Randomized masked controlled clinical trial to compare 7-day and 14-day course length of doxycycline in the treatment of Mycoplasma felis infection in shelter cats

The aim of this study was to compare the clinical and microbial efficacy of a 7-day or a 14-day course of doxycycline for the treatment of Mycoplasma felis-infected cats with clinical signs of upper respiratory tract disease (URTD) assessed using clinical scoring criteria. Cats were randomly allocated to either the Doxy-7 group (N = 20; 7-day course of oral doxycyline liquid followed by 7-days placebo); or the Doxy-14 group (N = 20; 14-day course of oral doxycycline). There were no significant differences in Mycoplasma load between groups at Day 1 or Day 7 (P > 0.05), but at Day 14 mean Mycoplasma load was lower in the Doxy-14 group (P = 0.01). Mycoplasma load reduced over Days 1–7 in each group (P < 0.01), but only the Doxy-14 group had a significantly reduced Mycoplasma load at Day 14 compared to Day 1 (P < 0.01). On Day 14, 11 (55%) cats in the Doxy-7 group and 5 (25%) cats in the Doxy-14 group had positive PCR results for M. felis. There was a statistically significant reduction within each group across the Day 1–7 period for ocular discharge, nasal discharge, demeanor, and food intake scores (P < 0.01 for each score category). Nasal discharge scores and sneezing scores were statistically lower in the Doxy-14 group than in the Doxy-7 group on individual days during the Day 8–14 period (P < 0.05). We conclude that in M. felis-infected cats with clinical signs of URTD, a 14-day course of oral doxycycline produced superior microbial but not clinical results compared to a 7-day course of treatment.