Pharmacotherapeutics of Scabies
1.
Definition
- Scabies is a contagious parasitic skin
infestation caused by the mite Sarcoptes scabiei var. hominis.
- The female mite burrows into the outer layer of the skin and lays
eggs.
- It causes intense itching (pruritus) and a characteristic
skin rash.
- It spreads mainly through prolonged direct skin-to-skin contact.
2. Causative Agent
- Sarcoptes scabiei var. hominis.
- It is a microscopic mite that lives and reproduces in human skin.
3. Mode of Transmission
Scabies commonly spreads through:
- Prolonged skin-to-skin contact.
- Close household contact.
- Sexual contact.
- Sharing bedding or clothing, especially in cases of heavy
infestation such as crusted scabies.
4. Risk Factors
- Close contact with an infected person.
- Overcrowded living conditions.
- Poor access to treatment.
- Institutional living environments.
- Delayed diagnosis and treatment.
- Immunosuppression, particularly for crusted scabies.
5. Pathophysiology
- The female mite enters the stratum corneum of the skin.
- It forms tunnels or burrows.
- The mite lays eggs within these burrows.
- Eggs hatch and develop into adult mites.
- The body's immune response develops against the mites, eggs, and
their products.
- This immune reaction causes:
- Intense itching.
- Redness.
- Papules.
- Rash.
- Itching may become more severe at night.
- Symptoms can persist temporarily even after successful treatment
because the immune reaction may continue.
6. Clinical Manifestations
Common symptoms include:
- Severe itching, especially at night.
- Small red papules.
- Skin rash.
- Burrows in the skin.
- Excoriations due to scratching.
Common
sites of involvement
- Between the fingers.
- Wrists.
- Elbows.
- Armpits.
- Waistline.
- Buttocks.
- Genital area.
In
infants and young children
Other areas may also be affected, including:
- Palms.
- Soles.
- Scalp.
- Face.
7. Diagnosis
Diagnosis is mainly based on:
- Clinical symptoms.
- Characteristic intense nocturnal itching.
- Typical distribution of lesions.
- History of close contact with an affected person.
- Identification of burrows.
Confirmation, when needed, may involve:
- Skin scraping and microscopic examination.
- Dermoscopy.
- Identification of mites, eggs, or mite feces.
8. Goals of Pharmacotherapy
The main goals are:
- Eliminate mites and eggs as effectively as possible.
- Relieve itching and skin inflammation.
- Prevent secondary bacterial infection.
- Prevent reinfestation.
- Treat close contacts simultaneously.
- Control outbreaks, especially in households and institutions.
9. Major Drugs Used in Scabies
Common scabicidal treatments include:
- Permethrin 5% cream.
- Oral ivermectin.
- Benzyl benzoate.
- Sulfur preparations.
- Crotamiton in some settings.
The choice depends on:
- Age.
- Pregnancy status.
- Severity of infestation.
- Presence of crusted scabies.
- Local availability and guidelines.
10. Permethrin 5% Cream
Drug
class
- Topical pyrethroid.
- It is generally considered a first-line treatment for typical
scabies.
Mechanism
of Action
- Permethrin affects sodium channels in the nervous system of the
mite.
- This causes abnormal nerve activity.
- The mite becomes paralyzed and dies.
How
it is generally used
- Applied according to the product instructions and medical guidance.
- In adults, treatment is generally applied thoroughly to the
appropriate skin areas, commonly from the neck downward.
- In infants, young children, elderly patients, and according to
clinical guidance, additional areas such as the scalp may require treatment.
- A repeat application may be recommended after about 7 days,
depending on the regimen and clinical situation.
Adverse
Effects
- Mild burning.
- Stinging.
- Skin irritation.
- Temporary itching.
- Redness.
11. Ivermectin
Drug
class
- Antiparasitic drug.
Mechanism
of Action
- Ivermectin binds to glutamate-gated chloride channels in the
parasite.
- This increases chloride ion movement.
- It causes paralysis and death of the mite.
Uses
- Alternative treatment for scabies.
- Useful when topical treatment is difficult.
- Important in crusted scabies, often as part of a combination
regimen.
- May be useful in outbreaks and situations requiring treatment of
many individuals.
Important
point
- A second dose is commonly required after an interval because
ivermectin has limited activity against eggs.
Adverse
Effects
- Headache.
- Dizziness.
- Nausea.
- Mild skin reactions.
Precautions
- Use in pregnancy and very young children should follow current
medical guidelines and specialist advice.
12. Benzyl Benzoate
Mechanism
- Has toxic effects on the nervous system of mites.
Uses
- Alternative topical treatment for scabies.
Adverse
Effects
- Burning sensation.
- Skin irritation.
- Dermatitis.
Precaution
- May be irritating, particularly in children and individuals with
sensitive skin.
13. Sulfur Preparations
Uses
- Alternative treatment when other treatments are unsuitable.
- May be considered in special situations depending on local medical
guidelines.
Advantages
- Has a long history of use.
Disadvantages
- Unpleasant odor.
- Messy application.
- May be inconvenient.
14. Crotamiton
Actions
- Has scabicidal and antipruritic properties.
Uses
- Alternative treatment in some settings.
Limitations
- May be less effective than first-line therapies.
- Availability varies.
15. Treatment of Itching
Even after successful eradication of mites:
- Itching may continue for several weeks.
- This does not necessarily mean that treatment has failed.
Symptomatic treatment may include:
- Oral antihistamines for itching when appropriate.
- Topical corticosteroids for post-scabetic inflammation when
prescribed.
- Emollients or moisturizers to reduce skin irritation.
16. Secondary Bacterial Infection
Continuous scratching can damage the skin and lead to
bacterial infection.
Signs may include:
- Pus.
- Increasing redness.
- Pain.
- Crusting.
- Fever in severe cases.
Management
- Appropriate medical evaluation.
- Antibiotics may be required if a bacterial infection is confirmed.
17. Crusted Scabies
Crusted scabies is a severe form of scabies.
Characteristics
- Thick crusted skin lesions.
- Very large numbers of mites.
- Highly contagious.
- More common in immunocompromised or vulnerable individuals.
Treatment
- Requires more intensive treatment.
- Combination therapy may include:
- Oral ivermectin.
- Topical permethrin or another topical scabicide.
- Multiple treatments may be required.
- Treatment should be supervised according to clinical guidelines.
18. Treatment of Close Contacts
This is an extremely important part of management.
- Close household contacts should be assessed and often treated
simultaneously according to public-health and clinical guidance.
- Sexual contacts may also require treatment.
- Simultaneous management helps prevent reinfestation.
Remember:
“Treat the patient + treat the close contacts = prevent
reinfection.”
19. Environmental Management
To prevent reinfestation:
- Wash recently used clothing and bedding according to recommended
hot-wash procedures when possible.
- Dry items using appropriate heat.
- Items that cannot be washed can be isolated in a sealed bag for the
recommended period.
- Follow local public-health guidance regarding environmental
cleaning.
20. Important Patient Counseling Points
- Apply treatment exactly as instructed.
- Treat close contacts as recommended.
- Do not assume persistent itching immediately means treatment
failure.
- Avoid close skin contact until treatment has been appropriately
completed.
- Keep fingernails short to reduce skin damage from scratching.
- Seek medical advice if symptoms persist, new burrows appear, or
secondary infection develops.
21. Simple Treatment Approach
Scabies suspected
⬇️
Confirm clinical diagnosis
⬇️
Typical scabies
➡️ Permethrin 5%
cream (common first-line option)
OR
➡️ Oral ivermectin
when clinically appropriate
⬇️
Treat close contacts simultaneously
⬇️
Manage itching and skin inflammation
⬇️
Environmental measures to prevent reinfestation
⬇️
Follow-up if symptoms persist or crusted scabies is
suspected
22. Summary
Scabies is a contagious skin infestation caused by Sarcoptes
scabiei. The main symptoms are intense nocturnal itching, papular rash,
and characteristic burrows. Permethrin 5% cream is commonly a first-line
treatment, while oral ivermectin is an important alternative and is
particularly useful in selected cases, including crusted scabies. Successful
management requires not only treatment of the patient but also simultaneous
management of close contacts and appropriate environmental measures.
Itching may persist after successful treatment and can be managed
symptomatically.