**2.** *Chlamydia pneumonia*

*Chlamydia pneumoniae* was first isolated in 1965 in Taiwan from a child's eye in a trachoma vaccine study and named TW-183 because it was the 183rd isolate in the laboratory. It was assumed that the isolate was a *C. trachomatis* serovar, but it was noticed that it did not share any features of *C. trachomatis* previously tested in laboratory or animal studies. After the micro-immunofluorescence (MIF) serologic test for *Chlamydia* was developed in 1972, TW-183 remained untypable [5]. These antibody studies concluded that TW-183 was probably not a cause of eye disease but a very common infection. Meanwhile, TW-183 was shown to be the probable cause of a pneumonitis outbreak in Finland [6]. AR-39 was the name of the isolate cultivated from the 39th student's throat in a university study, with similar characteristics to TW-183 [7]. While "TW-183/AR-39 like organisms" were continuing to be studied in respiratory tract infections, findings suggested that "TWAR" (as a shorthand for typing) strains were most likely a new human *C. psittaci* strain transmitted from human to human without a bird or animal reservoir [5]. They were shown to be responsible for 12% of pneumonia illnesses and 5% of bronchitis in a university outbreak over a 2.5-year period [5].

In 1989, the TWAR organism was officially announced as "*Chlamydia Pneumoniae*" as a new species by Grayston, based on distinct morphology, DNA sequence, and clinical disease spectrum in *Chlamydiae*, 50 years after Joseph E. Smadel first questioned the source of seven "psittacosis cases without known bird contact" [7].

More recently, a new taxonomic classification was proposed after genome sequencing, in which the genus *Chlamydia pneumoniae* would be replaced with a new genus name, *Chlamydophila pneumoniae* [8, 9]. Since the genus *Chlamydophila* would be composed of most veterinary chlamydiae and the proposal was not universally accepted, especially by the clinicians, and since both names are currently in use by different authors, the former, more widely recognized designation (*Chlamydia pneumoniae*), will be used in this review and will refer to *Chlamydophila pneumoniae*, biovar TWAR.

Even though *C. pneumoniae* is a common pathogen responsible for respiratory tract infections, the majority of recent *C. pneumoniae* research has focused on the pathogen's role as a cause of persistent infections in human chronic diseases and is considered a childhood infection with adult consequences. *C. pneumoniae* has been linked to a wide range of diseases, including cardiovascular disease, Alzheimer's disease, arthritis, ischaemic stroke, asthma, and lung cancer [10].

#### **2.1 Pathogenesis**

*Chlamydia pneumonia* appears to enter the human body via the respiratory mucosal epithelium, and it has been shown that, after infecting lung epithelial cells and alveolar macrophages, they can infiltrate different cell types such as monocytes, macrophages, monocyte-derived dendritic cells (DCs), lymphocytes, and neutrophils [10]. When *C. pneumonia* is not eradicated, RBs, keen on hiding from the host immune system within inclusion bodies, can persist intracellularly for long periods of time and cause chronic infection. Chronically infected monocytes can spread all over the body via the lower respiratory tract after intranasal CP inoculation in mice [11].

#### **2.2 Epidemiology**

The mode of transmission of the bacteria in children is not well understood, but it is thought to be similar to other respiratory infections, and transmission occurs from human to human through infected respiratory tract secretions.

It is challenging to describe the epidemiology of *C. pneumonia* precisely. The studies published after the initial identification, during the 1990s, defined the organism as being associated with 6–22% of lower respiratory tract infections in children and adults [12–15]. After the consideration of *C. pneumoniae*'s long-term effects, studies continued with an acceleration number in various populations, with different and more sensitive diagnostic methods. Heterogeneity in the serological methods and study population (hospitalized/outpatient, ages, used criteria in the diagnosis) makes it difficult to compare the results.

Serological studies showed that *C. pneumoniae* is common, with a seroprevalence of over 50% among adults, indicating previous infection [16–19]. In addition, prevalence is relatively low in children under five to ten years (7–8%), sharply increases by the age of 20 (%40–55), and continues to increase gradually to rise in the elderly to 70–80% [16, 20].

*C. pneumoniae* is considered to affect mostly school-aged children, and initial infection time peaks between 5 and 15 years of age; however, with expanding knowledge, this statement may alter as well. Some studies have shown that the prevalence rate of infection in younger children may be similar to that in older ones. A population-based seroepidemiological survey of students from Italy showed that 23% of the first graders had early exposure to *C. pneumoniae* infection in the community [21]. It seems that the age of primary infection due to *C. pneumonia* seems to differ due to the diagnostic test used. Most young children are not able to develop specific antibodies when culture or PCR testing is used. Colonization or possible infection in younger children would also be identified in the absence of a detectable antibody response.

There is no evidence of seasonality. The mean incubation period is 21 days.

#### **2.3 Clinical presentation**

*Chlamydia pneumoniae* infection is frequently asymptomatic or manifests with mild symptoms. *C. pneumonia*, infecting the upper respiratory epithelium in children, can cause or contribute to acute otitis media and sinusitis, as well as protracted cough illnesses and community-acquired pneumonia.

Upper respiratory tract infections caused by *C. pneumoniae* actually do not have a distinctive clinical picture, and patients may be asymptomatic, mildly, or moderately ill, with non-specific respiratory complaints. In general, signs and symptoms of respiratory infections have little value in the diagnosis of *C. pneumoniae*.

Clinical manifestations of the *C. pneumoniae* infection as upper respiratory tract infections were established as pharyngitis, otitis, and sinusitis, with an incidence of 5–10% in the initial studies [7, 22]. Nasopharyngitis (46%) was the most common clinical presentation in school-aged respiratory infections related to *C. pneumoniae*, documented by PCR positivity [23]. In a recent study, *C. pneumoniae* was detected by PCR in 38% of children under 10 years diagnosed with an upper respiratory infection in Brazil [24].

"Sore throat" and "hoarseness" are particular symptoms commonly mentioned in the studies in which either the upper or lower respiratory tract was included [22, 23]. Although fever and related respiratory symptoms can be self-limited, the cough usually follows pharyngitis. The clinical progress can be biphasic and end up in atypical pneumonia. The infection is often associated with a persistent cough when the lower respiratory tract is included. *C. pneumoniae* was isolated in 17% of the infants with prolonged coughing [25]. The mean duration of cough associated with a *C. pneumoniae* infection has been reported as 25–30 days.

Clinical features of *C. pneumoniae* pneumonia are similar to those of other community-acquired pneumonia (CAP), including fever, cough, tachypnea, and shortness of breath. Physical examination may reveal nonexudative pharyngitis, pulmonary rales, and bronchospasm. It is not feasible to distinguish the causative agent according to clinical or routine laboratory tests [26, 27]. Chest radiograph findings generally are nonspecific and include patchy subsegmental infiltration, bilateral infiltrates, segmental and lobar consolidation, and even pleural effusion [27–30].

Data gained from the epidemics painted a clinical picture of mild but long-lasting pneumonia in previously healthy young adults [6]. With our expanding knowledge, it seems that the course of the disease may vary with the patient's age, the presence of co-pathogens, or the existence of comorbidities. Severe and life-threatening infections have been reported [31]. Coinfection with other pathogens is possible and may affect clinical presentation [32].

*C. pneumoniae* can manifest as severe community-acquired pneumonia in immunocompromised hosts and has been associated with the onset or acute exacerbation of respiratory symptoms in patients with asthma, cystic fibrosis, and acute chest syndrome in children with sickle cell disease; rare cases of meningoencephalitis and myocarditis have also been attributed to the pathogen [33].

#### **2.4 Diagnosis**

The micro immunofluorescent (MIF) antibody test is the most sensitive and specific serologic test for acute infections, but it cannot be used to make an instant diagnosis. A fourfold increase in IgG levels in acute and convalescent sera is diagnostic. IgM titers greater than 1:16 are indicative of acute infection. IgM increases 1–2 weeks after the onset of primary infection, but not upon reinfection. It should be considered that early antibiotic treatment may suppress the antibody response.

NAATs, such as real-time polymerase chain reaction (PCR) assays, can detect the organism on nasopharyngeal swabs, bronchoalveolar lavages, and sputum samples with high sensitivity and specificity and are useful for rapid and accurate diagnosis [33]. *C. pneumoniae* can be isolated from swab specimens, sputum,

bronchoalveolar lavage, and tissue biopsy specimens, but the organism is relatively hard to culture.

#### **2.5 Treatment**

*Chlamydia pneumoniae* appears sensitive to tetracyclines, macrolides, ketolides, and the majority of fluoroquinolones (e.g., levofloxacin and moxifloxacin but not ciprofloxacin). A total of 70–90% of children with *C. pneumoniae* pneumonia eradicate the organism from the nasopharynx after a 10-day course of erythromycin, clarithromycin, or a 5-day course of azithromycin [26, 34, 35]. When tetracycline or doxycycline is prescribed, typical treatment regimens consist of 14–21 days: 14 days for erythromycin, 7–14 days for fluoroquinolones or clarithromycin, and 5 days for azithromycin.

Clinical improvement occurs in a high proportion of children even if they are untreated or given beta-lactam antibiotics which are thought to be ineffective [36]. On the other hand, despite 10–30 days of appropriate treatment, ongoing isolation of the organism persists in some patients [37].
