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 Chlamydophila caviae (guinea pigs), Chlamydia suis (pigs), Chlamydophila psittaci (birds), and Chlamydophila 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 Chlamydophila felis. Chlamydophila psittaci has been isolated from dogs with keratoconjunctivitis and dyspnea in a dog breeding facility. Trachoma and inclusion conjunctivitis in people are caused by Chlamydia trachomatis. Recently, 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 currently unclear, although a recent study in guinea pigs suggested an association between infection and ocular disease.
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 Chlamydophila 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 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.
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 Chlamydophila 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. Corneal disease is rare, and if present, may be the result of co-infection 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 can be confirmed by demonstration of intracytoplasmic chlamydial inclusions in exfoliative cytologic preparations, by isolation of the chlamydial organism in cell culture, or by PCR for chlamydial DNA on conjunctival swabs. 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.
Conjunctival cytology from guinea pigs generally reveals a neutrophilic inflammatory response. Chlamydial inclusions, which contain reticulate bodies, are round and generally stain purple with Romanowsky stains. Inclusions are generally only visible 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. Scrapings or conjunctival swabs may be submitted for isolation of chlamydiae in cell culture or for PCR testing at specialized diagnostic laboratories. Although acute and convalescent phase serology has been used to detect the antibody response to chlamydial infection, it is generally not used clinically for diagnosis of chlamydial conjunctivitis.
Prevention and Treatment
Vaccines are available for chlamydiosis in cats but not for other species. The feline chlamydial vaccine does not completely protect the cat but may reduce disease severity and infection rates. It may be considered for use in catteries where chlamydiosis is endemic.
All Chlamydophila psittaci isolates are susceptible to tetracyclines. The treatment of choice is doxycycline (10 mg/kg, sid) 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.
On rare occasions, Chlamydophila felis and Chlamydophila caviae have been isolated from people living with infected cats and guinea pigs. Follicular conjunctivitis was described in a single immunocompromised person that 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 approximately 200 diseased guinea pigs. C caviae was also detected in conjunctival swabs of the man'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.
Last full review/revision April 2012 by Jane E. Sykes, BVSc (Hons), PhD, DACVIM