Use of quantitative real-time PCR to monitor the response of Chlamydophila felis infection to doxycycline treatment

Abstract

Fifteen cats infected with Chlamydophila felis were monitored for the presence of C. felis DNA on ocular swabs by using real-time PCR and for clinical signs of disease. The cats were assigned to three groups: oral doxycycline at 10 mg/kg of body weight/day for 7 days (six cats), oral doxycycline at 10 mg/kg/day for 14 days (five cats), and an untreated control group (four cats). The untreated cats remained positive for C. felis throughout the trial; clinical signs were most severe on days 14 to 21 postinfection, and then they declined. Treatment with 7 and 14 days of doxycycline decreased C. felis relative copy numbers and clinical signs rapidly. C. felis became undetectable in some of the cats during or after treatment. However, after the cessation of treatment, a recurrence of high relative copy numbers of C. felis and severe clinical signs in all cats was seen. Rescue treatment with 21 days of doxycycline was successful at eliminating infection in eight of the cats; a further 28 days of doxycycline was required to eliminate infection in the remaining three cats. It was concluded that 7, 14, and, in some cases, 21 days of treatment with oral doxycycline will not eliminate C. felis infection. At least 28 days of treatment with doxycycline is required to ensure elimination of the organism. Real-time PCR is a sensitive technique for monitoring C. felis infection and the response to antibiotic treatment.

FIG. 1.

Effect of time on C. felis relative copy numbers of the untreated control cats in group A. Each line represents an individual cat.

FIG. 2.

Effect of 7 days of treatment with 10 mg of doxycycline/kg on C. felis relative copy number for group B. The initial 7-day treatment period is delineated by the first shaded area of the figure, from day 7 to day 14. Rescue treatment with 21 days of 10 mg of doxycycline/kg was initiated on day 32 for all cats and is delineated by the second shaded area on the figure.

FIG. 3.

Effect of 14 days of treatment with 10 mg of doxycycline/kg on C. felis relative copy number for group C. The initial 14-day treatment period is delineated by the first shaded area of the figure, from day 7 to day 21. Rescue treatment with 21 days of 10 mg of doxycycline/kg was initiated in all cats from day 39 to day 46 and is delineated by the second shaded area. Treatment was commenced on day 39 (start of the shaded area) for cats C1, C2, and C4; on day 42 for cat C5 (annotated with a T); and on day 46 for cat C3 (annotated with a T).

FIG. 4.

Effect of time on clinical scores of the untreated control cats in group A. Each line represents an individual cat.

FIG. 5.

Effect of 7 days of treatment with 10 mg of doxycycline/kg on clinical score for group B. The initial 7-day treatment period is delineated by the first shaded area of the figure, from day 7 to day 14. Rescue treatment with 21 days of 10 mg of doxycycline/kg was initiated on day 32 for all cats and is delineated by the second shaded area on the figure.

FIG. 6.

Effect of 14 days of treatment with 10 mg of doxycycline/kg on clinical score for group C. The initial 14-day treatment period is delineated by the first shaded area of the figure, from day 7 to day 21. Rescue treatment with 21 days of 10 mg of doxycycline/kg was initiated in all cats from day 39 to day 46 and is delineated by the second shaded area. Treatment was commenced on day 39 (start of the shaded area) in cats C1, C2, and C4; on day 42 in cat C5 (annotated with a T); and on day 46 in cat C3 (annotated with a T).