The use of live oocyst vaccines is becoming increasingly important in the control of avian coccidiosis in broilers. Knowledge of the mechanisms employed when chicks uptake oocysts and become immune is important for optimizing delivery of live vaccines. The current study tests the hypothesis that chicks not initially immunized may ingest oocysts by contact with litter containing oocysts shed by immunized cohorts. In Experiment 1, day-old broiler chicks were housed in pens containing clean litter. In Trial 1, 100% of chicks in some pens were immunized with 2.5 × 103 Eimeria acervulina oocysts while in other pens only 75% of chicks were immunized and remaining cohorts within the pens were not immunized. Other pens contained chicks that served as nonimmunized nonchallenged controls or nonimmunized challenged controls (NIC). On day 21, birds were given a homologous challenge of 6 × 105 oocysts. A second identical trial was conducted, except birds were immunized with 500 Eimeria maxima oocysts and were challenged with 3 × 103 E. maxima oocysts. In Experiment 2, 100% of chicks in some pens were immunized with 500 E. acervulina oocysts while in other pens either 75% or 50% of the birds were immunized. On day 14, birds were challenged with 1 × 106 oocysts. Trial 2 was identical to Trial 1 except that birds were immunized with 100 E. maxima oocysts and challenged with 1 × 106 oocysts. For all experiments weight gain, feed conversion ratio (FCR), plasma carotenoids, and litter oocyst counts were measured. In Experiment 1, the level of protection in groups containing 25% nonimmunized cohorts, as measured by weight gain, carotenoid level, FCR, and oocyst litter counts, was identical to groups containing 100% immunized chicks. In Experiment 2, pens where 50% or 75% of birds were immunized with either E. maxima or E. acervulina were not well protected from decreases in weight gain and plasma carotenoids nor from increases in litter oocyst counts following a challenge infection administered on day 14 relative to NIC. In addition, pens of birds where 100% of chicks were immunized were not well protected compared to NIC, and resistance to coccidiosis infection in immunized chicks was less than resistance in chicks challenged at 21 days. These results in total suggest that, when birds are challenged after 21 days, cohorts are protected from detrimental effects of challenge infection. However, when challenge infection is given at 14 days, cohorts are not well protected. The results support a conclusion that protection to coccidiosis is conveyed to cohorts by contact with oocysts shed into the litter by immunized chicks, but this resistance may take 14 days to develop.
Protección en pollos de engorde alojados con cohortes inmunizados contra Eimeria maxima y E. acervulina.
El uso de vacunas con ooquistes vivos es cada vez más importante en el control de la coccidiosis aviar en pollos de engorde. El conocimiento de los mecanismos empleados cuando los pollos ingieren los ooquistes y adquieren inmunidad es importante para optimizar los métodos de aplicación de vacunas vivas. El presente estudio pone a prueba la hipótesis de que pollos no inmunizados previamente pueden ingerir ooquistes mediante el contacto con cama con ooquistes eliminados por pollos cohortes inmunizados. En el experimento 1, pollos de engorde de un día fueron alojados en corrales que contenían cama limpia. En el Ensayo 1, el 100% de los pollos en algunos de los corrales fueron inmunizados con 2.5 × 103 ooquistes de Eimeria acervulina, mientras que en otros corrales se inmunizó sólo al 75% de los pollos y el resto de cohortes dentro de los corrales no fueron inmunizados. Otros corrales contenían pollos que sirvieron como controles no inmunizados y no desafiados, o como controles no inmuniz
