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Fifteen Eimeria spp. field isolates sampled on Dutch broiler farms were subjected to an Anticoccidial Sensitivity Test (AST) in a battery cage study. Four isolates dated from 1996, another four from 1999 and the last seven isolates from 2001. The selected anticoccidial drugs were monensin, narasin, salinomycin, lasalocid, nicarbazin, diclazuril, halofuginone, maduramicin and meticlorpindol/methylbenzoquate. Maduramicin and halofuginone were not included in the ASTs of 1999 and 2001, while meticlorpindol/methylbenzoquate was not tested in 1996 and 1999. Eimeria acervulina present in each of the four 1996 field isolates showed resistance for almost all products tested except maduramicin (1/4) and salinomycin (114), which appeared to be reduced sensitive. In 1999 the same species presented a similar resistance pattern for most products, although reduced sensitivity occurred for salinomycin (1/4), and sensitivity was found for diclazuril (2/4), monensin (1/4) and narasin (1/4). In the year 2001 increased sensitivity to various products was found. Higher sensitivity was found for meticlorpindol/ methylbenzoquate (7/7) and salinomycin and narasin (both 4/7), followed by nicarbazin (3/7) and monensin (2/7). Reduced sensitivity was found for monensin (3/7), lasalocid (2/7), salinomycin and narasin (1/7). E. maxima was only found in one field isolate per year. The E. maxima from 1996 was resistant to all products except narasin (sensitive) and halofuginone (reduced sensitive). In 1999 this species was reduced sensitive to narasin and lasalocid, showing resistance for the other products. The strain originating from the 2001 isolate was reduced sensitive to most products except monensin and narasin (resistant). Full sensitivity was found for meticlorpindol/ methylbenzoquate. E. tenella was present in one isolate of 1996, two of 1999 and four isolates of 2001. The AST of 1996 showed reduced sensitivity for nicarbazin, and sensitive to narasin, maduramicin and halofuginone. All other products showed resistance. In 1999 both strains showed resistance to all products tested. For the year 2001 full sensitivity was found to meticlorpindol/methylbenzoquate. Sensitivity was also found for salinomycin (1/4), nicarbazin (2/4), diclazuril (2/4) and lasalocid (2/4), monensin (1/4) and narasin (1/4). Reduced sensitivity was found for nicarbazin (1/4), lasalocid (1/4) and narasin (1/4). The different resistance patterns of Dutch coccidiosis isolates and resistance of coccidia in general is discussed.

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Twenty European Eimeria spp. field isolates were subjected to an anticoccidial sensitivity test (AST). The anticoccidial drugs tested were diclazuril (Clinacox) and monensin (Elancoban). The assay was performed in a battery cage trial. Infected medicated birds were compared with an unmedicated control group. Coccidial lesion scores and oocyst shedding were used as parameters. The results of the AST show that resistance is common amongst coccidiosis field isolates, especially Eimeria acervulina (68% and 53% resistance for diclazuril and monensin, respectively). Resistance is less frequent amongst Eimeria maxima (38% and 50% resistance for diclazuril and monensin, respectively) and Eimeria tenella isolates (23% and 38% resistance for diclazuril and monensin, respectively). A highly significant influence of the coccidiosis prevention program (live coccidiosis vaccination with Paracox-5 vs. anticoccidial drugs in feed) on the sensitivity patterns of Eimeria spp. field isolates for both diclazuril (P= 0.000) and monensin (P= 0.001) was found. Further, when looking at the single species and each anticoccidial drug level, significantly more sensitivity of E. acervulina for monensin (P= 0.018), E. maxima for diclazuril (P = 0.009), and E. tenella for diclazuril (P = 0.007) was found in isolates originating from vaccinated flocks. Moreover, for E. acervulina and diclazuril, E. maxima and monensin, and E. tenella and monensin a trend toward higher sensitivity of isolates for these products was found when live coccidiosis vaccination was applied. The present study shows that sensitivity for the anticoccidial drugs diclazuril and monensin is more frequent in Eimeria spp. field isolates originating from broiler farms where a coccidiosis vaccination policy is followed.