Inclusion conjunctivitis is an inflammation of the conjunctiva that usually presents as acute follicular conjunctivitis with a foreign body sensation in the eye. The causal organism is Chlamydia trachomatis, a gram-negative bacterial intracellular obligate pathogen that only affects humans and is divided into different serotypes. Serovars D through K cause chlamydial genital infection, a common sexually transmitted infection (STI), and inclusion conjunctivitis.

Transmission primarily occurs through contact between contaminated genital secretions and the eye(s), either via autoinoculation (hand-to-eye transmission) or from an infected sexual partner during sexual intercourse; however, case reports have documented transmission from poorly chlorinated swimming pools or hot tubs.

Inclusion conjunctivitis is classically seen in tepid latitudes of the globe, with incidence in industrialized countries ranging from 1.5%-25% of all sexually active adults. Annually, approximately 2.5-3 million adults in the United States are diagnosed with inclusion conjunctivitis.

Teenagers and young adults (aged 16-20 years) who engage in risky sexual activity are at highest risk for infection with C trachomatis. Approximately 25% of males with chlamydia are not aware of their infection. More than 50% cases of adult inclusion conjunctivitis cases have concurrent chlamydial urogenital infections, while less than 1% of individuals (1 per 300 patients) with chlamydial genital tract infection have co-infection with inclusion conjunctivitis.

Symptoms typically start within 48 hours but may not appear for up to 20 days. Depending on the degree of infection, patients may present with a viscous discharge in one or both eyes, which are moderately red and irritated, swollen, and considered slightly sticky. Infection is usually unilateral. The lower lid may show large follicles when pulled downward. Lymph nodes (specifically, preauricular) may be swollen, known as oculoglandular syndrome of Parinaud.

Patients with inclusion conjunctivitis should be screened for chlamydial urogenital infection because genital infections are often asymptomatic; therefore, a proper assessment is required, or diagnosis may be delayed.

Special considerations in infants:
Neonates may become infected during delivery as the baby passes through the birth canal of an infected mother who has not been diagnosed and/or has not received adequate treatment. Infection is acquired from secretions of the mother's infected cervix.

The condition typically manifests 1-2 weeks after birth with a viscous discharge in one or both eyes, which are moderately red and irritated, swollen, and considered slightly sticky (similar to adult infection). Since the lymphatic system is not well developed in neonates, it is unusual to find follicles and enlarged lymph nodes as in adults. Tearing along with a purulent ocular discharge and swollen eyelids are the main physical findings.

Up to 15% of pregnant individuals harbor C trachomatis, with an estimated 30%-50% of their newborns acquiring neonatal inclusion conjunctivitis during childbirth. A chlamydial etiology should be considered for all infants aged 30 days or younger who experience conjunctivitis, especially if the mother has a history of chlamydial infection. These infants should receive evaluation and age-appropriate care and treatment.