Chlamydiosis in Animals

Bacteria that cause chlamydiosis belong to the order Chlamydiales, which consists of gram-negative, obligate intracellular bacteria with a biphasic developmental cycle that can infect eukaryotic hosts.

The family Chlamydiaceae contains a single genus, Chlamydia, which has 14 recognized species: C abortus, C psittaci,Chlamydia avium, C buteonis, C caviae, C felis, C gallinacea, C muridarum, C pecorum, C pneumoniae, C poikilotherma, C serpentis, C suis, and C trachomatis. There are also three known closely related Candidatus species (ie, uncultured taxa): Candidatus Chlamydia ibidis, Candidatus Chlamydia sanzinia, and Candidatus Chlamydia corallus.

Chlamydial infections are found in most animals and can come from several species, occasionally concurrently. Although many species have a natural host or reservoir, many have been shown to cross natural host barriers. Research has identified one of the genes that allows chlamydial species to obtain new DNA from its surrounding environment to protect itself from host defenses while also replicating in large numbers so that it can spread to surrounding cells.

Chlamydiosis has a worldwide distribution, causing a wide range of illness in humans, food-producing animals, companion animals, mammalian wildlife, avian, reptile, amphibian, and aquatic species. Organisms may be transmitted by handling infected animals and tissues directly, breathing in aerosolized dried feces or respiratory secretions, or other exposure.

Direct zoonotic transmission of chlamydial species C abortus and C psittaci is well recognized. The species C suis, C caviae, C felis, C pecorum, and C gallinacea have also been isolated in humans, although the importance of the findings in relation to clinical signs of chlamydiosis remains undetermined.

In humans, C trachomatis is the leading cause of infectious blindness and sexually acquired infections, and there is support for its causing cardiovascular disease, neurodegenerative disorders, and respiratory illnesses. C pneumoniae is found in human populations worldwide and causes respiratory disease with human-to-human transmission via aerosol.

C abortus is the most common causative agent for enzootic abortion in sheep and goats, its primary reservoir hosts. It has been reported in numerous species, including cats, cattle, deer, fox, horses, llama, mink, pigs, rabbits, raccoon dogs (Nyctereutes procyonoides), water buffalo (Bubalus bubalis), yaks (Bos grunniens), Polish wildfowl, snakes, green sea turtles, and frogs.

Chlamydia avium has been isolated in feral pigeons and psittacine birds in Europe and in coinfections of feral pigeons with C psittaci. C avium has also been found in Picazuro pigeons (Patagioenas picazuro) in an aviary in the Netherlands. Routes of transmission for this species have not been investigated. Direct contact with infected animals and tissues, breathing in aerosolized organisms from dried feces or respiratory secretions, and other means of exposure are possible. The potential for zoonotic infection of humans is unknown.

Chlamydia buteonis strains have been detected in raptors in Europe and North America. The agent has been found in tissue in the conjunctiva and cloaca. Routes of transmission from this species have not been investigated. Direct contact with infected animals and tissues, breathing in aerosolized organisms from dried feces or respiratory secretions, and other means of exposure are possible. The potential for zoonotic infection of humans is unknown.

Chlamydia caviae is mostly found in guinea pigs and causes ocular and urogenital infections. It has also been detected in rabbits, horses, cats, dogs, and corn snakes (Pantherophis guttatus). Nucleic acids were isolated in the eyes of a person who owned guinea pigs infected with Chlamydia caviae. Mild serous ocular discharge was the only symptom that could have been attributed to something other than C caviae. Routes of transmission have not been investigated. Direct contact with infected animals and tissues, breathing in aerosolized organisms from dried feces or respiratory secretions, and other means of exposure are possible.

Chlamydia felis is associated with both stray and domestic cats. It has also been reported in humans, dogs, and iguanas (Iguana iguana) and is suspected to cause clinical signs of chlamydiosis in dogs. In cats, it is commonly associated with acute or chronic conjunctivitis, rhinitis, and bronchopneumonia. It has been found that natural transmission of C felis occurs primarily via close association with infected cats, aerosolized organisms, and fomites. C felis acquired from cats can cause keratoconjunctivitis in humans. The organism is labile outside of the host.

Chlamydia gallinacea has been reported in four European countries as well as the United States, Argentina, Australia, and China. It is found primarily in production and backyard chickens, guinea fowl, turkeys, and ducks, some of which are subclinically affected. It has also been isolated in woodcocks (Scolopax rusticola) in South Korea, dairy cattle in China, and a galah (Eolophus roseicapilla) in Australia. Studies indicate that the prevalence of C gallinacea may be greater than that of C psittaci in some poultry flocks. Slaughterhouse workers in France who were exposed to chickens infected with C gallinacea developed atypical pneumonia, suggesting a potential zoonotic risk.

Chlamydia muridarum is found in mice and other rodents, and it has been isolated in chickens, ducks, and geese in China and a Mexican green rattler (Crotalus basiliscus) in Europe. C muridarum is primarily used in mouse models in studies of human female genital infections. Transmission between animals and humans has not been documented.

Chlamydia pecorum is associated with disease in sheep, goats, fox, cattle, water buffalo, horses, pigs, and alpine chamois (Rupicapra rupicapra). C pecorum infection has a high prevalence in koalas (Phascolarctos cinereus). It has also been found in many other Australian wildlife such as the greater glider (Petauroides volans), mountain brushtail possum (Trichosurus cunninghami), common brushtail possum (Trichosurus vulpecula), squirrel glider (Petaurus norfolcensis), spotted-tail quoll (Dasyurus maculatus), and western barred bandicoot (Perameles bouganville). The common routes of transmission are direct contact and contaminated feed and water. This species was identified in ocular infections in a small percentage of human C trachomatis patients in a trachoma-endemic region of Nepal. The clinical importance of these findings is unknown. The route of transmission was not determined but is consistent with direct contact with potentially infected domesticated animals. Mother-offspring transmission is a major transmission route among koalas.

Chlamydia pneumoniae is a common respiratory pathogen of humans. This species has been detected in horses, cattle, cats, dogs, wild ruminants and cervids, koalas, bandicoots, and potoroo as well as reptiles and amphibians. Genomic and phylogenetic evidence suggests humans were originally infected by an animal isolate of C pneumoniae. Humans are the natural host for this species, and it is widespread in human populations, causing acute respiratory disease with human-to-human spread by aerosol transmission. Transmission between animals and humans has not been documented.

Chlamydia poikilotherma has been isolated from a captive corn snake (Pantherophis guttatus) in Europe. The agent was recovered from choanal and cloacal swabs. Routes of transmission for this species have not been investigated. Direct contact with infected animals and tissues, breathing in aerosolized organisms from dried feces or respiratory secretions, and other means of exposure are possible. The potential for zoonotic infection of humans is unknown.

C psittaci is associated with more than 465 species of domestic, companion, and wild birds. It has also been found in mammalian species such as cattle, swine, horses, small ruminants, and rodents and is a well-known pathogen in reptiles. Human psittacosis cases are most associated with those work with companion birds, poultry workers, veterinarians, pet bird owners, and gardeners in areas where C psittaci is epizootic in the native bird population. A University of Georgia 5-year retrospective study of avian diseases included 153 captive bird species. The highest percentage of bacterial infections was due to C psittaci, with the infections most often reported in Psittaciformes but also found in Passeriformes, Galliformes, Columbiformes, and Anseriformes. C psittaci infection in birds may be systemic and even fatal; however, older psittacine birds and poultry especially may shed the organisms for some time but display no clinical signs. Birds are the major zoonotic reservoir for this species. Human infections typically occur via breathing in aerosolized organisms from dried feces or respiratory secretions of infected birds. Human psittacosis infections have occurred in the absence of direct bird exposure, and epidemiological studies associate aerosolization of infectious particles shed by birds via lawn mowing.

Transmission of avian C psittaci strains to people may result in atypical pneumonia or even life-threatening acute illness. There is also limited evidence of person-to-person transmission. Zoonotic psittacosis cases are found most among people such as bird keepers, poultry workers, and veterinarians, whose work brings them into contact with avian species. Five cases of psittacosis in veterinary students and staff in New South Wales, Australia, have been associated with direct exposure to the infected equine fetal membranes of a mare. Horses appear to only be occasional hosts of C psittaci, which can cause respiratory disease and fetal abortion.

Chlamydia serpentis has been recovered from both choanal and cloacal swabs in snakes belonging to the families Colubridae and Viperidae and isolated from captive subclinically affected corn snakes (Pantherophis guttatus) and an African bush viper (Atheris squamigera). The natural route of transmission and potential reservoirs are unknown. Routes of transmission for this species have not been investigated. Direct contact with infected animals and tissues, breathing in aerosolized organisms from dried feces or respiratory secretions, and other means of exposure are possible. The potential for zoonotic infection of humans is unknown.

Chlamydia suis infections are endemic in domestic pigs worldwide and can lead to conjunctivitis, pneumonia, enteritis, and reproductive failure. They have also been reported in humans, cattle, sheep, horses, cat, poultry (chickens, ducks, and geese in isolated flocks in China), and frogs. This species was found in one case of community-acquired pneumonia in Germany. Nucleic acids of C suis have also been detected in the eyes of people with trachomatous ocular inflammation in Nepal as well as in upper respiratory, ocular, and stool samples from asymptomatic Belgian farmers working with pigs. In addition, C suis was isolated in samples from pigs, the air, contact surfaces, and eye swabs of asymptomatic employees at a Belgian pig slaughterhouse. Additional studies are needed to understand transmission routes and the clinical importance of C suis in humans.

Chlamydia trachomatis occurs almost exclusively in humans and causes a variety of diseases, such as trachoma, urogenital infection, and lymphogranuloma venereum. Humans are the natural host for this species, and it is the most common bacterial sexually transmitted infection worldwide, resulting in substantial morbidity and economic costs. It has been found in the reproductive organs of culled breeder pigs and has also been isolated in pigs, pigeons, and wild Eurasian coots (Fulica atra).

Chlamydiosis lacks a typical clinical picture. Infections affect multiple organs and can generate a diversity of clinical manifestations, ranging from acute to chronic inflammation and severe to mild disease. Organisms may be undetectable for some time. Common clinical findings include arthritis or polyarthritis, blepharitis, bronchopneumonia, conjunctivitis, enteritis, pericarditis, and rhinitis as well as reproductive system disease such as endometritis, metritis, orchitis, epididymitis, urethritis, abortion, stillbirth, and infertility or poor reproductive performance.

Mixed infections are common in herds and even in individual animals. Clinical pictures of bovine, ovine, and porcine chlamydiosis are highly variable, although most infections remain clinically inapparent. In many cases, these infections manifest clinically when they coincide with additional risk factors. Clinically inapparent chlamydial infections are probably economically more important than rare outbreaks of severe chlamydial disease because they may decrease animal productivity and lower birth rates.

C abortus is the species responsible for enzootic abortion of ewes (EAE; also known as ovine enzootic abortion [OEA]), the single largest infectious cause of ovine abortion in the UK and Northern Europe. Lambs delivered by ewes infected with C abortus may develop acute pneumonia; become febrile, lethargic, and dyspneic; and develop a nasal discharge.

C abortus has been identified as a causative agent for equine recurrent airway obstruction. In acute infection, clinical signs include fever and lethargy. Respiratory disorders may affect upper airways as well as the lower respiratory tract and result in pneumonia. Bronchopneumonia may be accompanied by abortions in mares, polyarthritis in foals, hepatitis, and fatal cases of encephalomyelitis. The relationship between C abortus infection and reproductive disorders in foxes and raccoon dogs has not been determined.

C avium infections have been found in Psittacine birds and pigeons that were clinically normal or had respiratory clinical signs or enteritis. Although only limited data link this species to respiratory disease in parrots and pigeons, two other species, C gallinacea and C avium, have been found concurrently with C psittaci in the same flock and even in the same bird.

C buteonis has been shown to cause conjunctivitis and respiratory disease. Systemic disease in birds and even death may follow.

In guinea pigs, clinical findings of C caviae infection can range from subclinical to the common clinical manifestations of follicular conjunctivitis, keratitis, rhinitis, nasal discharge, lower respiratory tract disease, vaginal discharge, and abortion. In rabbits, C caviae can cause a mild seromucous discharge and conjunctivitis. A cat with C caviae had no clinical signs.

In cats, infections with C felis result in rhinitis, conjunctivitis, or bronchopneumonia, and seropositive cats can be subclinically affected. Although C felis has been found in dogs, the clinical importance is unknown because many dogs are clinically normal.

C gallinacea has been isolated from clinically affected domestic poultry, guinea fowl, turkeys, and ducks. Experimentally infected chickens appear clinically normal but lose body weight. Clinically normal dairy and beef cattle were detected with C psittaci and C gallinacea from whole blood samples, milk, feces, and vaginal swabs.

In mice, C muridarum can cause severe acute infection characterized by ruffled fur, hunched posture, and labored respiration due to interstitial pneumonitis and death in 24 hours. Mice may develop progressive emaciation and cyanosis of the ears and tail with chronic infections.

C pecorum penetrates the gastrointestinal tract and migrates to joints and synovial membranes as well as the conjunctiva. Lambs and calves commonly develop polyarthritis and keratoconjunctivitis due to C pecorum. Adult sheep and beef cattle are primarily subclinically affected with shedding. In mature animals, C pecorum infection can manifest clinically as conjunctivitis, encephalomyelitis, keratoconjunctivitis, mastitis, metritis, polyarthritis, and pneumonia.

C pecorum can be present subclinically in infected Australian marsupials, deer, alpine ibex (Capra ibex), wild pigeons, pet birds, and chickens. It can also be carried in the intestinal tract in clinically normal ruminants and other species. Clinical signs of chlamydiosis in koalas manifests as ocular and urogenital disease as well as rhinitis, pneumonia, and arthritis affecting one or more joints.

C pneumoniaeis the most commonly reported etiologic agent in reptiles. In most mammals, infections with this species appear to be subclinical, discovered as a concurrent infection or an incidental finding. Infections in amphibians range from subclinical to generalized edema with epidermal ulceration, with high mortality rates. In reptiles, clinical signs relating to cardiac disease have been noted along with generalized signs of lethargy, anorexia, inability to digest, and respiratory disease.

C pneumoniae has been isolated from koalas with rhinitis and pneumonia, although a direct causal relationship has not been proven and most koalas appear to be subclinically affected. In laboratory rats, coinfections of C pneumoniae and Mycoplasma pulmonis have caused clinical signs related to respiratory disease.

The snake species in which C poikilotherma was isolated were subclinically affected.

Clinical signs of avian chlamydiosis due to C psittaci may be subtle and nonspecific and include anorexia, conjunctivitis, fibrinous pericarditis, lethargy, ocular or nasal discharge, ruffled feathers, and other clinical signs of upper respiratory disease, diarrhea, and signs of liver disease such as excretion of green to yellow-green urates. Clinical signs in dogs may manifest as bronchopneumonia, including fever, dry cough, keratoconjunctivitis, vomiting, diarrhea, and even neurologic signs. Clinical signs of chlamydiosis in horses are associated with respiratory disease and can cause a mild, diffuse, interstitial placentitis in mares and weak foals as well as equine abortion.

The snakes that were found to be carriers of C serpentis were subclinically affected; however, a facultative pathogenic role has been suggested.

C suis is commonly associated with pigs with endemic subclinical infections. When clinical signs of disease are apparent, they present as conjunctivitis, respiratory infections, reproductive disorders, and enteritis. In pig herds, C suis dominates, but C abortus, C pecorum, C psittaci, and C trachomatis have been found in conjunction with C suis.

Gnotobiotic pigs infected with C trachomatis demonstrate clinical signs ranging from mild to severe dyspnea after nasal and intralaryngeal inoculation. C trachomatis has also been shown to cause diarrhea in gnotobiotic pigs.

• Collection of samples

• Laboratory testing

Chlamydiosis affects many different hosts and tissues, making diagnosis complicated. Confirmation of chlamydial infection requires the collection of appropriate samples along with direct detection of the organism via appropriate diagnostic tests. In vivo, swabs (nasal, ocular, rectal, vaginal, choanal, or cloacal), tracheal washing or bronchoalveolar lavage fluid, and biopsies are useful. Because chlamydial infections arise in numerous animal hosts with different clinical pictures and are often diagnosed with other infectious agents, only clinical evaluation and laboratory testing can confirm the Chlamydia sp involved.

Chlamydiae need host metabolites for survival and do not grow on agar plates; they depend on cell culture or embryonated chicken eggs for their growth and isolation.

Antigen detection on tissue samples can confirm chlamydial organisms; however, individual chlamydial species or serotype and genotype cannot be identified. Insufficient antigen or intermittent shedding may yield false-negative results.

Direct complement fixation is more sensitive than agglutination testing and may produce false-negative results Even after treatment, high titers may complicate interpretation of subsequent tests.

Modified Machiavello, modified Gimenez, Giemsa, Castaneda, and modified Ziehl-Neelsen stain are favored to identify chlamydiae in direct smears.

Immunoglobulin M antibodies reach the highest titers at the beginning of an infection and are preferably detected by elementary body agglutination assays. Titers may remain high even after treatment; a titer of 10:1 or above indicates a seropositive result.

Monoclonal or polyclonal antibodies, fluorescein staining, and fluorescent microscopy are preferred methods to detect the organism in samples. Because they cross-react with nonchlamydial epitopes, the commercial antibodies used for fluorescent antibody staining are not diagnostic.

Nucleic acid amplification tests are currently considered the first-choice method for diagnosing chlamydial infections. Amplification of DNA via PCR assay may be sensitive and specific to target DNA sequences, although individual results may vary, given the lack of standardized PCR primers and laboratory techniques. Methods using PCR assay can also detect chlamydial DNA in formalin-fixed tissue specimens. However, because PCR assay does not distinguish between viable and nonviable microorganisms, results should be interpreted with care.

Polyclonal secondary antibodies are used to detect host antibodies and can vary in sensitivity and specificity based on their immunoreactivity to different species. Nonspecific reactivity may produce low titers.

Serologic testing can demonstrate that an animal was infected by a chlamydial species but might not indicate an active infection. False-negative results in animals with acute infection can also result from specimens being collected before seroconversion.

Not all infected birds will shed detectable quantities of C psittaci in feces, depending on the stage of infection and affected tissue. If feces are chosen for testing, 3–5 days of specimens should be pooled. If birds exhibit clinical signs of chlamydiosis, a combined conjunctival, choanal, and cloacal swab specimen or liver biopsy specimen should be used for testing. To detect nucleic acid in subclinically infected birds, swabs of conjunctival and choanal tissues may be most sensitive.

• Varies by species

• Doxycycline for birds

• Limited vaccination

Treatment for chlamydiosis varies across species. In Australia, long-acting oxytetracycline (300 mg/mL) is administered for treatment of C pecorum disease in lambs with polyarthritis and conjunctivitis, at a dose of 30 mg/kg (1 mL/10 kg) for prolonged duration of activity (5-6 days) or as directed by the treating veterinarian. Among chlamydial species, only C suis has naturally acquired genes encoding for tetracycline resistance in fattening pig herds. Despite the evidence for tetracycline resistance, treatment of chlamydiosis in pigs is still limited to tetracyclines.

C serpentis and C poikilotherma appear to be susceptible to tetracycline and moxifloxacin but have intermediate susceptibility or are resistant to azithromycin.

A limited number of antimicrobials, such as tetracyclines, macrolides (eg, erythromycin and azithromycin), and fluoroquinolones have shown good efficacy against chlamydiae. Chloramphenicol has been used successfully for treating koalas. In general, antimicrobial treatment is continued for several weeks after clinical resolution.

For birds with avian chlamydiosis, doxycycline is both absorbed better and eliminated more slowly than other tetracyclines (see table for dosage). Birds should be monitored for toxicosis. Although the duration of treatment for avian chlamydiosis has not been established, historically 45 days has been standard for most species. Thirty days of treatment can be effective in budgerigars.

For birds that regurgitate or do not tolerate the drug, other methods of administration may be used.

Budgerigars and cockatiels can be treated with doxycycline-medicated feed according to the following treatment protocol:

1. Mix cracked steel-cut oats with hulled millet seed (1:3 ratio by volume).

2. Add 5–6 mL of sunflower oil to each kg of oat-millet mixture and combine thoroughly.

3. Add 300 mg of doxycycline hyclate (from capsules) per kg of oat-millet-oil mixture and mix to coat.

Medication for infected birds may also be added to water in the following proportions to maintain therapeutic concentrations:

• Cockatiels: doxycycline hyclate at 200–400 mg/L

• Goffin cockatoos: 400–600 mg/L

• African grey parrots: 800 mg/L

• Other psittacine species: 400 mg/L

Budgerigars will not maintain therapeutic concentrations when provided medicated water.

Doxycycline can be administered parenterally at a dosage of 75–100 mg/kg, IM, every 5–7 days for the first 4 weeks and subsequently every 5 days for the duration of treatment. Irritation sometimes develops at the injection site; however, the injection is usually tolerated.

The attempt to develop an effective chlamydiae vaccine has spanned more than 70 years. There have been many advances in techniques and knowledge of the target species; however, to date no single antigen type or target, adjuvant, or route of administration has been established as a clear front-runner for effective vaccination. However, therapeutic vaccination may currently provide substantial health and economic benefits in some instances.

To prevent abortion in small ruminants, C abortus live vaccines are available. However, there is evidence that the live C abortus vaccine strain 1B is not attenuated and has the potential to cause disease.

Vaccines against C felis are available for pet cats. This vaccine has been shown to decrease the severity and incidence of clinical signs but is not completely protective. This is considered a noncore vaccine and may potentially be considered part of a control regime for cats in multiple-cat environments where infections associated with clinical signs of chlamydiosis have been confirmed.

There is evidence of ongoing successful vaccine trials using both recombinant and peptide C pecorum vaccine formulations in wild and captive koalas. The vaccine appears to be safe to use in both healthy and infected koalas.

1. Balsamo G, Maxted AM, Midla JW, et al. Compendium of measures to control Chlamydia psittaci infection among humans (psittacosis) and pet birds (avian chlamydiosis). J Avian Med Surg. 2017;31(3):262-282. https://doi.org/10.1647/217-265

2. 16S rRNA gene-based phylogenetic tree depicting evolutionary relationships of Chlamydiales. Science Direct. Accessed January 10, 2023. https://www.sciencedirect.com/science/article/pii/S2052297517300343#fig1