What could be causing itchiness after sex in a patient, potentially indicating a sexually transmitted infection (STI) or inflammatory condition?

Post-Coital Itchiness: Differential Diagnosis and Management

The most likely causes of itchiness after sex include seminal plasma hypersensitivity, contact dermatitis (including latex allergy), sexually transmitted infections (particularly trichomoniasis and pubic lice), and vulvovaginal candidiasis, with management requiring systematic exclusion of each etiology. 1, 2

Immediate Diagnostic Considerations

Seminal Plasma Hypersensitivity

• This is a rare but important IgE-mediated allergic reaction to proteins in seminal fluid that presents with itching, urticaria, and vaginal burning within seconds to minutes after ejaculation. 1
• Localized vulvar and vaginal burning with itching and swelling after ejaculation are the hallmark isolated symptoms. 1
• The diagnosis is essentially one of exclusion after ruling out sexually transmitted diseases, latex sensitivity, and transfer of food or drug proteins from the male partner. 1
• A key diagnostic clue is that correct condom use effectively prevents reactions—if condoms fail to prevent symptoms, this suggests either incorrect technique or concurrent latex allergy. 1
• Patients with allergic asthma or atopic dermatitis are at highest risk for this condition. 1

Contact Dermatitis and Latex Allergy

• Latex condom sensitivity must be excluded, as it can present with identical symptoms of vulvar itching and irritation. 1
• Other contactants to consider include fragrant sanitary napkins, lubricants, and spermicides. 1

Sexually Transmitted Infections

Trichomoniasis:

• Characterized by itching, yellow-green discharge, malodor, and vaginal irritation caused by Trichomonas vaginalis. 3, 4
• Diagnosed by wet mount examination showing motile trichomonads when vaginal discharge is present. 2, 3
• Treatment requires metronidazole with mandatory partner treatment. 3, 4

Pediculosis Pubis (Pubic Lice):

• Patients present primarily with pruritus and notice lice on pubic hair. 1
• First-line treatment is permethrin 1% cream rinse applied to affected areas and washed off after 10 minutes. 1, 2
• Lindane 1% shampoo for 4 minutes is an alternative but not recommended for pregnant/lactating women or children under 2 years. 1
• Sexual partners within the last month must be treated. 1

Scabies:

• Presents with severe pruritus, papular or vesicular rash, and linear burrows in finger webs and genital area. 1, 4
• Diagnosis confirmed by scraping burrows and identifying the mite Sarcoptes scabiei microscopically. 1, 4
• Treatment with permethrin or lindane 1% as for pediculosis pubis. 1, 2

Vulvovaginal Candidiasis

• Characterized by pruritus, erythema, and white discharge; commonly caused by Candida albicans. 3
• Normal vaginal pH (≤4.5) is typical, distinguishing it from bacterial vaginosis or trichomoniasis. 3
• Empiric treatment with topical antifungal therapy (clotrimazole 1% cream or miconazole 2% cream) is appropriate first-line treatment. 2, 3, 5
• For recurrent cases (≥4 episodes/year), maintenance fluconazole 150 mg weekly for 6 months is recommended. 3

Systematic Diagnostic Algorithm

Step 1: Timing and Pattern Assessment

• If symptoms occur within seconds to minutes after ejaculation and are prevented by condom use, strongly suspect seminal plasma hypersensitivity. 1
• If symptoms occur regardless of condom use, consider STIs, candidiasis, or other contact allergens. 1

Step 2: Physical Examination

• Look for visible lice or nits in coarse hair areas (pediculosis pubis). 1
• Examine for linear burrows in finger webs and genital area (scabies). 1
• Assess for erythema, white discharge, and vaginal pH. 3

Step 3: Laboratory Testing

• Perform wet mount examination with saline to identify trichomonads or clue cells. 2, 3
• Measure vaginal pH: >4.5 suggests trichomoniasis or bacterial vaginosis; ≤4.5 suggests candidiasis. 3
• Consider nucleic acid amplification tests (NAATs) for N. gonorrhoeae and C. trachomatis from all sites of penetration. 2
• Syphilis serology and HIV testing should be performed in all cases of suspected STI. 1, 2

Step 4: Exclusion of Seminal Plasma Hypersensitivity

• This diagnosis requires exclusion of all other causes listed above. 1
• If suspected, skin prick testing with whole human seminal plasma from the male partner can confirm (after screening donor for hepatitis, syphilis, and HIV). 1
• Serum-specific IgE assays are less sensitive than skin testing and a negative result does not exclude sensitization. 1

Treatment Approach Based on Diagnosis

For Confirmed STIs:

• Treat according to specific pathogen identified with mandatory partner notification and treatment for contacts within 60 days. 1, 2
• Instruct patients to abstain from sexual activity until treatment completion and symptom resolution. 1, 2

For Candidiasis:

• Use topical antifungals as first-line; avoid treating asymptomatic colonization. 2, 3, 5
• Avoid vaginal douching as it disrupts normal flora. 3

For Ectoparasitic Infections:

• Decontaminate bedding and clothing by machine washing/drying with heat or removing from body contact for 72 hours. 1
• Fumigation of living areas is unnecessary. 1

For Seminal Plasma Hypersensitivity:

• Correct condom use is the primary prevention strategy. 1
• Desensitization protocols may be considered in select cases. 1

Critical Pitfalls to Avoid

• Do not rely solely on patient-reported sexual history to exclude STIs, as denial and recall bias are common. 6
• Do not delay treatment while waiting for culture results when STI is suspected. 6
• Do not assume condom failure rules out seminal plasma hypersensitivity—it may indicate incorrect technique or concurrent latex allergy. 1
• Do not self-medicate except for women previously diagnosed with candidiasis experiencing identical symptoms. 3
• Failure to improve within 3 days requires reevaluation of both diagnosis and therapy. 1