Which Antibacterial Agent Performs Best for the Treatment and Clearance of Chlamydophila Felis Infection in Cats?

The evidence available indicates that a 14-day course is insufficient, whilst a 21-day course may be sufficient in acute-stage infections which the experimental trials model. However, in field-infected cats, especially those in which the duration of infection is unknown, such as strays, a longer course may be indicated. In the study by Dean et al (2005), all the cats required rescue treatment after 7 or 14-day courses of treatment, and could therefore be considered chronically infected – in these, a 28-day course was required to eliminate infection from all cats (though 73% or 8/11 cleared the infection with a 21-day course). In shelters without finances to test for organism clearance, a 28-day course may be considered more appropriate; equally, in owned cats with acute infection, a 21-day course is likely to suffice. However, without more trials with greater participant numbers involving naturally infected cats, firm conclusions cannot be drawn.

By way of alternatives to doxycycline, co-amoxiclav has shown similar efficacy to doxycycline in improving clinical signs (Sturgess et al 2001) and pradofloxacin and enrofloxacin have shown a mildly better rate of clinical improvement (Gerhardt et al 2006; Hartmann et al 2008). However, only co-amoxiclav has been shown to consistently (that is, in all cats) clear infection, though a 30-day course was required for chronically infected cats, and a 19-day course given to acutely-infected cats that showed a high rate (62.5%) of recurrence (Sturgess et al 2001). Whilst doxycycline should remain the first-line treatment, co-amoxiclav, if given for a sufficient length of time (30 days), is a reasonable alternative.

One of the main weaknesses with the evidence presented here is the use of experimental cats, as their use limits extrapolation of results to field-infected free-living cats. Field infections may occur with greater doses of infectious agent, of different pathogenecity, in the face of multiple infections with other agents or against a background of other disease processes, in cats with greater genetic variation and on a variable nutritional plane. Therefore disease presentation, severity, and response to treatment may not match that seen in the field. For example, in stray or unowned animals, the chronicity of disease is unknown and response to treatment in these cases may be suboptimal, especially where secondary sequelae such as tear-duct scarring has occurred. Cats commonly present with C felis in addition to other pathogens of the upper respiratory tract disease complex (Hartmann et al 2010); secondary bacterial infection may complicate the clinical picture and delay resolution especially as tetracyclines appear to have only moderate efficacy against studied isolates (Schulz et al 2006). However, doxycycline has well-documented anti-inflammatory effects (e.g. Krakauer and Buckley 2003) which may help with improvement of clinical signs associated with viral co-infections

Several side-effects of oral doxycycline have been described. In a retrospective study of 168 cats, 13% developed vomiting, 11% diarrhoea, and 6% decreased appetite, and 36-40% showed increases in ALT or ALP during therapy (Schulz et al 2013). Oesophagitis, oesophageal stricture and tooth discolouration, whilst described elsewhere, (e.g. McGrotty and Knottenbelt 2002; German et al 2005) were absent. Gastrointestinal side effects may be attributable to the use of the doxycycline-hyclate salt which is acidic in solution, and is the compound present in the UK licensed veterinary agent (Ronaxan; Merial). Administration with food, or providing food or water after administration may reduce potential gastrointestinal side effects, whilst others (liver enzyme increase, tooth discolouration) appear clinically insignificant and do not represent valid reasons for not using the drug.

[...]