Chapter 27: Chlamydia spp.

Chlamydiae that infect humans are divided into three species, Chlamydia trachomatis, Chlamydia pneumoniae, and Chlamydia psittaci, on the basis of antigenic composition, intracellular inclusions, sulfadiazine susceptibility, and disease production. The separation of the genus Chlamydia into the genera Chlamydia and Chlamydophila was controversial; in this chapter, the three chlamydiae that are pathogens of humans are considered to be in the genus Chlamydia in keeping with publications that do not support the new taxonomy. Other chlamydiae infect animals but rarely if ever infect humans. All chlamydiae exhibit similar morphologic features, share a common group antigen, and multiply in the cytoplasm of their host cells by a distinctive developmental cycle. The chlamydiae can be viewed as gram-negative bacteria that lack mechanisms for the production of metabolic energy and cannot synthesize adenosine triphosphate (ATP). This restricts them to an intracellular existence, where the host cell furnishes energy-rich intermediates. Thus, chlamydiae are obligate intracellular pathogens.

Developmental Cycle

All chlamydiae share a common and unique biphasic developmental cycle. The environmentally stable infectious particle (transmissible form) is a small cell called the elementary body (EB). These are about 0.3 μm in diameter (Figure 27-1) with an electron-dense nucleoid. The EB membrane proteins have highly cross-linked membrane proteins. The EBs have a high affinity for host epithelial cells and rapidly enter them. The first step in entry involves interaction between outer membrane proteins of the EB and heparin sulfate proteoglycan of the host cells. The second step involves additional and irreversible binding to a variety of other host cell receptors. There appear to be multiple adhesins, such as OmcB, the major outer membrane protein (MOMP), glycosylated MOMP, and other surface proteins. Following adherence, the mechanisms thought to mediate entry into the host cell also vary and involve cytoskeletal rearrangements and activation of type III secretion systems and other effectors. EBs are usually seen attached near the base of microvilli, where they are subsequently engulfed by the host cell. More than one mechanism appears to be functional: receptor-mediated endocytosis into clathrin-coated pits and pinocytosis via noncoated pits. Lysosomal fusion is inhibited, creating a protected membrane-bound environment around the chlamydiae. Shortly after entry into the host cell, the disulfide bonds of the EB membrane proteins are reduced (no longer cross-linked), and the EB is reorganized into a larger structure called a reticulate body (RB) [replicative form] measuring about 0.5–1 μm (see Figure 27-1) and devoid of an electron-dense nucleoid. Within the membrane-bound vacuole, the RB grows in size and divides repeatedly by binary fission. Eventually, the entire vacuole becomes filled with EBs derived from the RBs to form a cytoplasmic inclusion. The newly formed EBs may be liberated from the host cell to infect new cells. The developmental cycle takes 48–72 hours.

Structure and Chemical Composition

In chlamydiae, the outer cell wall resembles the cell wall of gram-negative bacteria. It has a relatively high lipid content including lipopolysaccharide of low endotoxic activity. It is rigid but does not contain a typical bacterial peptidoglycan. As mentioned above, another important structural component is the MOMP encoded by ompA. MOMP antigenic variants of C trachomatis are associated with different clinical syndromes. Penicillin-binding proteins occur in chlamydiae, and chlamydial cell wall formation is inhibited by penicillins and other drugs that inhibit transpeptidation of bacterial peptidoglycan. Lysozyme has no effect on chlamydial cell walls. N-acetylmuramic acid appears to be absent from chlamydial cell walls. Both DNA and RNA are present in EBs and RBs. The RBs contain about four times as much RNA as DNA, whereas the EBs contain about equal amounts of RNA and DNA. In EBs, most DNA is concentrated in the electron-dense central nucleoid. Most RNA exists in ribosomes. The circular genome of chlamydiae is 1.04 megabases in length, encodes 900 genes, and is one of the smallest bacterial genomes.

Multiple chlamydial genomes have been sequenced, providing insight into the basic biology of the organisms. For example, chlamydiae have a type III secretion system, which may allow them to inject effector proteins into host cells as part of the infectious process (see discussion above under Developmental Cycle).

Staining Properties

Chlamydiae have distinctive staining properties (similar to those of rickettsiae). Elementary bodies stain purple with Giemsa stain—in contrast to the blue of host cell cytoplasm. The larger, noninfective RBs stain blue with Giemsa stain. The Gram reaction of chlamydiae is negative or variable and is not useful in identification of the agents. Chlamydial particles and inclusions stain brightly by immunofluorescence, with group-specific, species-specific, or serovar-specific antibodies.

Fully formed, mature intracellular inclusions of C trachomatis are compact masses near the nucleus that are dark purple when stained with Giemsa stain because of the densely packed mature particles. If stained with dilute Lugol’s iodine solution, some of the inclusions of C trachomatis (but not C pneumoniae or C psittaci) appear brown because of the glycogen matrix that surrounds the particles (see Figure 27-1). In contrast, inclusions of C psittaci appear as diffuse intracytoplasmic aggregates.

Antigens

Chlamydiae possess shared group (genus)–specific antigens. These are heat-stable lipopolysaccharides with 2-keto-3-deoxyoctanoic acid as an immunodominant component. Antibody to these genus-specific antigens can be detected by complement fixation (CF) and immunofluorescence. Species-specific or serovar-specific antigens are mainly outer membrane proteins. Specific antigens can best be detected by immunofluorescence, particularly using monoclonal antibodies. Specific antigens are shared by only a limited number of chlamydiae, but a given organism may contain several specific antigens. There are at least 15 serovars of C trachomatis that are separated into two biovariants that cause different clinical syndromes. The trachoma biovar includes serovars A, B, Ba, and C as well as the genital tract serovars D–K. The lymphogranuloma venereum (LGV) biovar includes serovars L1, L2, and L3. Several serovars of C psittaci can be demonstrated by CF and microimmunofluorescence (MIF) tests. Only one serovar of C pneumoniae has been described.

Growth and Metabolism

Chlamydiae require an intracellular habitat because of the small genome size, which make them dependent upon host cells for their development and for energy requirements. Chlamydiae grow in cultures of a variety of eukaryotic cells lines. McCoy cells treated with cycloheximide commonly are used to isolate chlamydiae; C pneumoniae grows better in HL or HEp-2 cells. All types of chlamydiae proliferate in embryonated eggs, particularly in the yolk sac.

Some chlamydiae have an endogenous metabolism similar to other bacteria. They can liberate CO₂ from glucose, pyruvate, and glutamate; they also contain dehydrogenases. Nevertheless, they require energy-rich intermediates from the host cell to carry out their biosynthetic activities.

The replication of chlamydiae can be inhibited by many antibacterial drugs. Cell wall inhibitors such as penicillins and cephalosporins result in the production of morphologically defective forms but are not effective in treatment of clinical diseases. Inhibitors of protein synthesis (tetracyclines, erythromycins) are effective in most clinical infections. C trachomatis strains synthesize folates and are susceptible to inhibition by sulfonamides. Aminoglycosides are noninhibitory.

Characteristics of Host–Parasite Relationship

The outstanding biologic feature of infection by chlamydiae is the balance that is often reached between host and parasite, resulting in prolonged persistence of infection. Subclinical infection is the rule—and overt disease the exception—in the natural hosts of these agents. Spread from one species to another (eg, birds to humans, as in psittacosis) more frequently leads to disease. Antibodies to several antigens of chlamydiae are regularly produced by the infected host. These antibodies have little protective effect against reinfection. The infectious agent commonly persists in the presence of high antibody titers. Treatment with effective antimicrobial drugs (eg, tetracyclines) for prolonged periods may eliminate the chlamydiae from the infected host. Very early, intensive treatment may suppress antibody formation. Late treatment with antimicrobial drugs in moderate doses may suppress disease but permit persistence of the infecting agent in tissues.

The immunization of humans has been singularly unsuccessful in protecting against reinfection. Prior infection or immunization at most tends to result in milder disease upon reinfection, but at times, the accompanying hypersensitization aggravates inflammation and scarring (eg, in trachoma).

Classification

Chlamydiae are classified according to their pathogenic potential, host range, antigenic differences, and other methods. Three species that infect humans have been characterized (Table 27-1).

A. Chlamydia trachomatis

This species produces compact intracytoplasmic inclusions that contain glycogen; it is usually inhibited by sulfonamides. It includes agents of human disorders such as trachoma, inclusion conjunctivitis, nongonococcal urethritis, salpingitis, cervicitis, pneumonitis of infants, and LGV.

B. Chlamydia pneumoniae

This species produces intracytoplasmic inclusions that lack glycogen; it is usually resistant to sulfonamides. It causes respiratory tract infections in humans.

C. Chlamydia psittaci

This species produces diffuse intracytoplasmic inclusions that lack glycogen; it is usually resistant to sulfonamides. It includes agents of psittacosis in humans, ornithosis in birds, feline pneumonitis, and other animal diseases.

Humans are the natural host for C trachomatis. Monkeys and chimpanzees can be infected in the eye and genital tract. C trachomatis also replicates in cells in tissue culture. C trachomatis of different serovars replicates differently. Isolates from trachoma do not grow as well as those from LGV or genital infections. Intracytoplasmic replication results in the formation of compact inclusions with a glycogen matrix in which EBs are embedded.

Trachoma is an ancient eye disease, well described in the Ebers Papyrus, which was written in Egypt 3800 years ago. It is a chronic keratoconjunctivitis that begins with acute inflammatory changes in the conjunctiva and cornea and progresses to scarring and blindness. The C trachomatis serovars A, B, Ba, and C are associated with clinical trachoma.

Clinical Findings

The incubation period for chlamydial conjunctival infection is 3–10 days. In endemic areas, initial infection occurs in early childhood, and the onset of the long-term consequence, trachoma, is insidious. Chlamydial infection is often mixed with bacterial conjunctivitis in endemic areas, and the two together produce the clinical picture. The earliest symptoms of trachoma are lacrimation, mucopurulent discharge, conjunctival hyperemia, and follicular hypertrophy. Microscopic examination of the cornea reveals epithelial keratitis, subepithelial infiltrates, and extension of limbal vessels into the cornea (pannus). As the pannus extends downward across the cornea, there are scarring of the conjunctiva, eyelid deformities (entropion, trichiasis), and an added insult caused by eyelashes sweeping across the cornea (trichiasis). With secondary bacterial infection, loss of vision progresses over a period of years. There are, however, no systemic symptoms or signs of infection. The World Health Organization has a grading scheme for assessment of trachoma (see reference by Batteiger and Tan).

Laboratory Diagnosis

The laboratory diagnosis of chlamydial infections is also discussed in Chapter 47.

A. Culture

Typical cytoplasmic inclusions are found in epithelial cells of conjunctival scrapings stained with fluorescent antibody or by the Giemsa method. These occur most frequently in the early stages of the disease and on the upper tarsal conjunctiva.

Inoculation of conjunctival scrapings into cycloheximide-treated McCoy cell cultures permits growth of C trachomatis if the number of viable infectious particles is sufficiently large. Centrifugation of the inoculum into the cells increases the sensitivity of the method. The diagnosis can sometimes be made in the first passage after 2–3 days of incubation by looking for inclusions by immunofluorescence or staining with iodine or Giemsa stain.

B. Serology

Infected individuals often develop both group antibodies and serovar-specific antibodies in serum and in eye secretions. Immunofluorescence is the most sensitive method for their detection. Neither ocular nor serum antibodies confer significant resistance to reinfection.

C. Molecular Methods

Developing countries, where trachoma is endemic, generally do not have the resources to apply polymerase chain reaction (PCR) or other molecular methods to the diagnosis of C trachomatis infections of the eye. Developed countries have relatively little trachoma and little need for such tests. Thus, the molecular methods have been developed for the diagnosis of genital infections. Only research projects have used PCR in studies of trachoma.

Treatment

Clinical trials in villages with endemic trachoma using mass azithromycin treatment show that infection and clinical disease are greatly decreased at 6 and 12 months after therapy; this is true even with single-dose therapy. Thus, azithromycin has replaced erythromycin and doxycycline in the mass treatment of endemic trachoma. Topical therapy is of little value.

Epidemiology and Control

It is believed that more than 400 million people throughout the world have trachoma and that 20 million are blinded by it. The disease is most prevalent in sub-Saharan Africa, Asia, and the Mediterranean basin, where hygienic conditions are poor and water is scarce. In such hyperendemic areas, childhood infection may be universal, and severe blinding disease (resulting from frequent bacterial superinfection) is common. In the United States, trachoma occurs sporadically in some areas, and endemic foci persist.

The World Health Organization has initiated the S-A-F-E program to eliminate blinding trachoma and at least markedly reduce clinically active disease. The S-A-F-E program is as follows: surgery for deformed eyelids, periodic azithromycin therapy, face washing and hygiene, and environmental improvement such as building latrines and decreasing the number of flies that feed on conjunctival exudates. It is clear that improved socioeconomic conditions enhance the disappearance of endemic trachoma.

C trachomatis serovars D–K cause sexually transmitted diseases, especially in developed countries, and may also produce infection of the eye (inclusion conjunctivitis). In sexually active men, C trachomatis causes nongonococcal urethritis and, occasionally, epididymitis. In women, C trachomatis causes urethritis, cervicitis, and pelvic inflammatory disease, which can lead to sterility and predispose to ectopic pregnancy. Proctitis and proctocolitis may occur in men and women, although these infections appear to be most common in men who have sex with men. Any of these anatomic sites of infection may give rise to symptoms and signs, or the infection may remain asymptomatic but communicable to sex partners. Up to 50% of nongonococcal urethritis (men) or the urethral syndrome (women) is attributed to chlamydiae and produces dysuria, nonpurulent discharge, and frequency of urination. Genital secretions of infected adults can be self-inoculated into the conjunctiva, resulting in inclusion conjunctivitis, an ocular infection that closely resembles acute trachoma.

The newborn acquires the infection during passage through an infected birth canal. Probably 30–50% of infants of infected mothers acquire the infection, with 15–20% of infected infants manifesting eye symptoms and 10–40% manifesting respiratory tract involvement. Inclusion conjunctivitis of the newborn begins as a mucopurulent conjunctivitis 5–12 days after delivery. It tends to subside with erythromycin or tetracycline treatment or spontaneously after weeks or months. Occasionally, inclusion conjunctivitis persists as a chronic chlamydial infection with a clinical picture indistinguishable from that of subacute or chronic childhood trachoma in nonendemic areas and usually not associated with bacterial conjunctivitis.

Laboratory Diagnosis

A. Specimen Collection

Proper specimen collection is the key to the laboratory diagnosis of chlamydia infection. Because the chlamydiae are obligate intracellular bacteria, it is important that the specimens contain infected human cells as well as the extracellular material where they might also be present. Endocervical specimens should be collected after removal of discharge and secretions from the cervix. A swab or cytology brush is used to scrape epithelial cells from 1–2 cm deep into the endocervix. Dacron, cotton, or rayon on a plastic shaft should be used to collect the specimen; some other swab materials (calcium alginate) and wooden shafts are toxic to chlamydiae. A similar method is used to collect specimens from the vagina, urethra, or conjunctiva. The commercial diagnostic nonculture tests for chlamydia do not require viable organisms. In general, these proprietary tests include the specimen collection swabs and transport tubes that have been demonstrated to be suitable for the specific tests. For culture, the swab specimens should be placed in a chlamydiae transport medium, such as 2-sucrose phosphate supplemented with bovine serum and antibiotics that inhibit normal microbiota, and kept at refrigerator temperature before transport to the laboratory.

Urine can be tested for the presence of chlamydial nucleic acid. Only the first 20 mL of the void should be collected because a larger volume of bladder urine would dilute the initial urine that passes through the urethra; this could result in a negative test result because of the dilution.

B. Nucleic Acid Detection

Nonamplified probe assays—In one nucleic acid hybridization test, a DNA probe hybridizes to a specific sequence of C trachomatis 16S rRNA; chlamydiae have up to 10⁴ copies of the 16S rRNA. After the hybrids are formed, they are absorbed onto beads, and the amount of hybrid is detected by chemiluminescence. This assay is no longer commercially available in the United States. Another hybridization assay used RNA probes to detect chlamydiae DNA sequences. The overall sensitivity and specificity of these tests are good but are not as good as the nucleic acid amplification tests (NAATs). The hybridization assays, however, are less expensive than the NAATs.

Nucleic acid amplification tests—NAATs are the tests of choice for the diagnosis of genital C trachomatis infections. There are at least five U.S. Food and Drug Administration (FDA)-cleared assays in the United States. They use a variety of molecular methods that target the C trachomatis cryptic plasmid or 23SrRNA, including PCR, strand displacement, and transcription-mediated amplification. These tests have become widely used and have replaced most of the nonamplification methods. Although they are highly sensitive and specific, they are not perfect. New assays to diagnose chlamydiae infection can be compared to combined results from two NAATs as the reference standard.

Specimen types that are appropriate for testing by NAATs include first void urine from males and females and vaginal, cervical, and urethral swabs. Some of the commercial companies that market these platforms are in the process of validating or have validated extragenital sources such as conjunctival, oropharyngeal, and rectal samples. The nucleic acid detection tests have been adapted to simultaneously detect Neisseria gonorrhoeae.

C. Direct Cytologic Examination (Direct Fluorescent Antibody) and Enzyme-Linked Immunoassay

Commercially available direct fluorescent antibody (DFA) and enzyme-linked immunoassay (EIA) assays to detect C trachomatis continue to be marketed. The DFA uses monoclonal antibodies directed against a species-specific antigen on the chlamydial MOMP. The EIA detects the presence of genus-specific antigens extracted from EBs in the specimen. DFA remains useful for detection of chlamydiae in extragenital samples, such as conjunctival swabs. Because of their very low sensitivity and the widespread availability of the more sensitive NAATs, EIAs are being phased out as acceptable methods for screening for both chlamydia and gonorrhea.

D. Culture

Culture of C trachomatis has historically been used to diagnose chlamydia infections. Culture, however, is costly and arduous. Results are delayed compared with the timeliness of NAATs and other tests. Culture is generally much less sensitive than NAATs; the degree of lower sensitivity is largely dependent on the culture method used. Culture is now done in a limited number of reference laboratories. A number of susceptible cell lines can be used, most often McCoy, HeLa 229, or HEp-2. The cells are grown in monolayers on coverslips in dram or shell vials. Some laboratories use flat-bottomed microdilution trays, but cultures by this method are not as sensitive as those achieved with the shell vial method. The cells are treated with cycloheximide to inhibit metabolism and increase the sensitivity of isolation of the chlamydiae. The inoculum from the swab specimen is centrifuged onto the monolayer and incubated at 35–37°C for 48–72 hours. A second monolayer can be inoculated, and after incubation, it can be sonicated and passaged to another monolayer to enhance sensitivity. The monolayers are examined by direct immunofluorescence to visualize the cytoplasmic inclusions. Chlamydial cultures by this method are about 80% sensitive but 100% specific.

E. Serology

Because of the relatively great antigenic mass of chlamydiae in genital tract infections, serum antibodies occur much more commonly than in trachoma and are of higher titer. A titer rise occurs during and after acute chlamydial infection. Because of the high prevalence of chlamydial genital tract infections in some societies, there is a high background of antichlamydial antibodies in the population; serologic tests to diagnose genital tract chlamydial infections generally are not useful.

In genital secretions (eg, cervical), antibody can be detected during active infection and is directed against the infecting immunotype (serovar).

Treatment

It is essential that chlamydial infections be treated simultaneously in both sex partners and in offspring to prevent reinfection. Tetracyclines (eg, doxycycline) are commonly used in nongonococcal urethritis and in nonpregnant infected women. Azithromycin is effective and can be given to pregnant women. Topical tetracycline or erythromycin is used for neonatal N gonorrhoeae infections but may not effectively prevent neonatal C trachomatis infection. Systemic therapy should be used for inclusion conjunctivitis because topical therapy may not cure the eye infections or prevent respiratory disease.

Epidemiology and Control

Genital chlamydial infection and inclusion conjunctivitis are sexually transmitted diseases that are spread by contact with infected sex partners. Neonatal inclusion conjunctivitis originates in the mother’s infected genital tract. Prevention of neonatal eye disease depends on diagnosis and treatment of the pregnant woman and her sex partner. As in all sexually transmitted diseases, the presence of multiple etiologic agents (eg, gonococci, treponemes, trichomonads, herpes) must be considered. Instillation of erythromycin or tetracycline into the newborn’s eyes does not prevent development of chlamydial conjunctivitis. The ultimate control of this—and all—sexually transmitted disease depends on safe sex practices and on early diagnosis and treatment of infected persons. To accomplish the latter, the Centers for Disease Control and Prevention recommends annual screening of all sexually active women ages 25 years and younger.

Of newborns infected by the mother, 10–20% may develop respiratory tract involvement 2–12 weeks after birth, culminating in pneumonia. Affected newborns have nasal obstruction or discharge, striking tachypnea, a characteristic paroxysmal staccato cough, an absence of fever, and eosinophilia. Interstitial infiltrates and hyperinflation can be seen on radiographs. The diagnosis should be suspected if pneumonitis develops in a newborn who has inclusion conjunctivitis and can be established by isolation of C trachomatis from respiratory secretions. In such neonatal pneumonia, an immunoglobulin M (IgM) antibody titer to C trachomatis of 1:32 or more is considered diagnostic. Oral erythromycin for 14 days is recommended; systemic erythromycin is effective treatment in severe cases.

LGV is a sexually transmitted disease caused by C trachomatis and is characterized by suppurative inguinal adenitis; it is most common in tropical climates.

Properties of the Agent

The particles contain CF heat-stable chlamydial group antigens that are shared with all other chlamydiae. They also contain one of three serovar antigens (L1–L3), which can be defined by immunofluorescence.

Clinical Findings

Several days to several weeks after exposure, a small, evanescent papule or vesicle develops on any part of the external genitalia, anus, rectum, or elsewhere. The lesion may ulcerate, but usually it remains unnoticed and heals in a few days. Days to weeks later, the regional lymph nodes enlarge and tend to become matted and painful. In men, inguinal nodes are most commonly involved both above and below Poupart’s ligament and the overlying skin often turns purplish as the nodes suppurate (bubo formation) and eventually discharge pus through multiple sinus tracts. In women and in homosexual men, the perirectal nodes are prominently involved, with proctitis and a bloody mucopurulent anal discharge. During the stage of active lymphadenitis, there are often marked systemic symptoms, including fever, headaches, meningismus, conjunctivitis, skin rashes, nausea and vomiting, and arthralgias. Meningitis, arthritis, and pericarditis occur rarely. Unless effective antimicrobial drug treatment is given at that stage, the chronic inflammatory process progresses to fibrosis, lymphatic obstruction, and rectal strictures. The lymphatic obstruction may lead to elephantiasis of the penis, scrotum, or vulva. The chronic proctitis of women or homosexual men may lead to progressive rectal strictures, rectosigmoid obstruction, and fistula formation.

Laboratory Diagnosis

A. Smears

Pus, buboes, or biopsy material may be stained, but particles are rarely recognized.

B. Nucleic Acid Amplification Tests

All of the commercial NAATs detect all of the LGV serovars but cannot differentiate them from other C trachomatis serovars.

C. Culture

Suspected material is inoculated into McCoy cell cultures. The inoculum can be treated with an aminoglycoside (but not with penicillin) to lessen bacterial contamination. The agent is identified by morphology and serologic tests.

D. Serology

Antibodies are commonly demonstrated by the CF reaction. The test becomes positive 2–4 weeks after onset of illness. In a clinically compatible case, a rising antibody level or a single titer of more than 1:64 is good evidence of active infection. If treatment has eradicated the LGV infection, the CF titer falls. Serologic diagnosis of LGV can use immunofluorescence, but the antibody is broadly reactive with many chlamydial antigens.

Immunity

Untreated infections tend to be chronic, with persistence of the agent for many years. Little is known about active immunity. The coexistence of latent infection, antibodies, and cell-mediated reactions is typical of many chlamydial infections.

Treatment

The sulfonamides and tetracyclines have been used with good results, especially in the early stages. Some drug-treated persons have a marked decline in complement-fixing antibodies, which may indicate that the infective agent has been eliminated from the body. Late stages require surgery.

Epidemiology and Control

Although the highest incidence of LGV has been reported from subtropical and tropical areas, the infection occurs all over the world. The disease is most often spread by sexual contact but not exclusively so. The portal of entry may sometimes be the eye (conjunctivitis with an oculoglandular syndrome). The genital tracts and rectums of chronically infected (but at times asymptomatic) persons serve as reservoirs of infection. Laboratory personnel exposed to aerosols of C trachomatis serovars L1–L3 can develop a chlamydial pneumonitis with mediastinal and hilar adenopathy. If the infection is recognized, treatment with tetracycline or erythromycin is effective.

The measures used for the control of other sexually transmitted diseases also apply to the control of LGV. Case finding and early treatment and control of infected persons are essential.

The first C pneumoniae strain was obtained in the 1960s in chick embryo yolk sac culture. After the development of cell culture methods, this initial strain was thought to be a member of the species C psittaci. Subsequently, C pneumoniae was firmly established as a new species that causes respiratory disease in humans and nonhuman species.

Properties of the Agent

C pneumoniae produces round, dense, glycogen-negative inclusions that are sulfonamide resistant, similar to C psittaci (see Table 27-1). The EBs sometimes have a pear-shaped appearance. The genetic relatedness of C pneumoniae isolates is greater than 95%. Only one serovar has been demonstrated.

Clinical Findings

Most infections with C pneumoniae are asymptomatic or associated with mild illness, but severe disease has been reported. There are no signs or symptoms that specifically differentiate C pneumoniae infections from those caused by many other agents. Both upper and lower airway diseases occur. Pharyngitis is common. Sinusitis and otitis media may occur and be accompanied by lower airway disease. An atypical pneumonia similar to that caused by Mycoplasma pneumoniae is the primary recognized illness. The proportion of cases of community-acquired pneumonia caused by C pneumoniae varies in the literature from 0% to 40%, but seems to be lower in more recent series (<5%).

Laboratory Diagnosis

A. Smears

Direct detection of EBs in clinical specimens using fluorescent antibody techniques is insensitive. Other stains do not effectively demonstrate the organism.

B. Culture

Swab specimens of the pharynx should be put into a chlamydiae transport medium and placed at 4°C; C pneumoniae is rapidly inactivated at room temperature. It grows poorly in cell culture, forming inclusions smaller than those formed by the other chlamydiae. C pneumoniae grows better in HL and HEp-2 cells than in HeLa 229 or McCoy cells; the McCoy cells are widely used to culture C trachomatis. The sensitivity of the culture is increased by incorporation of cycloheximide into the cell culture medium to inhibit the eukaryotic cell metabolism and by centrifugation of the inoculum onto the cell layer. Growth is better at 35°C than 37°C. After 3 days’ incubation, the cells are fixed and inclusions detected by fluorescent antibody staining with genus- or species-specific antibody or, preferably, with a C pneumoniae–specific monoclonal antibody conjugated with fluorescein. Giemsa staining is insensitive, and the glycogen-negative inclusions do not stain with iodine. It is moderately difficult to grow C pneumoniae—as evidenced by the number of isolates described compared with the incidence of infection.

C. Serology

Serology using the MIF test is the most sensitive method for diagnosis of C pneumoniae infection. The test is species specific and can detect IgG or IgM antibodies by using the appropriate reagents. Primary infection yields IgM antibody after about 3 weeks followed by IgG antibody at 6–8 weeks. In reinfection, the IgM response may be absent or minimal, and the IgG response occurs in 1–2 weeks. The following criteria have been suggested for the serologic diagnosis of C pneumoniae infection: a single IgM titer of 1:16 or greater, a single IgG titer of 1:512 or greater, and a fourfold rise in either the IgM or IgG titers.

The CF test can be used, but it is group reacting, does not differentiate C pneumoniae infection from psittacosis or LGV, and is less sensitive than the MIF test.

D. Nucleic Acid Amplification Methods

Although many research and reference laboratories have attempted to develop molecular assays targeting genes such as the 16SrRNA gene and the ompA gene, among others, progress has been hampered by the lack of a reliable gold standard. However, recently BioFire Diagnostics, Inc. (Salt Lake City, UT) received FDA approval for the addition of C pneumoniae to its FilmArray Respiratory panel. Such tests are needed so that the true contribution of C pneumoniae to clinical disease can be fully determined.

Immunity

Little is known about active or potentially protective immunity. Prolonged infections can occur with C pneumoniae, and asymptomatic carriage may be common.

Treatment

C pneumoniae is susceptible to the macrolides and tetracyclines and to some fluoroquinolones. Treatment with doxycycline, azithromycin, or clarithromycin, levofloxacin or moxifloxacin, appears to significantly benefit patients with C pneumoniae infection, but there are only limited data on the efficacy of antibiotic treatment. Reports indicate that the symptoms may continue or recur after routine courses of therapy with erythromycin, doxycycline, or tetracycline, and these drugs should be given for 10- to 14-day courses.

Epidemiology

Infection with C pneumoniae is common. Worldwide, 30–50% of people have antibody to C pneumoniae. Few young children have antibody, but after the age of 6–8 years, the prevalence of antibody increases through young adulthood. Infection is both endemic and epidemic, with multiple outbreaks attributed to C pneumoniae. There is no known animal reservoir, and transmission is presumed to be from person to person, predominantly by the airborne route.

Lines of evidence suggesting that C pneumoniae is associated with atherosclerotic coronary artery and cerebrovascular disease consist of seroepidemiologic studies, detection of C pneumoniae in atherosclerotic tissues, cell culture studies, animal models, and trials of prevention using antibiotic agents. However, other studies have shown no association. The possible link between C pneumoniae infection and coronary artery disease remains controversial.

The term psittacosis is applied to the human C psittaci disease acquired from contact with birds and also the infection of psittacine birds (eg, parrots, parakeets, cockatoos). The term ornithosis is applied to infection with similar agents in all types of domestic birds (eg, pigeons, chickens, ducks, geese, turkeys) and free-living birds (eg, gulls, egrets, petrels). In humans, C psittaci produces a spectrum of clinical manifestations ranging from severe pneumonia and sepsis with a high mortality rate to a mild inapparent infection.

Properties of the Agent

C psittaci can be propagated in embryonated eggs, in mice and other animals, and in some cell cultures. The heat-stable group-reactive CF antigen resists proteolytic enzymes and appears to be a lipopolysaccharide. Treatment of C psittaci infection with deoxycholate and trypsin yields extracts that contain group-reactive CF antigens, but the cell walls retain the species-specific antigen. Antibodies to the species-specific antigen are able to neutralize toxicity and infectivity. Specific serovars characteristic for certain mammalian and avian species may be demonstrated by immunofluorescence typing. Neutralization of infectivity of the agent by specific antibody or cross-protection of immunized animals can also be used for serotyping, and the results parallel those of immunofluorescence typing.

Pathogenesis and Pathology

The agent enters through the respiratory tract, is found in the blood during the first 2 weeks of the disease, and may be found in the sputum at the time the lung is involved.

Psittacosis causes a patchy inflammation of the lungs in which consolidated areas are sharply demarcated. The exudates are predominantly mononuclear. Only minor changes occur in the large bronchioles and bronchi. The lesions are similar to those found in pneumonitis caused by some viruses and mycoplasmas. The liver, spleen, heart, and kidney are often enlarged and congested.

Clinical Findings

A sudden onset of illness taking the form of influenza or nonbacterial pneumonia in a person exposed to birds is suggestive of psittacosis. The incubation period averages 10 days. The onset is usually sudden, but can be insidious, with malaise, fever, anorexia, sore throat, photophobia, and severe headache. The disease may progress no further, and the patient may improve in a few days. In severe cases, the signs and symptoms of bronchial pneumonia appear at the end of the first week of the disease. The clinical picture often resembles that of influenza, nonbacterial pneumonia, or typhoid fever. The mortality rate may be as high as 20% in untreated cases, especially in elderly adults.

Laboratory Diagnosis

A. Culture

Culture of C psittaci can be dangerous, and detection of the organism using immunoassays or PCR is preferred. If necessary, C psittaci can be cultured from blood or sputum or from lung tissue by culture in tissue culture cells, embryonated eggs, or mice in an appropriate biosafety level-3 laboratory. Isolation of C psittaci is confirmed by the serial transmission, its microscopic demonstration, and serologic identification.

B. Antigen Detection of Chlamydia psittaci

Antigen detection by DFA staining or by immunoassay or molecular diagnosis by PCR is done in reference or research laboratories.

C. Serology

A diagnosis of psittacosis is usually confirmed by demonstrating complement-fixing or microimmunofluorescent antibodies in serum specimens. A confirmed case is one with a positive culture result or associated with a compatible clinical illness plus a fourfold or greater change in antibody titer to at least 1:32 or a single MIF IgM titer of at least 1:16. A probable case is one associated with a compatible illness linked epidemiologically with a confirmed case or a titer of at least 1:32 in a single specimen. The CF test is cross-reactive with C trachomatis and C pneumoniae. The MIF test is more sensitive and specific than the CF test, but cross-reactions do occur. MIF allows detection of IgM and IgG. Although antibodies usually develop within 10 days, the use of antibiotics may delay their development for 20–40 days or suppress it altogether.

In live birds, infection is suggested by a positive CF test result and an enlarged spleen or liver. This can be confirmed by demonstration of particles in smears or sections of organs and by passage of the agent in mice and eggs.

D. Molecular Methods

Multiple PCR assays have been developed to detect C psittaci in respiratory tract specimens, vascular tissues, serum, and mononuclear cells from peripheral blood. These tests are done in reference or research laboratories.

Immunity

Immunity in animals and humans is incomplete. A carrier state in humans can persist for 10 years after recovery. During this period, the agent may continue to be excreted in the sputum.

Live or inactivated vaccines induce only partial resistance in animals. They have not been used in humans.

Treatment

Because of the difficulty in obtaining laboratory confirmation of C psittaci infection, most infections are treated based only on the clinical diagnosis. Information on therapeutic efficacy comes from several clinical trials. Doxycycline and tetracycline are the preferred agents for treatment; macrolides and fluoroquinolones may be alternatives.

Epidemiology and Control

Outbreaks of human disease can occur whenever there is close and continued contact between humans and infected birds that excrete or shed large amounts of infectious agent. Birds often acquire infection as fledglings in the nest, may develop diarrheal illness or no illness, and often carry the infectious agent for their normal lifespan. When subjected to stress (eg, malnutrition, shipping), birds may become sick and die. The agent is present in tissues (eg, the spleen) and is often excreted in feces by healthy birds. The inhalation of infected dried bird feces is a common method of human infection. Another source of infection is the handling of infected tissues (eg, in poultry rendering plants) and inhalation of an infected aerosol.

Birds kept as pets have been an important source of human infection. Foremost among these were the many imported psittacine birds. Latent infections often flared up in these birds during transport and crowding, and sick birds excreted exceedingly large quantities of infectious agent. Control of bird shipment, quarantine, testing of imported birds for psittacosis infection, and prophylactic tetracyclines in bird feed have helped to control this source. Pigeons kept for racing or as pets or raised for squab meat have been important sources of infection. Pigeons populating buildings and thoroughfares in many cities, if infected, shed relatively small quantities of agent.

1. Chlamydiae are small organisms that multiply in the cytoplasm of their host cells using unique biphasic developmental cycles.

2. The EB is the infectious particle that is environmentally stable. The RB is the metabolically active form that divides by binary fission within a membrane-bound vacuole.

3. There are three species of Chlamydia that cause disease in humans: C trachomatis, C pneumoniae, and C psittaci.

4. C trachomatis is responsible for sexually transmitted diseases that include cervicitis, pelvic inflammatory disease, urethritis, epididymitis, LGV, and proctitis, and when transmitted to infants of infected pregnant women, infant inclusion conjunctivitis and eosinophilic pneumonia.

5. Diagnosis of C trachomatis urogenital infections is made most readily by NAATs; culture or DFA is required to diagnose pediatric syndromes. Treatment of infections caused by C trachomatis requires doxycycline or azithromycin.

6. C pneumoniae causes a variety of upper and lower respiratory infections. Pharyngitis is common, and atypical pneumonia resembling that of M pneumoniae is responsible for approximately 5% of cases of community-acquired pneumonia.

7. Serology using MIF is the most sensitive means of diagnosing C pneumoniae. NAATs are available in research and reference laboratories but vary in their performance. There is one FDA-approved commercial assay that detects C pneumoniae.

8. C psittaci is acquired from contact with birds such as parrots, pigeons, and various domestic poultry.

9. The disease psittacosis may be inapparent or mild, but severe pneumonia and sepsis associated with a high mortality rate have also been described.

10. Diagnosis is made serologically; doxycycline is used for treatment.