The effect of some anticoccidial drugs on the development of immunity to coccidiosis in field and laboratory conditions

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Abstract
In a laboratory trial, 2 wk old chickens were given immunizing doses of 6 Eimeria spp. and the effect of medication (monensin or arprinocid) for 0-10 wk was determined by challenge inoculation. Monensin in the food at 60-100 ppm resulted in depression of immunity but concentrations of 40-50 ppm only slightly affected immunity to challenge inoculation. Arprinocid at 50 ppm, but not at 20 ppm, greatly reduced the development of resistance to infection. In a field experiment where chickens were exposed only to naturally occurring coccidial populations, the feeding of monensin (45 ppm) or dinitolmide (125 ppm) for 14 wk did not affect the development of immunity to the indigenous coccidial populations. Monensin-treated birds appeared to be slightly better protected. Chickens treated with monensin reached their target body weight earlier and maintained it better than dinitolmide-treated chickens. Large numbers of oocysts were present in the litter of birds treated with dinitolmide. Oocyst numbers reached a peak at 3 wk and remained high until the birds were 10 wk of age. Treatment with monensin at 45 ppm resulted in low oocyst counts in the litter until skip-a-day treatment was initiated. The inclusion of monensin at 40-50 ppm in the food may be suitable for the rearing of replacement chickens.