Anticoccidial Resistance in Eimeria spp. From Thai Broiler Farms Using Shuttle Programs

Coccidiosis is a widespread poultry disease caused by Eimeria species and significantly affects performance and productivity. In the field, co-infections with multiple Eimeria species are commonly observed. While prophylactic coccidiostats are commonly used in feed for disease control, their extensive application has led to drug resistance, especially with the widespread use of shuttle programs in broiler production in Thailand. Understanding the resistance profiles of commonly used anticoccidials is crucial for developing effective prevention strategies. This study assessed the sensitivity of field isolates of Eimeria acervulina (E. acervulina), E. maxima, and E. tenella from broiler farms in Thailand using four anticoccidial sensitivity tests (ASTs). A dose-titration trial was conducted to determine effective inoculation doses, focusing on lesion induction and reduced weight gain while minimizing mortality. A total of 112-day-old broiler chicks were used to evaluate sensitivity to three widely used anticoccidial agents: narasin combined with nicarbazin, salinomycin, and maduramicin. Sensitivity assessment was conducted using four key indices: the anticoccidial index (ACI), the percentage of optimum anticoccidial activity (POAA), the reduction of lesion scores (RLS), and the relative oocyst production (ROP). During the study, broilers were fed the same combination of narasin + nicarbazin in the starter diet, while the in-feed anticoccidial in the grower phase was either maduramicin or salinomycin. Following inoculation with oral doses of sporulated oocysts of E. acervulina (2 × 10⁵), E. maxima (1 × 10⁵), and E. tenella (1.5 × 10⁴), both medicated and infected-unmedicated groups showed significantly lower weight gain compared to uninfected controls (p < 0.05). While both salinomycin and maduramicin treatments significantly reduced intestinal lesion scores and fecal oocyst shedding compared with the infected-unmedicated group (p < 0.05), the reductions were incomplete. Notably, the calculated indices yielded discordant results: ACI and ROP values initially suggested sensitivity, whereas POAA and RLS indicated reduced efficacy. According to the study's composite point system-where resistance in two indices signifies moderate resistance-the field isolates were classified as moderately resistant to both salinomycin and maduramicin. Histopathological analysis supported this, confirming that while medication reduced the severity of infection, it did not entirely prevent Eimeria colonization. This study provides the first systematic AST evaluation of Thai Eimeria isolates under shuttle-program conditions. The findings highlight the emergence of moderate drug resistance even within these structured programs, emphasizing a critical practical relevance for the poultry industry. To sustain long-term efficacy in Thai broiler production, there is an urgent need for strategic drug rotation and the integration of alternative control measures, such as vaccination and bio-shuttle strategies.