Bacteria of the order Chlamydiales are obligate intracellular pathogens. Virtually any chlamydial organism can infect any eukaryotic host cell, resulting in various infections with these ubiquitous gram-negative bacteria.

Etiology and Epidemiology

Traditional classification of chlamydiae was based on host and/or disease association, without a high degree of consistency. Different efficiencies in infectivity and replication determine consistent, but not absolute, associations between chlamydial strain, host, and disease manifestation. According to current thought, there are 9 chlamydial species; the following are known for involvement in respiratory tract infections.

Chlamydophila psittaci in birds (former avian serovars of Chlamydia psittaci) is the causative agent of a zoonotic disease named either psittacosis or ornithosis, when determined in psittacine birds or poultry/fowl, respectively. Transmission to people may result in atypical pneumonia or even life-threatening acute illness. Chlamydophila psittaci has also been found in numerous mammalian species (eg, cattle, swine, horses, dogs), in which both the pathogenetic relevance and the zoonotic potential have yet to be defined. Chlamydophila felis is associated with respiratory tract infection in cats. Transmission to people may result in atypical pneumonia or conjunctivitis. Chlamydophila pneumoniae is mainly a human pathogen, but it also infects koalas, horses, and frogs. Chlamydophila abortus (formerly Chlamydia psittaci serotype 1) is an agent causing primarily abortion in small ruminants (ovine chlamydiosis with zoonotic risks for pregnant women) but has also been found in the respiratory tract of cattle, swine, and horses. Infections with Chlamydophila pecorum are ubiquitous in cattle and are often accompanied by Chlamydophila abortus. The clinical picture of bovine chlamydiosis is highly variable and may include multiple organ manifestation (respiratory disorders, reproductive failure, mastitis, arthritis, enteritis, and diarrhea). Chlamydia muridarum was isolated from a mouse colony with pneumonia. In swine, Chlamydia suis is the most prevalent chlamydial agent that might be involved in multiple infection manifestations, including respiratory disorders.

With the availability of molecular diagnostic tools, the presence of chlamydiae has been frequently noticed in clinically inconspicuous animals (pets and farm animals). Epidemiologic data indicate that chlamydial infections are disseminated worldwide, but the epidemiologic importance of these findings is still unknown.

Clinical Findings

Chlamydial infections of the respiratory system do not present a typical clinical picture. In general, chlamydial infections can generate a variety of clinical manifestations ranging from acute to chronic inflammation and from a severe to a mild or even subclinical course.

In birds, clinical illness caused by Chlamydophila psittaci infection (avian chlamydiosis) is accompanied by conjunctivitis, serositis, fibrinopericarditis, hepato- or splenomegaly, anemia, and leukocytosis or monocytosis. Dogs infected by Chlamydophila psittaci (most likely transmitted from birds) present a clinical picture of bronchopneumonia that may include fever and dry cough, but also keratoconjunctivitis, GI signs (vomiting, diarrhea), and even neurologic signs. In cats, infections with Chlamydophila felis clinically result in rhinitis, conjunctivitis, and/or broncho-pneumonia, but seropositive cats are often asymptomatic. Lambs delivered by ewes with Chlamydophila abortus infection may develop acute chlamydial pneumonia. They become febrile, lethargic, and dyspneic, and develop a serous and later mucopurulent nasal discharge.

In cattle and pigs, chlamydial infections must be regarded as widespread but often underdiagnosed. Chlamydiae detected so far in these farm animals include Chlamydophila pecorum, Chlamydophila abortus, and Chlamydophila psittaci in cattle, and Chlamydia suis (former porcine serovar of Chlamydia trachomatis), C psittaci, C abortus, and C pecorum in pigs. Mixed infections are common in herds and even in individual animals. Acute respiratory chlamydial infections may affect upper airways as well as the lower respiratory tract. Clinical symptoms include fever, depression, nasal secretions, dry hacking cough, and dyspnea. Furthermore, chlamydial infections in cattle and pigs might be associated with keratoconjunctivitis, encephalomyelitis, polyarthritis, pericarditis, enteritis, abortion, and fertility disorders. In chlamydia-positive herds, newborns are free of chlamydiae but start to acquire chlamydial infections within 2 wk of birth. Thus, young animals develop more clinical signs than older ones.

Equine chlamydiosis has been described as variable. Bronchopneumonia may be accompanied by abortions in mares, poly-arthritis in foals, hepatitis, and fatal cases of encephalomyelitis. Recent data indicate a role of Chlamydophila psittaci and/or Chlamydophila abortus in equine recurrent airway obstruction as trigger factors of inflammation or indicators of severe disease.

Based on serologic data, most chlamydial infections in farm animals do not necessarily result in clinical illness. However, they may lead to chronic-persistent or recurrent chlamydial infections on a subclinical level. Because of the potential role of chlamydiae as bystanders, copathogens, or etiologic agents of latent persisting infections, clinically inapparent chlamydial infections are probably economically more important than rare outbreaks of severe chlamydial disease.

Lesions

Acute pulmonary lesions include bronchiolitis, severe focal pneumonia, and dystelectases. Dissemination of chlamydial bodies in lung tissue is usually accompanied by an influx of macrophages, granulocytes, and activated T cells. Pulmonary edema may occur. Disturbances in gas exchange and acid-base status (hypoxemia and/or respiratory acidosis) or even acute respiratory distress are attributed to multiple disorders in pulmonary functions.

Bronchointerstitial pneumonia and alveolitis may be accompanied by progressing to type II pneumocyte hyperplasia and interstitial thickening due to ingress of mixed inflammatory cells. Lymphocytic aggregates are frequently seen around airways and pulmonary vessels.

In chronic (often subclinical) chlamydial infections, macroscopic examination of the respiratory tract reveals only mild lesions or a few foci of atelectasis predominantly affecting the apical lobes. Histologic lesions may include neutrophil inflammation, follicular bronchiolitis, and active lymphoid tissues (tonsils, tracheobronchial and pulmonary lymph nodes, etc). Both activated bronchus-associated lymphoid tissue of bronchioles (bronchiolar cuffing) and hyperplastic bronchial and bronchiolar epithelium contribute to chronic small airway obstruction and persistent airflow limitation.

Diagnosis

Neither clinical signs nor lesions allow a definitive diagnosis of chlamydial pneumonia. Confirmation of chlamydial infection requires collection of an appropriate clinical sample from the animal, followed by direct detection of the organism using a suitable laboratory-based diagnostic test. Appropriate tests include direct impression smears and cytologic staining, cell culture isolation of the agent, immunofluorescence tests, enzyme immunoassays, and nucleic acid amplification-based tests (PCR and microarray techniques). In vivo nasal and ocular swabs, tracheal washing, or bronchoalveolar lavage fluid are useful. Chlamydial inclusion bodies may be detected in affected tissues.

Because most chlamydial infections do not elicit sufficiently high changes in antibody levels, serologic detection is generally more suitable for prevalence surveys than for the retrospective diagnosis of chlamydial infection. The standard method for detection of antibodies against Chlamydia or Chlamydophila spp in animals is the complement fixation test using crude or partially purified preparations of chlamydial lipopolysaccharide, but numerous ELISA methods have also been introduced.

Prevention and Treatment

The classic concept of prophylactic immunization that elicits sterilizing immunity and virtually 100% protection from disease does not apply to chlamydiae. However, therapeutic vaccination may nevertheless provide substantial health and economic benefits. Vaccines against Chlamydophila felis are available for pet cats, but little has been reported about their efficacy.

Several antimicrobials (eg, tetracyclines, quinolones, macrolides, lincosamides, rifamycins) can interfere with chlamydial replication. Tretracyclines or fluoroquinolones (eg, enrofloxacin) are generally the drugs of choice. Treatment must start as early as possible and continue for at least 5–7 days.

No antibiotic treatment is bactericidal. It is suspected that antibiotics frequently induce persistent chlamydial infections by reducing antichlamydial immunity due to suppression of antigen production while not completely eliminating chlamydiae.

Last full review/revision March 2012 by Petra Reinhold, DVM, PhD