Etiology and Epidemiology
Chlamydiae are obligate intracellular bacteria that form inclusions within the cytoplasm of epithelial cells. The developmental cycle of chlamydiae involves an alternation between the intracellular reticulate body and the extracellular elementary body, which is the infectious form of the organism. Chlamydiae infect the mucosa of a variety of anatomic sites, including the gastrointestinal tract, reproductive tract, and conjunctiva. Although some infection may be localized, animals are typically infected systemically, resulting in potential pathology and chlamydial shedding at various anatomic sites.
The conjunctiva is a typical site for chlamydial pathology and shedding. 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, sheep), and C pecorum (cattle, sheep, pigs, other ruminants such as reindeer, wildlife such as koalas, crocodiles, etc).
C pecorum is a ubiquitous cause of ocular infections in livestock, although the overall contribution to infectious conjunctivitis is unclear. Infection as a serious cause of conjunctivitis in koalas is well documented. C suis is a cause of infectious conjunctivitis in pigs. Chlamydial conjunctivitis in cats is caused by C 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. It has also been documented in the eyes of sheep. 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 years old, and cats 2–6 months old appear to be at highest risk of infection. Cats with conjunctivitis that are >5 years old are very unlikely to be infected, and cats <8 weeks 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. As with cats, young guinea pigs, especially those 1–2 months 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 a cat with chlamydial infection 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-week 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.
In livestock (pigs, sheep, cattle), infection of the eyes is often asymptomatic with the gastrointestinal tract serving as the primary site of infection. When chlamydial conjunctivitis develops, it may present with other well-recognized chlamydial pathologies, including polyarthritis. Conjunctivitis is typically characterized by the early development of bilateral epiphora, chemosis, and conjunctival hyperemia, with disease progressing to prominent conjunctival follicle formation and corneal neovascularization.
Guinea pigs may develop mild to severe conjunctivitis, with conjunctival hyperemia, chemosis, and mucopurulent ocular discharge.
Chlamydial PCR in cases of purulent conjunctivitis
Cytologic examination of conjunctival secretions
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; however, causality should be confirmed by exclusion of other well-known causes of infectious conjunctivitis in the host species affected. 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. Although not ideal, dry swabs may also be used to collect specimens for chlamydial PCR.
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.
Given issues with the sensitivity and specificity of most commercially viable serologic assays for chlamydial infections in animals, serology is not useful for the diagnosis of chlamydial conjunctivitis.
Prevention and Treatment
Systemic administration of tetracycline-class antibiotics
No vaccines except for cats
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.
Nearly all Chlamydia isolates are susceptible to tetracyclines. Systemic therapy is superior to topical therapy and is logical given that organisms are shed from sites other than the conjunctiva. Tetracycline resistance has recently been documented as a growing problem in C suis isolates from pigs.
In cats, the treatment of choice is doxycycline (10 mg/kg/day) for at least 4 weeks. Treatment for up to 6 weeks 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, C felis and C caviae have been isolated from people living in close contact 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. More recently, a cluster of cases of serious atypical pneumonia in humans was linked to C caviae infection of pet guinea pigs. C suis has been documented in the eyes of pig farmers and slaughterhouse workers who have come into contact with infected pigs. Routine hygiene practices, such as hand washing before and after handling sick animals, may reduce the potential for transmission of these organisms from affected animals to people.
Chlamydial infections are a common cause of infectious conjunctivitis in a variety of domesticated and wild animal species.
When chlamydial etiology is suspected, prompt treatment with systemic administration of tetracycline-class antibiotics is warranted.
Appropriate infection control, including handwashing after contact with infected animals, should be used to reduce the potential zoonotic risk.
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