Chicken coccidiosis can cause severe enteritis with high mortality, which causes serious economic losses to the global breeding industry each year. The most virulent species is Eimeria tenella (E. tenella), but the infectivity of different E. tenella varies among geographic strains. At present, there are no reports related to the pathogenicity and drug resistance of E. tenella in Yiwu, Zhejiang province, China. A total of 600 fecal samples were collected from 10 farms in Zhejiang province, the overall oocyst prevalence was 54.2% (325/600). The prevalence was significantly higher (P < 0.01) in chickens under 40 d (97.5%) than that in chickens between 60 and 85-days-old (40.5%) and chickens over 90-days-old (24.5%). E. tenella stain was isolated from fecal samples of chickens in Yiwu and the pathogenicity of this isolate was determined, and then we recorded the survival rate, bloody stool score, lesion score, average weight gain. The results showed that all of the chickens infected with 5 × 10⁵ sporulated oocysts of E. tenella died after the seventh day of infection, the bloody stool score and average lesion score of chickens from group 1 (5 × 10⁵), group 2 (5 × 10⁴), group 3 (5 × 10³) and group 4 (5 × 10²) decreased successively; the average weight gain (g) and relative weight gain (%) increased successively; the weight gain of the low-dose E. tenella infection groups (5 × 10³ and 5 × 10²) were higher than the other 2 groups (5 × 10⁵ and 5 × 10⁴) (P < 0.05). Finally, The E. tenella isolate was tested for sensitivity to 6 anticoccidial drugs (sulfachloropyrazine sodium, amproline, toltrazuril, clopidol, salinomycin, and nicarbazine) using 4 indexes including anticoccidial index(ACI), percent of optimum anticoccidial activity (POAA), reduction of lesion scores (RLS), and relative oocyst production (ROP). The results showed that this isolate has developed severe resistance to drugs of salinomycin and nicarbazine, moderate resistance to amproline and clopidol, slight resistance to toltrazuril, while the E. tenella isolate performed more sensitive to sulfachloropyrazine sodium.

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