Widespread use of nitroimidazoles in competition pigeons reduced their effectiveness against Trichomonas gallinae.

Trichomonas gallinae is a protozoan parasite of major concern in avian medicine, particularly in domestic pigeons (Columba livia). This study investigated the nitroimidazole susceptibility and T. gallinae occurrence in domesticated pigeons from Eastern Spain, kept for different competitions. A total of 220 pigeons from 11 lofts were sampled and examined by microscopy and culture, revealing 63.6% of infected birds. Genotyping identified genotype C as predominant, with occasional detection of genotype A, mixed A/C infections, and one isolate of Lineage III. In vitro susceptibility testing of 42 isolates showed a high occurrence (45.2%) of elevated metronidazole minimum lethal concentrations (MLC) values (MLC ≥40 μg/mL) ranging from 5 to >100 μg/mL in 9/11 pigeon lofts examined. MLC values were not associated with sex, age, or reproductive status. In contrast, higher IC50 values were observed in isolates obtained from pigeons bred for Pica competitions, and metronidazole exposure was associated with higher MLC values compared with ronidazole. No significant associations were found between in vitro nitroimidazole susceptibility values and environmental or management parameters, although suboptimal hygiene and high bird density were common in lofts with strains with reduced susceptibility to metronidazole. The findings of this study underscore the urgent need to establish evidence-based therapeutic protocols, strengthen biosecurity measures, and promote the development of novel therapeutic alternatives for the control of avian trichomonosis, in order to mitigate the spread of T. gallinae strains resistant to currently available drugs in pigeon populations.