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Overview of Chlamydial Conjunctivitis

By Jane E. Sykes, BVSc (Hons), PhD, DACVIM, Department of Medicine and Epidemiology, School of Veterinary Medicine, University of California-Davis

Etiology and Epidemiology:

Chlamydiae are obligate intracellular bacteria that form inclusions within the cytoplasm of epithelial cells. The life cycle of chlamydiae involves an alternation between the intracellular reticulate body and the extracellular elementary body, which is the infectious form of the organism. Several members of the family Chlamydiaceae have been associated with conjunctivitis in the host species they infect, including Chlamydia caviae (guinea pigs), C suis (pigs), C psittaci (birds), and C pecorum (cattle and sheep). Although chlamydial infection has been associated with keratoconjunctivitis in sheep and goats, a study that used molecular techniques to detect chlamydiae in sheep did not find a clear association between infection and disease. Chlamydial conjunctivitis in cats is caused by C felis (formerly Chlamydophila felis). C pneumoniae has also been detected in cats with conjunctivitis using molecular methods. C psittaci has been isolated from dogs with keratoconjunctivitis and respiratory signs in a dog breeding facility. Trachoma and inclusion conjunctivitis in people are caused by C trachomatis. Chlamydia-like organisms (Parachlamydia acanthamoebae) that reside and proliferate within free-living amoeba have been detected in the eyes of cats, guinea pigs, pigs, and sheep with conjunctivitis. The pathogenic role of these organisms and their amoebic hosts is unclear.

Although the disease in cats has been referred to as feline pneumonitis, chlamydiae rarely cause pneumonia in cats. The infection always involves the eye, occasionally causing signs of rhinitis, with sneezing and nasal discharge. Although antibody titers to C felis are common in some cat populations, the organism is rarely isolated from clinically healthy cats. Cats with chlamydial conjunctivitis are generally <1 yr old, and cats 2–6 mo old appear to be at highest risk of infection. Cats with conjunctivitis that are >5 yr old are very unlikely to be infected, and cats <8 wk old may be less at risk because of the presence of maternal antibody. Transmission occurs as a result of direct, close contact between cats, because the organism survives poorly in the environment. Infected cats also shed chlamydiae from their rectum and vagina, although whether venereal transmission may occur has not been confirmed. There is weak evidence that chlamydiae may be capable of causing reproductive disease and lameness in cats, although these associations have not been definitively documented.

Chlamydial infection is one of the most common causes of conjunctivitis in guinea pig populations, in which it is also known as guinea pig inclusion conjunctivitis (see Rodents:Bacterial Infections). As with cats, young guinea pigs, especially those 1–2 mo old, are predisposed. Subclinical disease may also occur. Rhinitis, lower respiratory tract disease, and genital infections, causing salpingitis and cystitis in female guinea pigs, and urethritis in males, may also occur.

Clinical Findings:

In cats, the incubation period after exposure to an infected cat ranges from 3 to 10 days. Signs can include serous to mucopurulent conjunctivitis, nasal discharge, and sneezing. Cats with signs of rhinitis in the absence of conjunctivitis are unlikely to be infected with C felis. Early signs include unilateral or bilateral conjunctival hyperemia, chemosis, and serous ocular discharge, with prominent follicles on the inside of the third eyelid in more severe cases. Keratitis is rare, and if present, may be the result of coinfection with organisms such as feline herpesvirus 1. The signs are most severe 9–13 days after onset and then become mild over a 2- to 3-wk period. In some cats, clinical signs can last for weeks despite treatment, and recurrence of signs is not uncommon. Untreated cats may harbor the organism for months after infection.

Guinea pigs may develop mild to severe conjunctivitis, with conjunctival hyperemia, chemosis, and mucopurulent ocular discharge.


Chlamydial conjunctivitis in cats should be differentiated from conjunctivitis caused by feline herpesvirus 1 and feline calicivirus, and in guinea pigs from mycoplasmal and other bacterial infections (eg, “pinkeye”). Diagnosis is best confirmed using PCR for chlamydial DNA on conjunctival swabs. Cell culture for Chlamydia is sensitive and specific but not widely available or practical for routine diagnostic purposes. Special chlamydial transport media is required for transport of specimens for culture.

A diagnosis of ocular chlamydiosis can also be made by demonstration of intracytoplasmic chlamydial inclusions in exfoliative cytologic preparations. Scrapings for cytologic examination are prepared by lightly but firmly moving a spatula over the conjunctiva and smearing the scraped material onto a glass slide; the preparation is air-dried and stained. Chlamydial inclusions, which contain reticulate bodies, are round and generally stain purple with Romanowsky stains. Conjunctival cytology from guinea pigs generally reveals a neutrophilic inflammatory response. Inclusions are generally visible only early in the course of infection and sometimes not at all. Melanin granules and remnants of some ophthalmic preparations may be mistaken for inclusions, leading to false-positives, so other diagnostic tests are recommended to confirm the diagnosis.

Prevention and Treatment:

Vaccines are available for chlamydiosis in cats but not for other species. Feline chlamydial vaccines do not provide complete protection from infection but may reduce disease severity and infection rates. Their use may be considered in catteries where chlamydiosis is endemic.

All Chlamydia isolates are susceptible to tetracyclines. The treatment of choice is doxycycline (10 mg/kg/day) for at least 4 wk. Systemic therapy is superior to topical therapy and is logical given that organisms are shed from sites other than the conjunctiva. Treatment for up to 6 wk has been required to eliminate infection in some cats. All cats in the household must be treated. Fluoroquinolones, such as enrofloxacin and pradofloxacin, and amoxicillin-clavulanic acid, also have been used to successfully treat feline chlamydiosis, although their efficacy may be less than that of doxycycline. Azithromycin does not appear to be effective.

Zoonotic Risk:

On rare occasions, C felis and C caviae have been isolated from people living with infected cats and guinea pigs. Follicular conjunctivitis was described in a single immunocompromised person who was found to be infected with C felis. There was one report of detection of C caviae in a person with serous ocular discharge who worked with ~200 diseased guinea pigs. C caviae was also detected in conjunctival swabs of this person’s cat and rabbit, the latter of which had signs of mild conjunctivitis. Routine hygiene practices, such as hand washing before and after handling sick pets, may reduce the potential for transmission of these organisms from affected animals to people.

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