Introduction and classification
The life-cycle and classification of Chlamydiae have been reviewed by Caul & Sillis (1998). Chlamydiae are small (elementary bodies being 0.3 mikro meter diameter) RNA- and DNA-containing obligate intracellular prokaryote parasites. Elementary bodies (containing a cell wall like conventional bacteria) bind to and enter host cells and differentiate into larger (1.0 mikro meter diameter) reticulate bodies. Division occurs by binary fission within a membrane-bound vacuole, and there is differentiation back into elementary bodies prior to release of particles from the infected cell. Metabolically the Chlamydiae are distinct from free-living bacteria in that they require a source of ATP from their host.
Chlamydia currently form a single genus within the Eubacteria. The order Chlamydiales has a single family Chlamydiaceae containing this single genus. Four species of Chlamydia are recognised: C psittaci, C trachomatis, C pneumoniae and C pecorum. Antigenic differences are present within the Chamydia species leading to recognition of different strains. Within C psittaci at least five strains have been identified - avian, ovine abortion, ruminant non-abortion, feline and guinea pig and many of these strains appear to have developed a strong host-adaptation. Indeed recent genetic analysis suggests that some of these strains should be re-classified as separate species (see below).
Chlamydia trachomatis and C pneumoniae are recognised as human pathogens, whereas C pecorum is an animal (primarily ruminant) pathogen. Chlamydia psittaci is also an animal (mammalian and avian) pathogen with different strains having a predilection for different species. However, C psittaci is also recognised as a zoonotic agent, although its ability to infect humans clearly varies considerably depending on the strain involved.
Genetic analysis of Chlamydia psittaci strains
Overwhelming evidence exists to support the contention that the C psittaci strain that infects cats is distinct from other C psittaci isolates. There is only a single case report of suspected transmission of C psittaci from a non-feline species to a cat (Lipman et al 1994). In this report, C psittaci transmission from a Macaw to a cat was suggested, and the organism was isolated from both animals. However, the evidence provided was only circumstantial, as the chlamydial strains isolated could not be maintained in culture for further analysis.
In contrast, there is a wealth of literature to demonstrate the existence of different strains of C psittaci and to show that feline isolates form a distinct group. This evidence is based partly on the antigenic nature of isolates – gel electrophoresis of chlamydial polypeptides showed a profile for feline isolates distinct to those isolated from other species (McClenaghan et al 1991), and monoclonal antibodies to major outer membrane protein (MOMP) epitopes of feline C psittaci isolates did not to react with other C psittaci strains (ferret, guinea pig, mouse, cow, and sheep) or with C trachomatis (Kurodakitagawa et al 1993, Tsao & Magee 1994). However, these observations are also supported by numerous genetic studies.
In 1989, Fukushi & Hirai reported that feline C psittaci strains possessed considerable genetic variation to those isolated from a variety of other species (avian, human, ferret, sheep, cattle, and muskrat), based on DNA fingerprinting using restriction endonucleases, and southern blots using DNA probes. McClenaghan et al (1991) reported similar findings using restriction endonuclease analysis of chalmaydial DNA isolated from various species.
Random amplification of polymorphic DNA was used by Pudjiatmoko et al (1997a) to analyse feline, avian, ovine and guinea pig isolates of C psittaci. Using this technique, the authors demonstrated that feline isolates had a unique ‘fingerprint’ in comparison to isolates from other species, and that the six feline isolates studied appeared to form two distinct patterns.
Other studies (Sayada et al 1994, Sykes et al 1997) have amplified the ompA gene from feline and other (avian and guinea pig) isolates of C psittaci and used various restriction endonuclease analyses to compare the isolates. These studies also demonstrated that feline isolates had distinctly different patterns from avian and guinea pig isolates, and that all the feline isolates shared the same restriction pattern. The homogeneity of the feline isolates is interesting, as in the study by Sayada et al (1994), their feline isolates were collected from three different countries (France, UK and USA) over a 50 year period.
Restriction enzyme analysis and gene sequence analysis of 16S and/or 23S ribosomal RNA also yield patterns that identify feline isolates as a distinct C psittaci group (Meijer et al 1997, Everett & Andersen 1997, Pudjiatmoko et al 1997b).
These studies are all entirely consistent in suggesting that isolates of C psittaci that infect cats form a distinct and separate group. The lack of similarity between feline isolates and those from a variety of other species would suggest that cross-species transmission of this strain is extremely rare or, potentially, may not occur at all. The wealth of new information on the genetic analysis of chlamydial isolates has recently led to a new classification scheme being proposed for the order Chlamydiales (Everrett et al 1999). Under this proposal, the Chlamydiaceae form one of four families in the order (the others being Parachlamydiaceae, Simkaniaceae and an as yet unnamed family). The Chalmydiaceae are split into two genera under the new proposal – Chlamydia and Chlamydophila. The Chlamydia genus comprises three species - C suis, C murridarum and C trachomatis; while the Chlamydophila genus also comprises three species, all derived from the previous C. psittaci species: Ch abortus, Ch caviae, and Ch felis. This suggestion that they may form a separate species is further recognition of the distinct nature of feline isolates.
Feline infection with Chlamydophila felis
In both the
and elsewhere, C felis has been established as a common feline pathogen. Baker first reported the isolation of C psittaci from cats in 1942, when it was described as an agent causing both upper and lower respiratory tract disease (‘feline pneumonitis’). Initially, the organism was thought to be responsible for most cases of acute upper respiratory tract infection in cats (Ott 1971, Studdert 1981). However, following further studies of the organism, and the subsequent isolation of feline herpesvirus and calicivirus and the recognition of their pathogenic role, it became clear that C psittaci (C felis) is primarily an ocular pathogen in cats (Cello 1971, Ott 1971, Hoover 1978, Studdert 1981) causing conjunctivitis as the primary clinical sign. UK
C. felis is now well established as an important and common cause of feline conjunctivitis worldwide (Cello 1971, Shewen et al 1978, Studdert et al 1981, Wills et al 1988, Danwitz & Rehman 1991, Pointin 1991, Beregi et al 1993, Nasise et al 1993, Gruffydd-Jones et al 1995, Gunn-Moore et al 1995). In the
, a study of over 750 pet cats with conjunctivitis demonstrated chlamydophilosis infection in 30% of these (Wills et al 1988). This study also demonstrated that the highest prevalence of infection was in cats aged five weeks to nine months. More recently, a serological study of healthy pet cats in the UK demonstrated greater than 9% had been infected with C felis (Gunn-Moore et al 1995). UK
The conjunctival epithelium appears to be the chief target for Chlamydophila infection in cats. Experimental studies have demonstrated that the incubation period, from infection to the onset of clinical signs is typically 3-10 days with clinical signs characterised by the appearance of blepharospasm, chemosis, conjunctivitis, mucopurulent ocular discharge, regional lymphadenopathy and occasional sneezing and mild nasal discharge (Hoover et al 1978, Wills et al 1987. O’Dair et al 1994, Sparkes et al 1999).
C felis is an unusual organism and was initially classified as a virus, it is now, however, regarded as highly specialised intracellular bacteria. This is an important distinction as it means that unlike viruses Chlamydophila are susceptible to a limited number of antibacterial agents.
Pathogenesis and clinical signs
The conjunctival epithelium appears to be the chief target for Chlamydophila infection. The organism replicates in the cytoplasm of the epithelial cells, eventually causing the cell to rupture, liberating infectious elementary bodies which then infect other epithelial cells.
Although cats of any age can be infected, the disease is seen most frequently in kittens and young adults. In endemic colonies, as the kitten's maternally derived antibody levels (from the queen's colostrum) declines, they become vulnerable to infection at 5-12 weeks of age. Recurrent episodes of Chlamydophila conjunctivitis are not uncommon in cats particularly during times of stress.
The incubation period, from infection to the onset of clinical signs is 4-10 days. In the early stages of the disease there is a marked serous ocular discharge (watery eye), blepharospasm (blinking) and the conjunctivae are reddened and swollen (chemotic). Initially only one eye may be affected, but both eyes eventually become involved 5-12 days after signs are first noticed. With time the conjunctivae become more reddened (hyperaemic) and the ocular discharge often becomes mucopurulent as secondary bacterial invaders become involved. On rare occasions, when this occurs, the surface of the eye (cornea) may become ulcerated and lead to permanent damage to the eye. Mild nasal discharge and sneezing are occasionally seen as well as mild fever for several days during the initial stages of the disease. Apart from the conjunctival discomfort affected cats are generally well and usually continue to eat.
In untreated cats, clinical signs may resolve after three weeks to three months (Hoover et al 1978, Wills et al 1987. O’Dair et al 1994). However, cats may continue to excrete infective C felis beyond the time when clinical lesions have resolved (Hoover et al 1978, Wills et al 1987). In addition to isolation of the organism from oculo-nasal discharges, C felis can also be isolated from rectal swabs of some infected cats (Wills et al 1987, O’Dair et al 1994) and also vaginal swabs (Wills et al 1987), indicating that infection with the organism is not confined to the conjunctival mucosa. Chlamydophila organisms have also been described infecting the gastric mucosa of cats (Hargis et al 1983, Gaillard et al 1984) and experimental exposure of cats with organisms recovered from the gastric mucosa appeared to produce typical signs of C felis infection (Gaillard et al 1984). The significance of extra-ocular sites of infection with C felis has not been fully established in cats, but persistence of the organism at these sites may serve as a reservoir of infection (Gaillard et al 1984) and it has been suggested that genital infections may be associated with reproductive failure in cats (Wills et al 1987).
Cats may continue to excrete Chlamydophila for long periods (which can be in excess of 8 months) even though clinical signs appear to have resolved. On close examination of these cats a very low grade conjunctivitis is usually still present representing persistent infection rather than a true carrier status. In cases where there is a significant upper respiratory tract component (sneezing, coughing, nasal discharge) co-infection with one of the respiratory viruses is likely.
Although Chlamydophila is the most common cause of contagious conjunctivitis in cats, it is not the only cause. In order to allow successful case management a positive diagnosis of chlamydophilosis needs to be made. A number of laboratory tests are available in order to help confirm the diagnosis:
1. Culture - this can be done by specialist laboratories. A firm conjunctival swab should be taken from the affected cat and placed in specially prepared transport media. Because the survival of Chlamydophila is poor outside of the host, the sample needs to arrive at the laboratory within 48 hours. Culture will establish a definite diagnosis in cats and is best done in cats that have been infected for less than 5-6 weeks and have not been treated with antibiotics. The use of topical local anaesthetic makes the procedure painless hence sedation is rarely necessary.
2. An ELISA test developed to look for Chlamydophila infection in people can be used to look for C felis in cats. The ELISA test can be carried out on a conjunctival swabs or conjunctival scrapings. Transport time is of less concern as the test will identify the Chlamydophila whether they are alive or dead. False positive results do occur, and the test is less reliable than isolation (culture) of the organism.
3. Antibodies titres to Chlamydophila can be evaluated, and demonstration of high titres in a group of cats indicates recent or current infection with Chlamydophila. This can be useful in colonies with chronic problems where few acute infections occur, or where the cats have been recently treated.
4. More recently specific and highly sensitive PCR-based assays have been developed for the detection of C felis. These assays are at least as sensitive as culture of the organism, and have the advantage that they do not have to rely on viable organisms being present in submitted samples
In individual cases of conjunctivitis there are numerous possible causes and a fairly extensive investigation may be required in order to establish a diagnosis. Reliable diagnosis is thus best achieved through isolation of the organism from conjunctivial swabs in cell culture (and its subsequent demonstration by immunofluorescence) or polymerase chain reaction (PCR) following DNA extraction from conjunctival swabs (McDonald et al 1998).
Although several antibacterial agents may have some effect in relieving clinical signs of Chlamydophila conjunctivitis in cats, the relief is usually temporary, as only certain groups of antibacterial agent are really effective. The tetracyclines are generally considered the drugs of choice for all Chlamydophila infections (O’Dair et al 1994, Sparkes et al 1999). Systemic treatment is optimal for therapy, but additional topical therapy has also been recommended. However, in a recent study, we found no evidence of any specific benefit from the addition of topical therapy to systemic treatment, although it may presumably provide some local ‘soothing’ effect for the inflamed conjunctiva. Doxycycline (Ronaxan; Merial) has the advantage of only requiring once daily treatment, although it is more expensive than oxytetracycline.
If Chlamydophila is isolated from a cat colony, treatment should involve all the cats in the colony regardless of age or apparent infection. Antibiotic therapy should continue for at least 4-6 consecutive weeks or for at least two weeks after all clinical signs have subsided, whichever is the longer. The major drawback of using tetracyclines in kittens is the risk of causing discolouration of their permanent teeth. Tetracyclines should not be given to pregnant queens. Recent work suggests that systemic therapy is superior to topical therapy with tetracyclines for therapy of ocular signs and in addition, systemic therapy may have the advantage of eliminating infection at extra-ocular sites (Sparkes et al 1999).
An alternative to tetracycline therapy, which we have shown to be valuable in feline chlamydophilosisis the combination of amoxycillin and clavulnate. This requires a longer treatment period than tetracyclines, but does not carry the same risks for use in kittens.
Recently, azithromycin (a macrolide derivative with an extremely long tissue half life) has been suggested to be of value in the treatment of chlamydophilosis. However, although widely accepted as a treatment, and commonly recommended, especially among cat breeders, recent evidence based on controlled studies suggests that azithromycin is NOT effective against feline chlamydophilosis.
Epidemiology and control
Chlamydophila felis is a labile organism (Cello 1971) and transmission between cats is thought to be primarily by direct contact. Although Chlamydophila can be present in vaginal fluid and faeces they are unlikely to be major sources of infection. The use of both inactivated and modified-live vaccines have been shown to be efficacious in controlling clinical disease (Wills et al 1987, Wasmoen et al 1992).
Once endemic in a colony, Chlamydophila can be very persistent and costly to eliminate. Not just in terms of the treatment, but also in the delays in being able to sell kittens and infertility in the queens. Natural immunity to the disease appears to be inefficient and recurrent episodes of the disease can occur in individual cats.
Control of endemic chlamydophilosisis best achieved using a combined approach. This should include:
i) Initial treatment of all cats with systemic antibiotics - doxycycline ideally
ii) Vaccination once the clinical signs have resolved. Both inactivated and modiified live Chlamydophila vaccines are available
iii) Management changes including kittening in isolation and then weaning the kittens at 4-5 weeks into isolation and good general hygiene.
iv) Vaccination of kittens or new adults before introduction into the colony.
The zoonotic implications of feline chlamydophilosis
Human infection with Chlamydia psittaci
Chlamydiosis in humans has been reviewed by Wills (1986) and Caul & Sillis (1998). Avian chlamydiosis is recognised in at least 130 species of birds and transmission of infection from birds to man is well-established. Psittacines, turkeys, ducks, pigeons, chickens, pheasants and sea-birds have all been incriminated with disease in humans being referred to as psittacosis (where infection is derived from psittacines) or ornithosis (where infection is derived from non-psittacine birds). Transmission to humans is mainly through aerosolised organisms from dried faeces or plumage. Direct contact and bite wounds are also a possibility. Signs in man include ‘flu’-like symptoms (fever, anorexia, sore throat) and in more severe cases pneumonia. Diarrhoea, nausea, vomiting have been reported as have endocarditis, myocarditis, conjunctivitis and renal involvement.
In contrast to avian disease, there have been few well-documented cases of transmission of C psittaci infection from mammals to man although transmission from infected sheep is well established. Infection in pregnant sheep causes abortion, stillborn and weak lambs (enzootic abortion of ewes - EAE) and C psittaci is the major cause of ovine abortion in the
. Transmission to humans has resulted in signs of disease similar to psittacosis including ‘flu’-like signs, pneumonia, conjunctivitis and abortion. Evidence for transmission from cattle and cats is controversial. Some evidence is simply based on serology which cannot reliably distinguish the origin of the strain. UK
Feline strains of Chlamydia psittaci causing human disease
Although feline C psittaci infections are commonly regarded as being potentially zoonotic, an extensive search of the literature has revealed only four case reports of suspected transmission from cats to humans.
- In 1969, Ostler et al reported a 21-year-old man who developed follicular conjunctivitis confined to the right eye. Chlamydia was isolated from conjunctival cultures and fluoresecent antibody tests for Chlamydia on conjunctival scrapings were postive. Complement fixing antibody titres also rose during the course of disease and no other pathogens were isolated. The man came from a boarding house where there were also 13 cats, and he reported close contact with a three-month-old kitten. Two weeks prior to the man’s illness, the cat was reported to have had a unilateral ocular discharge and subsequent conjunctival scrapings and nasal washings yielded Chlamydia organisms from the cat. Chlamydia was also isolated from another cat in the group that developed conjunctivitis two months later. Cultured Chlamydia from the human patient were inoculated into the eyes of four kittens, and conjunctivitis developed in these kittens with inclusion bodies seen on conjunctival scrapings. No other humans in the household exhibited evidence of infection, and the authors’ suggested that this may have been due to overwhelming infection in the patient that did develop disease.
- In 1978, Griffiths et al reported malaise, pyrexia and hepatosplenomegaly in a 38-year-old woman who had received a renal transplant two years earlier. Various diagnostic investigations were negative, but the woman developed a rising antibody titre to C psittaci. Antibodies were detected against both feline and avian strains. The woman was reported to have contact with birds at her local pet shop, but all had been asymptomatic. She also owned three cats but these also were asymptomatic. The authors concluded that the cats were the most likely source of infection, despite the fact that all of the cats proved to be seronegative for C psittaci antibodies. Cause and effect was therefore not established.
- In 1979, Regan et al reported on a 40-year-old man who developed endocarditis and secondary glomerulonephritis. An aetiological agent was not cultured from the patients blood, however antibodies were demonstrated to feline C psittaci (both IgM and IgG), which declined in magnitude as the patient responded to therapy (erythromycin). Although the patient reported close contact with cats, no investigations were performed on these cats, and antibody titres to other C psittaci strains were not reported.
- In 1987, Schmeer et al reported acute follicular conjunctivitis in a 25-year-old woman. C psittaci-specific IgM antibodies were detected in serum samples from the woman and her cat which was also showing signs of conjunctivitis. Other microbial investigations were negative, but C psittaci could not be isolated from either cat or patient (possibly due to prior antibiotic therapy). Transmission of infection from cat to human was suggested.
In two of these four reports (Griffiths et al 1978, Regan et al 1979) the evidence for transmission of C psittaci from cats to humans is weak. Only serological data was presented to support evidence of infection in the human patients, and the cats which were the proposed source of infection either showed no evidence of infection (Griffiths et al 1978) or were not investigated at all (Regan et al 1979). In contrast, the earlier report by Ostler et al (1969) does provide strong evidence of human C psittaci conjunctivits being acquired from a cat, and the report by Schmeer et al (1987) is also suggestive of this.
Summary and conclusions
The results of numerous studies of C psittaci isolates from a variety of different species are consistent in suggesting that feline isolates are distinct from those infecting other species. This provides very strong evidence that the feline strain is a very highly host-adapted and that if it infects other species this is a very rare event, and similarly other strains of C psittaci rarely, if ever, infect cats.
Although widely regarded as having zoonotic potential, only four case reports could be found suggesting transmission of C psittaci from cats to humans. Two of these reports must be regarded with some scepticism as the evidence reported in the papers was very weak. Nevertheless in the remaining two cases evidence was suggestive of human chlamydial conjunctivitis being acquired from a cat. In both these cases the human patient responded rapidly to therapy and no other adverse effects were reported.
While it is clearly prudent to advise caution when handing and medicating cats known to be infected with C psittaci it is clear that this common feline pathogen is extremely unlikely to be transmitted to humans. Indeed Wills (1986) reported no evidence of human disease in the course of investigating over 150 clinical cases of feline C psittaci infections.
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*) The Feline Unit
Centre for Small Animal Studies
Animal Health Trust
Kentford, Suffolk, UK
Centre for Small Animal Studies
Animal Health Trust
Kentford, Suffolk, UK