Effects of Monensin-Feeding Regimens on Flock Immunity to Coccidiosis

ABSTRACT Feed medication with monensin caused delays in development of immunity in two floor-pen experiments which simulated commercial broiler production. Development of immunity was retarded with the higher level of monensin (120 p.p.m.) but was progressively less delayed as the monensin level was decreased (100, 60 or 0 p.p.m.). Delay was greatest with Eimeria tenella, but also occured with intestinal species including E. acervulina, E. brunetti, E. maxima, E. mivati, and E. necatrix. Drug withdrawal permitted earlier development of immunity. Plans involving use of monensin on layer replacements which later will be maintained on the floor without medication should consider: 1) prevalence of infective oocyst exposure in the area, 2) the lowest possible level of drug required for protection, and 3) its use for the shortest possible period of time. POULTRY SCIENCE 56: 66-71, 1977 ALTHOUGH new preventive anticocci-dials are first introduced to protect meat strains of birds for short growing periods, the question inevitably arises as to effects the drug may have on development of immu-nity in layer or breeder replacements. Since any anticoccidial may affect cycling of the parasite, its use may retard development of flock immunity (Reid et al, 1960, 1968). Can monensin, which is now established as a broiler anticoccidial, provide protection for growing birds and yet permit immunity to develop before the drug is withdrawn or egg production is started? Callender and Shumard (1973) concluded that birds develop immunity earlier when lower levels of monensin (60 or 100 p.p.m.) were fed than with a higher level (120 p.p.m.). This immunity developed more rapidly as measured by severity of lesions after chal-lenge if flocks were exposed to "direct" litter-seeding than if by "indirect " exposure (terminology of Callender and Tonkinson, 1971). Their experimental design did not in