Manuscripts documenting the occurrence of resistance against all commonly used anticoccidial drugs abroad, together with the high incidence of clinical coccidiosis in the field (60-90% of flocks) in the Netherlands, were the reasons to start investigations on the occurrence of anticoccidial drug resistance of Dutch and other European Eimeria spp. field isolates. The results showed a high degree of resistance against most tested anticoccidial drugs. The research performed in this thesis was preceded by an extensive literature review focusing on anticoccidial products and alternative strategies. Based on this literature study one alternative coccidiosis control strategy, namely a mannanoligosaccharide (a prebiotic), was chosen for further experimental studies in broiler chickens. Additionally, two other products i.e. ibuprofen (a non-steroidal anti-inflammatory drug (NSAID)) and a mucolytic enzyme (protease) were also used in experimental research aiming at assessing their anticoccidial potential. Ibuprofen was chosen because as a cyclooxygenase inhibitor it was expected to reduce the pathology of coccidiosis lesions through impairment of the production of prostaglandins, and because it was reported that another NSAID (indometacin) was able to reduce oocyst shedding. The mucolytic enzyme was studied as it was speculated that a protease might block the attachment of Eimeria to the mucus layer through its degradation. In view of the poor anticoccidial effect found in the studies performed with mannanoligosaccharide, ibuprofen and the mucolytic enzyme, further research focused on vaccination against chicken Eimeria, which despite some drawbacks has shown to be highly effective against outbreaks of clinical coccidiosis. Another reason to focus on vaccination was the inefficacy of rotation and shuttle programs to solve the coccidiosis resistance problem, which is explained by the fact that resistance is stable even in the absence of drug selection pressure. A welcome side effect of vaccination is the improved sensitivity of Eimeria spp. field isolates for anticoccidial drugs reported by some researchers. This phenomenon may play a key role in reducing the anticoccidial drug resistance problem. However, large-scale field studies documenting this were lacking. Therefore, the relation between the coccidiosis prevention program (vaccination with a live attenuated anticoccidial vaccine or anticoccidial drugs in feed) and anticoccidial drug sensitivity profiles of Eimeria spp. field isolates for diclazuril and monensin was studied. The results showed that use of vaccination was associated with an increased sensitivity of Eimeria spp. field isolates to anticoccidial drugs. Coccidiosis vaccines could be produced more efficiently if cross protection between Eimeria spp. would be relevant enough to reduce the number of species included in the vaccine and/or reduce the vaccination dose. Therefore, cross protection studies between an E. acervulina vaccine line and E. tenella, and E. maxima were performed. The results showed complete protection against E. acervulina, partial cross protection against E. tenella but no cross protection against E. maxima. The thesis is finalized with a summarizing discussion with conclusions and perspectives for future research
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