Live vaccines for coccidiosis control are infrequently used in broilers, mainly due to variability in efficacy and relatively high costs. More insight in transmission of vaccine and wild-type strains can facilitate optimization of vaccination strategies and might increase its use as an alternative for anticoccidial drugs. The aim of this study was to quantify transmission of a live Eimeria acervulina vaccine strain and to determine the degree of protection against a subsequent infection with a wild-type E. acervulina strain. An experiment was carried out with 4 groups of 22 SPF broilers. At 2 days of age, 11 birds of groups 2 to 4 were vaccinated directly by oral application of E. acervulina oocysts of the Paracox™ vaccine and 11 birds were placed in contact with these birds (contact-vaccinated). Birds in group 1 remained unvaccinated (controls) and were not exposed to vaccinated birds. At day 28 of age, 6 groups of 10 birds were formed, with 2 groups (duplo) for each treatment group, i.e. vaccinated, contact-vaccinated or unvaccinated control birds. Five birds of each group were orally inoculated with wild-type E. acervulina oocysts and five were contact-exposed. Single droppings were examined daily from days 5 to 49 of age for oocyst output and to determine the time of infection. The transmission rate of the vaccine strain was estimated to be 1.6 per day and of the wild-type strain 2.3, 8.7 and 20.8 per day for vaccinated, contact-vaccinated and unvaccinated birds, respectively. Although transmission of wild-type coccidia was not significantly reduced in vaccinated or contact-vaccinated groups, both groups were equally protected against high oocyst output after infection compared to unvaccinated groups. These results suggest that factors influencing transmission of live vaccine strains in flocks may be important targets for improvement of vaccine efficacy and warrant further research.
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