Comparison of the Polymerase Chain Reaction and Culture for the Detection of Feline Chlamydia psittaci in Untreated and Doxycycline-Treated Experimentally Infected Cats

Abstract

The diagnostic sensitivity of the polymerase chain reaction (PCR) was compared with that of culture on conjunctival swabs over the course of infection in 4 doxycycline-treated and 4 untreated cats that were experimentally infected with feline Chlamydia psittaci. Treated cats were given 25 mg (5 mg/kg) of doxycycline orally twice daily for 3 weeks from day 6 after challenge. Clinical signs improved within 3 days of institution of treatment. Culture remained positive for 1 day and PCR remained positive for up to 5 days after treatment was commenced. No recurrence of clinical signs occurred and the organism could not be detected by either PCR or culture for 2 weeks after cessation of therapy. In the 4 untreated cats, conjunctival swabs were taken daily to day 14 and every 2nd weekday to day 64 after challenge. PCR was significantly more sensitive than culture in untreated cats overall (PCR 85.7%, culture 72.9%, P approximately 0) and for cats with clinical signs (PCR 89.2%, culture 79.2%, P = .008). PCR and culture had equivalent sensitivity (100%) for cats showing clinical signs in the 1st month of infection, whereas PCR was considerably more sensitive than culture for cats showing clinical signs in the 2nd month (PCR 72.9%, culture 47.9%, P = .028). Organisms were not detected by PCR in blood or any tissue collected from treated or untreated cats at postmortem. Thus, effective treatment of chlamydiosis in cats is possible with much shorter treatment regimens than currently recommended, and PCR is the more sensitive diagnostic method in chronically infected cats.

Use of Quantitative Real-Time PCR To Monitor the Response of Chlamydophila felis Infection to Doxycycline Treatment

“…One study by Sykes et al (12) suggested that short courses of doxycycline may be sufficient at eliminating infection. In that study, all of the cats were negative by conventional PCR by day 6 of a 21-day treatment period and remained negative for 14 days after treatment, at which point monitoring was ceased.…”

“…A requirement for prolonged courses (4 to 6 weeks) of doxycycline to eliminate the organism has been advocated, as has the idea that all in-contact cats should be treated (3). However, one study has suggested that shorter courses of doxycycline may also be effective at eliminating infection (12).…”

“…In previous treatment studies, C. felis isolation and conventional PCR techniques have been used to monitor the response of infection to treatment (8,10,11,12,14). PCR has been shown to be more sensitive than isolation techniques (7,12), and real-time PCR has further advantages as it facilitates quantification of the organism (5).…”

“…PCR has been shown to be more sensitive than isolation techniques (7,12), and real-time PCR has further advantages as it facilitates quantification of the organism (5).…”

Detection of Chlamydophila felis and Feline Herpesvirus by Multiplex Real-Time PCR Analysis

“…Isolation has traditionally been used to identify these pathogens. Recently, PCR tests which are more sensitive than isolation have been developed for the detection of FHV and C. felis (1,(6)(7)(8)10).…”