Effect of diclazuril (Clinacox) on the development of protective immunity against Eimeria tenella: laboratory trial in broiler chickens.

The effect of diclazuril, fed to chickens at 1 ppm in the feed, was studied to determine whether the drug interfered with the development of immunity to Eimeria tenella. Group A was not treated, Groups B and C received diclazuril from Day 1 until Day 15, after which time the medicated feed was replaced by blank feed for the remainder of the experiment from Day 16 until Day 42. Immunization was performed in Groups A and B by artificial trickle infections with 2,000 sporulated oocysts per bird on Days 4, 6, 8, 11, and 13. On Day 29, a challenge infection was given using 200,000 oocysts per bird. The unmedicated birds (Group A) developed subclinical coccidiosis after the trickle infections with excretion of oocysts and a slightly decreased growth performance. At challenge, a good protective unimmunity was present, reflected by a good growth performance and a low oocyst excretion. The unimmunized birds (Group C) developed a severe clinical disease after challenge with high oocyst output, increased mortality, and poor growth performance. The diclazuril-medicated, trickle-infected birds (Group B) were well protected, both against the immunizing trickle infections and the challenge infection. After challenge, no clinical disease developed, although some lesions and oocyst excretion were present. It is concluded that, under the conditions of the trial, diclazuril did not significantly interfere with protective immunity formation against E. tenella.