synchronize treatment with family members and other contactsLack of treatment synchronization allows passage of the mite back and forth
Inadequate patient educationPatients should be educated on the pathophysiology of scabies, the need for treatment, aspects of contagion (eg, the mites cannot fly, cannot live for long off the human body), and the futility of pest control

The lack of microscopic confirmation lead to much confusion and needless suffering for this patient and his family. There are times that empirical treatment seems necessary but, even then, other family members are usually ignored as sources of recurrences, which virtually assures reinfestation.

Again, it is important to emphasize to patients that human scabies cannot be passed from or to pets and other animals. Animals can and do have their own types of scabies, but these cannot live on or multiply on human skin; however, these animal mites (from a cat, rabbit, or other cuddly pet) can reside on human skin for a short time and can cause mild itching.

Clinical Pearls

These final clinical pearls are offered to help guide nurse practitioners and physician assistants in the differential diagnosis and treatment of scabies in patients presenting with itching:

  • Pruritus of any cause will generally be worse at night, so beware of placing undue importance on this indication
  • It is true, however, that the itching from scabies is truly 24/7; most patients with scabies will be scratching as the provider enters the room, a valuable clue unto itself
  • A history of total treatment failure with the commonly used steroid creams, pills, and injections is suggestive of scabies
  • Scabies is vastly overdiagnosed by harried providers who do not take the time to confirm the diagnosis microscopically
  • In the appropriate age group, scabies can suggest sexual transmission, a key fact in terms of potential for reinfestation. A frank discussion of this possibility is necessary.
  • Scabies is common in wrestlers and other contact athletes
  • Do not be afraid to hold off on treating scabies empirically until a positive diagnosis is confirmed. In urgent care settings, this can be difficult, but it is best practice in the long run. Consider expediting a referral to a dermatologist in those cases

Joseph Monroe, MPAS, PA, is a graduate of the University of Oklahoma physician assistant program and has been practicing dermatology for over 35 years. He is also an educator and avid writer on dermatologic topics.

References

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