Scabies Infection Clinical Overview
I. The "On-Call" Snapshot
Clinical Significance in Malaysia: Scabies is endemic. You will see it in outpatient clinics, emergency departments, and as a cause of nosocomial outbreaks in wards. It is highly contagious and its presence is a marker for crowded living conditions. A key differential for any intensely pruritic rash.
High-Yield Definition: Scabies is a parasitic infestation of the skin caused by the mite Sarcoptes scabiei var. hominis. (Source: UpToDate, "Scabies: Epidemiology, clinical features, and diagnosis", 2024).
Clinical One-Liner: Basically, it's a microscopic mite that burrows into the skin, causing an insanely itchy rash that drives patients (and their families) crazy, especially at night.
II. Etiology & Risk Factors
Etiology: Infestation with the Sarcoptes scabiei mite. Transmission is via prolonged, direct skin-to-skin contact. Brief contact like a handshake is usually not enough. Fomites (bedding, clothing) are a significant route of transmission only in cases of crusted scabies.
Risk Factors:
Overcrowding: Hostels, long-term care facilities, prisons, day-care centres.
Immunocompromised state (e.g., HIV, transplant recipients, chronic steroid use) - risk for Crusted Scabies.
Poor socioeconomic conditions.
Close contact with an infected individual.
III. Quick Pathophysiology
The intense itching is not from the burrowing itself. It is a delayed type-IV hypersensitivity reaction to the mite, its eggs, and its scybala (faeces). This is why there's a 4-6 week incubation period in primary infestation before symptoms start. In a subsequent infestation, the itch can start within 1-2 days because the immune system is already sensitized.
IV. Clinical Assessment
Red Flags & Immediate Actions:
Widespread, crusted, scaly rash with minimal itch in an elderly or immunocompromised patient: This is Crusted (Norwegian) Scabies. The patient is extremely contagious. Immediately institute contact precautions, alert your senior and the dermatology team. This is a high-priority consult.
History:
Pathognomonic Symptom: Intense pruritus that is characteristically worse at night. This is a huge clue.
Contact History: Ask if any family members, housemates, or close contacts have a similar itchy rash. The answer is often yes.
Past History: Previous episodes of similar rash.
Physical Examination:
Lesions: Look for multiple, small, erythematous papules, vesicles, and nodules. Excoriations from scratching are almost always present.
Burrows: These are the classic sign. They appear as thin, greyish, reddish or brownish, serpiginous lines on the skin, a few millimetres to 1 cm long. A black dot may be visible at the end of the burrow (the mite itself).
Distribution (Classic Sites): The mite prefers areas with fewer hair follicles. Look carefully at:
Finger webs and sides of fingers
Flexor aspects of the wrists
Axillae
Areolae (in women)
Genitalia (penis and scrotum in men) - scabetic nodules here are very common.
Umbilicus
Belt line
Important: In adults and older children, the rash characteristically spares the head and neck. In infants, the scalp, face, palms, and soles are often involved.
Clinical Pearl: When a patient complains of an itch that is driving them mad at night and affecting their sleep, you must have scabies at the top of your differential list, regardless of how "clean" they appear.
V. Diagnostic Workflow
Differential Diagnosis:
Atopic Eczema:
Points For: Pruritic, papular rash.
Points Against: Eczema typically involves flexural creases (antecubital, popliteal fossae), has a history of atopy (asthma, allergic rhinitis), and lacks the characteristic burrows and nocturnal pruritus of scabies.
How to Differentiate: Clinical distribution and history.
Insect Bites (e.g., bed bugs):
Points For: Itchy papules.
Points Against: Bite marks are often in a linear "breakfast, lunch, and dinner" pattern and typically appear on exposed skin. They don't have burrows.
How to Differentiate: Distribution pattern and absence of burrows.
Dermatitis Herpetiformis:
Points For: Intensely pruritic vesicles.
Points Against: Symmetrically involves extensor surfaces (elbows, knees, buttocks). Associated with celiac disease.
How to Differentiate: Distribution and skin biopsy if diagnosis is unclear.
Investigations Plan:
Diagnosis is almost always clinical.
Confirmatory / Gold Standard:
Skin Scraping / Mineral Oil Prep: Use a scalpel blade to scrape a burrow onto a glass slide with mineral oil. Microscopic examination will reveal the mite, eggs, or faecal pellets. This confirms the diagnosis but is often not necessary in a classic clinical presentation and can be falsely negative.
VI. Staging & Severity Assessment
Classic Scabies: The typical presentation in an immunocompetent host with intense pruritus and a low mite burden (usually 10-15 mites on the entire body).
Crusted (Norwegian) Scabies: Occurs in immunocompromised, neurologically impaired, or elderly patients. Characterised by thick, crusted, warty lesions teeming with thousands to millions of mites. Pruritus may be minimal or absent. This form is highly contagious.
VII. Management Plan
Treatment failure is common. The key is meticulous patient education.
Patient Education is CRITICAL:
The lotion/cream must be applied to the entire body from the neck down, including under the fingernails and toenails, in the umbilicus, and in all skin folds.
The treatment must be left on for the recommended time (usually 8-14 hours) and then washed off.
Crucially, all household members and close personal contacts must be treated at the same time, even if they are not symptomatic. This is the most common reason for treatment failure (re-infestation).
Bedding, clothing, and towels used in the last 3 days should be machine-washed in hot water and dried in a hot dryer, or sealed in a plastic bag for at least 72 hours to kill the mites.
Pharmacological Treatment (Scabicides):
First-Line: Topical Permethrin 5% cream (e.g., A-Scabs lotion). It's effective and has a good safety profile. Apply as described above, leave on for 8-14 hours, then wash off. A second application one week later is recommended to kill any newly hatched mites.
Second-Line / Alternative: Topical Benzyl Benzoate 25% emulsion. Cheaper but more irritating to the skin. Often requires application on 2-3 consecutive days.
Crusted Scabies: This is an emergency for the dermatology team. It requires a combination of topical therapy (Permethrin) AND oral Ivermectin. Multiple doses of Ivermectin are usually needed, guided by a specialist. The patient must be isolated.
Symptomatic Relief:
Antihistamines: (e.g., Chlorpheniramine, Hydroxyzine) can help with the itch, especially at night.
Post-scabetic Pruritus: Inform the patient that the itching can persist for 2-4 weeks even after successful treatment as the dead mites and their products are cleared by the body. A mild topical steroid (e.g., hydrocortisone 1%) can be used for this.
VIII. Complications
Secondary Bacterial Infection: Staphylococcal or Streptococcal infection of excoriated lesions (impetigo, cellulitis). Look for crusting, pus, and increased erythema. Manage with appropriate antibiotics.
Post-streptococcal Glomerulonephritis: A known, though rare, sequela of secondary skin infection.
Post-scabetic Pruritus: Persistent itch after treatment. This is not a treatment failure if no new lesions appear.
IX. Prognosis
Excellent with correct and timely treatment of the patient and all contacts.
Recurrence is common due to improper application technique or failure to treat all contacts simultaneously.
X. How to Present to Your Senior
"Dr, for your review. This is patient [Name] in Bed 10, who was admitted for another issue but has been complaining of a severe itchy rash for the past month, which is much worse at night and disturbs his sleep.
On examination, he has multiple excoriated papules and several linear burrows in his finger webs and on his wrists. His cubicle-mate has also started complaining of an itch.
My main diagnosis is Scabies. I have counselled the patient on the application of Permethrin 5% lotion and the importance of contact tracing and environmental decontamination. I would like to prescribe the lotion for him and arrange for his close contacts (family and cubicle-mate) to be treated as well to prevent re-infestation."
XI. Summary & Further Reading
Top 3 Takeaways:
Itch worse at night + finger web/wrist involvement = Scabies. Always ask about contacts.
Treat the patient AND all close contacts simultaneously. Meticulous application technique is key.
In an immunocompromised or elderly patient, think of Crusted Scabies. Isolate, get help, and treat aggressively with combination therapy.
Key Resources:
Malaysian Guidelines: While there isn't a standalone CPG, scabies management is covered in the Malaysian National Formulary and dermatology manuals.
UpToDate: Search for "Scabies: Management".
StatPearls: Search for "Scabies".
