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A series of studies were conducted to assess the drug sensitivity of 26 coccidial field isolates to the anticoccidial effects of nicarbazin (NIC) and narasin + NIC (NAR + NIC). Isolates were collected from typical broiler farms in the United States from 2003 to 2006, propagated once in the absence of anticoccidial medication, and then used to inoculate broilers that were fed nonmedicated rations or those containing NIC 125 ppm or NAR + NIC 80 ppm. Results of these sensitivity trials indicated that 81% of these coccidial isolates were sensitive to the effects of NIC, but only 22% of these coccidia were controlled by NAR + NIC. Studies conducted to evaluate performance responses to these drugs demonstrated that birds fed NIC gained more weight and utilized feed more efficiently than those receiving NAR + NIC. The results of 2 floor pen tests, conducted to confirm the results of the above sensitivity trials, demonstrated that NIC provided a greater level of protection from coccidiosis than NAR + NIC. Lower lesion scores and improved performance were recorded for birds receiving NIC compared with NAR + NIC. Results of these studies revealed that changes in the susceptibility of Eimeria spp. to the activity of NAR + NIC are evident. These changes appear to be associated with the reduction in ionophore sensitivity that has been documented in most areas of the world.

Recent state and federal legislative actions and current recommendations from the World Health Organization seem to suggest that, when it comes to antimicrobial stewardship, use of antimicrobials for prevention, control, or treatment of disease can be ranked in order of appropriateness, which in turn has led, in some instances, to attempts to limit or specifically oppose the routine use of medically important antimicrobials for prevention of disease. In contrast, the AVMA Committee on Antimicrobials believes that attempts to evaluate the degree of antimicrobial stewardship on the basis of therapeutic intent are misguided and that use of antimicrobials for prevention, control, or treatment of disease may comply with the principles of antimicrobial stewardship. It is important that veterinarians and animal caretakers are clear about the reason they may be administering antimicrobials to animals in their care. Concise definitions of prevention, control, and treatment of individuals and populations are necessary to avoid confusion and to help veterinarians clearly communicate their intentions when prescribing or recommending antimicrobial use.

Experiments were conducted to investigate further an interrelationship between dietary composition and response of chicks to monensin. In three experiments with broiler chicks where a monensin level of 100 or 100 and 120 ppm was fed, no significant (P>.05) growth depression was observed regardless of dietary composition. In other experiments, 140 or 160 ppm monensin significantly depressed growth rate, and the depression with 160 ppm was significantly greater when the diets contained either 20% fish meal or poultry by-product meal than when a diet with no animal protein was fed. In another experiment chicks were fed to 4 weeks either a cornsoy diet (CS) or an isocaloric diet with 20% fish meal (FM) containing concentrations of either 0, 120, or 160 mg monensin/kg and either 0 or .3% added K (as carbonate). Growth rate of chicks fed the FM diet with the highest monensin level was significantly depressed below all other treatments, but adding K counteracted the growth depression. Another factorial experiment (2 X 2X2X3) was conducted comparing the CS and FM diets with monensin concentrations of 0, 120, and 160 mg/kg, added K concentrations of 0 and .3%, and dietary sodium levels of .18 and .28%. With respect to main effects on growth rate, monensin significantly depressed it and potassium increased it, but no significant effects were observed with diet or Na supplementation. A significant interaction between monensin and K was observed. Similar effects were observed with feed/gain ratios except that a significant diet times K interaction was obtained. These results suggest that the growth depression sometimes observed in commercial broilers fed diets containing monensin may be eliminated by dietary supplementation with potassium carbonate or other changes in ration formulation. (