Feline chlamydiosis is a common bacterial infection of cats caused by Chlamydia felis, an obligate intracellular bacterium that primarily targets the conjunctival epithelium. While it is rarely life-threatening in otherwise healthy adult cats, it is one of the most frequent causes of chronic or recurrent conjunctivitis in cats and a significant problem in multi-cat households, catteries, and shelters, where it can spread rapidly and persist for extended periods despite treatment.

Unlike most bacteria, Chlamydia felis cannot survive independently outside a host cell, which shapes both its transmission dynamics and the antibiotics needed to treat it. This article explains what feline chlamydiosis is, how it spreads, how to diagnose it, and how to treat it effectively.

What Is Chlamydia felis?

Chlamydia felis (previously Chlamydophila felis, and before that Chlamydia psittaci var. felis) is an obligate intracellular gram-negative bacterium. As an obligate intracellular organism, it can only replicate inside host cells — it cannot grow on standard bacteriological culture media, which is why routine bacterial culture will not detect it.

Chlamydiae have a distinctive two-stage life cycle. The elementary body (EB) is the infectious, metabolically inert extracellular form that enters host cells. Once inside, it transforms into the reticulate body (RB), the metabolically active intracellular form that replicates by binary fission before reorganising into new EBs, which are released to infect adjacent cells.

This intracellular lifestyle explains why Chlamydia felis is relatively fragile outside its host — elementary bodies are inactivated within hours by drying, heat, and many common disinfectants. Transmission therefore requires close direct contact between cats.

How Does Feline Chlamydiosis Spread?

Chlamydia felis is transmitted primarily through direct contact with infectious ocular and nasal secretions. The key transmission routes are:

• Direct cat-to-cat contact: Nose rubbing, mutual grooming, and close proximity allow direct transfer of infectious secretions. This is the dominant transmission route and explains why Chlamydia felis spreads so efficiently in densely populated environments.
• Fomite transmission: Contaminated hands, bedding, and shared equipment such as food bowls and grooming tools can carry infectious material. While C. felis is less environmentally robust than non-enveloped viruses like FCV, fomite transmission is clinically relevant in cattery settings.
• Queen to kitten: Transmission from infected queens to kittens during birth or early neonatal contact is well documented. Kittens typically develop signs from 5 to 12 weeks of age.

There is no evidence for flea-borne or airborne (long-distance aerosol) transmission. The organism does not persist in the environment for prolonged periods.

Zoonotic Potential

Chlamydia felis is considered to have low but non-zero zoonotic potential. Human conjunctivitis following exposure to infected cats has been reported, predominantly in immunocompromised individuals. The risk to healthy adults is considered very low, but basic hygiene precautions — particularly hand washing after handling infected cats or their secretions — are prudent. Pregnant women and immunocompromised individuals should take particular care.

Clinical Signs

Feline chlamydiosis most commonly presents as conjunctivitis. The respiratory signs are typically mild compared to FCV or FHV-1 infection, and systemic signs are usually absent or minimal.

Ocular Signs

Conjunctivitis is the hallmark of C. felis infection. A clinically useful distinguishing feature is its initial presentation: chlamydial conjunctivitis often begins unilaterally (affecting one eye first), then spreads to become bilateral within one to two weeks. This contrasts with FHV-1 and FCV, which more commonly cause bilateral ocular signs from the outset.

Typical ocular findings include:

• Serous to mucopurulent ocular discharge
• Conjunctival hyperemia (redness) and chemosis (swelling)
• Blepharospasm (squinting) due to ocular discomfort
• Follicular hypertrophy of the conjunctiva in chronic cases

Corneal ulceration, which is characteristic of FHV-1, is not a typical feature of C. felis infection.

Respiratory Signs

Mild rhinitis with serous nasal discharge and sneezing can accompany the conjunctivitis, but marked respiratory distress or pneumonia is not typical. In kittens, signs may be more severe, and secondary bacterial infections can complicate the clinical picture.

Systemic Signs

Unlike FCV-VSD or systemic FHV-1 in neonates, C. felis infection does not typically produce systemic disease. Fever, if present, is mild. Affected cats generally maintain normal appetite and demeanour unless severely debilitated by ocular discomfort or concurrent infection.

Diagnosis

Because the clinical signs of chlamydial conjunctivitis overlap significantly with those of FHV-1 and FCV, laboratory confirmation is important, particularly when treatment has failed, or in cattery outbreaks.

PCR Testing

PCR testing on conjunctival swabs is the diagnostic method of choice and the current gold standard. It is highly sensitive and specific, differentiates C. felis from other conjunctival pathogens, and is included on most commercial feline respiratory PCR panels. Swabs should be taken from the conjunctival fornix, using a dry or transport medium swab as specified by the testing laboratory.

Other Diagnostic Methods

Cytological examination of conjunctival scrapes may reveal intracytoplasmic inclusions within epithelial cells — the reticulate bodies of replicating Chlamydia. However, cytology is less sensitive than PCR and may miss infection, particularly in chronic or previously treated cases. Immunofluorescence and ELISA-based antigen detection have also been used but have largely been superseded by PCR in routine practice.

Antibiotic Treatment

Because Chlamydia felis is an obligate intracellular organism, treatment requires antibiotics that penetrate host cells and reach intracellular concentrations sufficient to inhibit bacterial replication. Beta-lactams (penicillins, cephalosporins) have poor intracellular penetration and are ineffective against Chlamydia.

Systemic Treatment (Preferred)

• Doxycycline (5–10 mg/kg q12–24h for a minimum of 4 weeks): The first-line antibiotic for feline chlamydiosis. Systemic treatment is preferred over topical alone because it addresses any occult systemic or respiratory infection in addition to the conjunctivitis. Always administer doxycycline tablets with food or a water bolus to prevent oesophageal stricture.
• Pradofloxacin (5–7.5 mg/kg q24h for 3 weeks): A third-generation fluoroquinolone with excellent intracellular penetration and proven efficacy against C. felis. Appropriate as a first-line alternative, particularly in cats where doxycycline is not tolerated or where concurrent Mycoplasma felis infection is suspected.
• Azithromycin: A macrolide antibiotic effective against Chlamydia. Less commonly used in cats than in human chlamydial infections, but can be considered as an alternative.

Topical Treatment

• Tetracycline or chlortetracycline ophthalmic ointment: Effective as topical treatment but requires very frequent application (3–4 times daily) and does not address systemic involvement. Generally used as an adjunct to systemic therapy rather than as a sole treatment.

Treatment duration is important: courses of less than 4 weeks are associated with treatment failure and relapse. In cattery outbreaks, all in-contact cats should be treated simultaneously to prevent reinfection of treated individuals.

Prevention and Vaccination

Hygiene and Infection Control

Given C. felis’s susceptibility to disinfectants, environmental decontamination is achievable. Affected premises should be cleaned thoroughly with quaternary ammonium compounds or bleach-based disinfectants, and bedding should be washed at high temperatures. Strict hand hygiene and the avoidance of shared equipment between infected and susceptible cats are the cornerstones of infection control.

Vaccination

A live attenuated C. felis vaccine is available in many countries as a non-core (optional) vaccine. It is recommended primarily for cats at increased risk due to multi-cat living arrangements or cattery exposure. The vaccine reduces the severity of clinical signs significantly but does not fully prevent infection. Vaccinated cats may still shed organism and develop mild signs on challenge.

Vaccination is generally incorporated into combination upper respiratory tract vaccines alongside FHV-1, FCV, and FPV components. It should be considered a component of the management program in any catteries or multi-cat households with documented C. felis infection.

Management in Multi-Cat Settings

Chlamydiosis can be frustratingly persistent in catteries if not managed comprehensively. The following approach is recommended for managing an outbreak:

• PCR testing all cats in the affected population to identify infected individuals
• Simultaneous treatment of all positive cats and all in-contact cats for a full 4-week course
• Isolation of newly introduced cats for a minimum of 2 weeks with PCR testing before integration
• Vaccination of all susceptible cats as part of the long-term control program
• Strict hygiene protocols including hand washing between handling individual cats
• Retesting at the end of the treatment course to confirm clearance

Key Takeaways

• Chlamydia felis is the leading infectious cause of conjunctivitis in cats and a significant cattery pathogen
• Unilateral onset of conjunctivitis that subsequently becomes bilateral is a clinically useful distinguishing feature
• PCR from conjunctival swabs is the diagnostic gold standard; standard bacterial culture does not detect Chlamydia
• Doxycycline for 4 weeks (systemic) is first-line treatment; shorter courses risk relapse
• In cattery outbreaks, all in-contact cats must be treated simultaneously
• Vaccination reduces disease severity but does not fully prevent infection; it is recommended as a non-core vaccine in at-risk populations

References

1. Lloret A et al. (2013). Chlamydiosis in cats: ABCD guidelines on prevention and management. J Feline Med Surg 15(7):622–32.

2. Thiry E et al. (2009). Upper respiratory tract disease in the cat: a European multicentre prospective study. J Feline Med Surg 11(7):555–60.

3. Dean R et al. (2005). Use of quantitative real-time PCR to monitor the response of Chlamydophila felis infection to doxycycline treatment. J Clin Microbiol 43(4):1858–64.

4. Browning GF (2004). Chlamydia psittaci in cats. Aust Vet J 82(7):446.

5. Gruffydd-Jones T et al. (2009). Chlamydophila felis infection: ABCD guidelines. J Feline Med Surg 11(7):605–9.

6. Sykes JE (2014). Canine and Feline Infectious Diseases. Elsevier Saunders.