What is the most common cause of genital infections in adults and what is the first‑line treatment?

Most Common Causes of Genital Infections in Adults

Chlamydia trachomatis is the most common bacterial cause of genital infections in sexually active adults in the United States, followed closely by Neisseria gonorrhoeae, while Candida albicans is the most common fungal cause of genital infections, particularly vulvovaginal candidiasis. 1

Bacterial Causes (Sexually Transmitted)

Chlamydia trachomatis

• Occurs frequently among sexually active adolescents and young adults, with asymptomatic infection being common in both men and women 1
• Presents as mucopurulent cervicitis (MPC) in women, characterized by yellow endocervical discharge visible in the endocervical canal 2
• Can lead to serious sequelae including pelvic inflammatory disease (PID), ectopic pregnancy, and infertility even when cervical infection appears uncomplicated 1
• Coinfection with gonorrhea occurs frequently, making dual testing and presumptive dual treatment appropriate 1, 2

Neisseria gonorrhoeae

• Second most common bacterial STI, frequently coexists with chlamydia 1, 2
• Also presents as mucopurulent cervicitis with yellow endocervical exudate 2
• Testing for both gonorrhea and chlamydia should always be performed together given high coinfection rates 2

Fungal Causes (Non-Sexually Transmitted)

Candida albicans

• Most common cause of vulvovaginal candidiasis (VVC), affecting approximately 75% of women at least once in their lifetime 3, 4
• Characterized by thick white "cottage cheese" discharge, severe vulvar pruritus and burning, with normal vaginal pH (≤4.5) 2
• Not traditionally considered a sexually transmitted disease, though sexual transmission can occasionally occur 5
• Risk factors include diabetes (especially poorly controlled), pregnancy, antibiotic use, corticosteroid use, and oral contraceptive use 6

Candida glabrata

• Prominent pathogen in women with type 2 diabetes mellitus, less virulent but also less susceptible to conventional antifungal treatment 6
• Equally distributed with C. albicans in chronic recurrent vulvovaginal candidosis 7

Other Common Causes

Bacterial Vaginosis

• Characterized by thin, white-to-gray homogeneous discharge with distinctive fishy odor, minimal pruritus, and pH >4.5 2
• Approximately 50% of women meeting clinical criteria are asymptomatic 2

Trichomonas vaginalis

• Characterized by profuse, yellow-green, frothy discharge with malodorous odor and moderate-to-severe vulvar irritation 2
• Approximately 70% of infections are asymptomatic 2
• Elevated vaginal pH (>4.5) with motile trichomonads on wet mount 2

First-Line Treatment Recommendations

For Chlamydia

Azithromycin 1 g orally in a single dose OR Doxycycline 100 mg orally twice daily for 7 days 1

• Both regimens are equally efficacious 1
• Azithromycin preferred when compliance is questionable due to single-dose, directly observed therapy 1

For Gonorrhea with Presumptive Chlamydia Treatment

Ceftriaxone 250 mg IM single dose PLUS Doxycycline 100 mg orally twice daily for 7-10 days 1

For Vulvovaginal Candidiasis

Fluconazole 150 mg orally as a single dose, achieving 97% clinical cure rates 8, 9

• Alternative: Topical azoles (clotrimazole, miconazole, terconazole) for 3-7 days with 80-90% cure rates 8
• For severe VVC: Fluconazole 150 mg repeated 72 hours later 8

For Bacterial Vaginosis

Metronidazole 500 mg orally twice daily for 7 days, achieving approximately 95% clinical cure 10

• Alternative: Metronidazole gel 0.75% intravaginally twice daily for 5 days (78-84% cure rate) 10

For Trichomoniasis

Metronidazole 2 g orally as a single dose 2

Critical Diagnostic Pitfalls

• MPC is not a sensitive predictor of chlamydia or gonorrhea infection; most women with these infections do not have MPC 1
• Clinical diagnosis of vaginal infections is unreliable without laboratory testing, leading to both false positives and false negatives 2
• Vaginal pH testing combined with microscopic examination provides rapid, accurate diagnosis: pH >4.5 with fishy odor and clue cells indicates bacterial vaginosis; pH >4.5 with motile trichomonads indicates trichomoniasis; pH ≤4.5 with yeast/pseudohyphae indicates candidiasis 2
• High blood glucose levels in diabetic patients promote yeast attachment and growth, increasing risk for both incident infection and recurrence 6