Bacteria of the order Chlamydiales are ubiquitous, obligate intracellular gram-negative bacteria. Within the host cell, they replicate via a unique developmental cycle competing with the host for intracellular nutrient pools. Virtually any chlamydial organism can infect any eukaryotic host cell, resulting in various infections.
Chlamydial taxonomy with two genera, Chlamydia and Chlamydophila, was proposed in 1999 but is no longer valid. A single genus, Chlamydia, is now used, as well as nine species (abortus, caviae, felis, muridarum, pecorum, pneumoniae, psittaci, suis, and trachomatis). Genome comparison revealed a high level of sequence conservation and synteny across taxa with the major exception of the human pathogen C trachomatis. Two additional new species have been discovered recently, ie, C avium and C gallinacea.
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. The Chlamydia species known so far were traditionally attributed to their main hosts.
C abortus (formerly C psittaci serotype 1) is an agent causing primarily abortion in small ruminants, mainly sheep (ovine enzootic abortion, now ovine chlamydiosis; see Enzootic Abortion of Ewes (EAE)) and goats (see Chlamydiosis (Enzootic Abortion)). C caviae was found in guinea pigs and was related to ocular and urogenital infections. C felis is associated with acute or chronic conjunctivitis (see Chlamydial Conjunctivitis), rhinitis, and bronchopneumonia in both stray and domestic cats. C muridarum was isolated from a mouse colony with pneumonia. Infections with C pecorum are ubiquitous in cattle herds with multiple organ manifestations (also see Abortion in Large Animals). C pneumoniae is a respiratory pathogen in people and is involved in community-acquired respiratory tract infections as well as exacerbations of chronic obstructive airway diseases. C psittaci is the causative agent of avian chlamydiosis (see Avian Chlamydiosis), formerly called psittacosis or ornithosis in psittacine birds or poultry/fowl, respectively. In swine, C suis (former porcine serovar of C trachomatis) is the most prevalent chlamydial agent that might be involved in multiple infection sites of the body. C trachomatis causes a variety of diseases in people, such as trachoma, urogenital infection, and lymphogranuloma venereum. With respect to the two new species, C avium has already been identified as a pathogen in pigeons and in psittacines, whereas the role of C gallinacea in poultry has yet to be defined.
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.
Recent studies indicate that host specificity of different species is not as clear as previously thought. Most members of the genus Chlamydia have shown to be transmissible among species, including people. Zoonotic transmission from animals to people is well known for C psittaci, C abortus, and C felis. Conversely, chlamydial species of people have been detected in numerous animal species.
Transmission of avian C psittaci strains to people may result in atypical pneumonia or even life-threatening acute illness (ie, psittacosis in people). Transmission between companion parrots and dogs or cats, respectively, has also been associated with clinical cases. Furthermore, C psittaci has been found in numerous other mammalian species (eg, cattle, swine, horses, small ruminants, rodents, wildlife). An association between C psittaci genotypes with host species has recently been detected, but the pathogenetic relevance as well as the zoonotic potential of non-avian C psittaci strains have yet to be defined.
C abortus may cause abortion and fetal death in pregnant women after transmission from goats or sheep. It has also been found in other animals (cattle, swine, wild suidae, horses, and birds). Zoonotic risks resulting from these hosts and the role of this pathogen in these hosts are unknown.
Natural transmission of C felis mostly occurs through close contact with other infected cats, their aerosol, and fomites, but the pathogen has also been found in dogs. With respect to people, there is clear evidence that C felis acquired from cats may occasionally cause keratoconjunctivitis. Reports are rare, attributing this pathogen to serious systemic disease or atypical pneumonia.
The known human pathogen C pneumoniae was detected in cats with conjunctivitis and also infects koalas, horses, and frogs. C trachomatis, the other human pathogen, was found in pigs and birds so far.
The risk of zoonotic transmission of other chlamydial species found in numerous animal species beside their main hosts, including the two new species C avium and C gallinacea, has yet to be defined.
Chlamydial infections do not present a typical clinical picture. In general, they affect multiple organs and can generate a variety of clinical manifestations, ranging from acute to chronic inflammation and from a severe to a mild or even subclinical course. Because aerosol transmission of the pathogen is one of the main infection routes, the respiratory system is often involved.
In birds, avian chlamydiosis is accompanied by conjunctivitis, serositis, fibrinopericarditis, hepato- or splenomegaly, anemia, and leukocytosis or monocytosis. Dogs infected by C 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 C felis clinically result in rhinitis, conjunctivitis, and/or bronchopneumonia, but seropositive cats are often asymptomatic. Lambs delivered by ewes with C 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 cattle herds include mainly C pecorum, but also C abortus, C psittaci, and C suis. In pig herds, C suis dominates, but C psittaci, C abortus, C perorum, and even C trachomatis have been found in parallel. Mixed infections are common in herds and even in individual animals. Clinical pictures of bovine and porcine chlamydioses are highly variable, although most infections remain clinically inapparent. Current knowledge suggests these infections manifest clinically when they coincide with additional risk factors. In acute infection, clinical symptoms include fever and depression. Respiratory disorders may affect upper airways as well as the lower respiratory tract and result in pneumonia with the typical signs (nasal secretions, dry hacking cough, and dyspnea). In reproducing cows and sows, chlamydial infections might be associated with abortion, disorders in fertility, and mastitis. Other disease manifestations of acute chlamydiosis include enteritis and diarrhea, polyarthritis (see Chlamydial Polyarthritis-Serositis in Large Animals), keratoconjunctivitis, encephalomyelitis (see Sporadic Bovine Encephalomyelitis), pericarditis, or hepatitis. 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, polyarthritis in foals, hepatitis, and fatal cases of encephalomyelitis. Recent data indicate a role of C psittaci and/or C 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.
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 progression 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.
Neither clinical signs nor lesions allow a definitive diagnosis of chlamydiosis. Detection of Chlamydia species is not part of routine veterinary bacteriologic diagnosis, and infections caused by these intracellular pathogens have long been underestimated because of the requirement for special laboratory facilities. Isolation of the pathogen strictly depends on cell culture techniques.
Confirmation of chlamydial infection requires collection of an appropriate clinical sample from the animal, followed by direct detection of the organism using a suitable diagnostic test. In vivo, swabs (nasal, ocular, rectal, vaginal), tracheal washing, or bronchoalveolar lavage fluid are useful. Chlamydial inclusion bodies may be detected in affected tissues. Appropriate tests include direct impression smears and cytologic staining, cell culture isolation of the agent, immunofluorescence tests, and nucleic acid amplification–based tests (PCR and microarray techniques).
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. Species-specific serologic tests are still lacking. Most commercially available ELISA methods may detect infections caused by the family Chlamydiaceae but do not differentiate between chlamydial species. Sensitivity and specificity of these serologic tests are much lower than direct detection of the antigen by PCR.
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. To prevent abortion in small ruminants, C abortus live vaccines are available. There is, however, an ongoing and controversial discussion whether the vaccine strain might even be involved in enzootic abortion. Vaccines against C 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 7 days.
No antibiotic treatment for chlamydiae 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.