Prevalence and anticoccidial drug sensitivity of Eimeria tenella isolated from commercial broiler farms in Thailand.

Background and aim: Coccidiosis, a parasitic disease caused by Eimeria species, remains a critical challenge for poultry production worldwide. Eimeria tenella is one of the most pathogenic species, causing intestinal lesions and compromising growth in chickens. This study aimed to estimate the prevalence of Eimeria species and evaluate the anticoccidial drug sensitivity of E. tenella field isolates in commercial broiler farms across Thailand.

Materials and methods: Four fecal samples were collected from each of the ten broiler farms in seven provinces of eastern and central Thailand. Oocysts were identified through morphological examination and confirmed through species-specific multiplex polymerase chain reaction. E. tenella isolates were propagated and subjected to in vivo anticoccidial sensitivity testing (AST) against four drugs: Nicarbazin (NIC), salinomycin (SAL), monensin (MON), and a combination of MON and NIC. Experimental infection trials were conducted on Ross 308 broiler chicks to assess weight gain, fecal oocyst shedding, lesion scores, and anticoccidial index (ACI) values.

Results: E. tenella and Eimeria praecox were the most prevalent species (40%), followed by Eimeria acervulina, Eimeria brunetti, and Eimeria mitis (20%). Mixed-species infections were detected in 50% of samples. The AST results showed that field isolates were sensitive to NIC, MON, and the MON + NIC combination, with ACI values of 172.51, 175.49, and 174.21, respectively. In contrast, SAL showed an ACI of 158.81, indicating resistance. All treated groups demonstrated reduced oocyst shedding and improved weight gain compared to untreated infected controls, though lesion score differences among treatments were not statistically significant.

Conclusion: This study constitutes the first comprehensive report on anticoccidial drug resistance in E. tenella from broiler farms in Thailand. The findings indicate high efficacy of NIC, MON, and their combination, while revealing emerging resistance to SAL. These results provide crucial insights for revising coccidiosis control strategies and support the need for ongoing monitoring and development of alternative therapeutics to mitigate resistance evolution.