# Sexually Transmitted Infections: Screening, Diagnosis, and Treatment

## Introduction

Sexually transmitted infections (STIs) remain a major public health challenge, with the CDC reporting over 26 million new infections annually in the United States. Family physicians are essential in STI prevention, screening, diagnosis, and treatment. Missed or delayed diagnoses lead to serious complications including infertility, chronic pain, and increased HIV transmission risk.

## Screening Recommendations

### Chlamydia and Gonorrhea

All sexually active women under 25 years should be screened annually. Women 25 and older should be screened if at increased risk due to new or multiple partners, inconsistent condom use, or prior STI. Men who have sex with men (MSM) should be screened at least annually at all exposed sites, including urethral, rectal, and pharyngeal. All pregnant women should be screened at the first prenatal visit, with rescreening in the third trimester if at risk. Nucleic acid amplification testing (NAAT) is the preferred method, and urine or self-collected vaginal swabs are acceptable specimens.

### Syphilis

All pregnant women should be screened at the first prenatal visit. MSM, persons with HIV, and individuals with risk factors should be screened at least annually. RPR or VDRL (non-treponemal tests) are used for screening, with confirmation by FTA-ABS or TP-PA (treponemal tests). The reverse screening algorithm, using a treponemal test first, is increasingly used by laboratories.

### HIV

Universal screening is recommended for all persons aged 15 to 65 at least once, with more frequent screening for those at increased risk. The fourth-generation antigen/antibody combination test is the standard.

### Hepatitis B and C

See Lecture 75 for detailed screening, vaccination, and treatment.

![STI screening recommendations by population and frequency](/images/sti-screening-recommendations.jpg)

## Chlamydia (Chlamydia trachomatis)

### Clinical Presentation

Chlamydia is often asymptomatic, with up to 70% of women and 50% of men showing no symptoms. In women, it may present as mucopurulent cervicitis, dysuria, abnormal bleeding, or pelvic pain. In men, it causes urethritis with discharge and dysuria, or epididymitis. Rectal infection presents as proctitis with pain, discharge, and bleeding. Complications include pelvic inflammatory disease (PID), tubal factor infertility, ectopic pregnancy, and reactive arthritis.

### Treatment

Doxycycline 100 mg twice daily for 7 days is now the preferred first-line treatment per the 2021 CDC guidelines. Azithromycin 1 g as a single dose is an alternative but is less effective for rectal chlamydia. Test of cure at 4 weeks is recommended for pregnant patients. All treated patients should be retested at 3 months to detect reinfection. Sexual partners from the prior 60 days should be treated, and expedited partner therapy (EPT) is legal in most states.

## Gonorrhea (Neisseria gonorrhoeae)

### Clinical Presentation

Women are often asymptomatic but may present with cervicitis, urethritis, or PID. Men typically present with purulent urethral discharge and dysuria and are usually symptomatic. Pharyngeal infection is usually asymptomatic but important to test for in MSM and those with oral sexual exposure. Disseminated gonococcal infection (DGI) presents with tenosynovitis, dermatitis, and septic arthritis.

### Treatment

Ceftriaxone 500 mg intramuscularly as a single dose is standard for patients under 150 kg, with 1 g for those 150 kg or greater. If chlamydia has not been excluded, doxycycline 100 mg twice daily for 7 days should be added. Test of cure is recommended for pharyngeal gonorrhea at 14 days after treatment using NAAT. Antimicrobial resistance is a growing concern, and current CDC guidelines should always be followed. Cases should be reported to public health as required.

## Syphilis (Treponema pallidum)

### Stages

Primary syphilis presents with a painless chancre at the inoculation site 3 to 90 days post-exposure, which heals spontaneously. Secondary syphilis produces a diffuse maculopapular rash characteristically involving the palms and soles, along with condylomata lata, mucous patches, lymphadenopathy, and constitutional symptoms. Latent syphilis is asymptomatic, divided into early latent (less than 1 year) and late latent (more than 1 year or unknown duration). Tertiary syphilis manifests as gummas, cardiovascular syphilis (aortitis), or neurosyphilis (tabes dorsalis, general paresis).

### Treatment

| STI | First-Line Treatment | Alternative | Notes |
|-----|---------------------|-------------|-------|
| Chlamydia | Doxycycline 100 mg BID x 7 days | Azithromycin 1 g single dose | Retest at 3 months; treat partners |
| Gonorrhea | Ceftriaxone 500 mg IM x 1 | 1 g if ≥150 kg | Add doxycycline if chlamydia not excluded |
| Syphilis (primary/secondary/early latent) | Benzathine PCN G 2.4 MU IM x 1 | Doxycycline 100 mg BID x 14 days (non-pregnant) | Follow RPR/VDRL titers |
| Syphilis (late latent/unknown) | Benzathine PCN G 2.4 MU IM weekly x 3 weeks | — | Rule out neurosyphilis first |
| Neurosyphilis | Aqueous PCN G 18-24 MU/day IV x 10-14 days | — | LP for CSF analysis |

Primary, secondary, and early latent syphilis are treated with benzathine penicillin G 2.4 million units intramuscularly as a single dose. Late latent syphilis or syphilis of unknown duration is treated with benzathine penicillin G 2.4 million units intramuscularly weekly for 3 weeks. Neurosyphilis requires aqueous penicillin G 18 to 24 million units per day intravenously for 10 to 14 days. The Jarisch-Herxheimer reaction, involving fever, myalgias, and headache within 24 hours of treatment, is self-limited, and patients should be warned. RPR or VDRL titers should be followed to confirm treatment response, with a fourfold decline expected by 6 to 12 months.

![Syphilis stages with clinical photographs and timeline](/images/syphilis-stages-clinical.jpg)

## Genital Herpes (HSV)

HSV-2 causes most genital herpes, though HSV-1 is increasingly responsible, especially in young adults. The primary outbreak presents with painful grouped vesicles and ulcers, dysuria, inguinal lymphadenopathy, and systemic symptoms. Recurrent outbreaks are typically milder and shorter. Diagnosis is made by HSV PCR (preferred) or viral culture from active lesions; type-specific serology (IgG) is available for asymptomatic screening but is not routinely recommended by the USPSTF. Treatment of primary infection is valacyclovir 1 g twice daily for 7 to 10 days, and for recurrent episodes, 500 mg twice daily for 3 days. Suppressive therapy with valacyclovir 500 mg to 1 g daily is appropriate for frequent recurrences or to reduce transmission to serodiscordant partners.

## Trichomoniasis (Trichomonas vaginalis)

Trichomoniasis is the most common non-viral STI worldwide. Women present with malodorous yellow-green vaginal discharge, vulvar irritation, and the characteristic "strawberry cervix" on exam. Men are usually asymptomatic but may have urethritis. Diagnosis is made by NAAT (most sensitive) or wet mount showing motile trichomonads (less sensitive). Treatment is metronidazole 500 mg twice daily for 7 days in women and a single 2 g dose in men. Partners should be treated simultaneously, and retesting at 3 months is recommended.

## Human Papillomavirus (HPV)

HPV is the most common STI, with the majority of sexually active individuals infected at some point. Low-risk types (6 and 11) cause genital warts (condylomata acuminata). High-risk types (16 and 18) cause cervical, anal, and oropharyngeal cancers. HPV vaccination with Gardasil 9 is recommended at age 11 to 12 with catch-up through age 26, and shared decision-making for ages 27 to 45. Genital warts are managed with cryotherapy, imiquimod, podophyllotoxin, or trichloroacetic acid. Cervical cancer screening follows current USPSTF and ASCCP guidelines.

## Pelvic Inflammatory Disease (PID)

PID is an ascending infection of the upper genital tract that is usually polymicrobial, involving chlamydia, gonorrhea, and anaerobes. The clinical diagnosis is based on pelvic or lower abdominal pain plus cervical motion tenderness, uterine tenderness, or adnexal tenderness. Outpatient treatment consists of ceftriaxone 500 mg intramuscularly as a single dose plus doxycycline 100 mg twice daily for 14 days, with or without metronidazole 500 mg twice daily for 14 days. Hospitalization is indicated when patients cannot tolerate oral medication, when a tubo-ovarian abscess is present, during pregnancy, or when symptoms fail to improve within 72 hours.

![PID diagnostic criteria and treatment algorithm](/images/pid-treatment-algorithm.jpg)

## Key Clinical Pearls

Doxycycline has replaced azithromycin as first-line treatment for chlamydia per updated CDC guidelines because it is more effective, especially for rectal infections. A rash on the palms and soles should always prompt consideration of secondary syphilis in the differential diagnosis. Expedited partner therapy, which involves treating partners without requiring them to be seen, is an effective public health strategy, and clinicians should know their state laws. Patients with one STI have a high probability of having another, making co-infection testing essential, especially for gonorrhea and chlamydia together. Retesting at 3 months after treatment for chlamydia, gonorrhea, and trichomoniasis detects reinfection, which is common.

## References

1. Workowski, K. A., et al. (2021). Sexually transmitted infections treatment guidelines, 2021. *MMWR Recommendations and Reports*, 70(4), 1-187.
2. US Preventive Services Task Force. (2019). Screening for chlamydia and gonorrhea: Recommendation statement. *JAMA*, 322(12), 1172-1180.
3. Hook, E. W. (2017). Syphilis. *The Lancet*, 389(10078), 1550-1557.
4. Meites, E., et al. (2019). Human papillomavirus vaccination for adults: Updated recommendations from ACIP. *MMWR*, 68(32), 698-702.