**3.** *Chlamydia Trachomatis*

*C. trachomatis* is the most prevalent sexually transmitted bacterium in the world [38]. It is a significant public health concern [39].

Although chlamydia typically affects sexually active adolescents or adults, this infection can also be transmitted vertically during delivery from their infected mothers.

Prepubertal infection of *C. trachomatis* may be a sign of probable child sexual abuse (CSA), necessitating a multidisciplinary thorough investigation.

In addition to its role in genital diseases and associated perinatal infections, trachoma is one of the world's leading preventable causes of blindness. *C. trachomatis* can also cause Lymphogranuloma venereum (LGV), a systemic, sexually transmitted disease characterized by genital ulceration and inguinal lymphadenopathy.

#### **3.1 Classification**

*C. trachomatis* encodes an abundant surface-exposed protein known as the major outer membrane protein (MOMP or OmpA), which is the primary determinant of serologic classification. Based on antigenic variation in the major OMPs (serovars) and clinical expression, *C. trachomatis* is subdivided into subgroups. Microimmunofluorescence and monoclonal antibody testing have revealed that there are over 18 serovars of *C. trachomatis* classified under three biovars [4].

In developing nations, the trachoma biovar (serovars A–C) is the leading cause of non-congenital blindness, whereas the genital tract biovar (serovars D–K) is the most common sexually transmitted bacterium, each of which is associated with a distinct clinical presentation [1, 3] Serovars A, B, Ba, and C cause trachoma; serovars B, Da, Ga, Ia, and D-K cause oculogenital and neonatal disease; and serovars L1, L2, L2a, and L3 cause lymphogranuloma venereum (LGV).

#### **3.2 Pathogenesis**

The major symptoms of *C. trachomatis* infection are not due to direct pathogen activity but rather to the host's immune response to infection. LGV serovars can

proliferate in lymph nodes and macrophages, whereas other types of *C. trachomatis* can only replicate in mucosal epithelial cells. After 7–21 days (on average, 10 days), various clinical symptoms manifest due to tissue degradation or the host's inflammatory response. Neutrophil infiltration distinguishes the initial stages of the primary infection. Although lymphocytes and plasma cells contribute to the initial response, they also play a role in the resolution of the infection [40, 41]. Plasma cells predominate in ocular and genital tract infections [42, 43], whereas eosinophils and neutrophils predominate in neonatal pneumonia [44]. Chlamydial infections can be self-limiting and asymptomatic or infections can be persistent for months or years, it is assumed that the host will develop some form of a protective immune response [45–47]. However, natural *C. trachomatis* infection is insufficient to prevent reinfection. Since the majority of chlamydial infections of the genital tract are asymptomatic, the risk of chronic, untreated infections is high.

#### **3.3 Epidemiology**

Different strains of *C. trachomatis* infect either the mucosa of the genital tract or the eye. In endemic regions, primarily in Africa and the Middle East, *C. trachomatis* causes trachoma, the leading preventable cause of blindness globally. Approximately 136 million people reside in trachoma-endemic areas in 44 countries [48].

*C. trachomatis* is the most common bacterial cause of sexually transmitted infections (STIs) worldwide. According to the World Health Organization's (WHO) global surveillance of STIs in 2018, the global estimate of new CT cases in 2016 was 127 million [49]. Genital Chlamydia infections are asymptomatic in 61% of women and 68% of men; consequently, they are frequently misdiagnosed and untreated, resulting in transmission to others.

The most strongly associated sociodemographic factor with chlamydial infection is young age (<20 years). *C. trachomatis* was found in 6.5% of high school students, with rates among girls being more than double those of boys (4.0% versus 9.7%), and rates of infection increased with age [50]. In a study of over 3000 sexually active middleschool-aged female adolescents, 29% of them had at least 1 positive test result, and the highest age-specific prevalence rate (28%) was found in 14-year-old females [51]. The prevalence of CT infection was 11.5% among adolescents and 6.2% among young adult women.
